Search

Although widely studied experimentally in the 1990s, the structure and properties of low-dimensional or high-pressure phases of fullerenes have recently been re-examined. Remarkably, recent experiments have shown that transparent, nearly pure amorphous sp3-bonded carbon phases can be obtained by heating a C60 molecular crystal at a high pressure. With the additional aim of testing the ability of three classical carbon potentials reactive empirical bond order, environment-dependent interatomic potential, and reactive force-field to reproduce these results, we investigate the details of the structural transformations undergone by fullerene crystals over a wide range of pressures and temperatures. All the potentials tested show that the initial polymerization of fullerenes is accompanied by negative thermal expansion, albeit in slightly different ranges. However, more significant differences in structural and mechanical properties are observed in the amorphous phases, in particular the sp3 carbon fraction and the existence of layered amorphous carbon. Overall, these results indicate to which extent classical reactive potentials can be used to explore phase transitions over a wide range of pressures and temperatures. [ABSTRACT FROM AUTHOR]

We explore the phase behaviors of simple molecular crystals in order to investigate the molecular basis of the stability mechanism relative to their liquid counterparts. The free energies of the face centered cubic crystals of Xe and CO2 are calculated as a collection of oscillators, and those of the liquids are from an equation of state via molecular dynamics simulations. The vibrational free energy in the solid is separated into the harmonic and anharmonic terms. The harmonic free energies decrease harshly with the expansion of the volume manifested as the large positive Grüneisen parameters, but the anharmonic free energies are positive and increase with volume, both of which originate from the deviation of the potential surface from the parabolic curve. The anharmonic free energies, though less significant in magnitude and destabilize the solids thermodynamically, serve to enhance their mechanical stability. The solid–liquid phase boundaries cannot be settled correctly without the exquisite balance between the two opposing contributions. A sharp contrast regarding the solid free energy is found in low-pressure ice, where the harmonic free energy does not decrease monotonically with volume and its anharmonic free energy is negative. [ABSTRACT FROM AUTHOR]

The probability of finding a spherical "hole" of a given radius r contains crucial structural information about many-body systems. Such hole statistics, including the void conditional nearest-neighbor probability functions GV(r), have been well studied for hard-sphere fluids in d-dimensional Euclidean space R d . However, little is known about these functions for hard-sphere crystals for values of r beyond the hard-sphere diameter, as large holes are extremely rare in crystal phases. To overcome these computational challenges, we introduce a biased-sampling scheme that accurately determines hole statistics for equilibrium hard spheres on ranges of r that far extend those that could be previously explored. We discover that GV(r) in crystal and hexatic states exhibits oscillations whose amplitudes increase rapidly with the packing fraction, which stands in contrast to GV(r) that monotonically increases with r for fluid states. The oscillations in GV(r) for 2D crystals are strongly correlated with the local orientational order metric in the vicinity of the holes, and variations in GV(r) for 3D states indicate a transition between tetrahedral and octahedral holes, demonstrating the power of GV(r) as a probe of local coordination geometry. To further study the statistics of interparticle spacing in hard-sphere systems, we compute the local packing fraction distribution f(ϕl) of Delaunay cells and find that, for d ≤ 3, the excess kurtosis of f(ϕl) switches sign at a certain transitional global packing fraction. Our accurate methods to access hole statistics in hard-sphere crystals at the challenging intermediate length scales reported here can be applied to understand the important problem of solvation and hydrophobicity in water at such length scales. [ABSTRACT FROM AUTHOR]

We have applied positron annihilation spectroscopy to study the vacancy-type defects in β-Ga2O3 single crystals. The three different types of crystals were prepared by Czochralski and edge-defined film-fed growth and doped with Fe, Mg, and Sn for semi-insulating and n-type characteristics. The crystals were also subjected to 6-MeV proton irradiation for controlled introduction of mono-vacancy defects. Positron lifetime and the details of the anisotropy of the Doppler broadening signals were measured as a function of temperature, and the results were compared with the annihilation signals predicted by theoretical calculations. We find Ga vacancies in all three basic split Ga vacancy configurations to dominate the positron data in the as-grown crystals. In contrast, unrelaxed Ga vacancies are found as the main defect introduced by the irradiation. [ABSTRACT FROM AUTHOR]

We have applied positron annihilation spectroscopy to study the vacancy-type defects in β-Ga2O3 single crystals. The three different types of crystals were prepared by Czochralski and edge-defined film-fed growth and doped with Fe, Mg, and Sn for semi-insulating and n-type characteristics. The crystals were also subjected to 6-MeV proton irradiation for controlled introduction of mono-vacancy defects. Positron lifetime and the details of the anisotropy of the Doppler broadening signals were measured as a function of temperature, and the results were compared with the annihilation signals predicted by theoretical calculations. We find Ga vacancies in all three basic split Ga vacancy configurations to dominate the positron data in the as-grown crystals. In contrast, unrelaxed Ga vacancies are found as the main defect introduced by the irradiation. [ABSTRACT FROM AUTHOR]

Colloidal and nanoparticle self-assembly enables the creation of ordered structures with a variety of electronic and photonic functionalities. The outcomes of the self-assembly processes used to synthesize such structures, however, strongly depend on the uniformity of the individual nanoparticles. Here, we explore the simplest form of particle size dispersity—bidispersity—and its impact on the self-assembly process. We investigate the robustness of self-assembling bcc-type crystals via isotropic interaction potentials in binary systems with increasingly disparate particle sizes by determining their terminal size ratio—the most extreme size ratio at which a mixed binary bcc crystal forms. Our findings show that two-well pair potentials produce bcc crystals that are more robust with respect to particle size ratio than one-well pair potentials. This suggests that an improved self-assembly process is accomplished with a second attractive length scale encoded in the particle–particle interaction, which stabilizes the second-nearest neighbor shell. In addition, we document qualitative differences in the process of ordering and disordering: in bidisperse systems of particles interacting via one-well potentials, we observe a breakdown of order prior to demixing, while in systems interacting via two-well potentials, demixing occurs first and bcc continues to form in parts of the droplet down to low size ratios. [ABSTRACT FROM AUTHOR]

