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Perovskites have long been materials of interest in solid‐state physics. Their structure leads them to exhibit various characteristics which have both theoretical and practical interest. Lattice dynamics can give a lot of insight into a material's nature such as phase transitions, thus the research presented here focuses on that, by doing a comprehensive vibrational and structural investigation of monocrystalline YAlO3 (YAP). Polarized Raman and infrared (IR) reflectance spectroscopy are used alongside ab initio and classical calculations. In the course of this work, complete Raman spectra have been obtained, confirming new vibrational modes, predicted by ab initio calculations but previously not observed experimentally. IR reflectance spectra have been obtained for the first time for this material. Classical molecular dynamics has been employed to calculate the phonon dispersion and density of states. [ABSTRACT FROM AUTHOR]

Perovskites have long been materials of interest in solid‐state physics. Their structure leads them to exhibit various characteristics which have both theoretical and practical interest. Lattice dynamics can give a lot of insight into a material's nature such as phase transitions, thus the research presented here focuses on that, by doing a comprehensive vibrational and structural investigation of monocrystalline YAlO3 (YAP). Polarized Raman and infrared (IR) reflectance spectroscopy are used alongside ab initio and classical calculations. In the course of this work, complete Raman spectra have been obtained, confirming new vibrational modes, predicted by ab initio calculations but previously not observed experimentally. IR reflectance spectra have been obtained for the first time for this material. Classical molecular dynamics has been employed to calculate the phonon dispersion and density of states. [ABSTRACT FROM AUTHOR]

To build a safe, sustainable, and reliable disposal system for radioactive waste from the nuclear industry, researchers need to understand the physical and chemical aspects of technetium (Tc)‐based oxides. Here, structure–property correlation studies of scheelite‐type AgTcO4 are performed using ab initio calculations. The full phonon dispersion studies reveal the dynamical stability of this compound. The partial phonon density of states, including the complete mode assignment of the vibrational frequencies, shows the maximum involvement of silver (Ag) atoms in the low‐frequency range, while high contribution of oxygen atoms and average contribution from Tc atoms are found throughout the frequency region. The electronic band structure shows that the main contribution to the bottom of the conduction band is Tc d states with a minor contribution from oxygen p states, while the top of the valence band is from Ag d states, which are responsible for the electronic transitions. The frequency‐dependent dielectric functions and computed optical properties show near‐isotropy behavior and, together with absorption peak in the low‐energy range, indicate that AgTcO4 may be a good phosphor when doped with a suitable activator. The estimations are grounded by the available experimental data and also deliver direction for distinct and futuristic applications of AgTcO4. [ABSTRACT FROM AUTHOR]

To build a safe, sustainable, and reliable disposal system for radioactive waste from the nuclear industry, researchers need to understand the physical and chemical aspects of technetium (Tc)‐based oxides. Here, structure–property correlation studies of scheelite‐type AgTcO4 are performed using ab initio calculations. The full phonon dispersion studies reveal the dynamical stability of this compound. The partial phonon density of states, including the complete mode assignment of the vibrational frequencies, shows the maximum involvement of silver (Ag) atoms in the low‐frequency range, while high contribution of oxygen atoms and average contribution from Tc atoms are found throughout the frequency region. The electronic band structure shows that the main contribution to the bottom of the conduction band is Tc d states with a minor contribution from oxygen p states, while the top of the valence band is from Ag d states, which are responsible for the electronic transitions. The frequency‐dependent dielectric functions and computed optical properties show near‐isotropy behavior and, together with absorption peak in the low‐energy range, indicate that AgTcO4 may be a good phosphor when doped with a suitable activator. The estimations are grounded by the available experimental data and also deliver direction for distinct and futuristic applications of AgTcO4. [ABSTRACT FROM AUTHOR]