Force Field X (FFX) is an open-source software package for atomic resolution modeling of genetic variants and organic crystals that leverages advanced potential energy functions and experimental data. FFX currently consists of nine modular packages with novel algorithms that include global optimization via a many-body expansion, acid–base chemistry using polarizable constant-pH molecular dynamics, estimation of free energy differences, generalized Kirkwood implicit solvent models, and many more. Applications of FFX focus on the use and development of a crystal structure prediction pipeline, biomolecular structure refinement against experimental datasets, and estimation of the thermodynamic effects of genetic variants on both proteins and nucleic acids. The use of Parallel Java and OpenMM combines to offer shared memory, message passing, and graphics processing unit parallelization for high performance simulations. Overall, the FFX platform serves as a computational microscope to study systems ranging from organic crystals to solvated biomolecular systems. [ABSTRACT FROM AUTHOR]

The development of new materials typically involves a process of trial and error, guided by insights from past experimental and theoretical findings. The inverse design approach for soft-matter systems has the potential to optimize specific physical parameters, such as particle interactions, particle shape, or composition and packing fraction. This optimization aims to facilitate the spontaneous formation of specific target structures through self-assembly. In this study, we expand upon a recently introduced inverse design protocol for monodisperse systems to identify the required conditions and interactions for assembling crystal and quasicrystal phases within a binary mixture of two distinct species. This method utilizes an evolution algorithm to identify the optimal state point and interaction parameters, enabling the self-assembly of the desired structure. In addition, we employ a convolutional neural network (CNN) that classifies different phases based on their diffraction patterns, serving as a fitness function for the desired structure. Using our protocol, we successfully inverse design two-dimensional crystalline structures, including a hexagonal lattice and a dodecagonal quasicrystal, within a non-additive binary mixture of hard disks. Finally, we introduce a symmetry-based order parameter that leverages the encoded symmetry within the diffraction pattern. This order parameter circumvents the need for training a CNN and is used as a fitness function to inverse design an octagonal quasicrystal. [ABSTRACT FROM AUTHOR]

Band structure and Dirac degeneracy are essential features of sonic crystals/acoustic metamaterials to achieve advanced control of exciting wave effects. In this work, we explore a deep learning approach for the design of phononic crystals with desired dispersion. A plane wave expansion method is utilized to establish the dataset relation between the structural parameters and the energy band features. Subsequently, a multilayer perceptron model trained using the dataset can yield accurate predictions of wave behavior. Based on the trained model, we further impose a re-learning process around a targeted frequency, by which Dirac degeneracy and double Dirac degeneracy can be embedded into the band structures. Our study enables the deep learning approach as a reliable design strategy for Dirac structures/metamaterials, opening up the possibilities for intriguing wave physics associated with Dirac cone. [ABSTRACT FROM AUTHOR]

Crystal surfaces play a pivotal role in governing various significant processes, such as adsorption, nucleation, wetting, friction, and wear. A fundamental property that influences these processes is the surface free energy, γ. We have directly calculated γ(T) for low-index faces of Lennard-Jones (LJ), germanium, and silicon crystals along their sublimation lines using the computational cleavage technique. Our calculations agree well with experimental values for Si(111) and Ge(111), highlighting the accuracy of the method and models used. For LJ crystals, we identified a premelting onset at Tpm = 0.75Tm, marked by a sharp increase in atom mobility within the second outermost surface layer. Notably, Tpm closely aligned with the endpoint of the LJ melting line at negative pressures, Tend = 0.76Tm. We hypothesize that the emergence and coexistence of a liquid film atop the LJ crystal at Tpm < T < Tm correspond to the metastable melting line under negative pressures experienced by stretched crystal surfaces. Furthermore, our study of thin LJ crystal slabs reveals that premelting-induced failure leads to recrystallization below the homogeneous freezing limit, offering a promising avenue to explore crystal nucleation and growth at extremely deep supercoolings. Finally, no evidence of premelting was detected in the model crystals of Ge and Si, which is consistent with the experimental observations. Overall, our findings offer valuable insights into crystal surface phenomena at the atomic scale. [ABSTRACT FROM AUTHOR]

One method for computationally determining phase boundaries is to explicitly simulate a direct coexistence between the two phases of interest. Although this approach works very well for fluid–fluid coexistences, it is often considered to be less useful for fluid–crystal transitions, as additional care must be taken to prevent the simulation boundaries from imposing unwanted strains on the crystal phase. Here, we present a simple adaptation to the direct coexistence method that nonetheless allows us to obtain highly accurate predictions of fluid–crystal coexistence conditions, assuming that a fluid–crystal interface can be readily simulated. We test our approach on hard spheres, the screened Coulomb potential, and a 2D patchy-particle model. In all cases, we find excellent agreement between the direct coexistence approach and (much more cumbersome) free-energy calculation methods. Moreover, the method is sufficiently accurate to resolve the (tiny) free-energy difference between the face-centered cubic and hexagonally close-packed crystal of hard spheres in the thermodynamic limit. The simplicity of this method also ensures that it can be trivially implemented in essentially any simulation method or package. Hence, this approach provides an excellent alternative to free-energy based methods for the precise determination of phase boundaries. [ABSTRACT FROM AUTHOR]