The structural, electronic, lattice dynamic, thermodynamic, and mechanical properties of disodium dimolybdate (Na2Mo2O7) are studied using first‐principles. First, for the crystal structure, the calculated structural parameters have a standard deviation of less than 1.7% compared with the experimental results. Then, the calculated electronic energy band and density of states reveal that Na2Mo2O7 is an insulator and its bandgap is about 2.87 eV. In addition, the linear response method is used to study the lattice dynamics, and the complete phonon dispersion curve, infrared spectra, Raman shift, Born effective charge, and longitudinal optical–transverse optical (LO–TO) splitting characteristic for Na2Mo2O7 are presented for the first time; its thermodynamic properties are worked out based on the phonon density of states (PDOS). Furthermore, the result of elastic anisotropy shows the title compound is a highly anisotropic material. The calculation results in this article are conducive to the development of Na2Mo2O7 and provide guidelines for further experimental research to a certain extent. [ABSTRACT FROM AUTHOR]

The dynamics of the 1D array of single‐walled carbon nanotubes (CNTs), which interact by van der Waals forces, is considered. The molecular dynamics simulation shows that both the mutual displacements of the nanotubes and the deformations of their walls occur in the low‐frequency oscillations domain. The composite model taking into account both types of the nanotubes' motions is developed in the framework of the thin elastic shell theory. Such an approach allows us to reduce the problem to the dynamics of the linear lattice with contact interaction. The dispersion relations are represented analytically and the multichannel propagation that results in phonon interference (the acoustical analog of the Fano resonance) is observed in the presence of the array's irregularities. The latter can be formed with redundant nanotubes, which are placed over the array in the grooves between neighbor sites. The calculations of the transmittance have been performed by the transfer matrix method for several typical configurations. [ABSTRACT FROM AUTHOR]

The electronic band structure, the optical properties, and the lattice dynamics of the quaternary Cu2ZnSnTe4 (CZTTe) semiconductor with kesterite crystallographic structure are calculated within first principles. The analysis of the phonon partial density of states shows that the highest frequency phonon mode involves mostly heavy Sn and Te atoms due to their exceptionally strong bonding. Experimental Raman spectra of Cu–Zn–Sn–Te nanocrystals, prepared by an aqueous colloidal synthesis, reveal a single characteristic line and its higher order overtones due to specific resonant conditions. In the case of Cu–Zn–Sn–(Te–S) alloy nanoparticles, other Raman features, related to Sn–S vibrations, appear in the spectra indicating a two‐mode type of transformation of their vibrational spectra. The effect of sulfur incorporation on the Te‐related frequency is opposite to the effect observed for the Se‐related frequency in CZTSSe alloys. [ABSTRACT FROM AUTHOR]

Lacunar spinels with a chemical formula of AM4X8 form a populous family of narrow‐gap semiconductors, which offer a fertile ground to explore correlation and quantum phenomena, including transition between Mott and spin‐orbit insulator states, ferro/antiferroelectricity driven by cluster Jahn‐Teller effect, and magnetoelectric response of magnetic skyrmions with polar dressing. The electronic and magnetic properties of lacunar spinels are determined to a large extent by their molecular‐crystal‐like structure. The interplay of electronic correlations with spin‐orbit and vibronic couplings leads to a complex electronic structure already on the single‐cluster level, which—together with weaker intercluster interactions—gives rise to a plethora of unconventional correlated states. This review primarily focuses on recent progresses in the field of optical, dielectric, and magnetoelectric properties on lacunar spinels. After introducing the main structural aspects, lattice dynamics and electronic structure of these compounds are discussed on the basis of optical spectroscopy measurements. Dielectric and polarization studies reveal the main characteristics of their low‐temperature ferro‐ or antiferroelectric phases as well as orbital fluctuations in their high‐temperature cubic state. Strong couplings between spin, lattice, and orbital degrees of freedom are manifested in singlet formation upon magnetostructural transitions, the emergence of various multiferroic phases, and exotic domain‐wall functionalities. [ABSTRACT FROM AUTHOR]

A systematic hybrid density functional theory study on the electronic and vibrational properties of MxCoO2 compounds with M = Li, Na and x=0,0.5,1 is reported. The used DFT‐PBE0 method describes the structural parameters of the studied compounds well in comparison to experimental data. All studied magnetic species are treated as ferromagnets and the Co(IV) atoms possess a magnetic moment of 1.2 μB. At 0 K, CoO2 favors a monoclinic structure very close to trigonal symmetry and behaves as a Mott insulator. The electronic bandgap increases as x increases from 0 to 1. The simulated infrared and Raman spectra together with full phonon dispersion relations show that the intercalation of Li and Na ions affects the lattice dynamics of CoO2 in a different way. [ABSTRACT FROM AUTHOR]