Lennard-Jones potential parameters have been determined for the interlayered interaction in 12 elemental layered crystals. The layered crystals include corrugated borophene, buckled honeycomb structures with elements from the group IV (Si, Ge, and Sn) and group V (P, As, Sb, and Bi), as well as four puckered structures in the aw-phase (Sb and Bi) and sw-phase (P and As). We first calculated the minimal interfacial energy and the interlayer equilibrium distance for two typical shifted stackings, AA and AB, for every bilayer crystal using the density functional theory with a long-range dispersion correction. Then, we use the Lennard-Jones potential and the corresponding derivative to graphically match the calculated adhesion energy and the zero resultant force along the out-of-plane direction, determining the energy parameter and the size parameter for both stackings. The Lennard-Jones model with the obtained parameters can be in good agreement with the first-principles calculations for the AA and AB stackings in describing the relationship between the interfacial energy and the interlayer distance, and the Lennard-Jones parameters for the other stackings are directly determined according to Lorentz–Berthelot combining rules. The parameters can be applied in molecular dynamics simulations for the interfacial effect of elemental layered materials such as mechanical peeling and interlayer friction, as well as stacking formation, and the analytical method can be extended to determine the parameters of the Lennard-Jones potential for the compound layered crystals and the interlayer heterojunction structures. [ABSTRACT FROM AUTHOR]

Over 2300 years ago, the discovery of tourmaline led to the understanding of pyroelectric properties, which opened new doors to various applications of pyroelectric crystal, such as neutron and x-ray generation, energy harvesting, mass spectrometry, high-voltage sources, and more. In the last two decades, researchers have carried out extensive research and development to select components and materials and innovate the design and construction of the pyroelectric neutron generator (PNG). This manuscript investigates the process and history of the PNG's development. It explains the physics governing pyroelectric crystals and the method of producing neutrons in a comprehensive and straightforward manner. Although PNGs have a lower yield and shorter lifetime compared to other neutron generators, they are still significant for research purposes due to their lack of need for an external high-voltage power supply, lower cost, smaller size, and safety. The main objective of this manuscript is to bring more attention to the research and development of PNGs. In recent years, new methods have been introduced that reduce the amount of neutron flux required for various applications. This has raised hope for the progress of commercial and industrial use of PNGs in the near future. The manuscript mentions some research cases that represent the future perspective of PNG development. Furthermore, the challenges faced by PNGs can be handled more efficiently with the utilization of generative learning algorithms and improvements in the components/mechanisms used for PNG design. [ABSTRACT FROM AUTHOR]

Achieving the formation of target open crystalline lattices from colloidal particles is of paramount importance for their potential application in photonics. Examples of such desired structures are the diamond, tetrastack, and pyrochlore lattices. Here, we demonstrate that the self-assembly of tetravalent patchy particles results in the selective formation of cubic tetrastack crystals, both in the bulk and in the systems subjected to external fields exerted by the solid substrate. It is demonstrated that the presence of an external field allows for the formation of well-defined single crystals with a low density of defects. Moreover, depending on the strength of the applied external field, the mechanism of epitaxial growth changes. For weakly attractive external fields, the crystallization occurs in a similar manner as in the bulk, since the fluid does not wet the substrate. Nonetheless, the formed crystal is considerably better ordered than the crystals formed in bulk, since the surface induces the ordering in the first layer. On the other hand, it is demonstrated that the formation of well-ordered cubic tetrastack crystals is considerably enhanced by the increase in external field strength, and the formation of the thick crystalline film occurs via a series of layering transitions. [ABSTRACT FROM AUTHOR]

Unscreened (ungated) plasmons in large-area grating-gate AlGaN/GaN heterostructures were studied experimentally by Fourier-transform spectroscopy. Special attention was paid to the recently discovered THz plasmonic crystal modes observed at totally depleted gated regions when plasma oscillations were localized only in ungated parts of the grating-gate structures. The frequency of these modes is still gate voltage-dependent in the limited range due to the depletion of the ungated parts located close to the gate edges. Double gate structures with an additional bottom gate were fabricated and studied to improve the gate voltage tunability of the unscreened plasmons. Since this gate is located deep below the channel, the plasmons behaved as ungated ones, but their frequency still could be tuned by this bottom gate. We show that the combined effect of the top and bottom gates allows the efficient tuning of terahertz frequencies of unscreened modes in the grating-gate AlGaN/GaN plasmonic crystals. [ABSTRACT FROM AUTHOR]

III-nitrides have fomented a revolution in the lighting industry and are poised to make a huge impact in the field of power electronics. In the III-nitride family, the crystal growth and use of hexagonal BN (h-BN) as an ultrawide bandgap (UWBG) semiconductor are much less developed. Bulk crystals of h-BN produced by the high-temperature/high-pressure and the metal flux solution methods possess very high crystalline and optical qualities but are impractical to serve as substrates or for device implementation as their sizes are typically in millimeters. The development of crystal growth technologies for producing thick epitaxial films (or quasi-bulk or semi-bulk crystals) in large wafer sizes with high crystalline quality is a prerequisite for utilizing h-BN as an UWBG electronic material. Compared to traditional III-nitrides, BN has another unique application as solid-state neutron detectors, which however, also require the development of quasi-bulk crystals to provide high detection efficiencies because the theoretical efficiency (ηi) relates to the detector thickness (d) by η i = 1 − e − d λ , where λ denotes the thermal neutron absorption length which is 47 μm (237 μm) for 10B-enriched (natural) h-BN. We provide an overview and recent progress toward the development of h-BN quasi-bulk crystals via hydride vapor phase epitaxy (HVPE) growth and the attainment of thermal neutron detectors based on 100 μm thick 10B-enriched h-BN with a record efficiency of 60%. The thermal neutron detection efficiency was shown to enhance at elevated temperatures. Benchmarking the crystalline and optical qualities of h-BN quasi-bulk crystals with the state-of-the-art mm-sized bulk crystal flakes and 0.5 μm thick epitaxial films identified that reducing the density of native defects such as vacancies remains the most critical task for h-BN quasi-bulk crystal growth by HVPE. [ABSTRACT FROM AUTHOR]

Since the 1940s, it has been known that diffusion in crystalline solids occurs due to lattice defects. The diffusion of defects can have a great impact on the processing and heat treatment of materials as the microstructural changes caused by diffusion can influence the material qualities and properties. It is, therefore, vital to be able to control the diffusion. This implies that we need a deep understanding of the interactions between impurities, matrix atoms, and intrinsic defects. The role of density functional theory (DFT) calculations in solid-state diffusion studies has become considerable. The main parameters to obtain in defect diffusion studies with DFT are formation energies, binding energies, and migration barriers. In particular, the utilization of the nudged elastic band and the dimer methods has improved the accuracy of these parameters. In systematic diffusion studies, the combination of experimentally obtained results and theoretical predictions can reveal information about the atomic diffusion processes. The combination of the theoretical predictions and the experimental results gives a unique opportunity to compare parameters found from the different methods and gain knowledge about atomic migration. In this Perspective paper, we present case studies on defect diffusion in wide bandgap semiconductors. The case studies cover examples from the three diffusion models: free diffusion, trap-limited diffusion, and reaction diffusion. We focus on the role of DFT in these studies combined with results obtained with the experimental techniques secondary ion mass spectrometry and deep-level transient spectroscopy combined with diffusion simulations. [ABSTRACT FROM AUTHOR]

Many solid crystals exhibit a structural phase transition where a subset of its ions or entire molecules become orientationally ordered. As to why such ordering occurs remains mostly unresolved. We consider the extremely weak magnetic elements arising from the reorientations of the molecules experiencing mutual resonance to play the chief role. Two new features are identified in d-camphor: (1) the magnetic susceptibility abruptly changes when crossing the order–disorder phase transition at TII–III = 239.8 K during cooling and at TIII–II = 245.2 K during warming and (2) the complex dielectric constant exhibits two successive discontinuities only 0.2 K apart near the critical temperatures when the sweeping rate is only 0.1 K/min. We discuss how the change in entropy associated with order–disorder transitions in plastic crystals represents temporal changes rather than spatial changes in the system. Our findings may be extended to study why many other crystalline solids exhibit orientational ordering and irreversibility. [ABSTRACT FROM AUTHOR]

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert Se P + donors to Se P 0 donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior. [ABSTRACT FROM AUTHOR]

A hybrid quantum mechanics/molecular mechanics setup was used to model electronically excited pentacene in the crystal phase. Particularly interesting in the context of singlet fission (SF) is the energetic location of the antiferromagnetically coupled multiexcitonic singlet state, 1(TT), and the ferromagnetically coupled analog in relation to the optically bright singlet state. To provide photophysical properties of the accessible spin manifold, combined density functional theory and multi-reference configuration interaction calculations were performed on pentacene dimers and a trimer, electrostatically embedded in the crystal. The likelihood of a quintet intermediate in the SF process was estimated by computing singlet–quintet electron spin–spin couplings employing the Breit–Pauli Hamiltonian. The performance of the applied methods was assessed on the pentacene monomer. The character of the optically bright state and the energetic location of the 1(TT) state depend strongly on the relative orientation of the pentacene units. In the V-shaped dimers and in the trimer, the optically bright state is dominated by local and charge transfer (CT) excitations, with admixtures of doubly excited configurations. The CT excitations gain weight upon geometry relaxation, thus supporting a CT-mediated SF mechanism as the primary step of the SF process. For the slip-stacked dimer, the energetic order of the bright and the 1(TT) states swaps upon geometry relaxation, indicating strong nonadiabatic coupling close to the Franck–Condon region—a prerequisite for a coherent SF process. The multiexcitonic singlet, triplet, and quintet states are energetically too far apart and their spin–spin couplings are too small to bring about a noteworthy multiplicity mixing. [ABSTRACT FROM AUTHOR]

The trap centers have a significant impact on the electronic properties of lead tungstate (PbWO4), suggesting their crucial role in optoelectronic applications. In the present study, we investigated and revealed the presence of shallow trap centers in PbWO4 crystals through the utilization of the thermally stimulated current (TSC) method. TSC experiments were performed in the 10–280 K range by applying a constant heating rate. The TSC spectrum showed the presence of a total of four peaks, two of which were overlapped. As a result of analyzing the TSC spectrum using the curve fit method, the activation energies of revealed centers were found as 0.03, 0.11, 0.16, and 0.35 eV. The trapping centers were associated with hole centers according to the comparison of TSC peak intensities recorded by illuminating the opposite polarity contacts. Our findings not only contribute to the fundamental understanding of the charge transport mechanisms in PbWO4 crystals but also hold great promise for enhancing their optoelectronic device performance. The identification and characterization of these shallow trap centers provide valuable insights for optimizing the design and fabrication of future optoelectronic devices based on PbWO4. [ABSTRACT FROM AUTHOR]