In the past decades, the research on layered materials has attracted much attention, not only because they have many peculiar and anisotropic physical properties but also because they are the precursors of many fascinating 2D materials. Herein, theoretical studies on the elastic, electronic, and phononic properties of the layered material Sc2Al2C3, which has been synthesized in a recent experiment, are systematically presented. The calculations find that Sc2Al2C3 is a brittle semiconductor with a small indirect bandgap of 0.5 eV and a high bulk modulus of 164 GPa. The phonon calculations indicate that there are six Raman‐active modes (three Eg and three A1g) and six infrared‐active modes (three Eu and three A2u) in Sc2Al2C3. This work can be helpful in future studies of this layered material and its possible 2D derivatives. [ABSTRACT FROM AUTHOR]

In the past decades, the research on layered materials has attracted much attention, not only because they have many peculiar and anisotropic physical properties but also because they are the precursors of many fascinating 2D materials. Herein, theoretical studies on the elastic, electronic, and phononic properties of the layered material Sc2Al2C3, which has been synthesized in a recent experiment, are systematically presented. The calculations find that Sc2Al2C3 is a brittle semiconductor with a small indirect bandgap of 0.5 eV and a high bulk modulus of 164 GPa. The phonon calculations indicate that there are six Raman‐active modes (three Eg and three A1g) and six infrared‐active modes (three Eu and three A2u) in Sc2Al2C3. This work can be helpful in future studies of this layered material and its possible 2D derivatives. [ABSTRACT FROM AUTHOR]

Herein, optical (e.g., UV–Vis reflectance spectra, infrared and Raman spectroscopy) and magnetization properties of the double‐perovskite compounds Ba2DySbO6 (BDS) and Ba2GdSbO6 (BGS) are reported. Optical bandgap is determined to be 4.524 eV for BDS and 4.465 eV for BGS using Kubelka–Munk function fit to the UV–Vis spectra. The lattice dynamical calculations on the force‐field model are undertaken for assigning the observed infrared and Raman modes. The dc susceptibility does not exhibit magnetic ordering down to 2 K for the two compounds. The dc magnetization at T = 2 K does not saturate for BDS even at a magnetic field of 11 T, but saturates at 5 T to a value of 7μB/Gd3+ for BGS. A frequency‐dependent cusp in the ac susceptibility appears at 3.3 K, that is unlikely generic spin‐glass‐like freezing, and may be attributed to the short‐range correlations among Dy3+‐moments in BDS. Crystal‐field (CF) computations are performed to analyze the magnetic data for BDS. The obtained CF level scheme of Dy3+ ions demonstrates the observed susceptibility and isothermal magnetization curves very well. The field‐dependence of the two lowest CF levels along different principal axes does not exhibit any level crossing, consistent with the lack of ordering in Ba2DySbO6. [ABSTRACT FROM AUTHOR]

A theoretical study of many physical properties, namely structural stability, electronic, mechanical stability, and phonon properties of the nonmagnetic intermetallic compound YSn3 is realized using the density functional theory (DFT) calculations. The structural properties are calculated in many phases, specifically, Cu3Au, Al3Ti, GdSn2.75, and AgSbTe2 structures. The results show that YSn3 crystallizes in the Cu3Au structure at ambient conditions, whereas the energy difference between the Cu3Au and Al3Ti structures is very weak and in the order of thermal energy at room temperature, which suggests the YSn3 may also coexist in this structure. The single‐crystal elastic constants for different phases at zero pressure are computed, and main mechanical quantities are derived. The mechanical stability criteria predict that the crystal in the Cu3Au structure is stable when pressure is below 18.68 and 4.61 GPa for the Cu3Au and Al3Ti phases of YSn3. The phonon dispersion relations and their projected density of states are investigated. The titled compound is found to be dynamically and mechanically stable in the Cu3Au and Al3Ti prototype phases under ambient conditions. As far as the authors know, high‐pressure features of YSn3 have not yet been explored neither experimentally nor theoretically. [ABSTRACT FROM AUTHOR]