We explore piezooptic and photoelastic properties of Ca3Ga2Ge4O14 (CGG) crystals. A Mach–Zehnder interferometry technique is utilized to determine a complete set of the piezooptic tensor constants πim, with particular attention to the measurement accuracy, reliability, and uncertainty resolution in the piezooptic measurements. For these reasons, independent piezooptic constants or their combinations were determined by performing interferometric measurements for different geometries of piezooptic coupling using samples of both principal (direct) and non-principal (X/45°) crystallographic cuts. Basing on the results of piezooptic measurements, a complete set of photoelastic tensor constants pim and the acousto-optic performance (figure of merit M2) of CGG crystals were evaluated. The obtained piezooptic, photoelastic, and acousto-optic characteristics of CGG crystals are compared with those of other crystals of the langasite group, langasite and CTGS (Ca3TaGa3Si2O14). A comparative analysis reveals a correlation between the values of the piezooptic or photoelastic constants and the lattice parameters for this group of crystals. High optical quality and optical transparency in the UV region of the spectrum up to 260 nm together with extremely high mechanical resistance to uniaxial compression (≤2000 kg/cm2) opens up the possibility for specific application of CGG crystal as an optical material for piezooptic sensors of high uniaxial compression. The presented piezooptic interference technique along with methodological and metrological approaches paves the way for accurate and unambiguous characterization of photoelastic crystal materials of trigonal symmetry. [ABSTRACT FROM AUTHOR]

In this study, the basic properties of Ba2AgSi3 were investigated in detail from both experimental and computational viewpoints. Polycrystalline Ba2AgSi3 formed by an arc-melting apparatus under an argon atmosphere was ground into powders, and then powder samples were sintered using the spark plasma sintering method. Both n-type and p-type samples were obtained. This may be due to a slight deviation from the stoichiometric composition. The energy bandgap of Ba2AgSi3 was measured to be around 0.17 eV from the temperature dependence of electrical conductivity and was in agreement with that by first-principles calculations. Sintered samples exhibited a high Seebeck coefficient of −273 μV K−1 and a high power factor of 0.38 mW m−1 K−2 at 307 K for n-type samples. They were 217 μV K−1 and 0.23 mW m−1 K−2, respectively, at 320 K for p-type samples. The electronic structures of impurity-doped Ba2AgSi3 were also discussed using first-principles calculations to investigate the insertion site of impurity atoms. The calculations suggest that the substitution of B (P) at any Si site shifts the Fermi level and transforms it into p-type (n-type) semiconductors. On the other hand, substitution of Ba or Ag sites with B or P is unlikely to occur in terms of formation energy. [ABSTRACT FROM AUTHOR]

When a cubic lattice is confined by a surface layer, the effective thermoelastic properties can be tailored by the prestress produced by the surface. The thermal expansion coefficient, temperature derivative of elasticity, and the equation of state (EOS) of the solid depend on the potential of each bond and the lattice structure, which can be predicted by the recently developed singum model. This paper first uses a granular lattice confined by a spherical shell to demonstrate singum modeling of the thermoelastic behavior of the cubic lattices and then extends it to atomic crystal lattices by considering the surface tension and long-range interactions. Given the elasticity and the EOS of a cubic crystal, the interatomic potential can be inversely derived. As the bond length changes with thermal expansion and pressure, the singum model predicts the temperature- and pressure-dependent elasticity. Using the orientational average, isotropic elastic constants can be obtained for polycrystals. The case study of copper (Cu) demonstrates the versatility of the model for different cubic lattices and predicts the experimental results of pressure- and temperature-dependent elasticity. The singum model is general for different lattice types and EOS forms and provides clear physical and mechanical meanings to correlate the interatomic potential, EOS, and elasticity in the closed-form formulation, which is very useful in engineering design and analysis of metal structural members in fire, geothermal, and space applications without the needs of large-scale numerical simulations. [ABSTRACT FROM AUTHOR]

TiO2 and SrTiO3 are stable and efficient materials for photoelectrochemical (PEC) water splitting. PEC activity is influenced by carrier recombination in crystals of these materials. In this study, we analyzed the effect of dislocations on carrier recombination in TiO2 and SrTiO3 using microwave photoconductive decay measurements on polished and unpolished faces. The apparent slow decay in the unpolished face implies that dislocations trap minority carriers. Based on the different dependences of the injected photon density and temperature, the recombination processes in the polished and unpolished faces differ. A high concentration of oxygen vacancies or hydroxyl in the unpolished face of SrTiO3 was observed by x-ray photoelectron spectroscopy, and dislocations in the unpolished faces of both materials were observed using transmission electron microscopy. Additionally, we found that the photocurrent duration in the unpolished faces was shorter than that in the polished faces for both TiO2 and SrTiO3, confirming that dislocations inhibited photoelectrochemical activity. [ABSTRACT FROM AUTHOR]

The dynamical properties of nuclei, carried by the concept of phonon quasiparticles , are central to the field of condensed matter. While the harmonic approximation can reproduce a number of properties observed in real crystals, the inclusion of anharmonicity in lattice dynamics is essential to accurately predict properties such as heat transport or thermal expansion. For highly anharmonic systems, non-perturbative approaches are needed, which result in renormalized theories of lattice dynamics. In this article, we apply the Mori–Zwanzig projector formalism to derive an exact generalized Langevin equation describing the quantum dynamics of nuclei in a crystal. By projecting this equation on quasiparticles in reciprocal space, and with results from linear response theory, we obtain a formulation of vibrational spectra that fully accounts for the anharmonicity. Using a mode-coupling approach, we construct a systematic perturbative expansion in which each new order is built to minimize the following ones. With a truncation to the lowest order, we show how to obtain a set of self-consistent equations that can describe the lineshapes of quasiparticles. The only inputs needed for the resulting set of equations are the static Kubo correlation functions, which can be computed using (fully quantum) path-integral molecular dynamics or approximated with (classical or ab initio) molecular dynamics. We illustrate the theory with an application on fcc 4He, an archetypal quantum crystal with very strong anharmonicity. [ABSTRACT FROM AUTHOR]

The potential of KD 2 PO 4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼ 200 − 300 μ J energy were converted to broadband (∼ 4 THz bandwidth) terahertz transients with the efficiency of ∼ 8 × 10 − 6 . The transients produced electro-optic signals of a ∼ 0.15 modulation depth and ∼ 10 3 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d 36 ≈ 3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d 36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r 63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation. [ABSTRACT FROM AUTHOR]

Nickel-doped β-Ga2O3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. The photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck–Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/−) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for NiGa, which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of ∼2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating β-Ga2O3 substrates due to the position of the acceptor level (below and close to the mid-bandgap). [ABSTRACT FROM AUTHOR]

When an impurity with a light mass is inserted into a crystal, it can undergo a high-frequency oscillation referred to as a local vibrational mode (LVM). A Fermi resonance may occur between the LVM and lower-frequency modes of the defect. The LVM may also interact with phonons or the electromagnetic field. Understanding these interactions can help model and control diffusion, defect reactions, and thermal conductivity. LVMs have been probed in semiconductors using pressure and alloying as experimental parameters, resulting in anticrossing between localized and extended vibrational modes. These types of vibrational interactions could play an important role in the stability and thermal properties of organic–inorganic hybrid semiconductors. The coupling between an LVM and electromagnetic wave yields an "LVM polariton," an excitation that has significant vibrational and electric-field amplitudes. [ABSTRACT FROM AUTHOR]

We report the results of measurements of thermal conductivity coefficient dependence on temperature of single crystals of SrIr4In2Ge4 and EuIr4In2Ge4. The measurements were carried out over the temperature range of ∼5–300 K. The EuIr4In2Ge4 crystal, unlike its strontium analog SrIr4In2Ge4, shows an amazing anisotropy: At low temperatures, it displays significantly smaller thermal conductivity in the ab plane than in the direction of c axis, while at the high ones the thermal conductivity in the direction perpendicular to the c axis increases well above that of in the c axis. The observed phenomena may be a result of the interaction of phonons with 1D chains of short-range ordered magnetic moment of europium atoms and the exchange energy between the chains in the paramagnetic phase of EuIr4In2Ge4. [ABSTRACT FROM AUTHOR]

Brillouin light scattering is a versatile measurement technique of the dispersion of bulk acoustic phonons in amorphous and crystalline solids. It allows contactless and non-destructive characterization of the relevant material tensors of optically transparent materials, provided that the optical refraction indices, and in addition the dielectric tensor for piezoelectric materials, are known beforehand. The complete estimation of the anisotropic material tensors is often performed using many different crystal orientations, and hence different samples of the same crystal. In this paper, we consider the problem of measuring those tensors using a single sample of an anisotropic single crystal with trigonal symmetry. A complete measurement requires sufficient experimental diversity in the phonon wavevectors when sampling the phonon velocity surfaces, hence the consideration of a large number of directions away from crystallographic axes. We estimate all six independent elastic constants of non-piezoelectric sapphire ( 3 ¯ m point group) with a single X-cut wafer and the six independent elastic constants together with the four independent piezoelectric constants of piezoelectric lithium niobate (3 m point group) with a single Y-cut wafer. The estimated tensors are in close agreement with those reported in the literature based on resonant ultrasonic techniques. [ABSTRACT FROM AUTHOR]

The combination of optical and mechanical properties of calcium lanthanum sulfide (CLS) makes it an attractive material for optical applications. CLS is a γ -phase La2S3/CaS solid solution above 50:50 La2S3/CaS. Experiments have characterized the effect of composition on the structural, thermal, and optical properties. However, the effect of impurities and other defects, such as porosity, on resulting properties is difficult to deconvolute. We used the density functional theory (DFT) simulations to characterize the effect of La2S3/CaS relative fractions on structural, electronic, mechanical, and vibrational properties of single crystalline CLS solid solutions. We find a decrease in the stiffness with CaS fraction and confirm the experimentally observed increase in the lattice parameter with La2S3 fraction. We also characterized the point defects, including sulfur vacancies, oxygen substituting sulfur, oxygen, and sulfur impurities on lanthanum sublattice vacant sites, and sulfur substituting calcium (vS, OS, OLa, and SLa, respectively). Our defect studies in 90:10 CLS find the neutral charge state configuration to be energetically favorable for all tested configurations except for the sulfur vacancies (vS). [ABSTRACT FROM AUTHOR]

The material dependence of phonon-polariton-based in-plane thermal conductance is investigated by examining systems composed of air and several wurtzite and zinc-blende crystals. Phonon-polariton-based thermal conductance varies by over an order of magnitude (∼ 0.5 – 60 nW/K), which is similar to the variation observed in the materials corresponding to bulk thermal conductivity. Regardless of the material, phonon-polaritons exhibit similar thermal conductance to that of phonons when layers become ultrathin (∼ 10 nm), suggesting the generality of the effect at these length-scales. A figure of merit is proposed to explain the large variation of in-plane polariton thermal conductance that is composed entirely of easily predicted and measured optical phonon energies and lifetimes. Using this figure of merit, in-plane phonon-polariton thermal conductance enlarges with increases in (1) optical phonon energies, (2) splitting between transverse and longitudinal mode pairs, and (3) phonon lifetimes. [ABSTRACT FROM AUTHOR]

A review of the structural, elastic, and thermodynamic properties of cubic and hexagonal ScxAl1−xN crystals over the range of possible random alloys is provided. Based on measured and simulated lattice and internal cell parameters of NaCl (B1), CsCl (B2), and α-ZnS (B3) type cubic ScxAl1−xN lattices as well as of β-ZnS (B4), lh-MgO (Bk), and NiAs (B81) type hexagonal ScxAl1−xN crystals, their atomic positions, distances to nearest neighbor atoms, geometric dimensions of crystal cells, mass density, as well as their average bond length and bond angles are presented in dependence on the alloy composition. The understanding gained about the crystal lattices is used to provide a model for the transitions from the β-ZnS to the lh-MgO or NaCl lattice induced by the alloying of AlN with ScN. Based on published data sets of stiffness coefficients, the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and the sound velocities are presented in relation to the orientation of representative crystal planes and axes for rock salt, layered hexagonal, and wurtzite ScxAl1−xN crystals. Particular attention is paid to the directional anisotropies of elastic properties of the different crystal lattices if Sc atoms substitute an increasing number of Al atoms. Based on sound velocities determined, an overview of the fundamental thermodynamic properties of cubic and hexagonal ScxAl1−xN alloys is provided, such as the Debye temperature, heat capacity, minimum heat conduction, and melting temperature. [ABSTRACT FROM AUTHOR]

Synchrotron radiation x-ray topography (XRT) in a transmission configuration based on the Borrmann effect (BE) was carried out to observe characteristic dislocation structures and three-dimensional distribution and propagation of dislocations in β-Ga2O3 grown via the edge-defined film-fed growth (EFG) method. Substrates with a range of surface orientations of (001), (010), and (2 ¯ 01) , cut perpendicular or parallel to the ⟨010⟩ growth direction of the EFG, were observed to understand the whole picture of dislocations distributed in the bulk crystals. Using the (001)-oriented substrate, we found characteristic dislocation structures such as dislocation helices, damage-related (001)-plane dislocation networks, and tangled dislocation complexes, which exist universally in EFG crystals but have rarely been reported before. A careful measurement of the dislocation length in BE-XRT images taken with different g-vectors allows us to determine the crystal plane on which a dislocation lies. The BE-XRTs taken from the (010)-oriented and (2 ¯ 01) -oriented substrates suggested that the dislocations propagating along the [010] growth direction were dominant. Most of these b-axis threading dislocations had a Burgers vector of [010] or [001], and they tended to align in the (100) plane. The BE-XRT observations in this study provide valuable knowledge for understanding the structure and character of dislocations in β-Ga2O3. [ABSTRACT FROM AUTHOR]

The interplay between crystal nucleation and the structure of the metastable fluid has been a topic of significant debate over recent years. In particular, it has been suggested that even in simple model systems such as hard or charged colloids, crystal nucleation might be foreshadowed by significant fluctuations in local structure around the location where the nucleus first arises. We investigate this using computer simulations of spontaneous nucleation events in both hard and charged colloidal systems. To detect local structural variations, we use both standard and unsupervised machine learning methods capable of finding hidden structures in the metastable fluid phase. We track numerous nucleation events for the face-centered cubic and body-centered cubic crystals on a local level and demonstrate that all signs of crystallinity emerge simultaneously from the very start of the nucleation process. We thus conclude that we observe no precursor for the crystal nucleation of hard and charged colloids. [ABSTRACT FROM AUTHOR]

Lattice simulations are an important class of problems in crystalline solids, surface science, alloys, adsorption, absorption, separation, catalysis, to name a few. We describe a fast computational method for performing lattice thermodynamic calculations that is based on the use of the reverse Monte Carlo (RMC) technique and multiple short-range order (SRO) parameters. The approach is comparable in accuracy to the Metropolis Monte Carlo (MC) method. The equilibrium configuration is determined in 5–10 Newton–Raphson iterations by solving a system of coupled nonlinear algebraic SRO growth rate equations. This makes the RMC-based method computationally more efficient than MC, given that MC typically requires sampling of millions of configurations. The technique is applied to the interacting 2D adsorption problem. Unlike grand canonical MC, RMC is found to be adept at tackling geometric frustration, as it is able to quickly and correctly provide the ordered c(2 × 2) adlayer configuration for Cl adsorbed on a Cu (100) surface. [ABSTRACT FROM AUTHOR]

Here, we successfully synthesized a pair of enantiomeric molecular piezoelectric materials by H/F substitution strategy. These compounds show a large piezoelectric coefficient d33 value of 25 pC N−1 measured by the quasi-static method. A simple energy harvesting device was fabricated based on this crystal, showing great potential in piezoelectric mechanical energy harvesters. [ABSTRACT FROM AUTHOR]

Finding a deep-ultraviolet (deep-UV, λ < 200 nm) nonlinear optical (NLO) crystal is vital to a number of advanced optical technologies, yet it remains a great challenge. Herein, we report a deep-UV NLO crystal, K2B3O3F4(OH) (KBOFH), composed of π-conjugated confined groups. It exhibits a relatively large birefringence of 0.054@550 nm and a wide transparency window with an absorption edge below 200 nm. Theoretical calculations reveal that the observed optical properties of KBOFH originate from the confined π-conjugated [B3O3F4(OH)] unit, in which the π-conjugated interactions are decoupled by the tetrahedral non-π-conjugated [BO2F2] unit. This work provides an opportunity to design deep-UV NLO crystals containing π-conjugated confined groups. [ABSTRACT FROM AUTHOR]

We reported a novel donor–acceptor complex with near-infrared emission of 735 nm. During the hierarchical self-assembly process, the supramolecular system twisted into a micro-helix with red emission, depending on both crystal thickness and used solvent, despite the nonpolar structure. It is revealed that the uniform alignment of cyano groups along the stacking columns and the solvent effect induces the twisting. This research provides an innovative approach for designing helical organic micro/nanostructures and near-infrared emission materials. [ABSTRACT FROM AUTHOR]

This year marks the 50th anniversary of a significant milestone in organic optoelectronics, i.e., the first account of metallic-like conductivity in an entirely organic material, tetrathiafulvalene–tetracyanoquinodimethane (TTF–TCNQ). Since the discovery of this prototypical TTF–TCNQ charge-transfer (CT) pair, countless studies, ranging from various aspects, have been done on this fascinating TTF–TCNQ and related CT complexes. More recently, the CT complex-based approach has emerged as an elegant way of designing various next-generation functional organic materials, viz. room temperature phosphorescent (RTP), thermally activated delayed fluorescent (TADF), near infra infrared (NIR) emissive, organic spin-systems, etc. This highlight discusses how the synergy between computational screening and crystal engineering principles can be used to obtain the desired functional output and modulate the CT complex's bulk optical and electronic features. Emphasis is given to some of the recent non-covalent CT complex-based approaches to designing functional optical materials like RTP, TADF, NIR-emissive, chiral CT complex, etc. [ABSTRACT FROM AUTHOR]

Single crystals of Ce-doped gadolinium scandium aluminum garnet (Gd3Sc2Al3O12; GSAG) with Ca2+, Mg2+ and Li+ co-dopants were grown by the Bridgman method. Regardless of melt composition, crystals tend to grow close to the congruent composition. Introduction of Ca2+ or Mg2+, as in other Ce-doped garnets, leads to an increase of absorption below 350 nm, which may indicate formation of Ce4+ states, while no effect is seen with Li+ co-doping. High Ce concentration and co-doping accelerate the scintillation rise and decay times; the codopants strongly significantly reduce the afterglow. [ABSTRACT FROM AUTHOR]

Three photoactive Cd(II) coordination polymers (CPs), [Cd (Fsbpe)(DBBA)2]·2DMF (CP1), [Cd(Fepbpe)(DBBA)2]·2DMF (CP2) and [Cd(Fsbpeb)(DBBA)2] (CP3) (DBBA = 3,5-dibromobenzoic acid, DMF = dimethyl formamide) with similar 1D chain motifs exhibited completely different photosalient behaviors (PS) in response to UV light. Mechanical motion was triggered by [2+2] photocycloaddition and regulated by positioning of the photoactive alkene centers relative to the crystal axis. This solid-state reaction was reversed by heating and photomechanical behaviour was repeated over several cycles. A simple photoactuating device was prepared using a CP3–PVA composite. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the soliton evolution dynamics influenced by the stimulated Raman scattering effect. The strong Raman gain ensures the breathing platicon and stable platicon formation at the cavity with normal quartic dispersion. The switching waves benefitted from the power transformation between pump mode and Raman gain spectrum could modulate the temporal waveform at the breathing platicon existence region. The oscillated characteristics of the temporal waveform mainly rely on the central frequency of the Raman gain spectrum. Meanwhile, at the anomalous quadratic dispersion regime, the Raman effects could capture the perfect soliton crystal formation which provides one novel method for the implementation of the harmonic mode-locking process. Our findings provide a viable way to demonstrate the cavity soliton dynamics and soliton-based applications. [ABSTRACT FROM AUTHOR]

Na 2 Ti 4 O 9 (NTO), an important chemical material, is used in chemical pigments, lithium ion batteries, catalyst carriers, and other fields. Nanoscale NTO crystal was firstly obtained by solvothermal process in this study, and the reactant conditions (time, temperature, solvent, and concentration of NaOH solution) for preparing NTO crystal were investigated. The result reveals that the as-prepared NTO crystal with one-dimensional (1D) rod-like morphology can be obtained in a solvent environment with H 2 O to C 2 H 5 OH via solvothermal process. And then investigating the formation mechanism of 1D NTO crystal based on the solvothermal soft chemical method. In addition, NTO crystal was utilized as the precursor for BaTiO 3 (BT) crystal at desired condition. The [010]-oriented BT polycrystals were directly obtained with NTO precursor via hydrothermal process in low concentration of Ba(OH) 2 solution. The transformation mechanism between BT polycrystals and NTO precursor was revealed, and the process of obtaining functional materials directly from NTO precursor was discussed. The study provides a novel way for the development of small-size barium titanate based functional materials. [ABSTRACT FROM AUTHOR]

Finding a deep-ultraviolet (deep-UV, λ < 200 nm) nonlinear optical (NLO) crystal is vital to a number of advanced optical technologies, yet it remains a great challenge. Herein, we report a deep-UV NLO crystal, K2B3O3F4(OH) (KBOFH), composed of π-conjugated confined groups. It exhibits a relatively large birefringence of 0.054@550 nm and a wide transparency window with an absorption edge below 200 nm. Theoretical calculations reveal that the observed optical properties of KBOFH originate from the confined π-conjugated [B3O3F4(OH)] unit, in which the π-conjugated interactions are decoupled by the tetrahedral non-π-conjugated [BO2F2] unit. This work provides an opportunity to design deep-UV NLO crystals containing π-conjugated confined groups. [ABSTRACT FROM AUTHOR]

We investigated the melting behavior of four CPs with one-dimensional structures from a thermodynamic point-of-view. The difference in melting points depending on the crystal structures is observed. The interactions within the crystals were analyzed using DFT calculations. These analyses suggest that entropic terms dominate the melting points. [ABSTRACT FROM AUTHOR]