A theoretical study of many physical properties, namely structural stability, electronic, mechanical stability, and phonon properties of the nonmagnetic intermetallic compound YSn3 is realized using the density functional theory (DFT) calculations. The structural properties are calculated in many phases, specifically, Cu3Au, Al3Ti, GdSn2.75, and AgSbTe2 structures. The results show that YSn3 crystallizes in the Cu3Au structure at ambient conditions, whereas the energy difference between the Cu3Au and Al3Ti structures is very weak and in the order of thermal energy at room temperature, which suggests the YSn3 may also coexist in this structure. The single‐crystal elastic constants for different phases at zero pressure are computed, and main mechanical quantities are derived. The mechanical stability criteria predict that the crystal in the Cu3Au structure is stable when pressure is below 18.68 and 4.61 GPa for the Cu3Au and Al3Ti phases of YSn3. The phonon dispersion relations and their projected density of states are investigated. The titled compound is found to be dynamically and mechanically stable in the Cu3Au and Al3Ti prototype phases under ambient conditions. As far as the authors know, high‐pressure features of YSn3 have not yet been explored neither experimentally nor theoretically. [ABSTRACT FROM AUTHOR]

The crystal structure and lattice dynamics of quantum paraelectric BaxSr1−xTiO3 (x = 0, 0.01, 0.02) solid solutions are studied using X‐ray diffraction (XRD), Raman and terahertz‐infrared (THz‐IR) spectroscopies in a temperature range of 4–300 K. XRD and Raman spectroscopy reveal the cubic‐to‐tetragonal nonpolar structural phase transition at about 100 K. At the same time, Raman spectra manifest the presence of polar modes, TO2 and TO4, normally prohibited in paraelectric phase. Emergence of these modes indicates the appearance of the polar nanoregions in a broad temperature range. The modes become more intensive at low temperatures, and temperature dependence of their intensities on cooling reveals the kink‐like change of the slope from flat to steep, indicating on activation of polar nanoregions. The transmission THz‐IR spectra show that squared frequency of the polar TO1 soft mode, responsible for ferroelectric transition, follows Cochran's behavior at high temperatures. However, at low temperatures, it doesn't vanish at extrapolated Curie temperature but saturates, demonstrating the plateau feature below 20 K. This behavior, coherent with known saturation of the dielectric constant, indicates that transition to ferroelectric phase in BaxSr1−xTiO3 is suppressed by quantum fluctuations and the system stays in quantum paraelectric state at very low temperatures. [ABSTRACT FROM AUTHOR]

Using first‐principles lattice dynamics calculations, new phase transitions driven by soft phonon modes in CsPbBr3, from the high‐symmetry cubic structure Pm3m to hypothetical structures, are predicted. The structural, electronic, and dielectric properties of the new phases are determined. A ferroelectric phase with Pmc21 symmetry is found to be more stable than the experimentally observed phases. It is suggested that it is a good candidate as a ground state. It is demonstrated that the out‐of‐phase rotations of the PbBr6 octahedra determine a more stable crystal lattice than the in‐phase rotations. All predicted structures show semiconductor character, with bandgap values between 1.44 and 1.87 eV. The bandgap increases with increasing PbBr bond length and the angle of rotation of the PbBr6 octahedron. The dielectric tensor, Born effective charges, and spontaneous polarization are also calculated. The optical dielectric constant shows low sensitivity to unit cell volume and symmetry of the structures. The Born effective charges are sensitive to lattice symmetry, as they are related to the directions of charge transfer between ions, whereas the cell volume has no significant effect on their values. The infrared and Raman vibration modes of the three structures are also determined. [ABSTRACT FROM AUTHOR]

Herein, the symmetry of the experimentally observed soft phonons in the body‐centered cubic β‐phase (Im3¯m) of titanium is analyzed. The harmonic phonon dispersion relations are calculated using the first‐principles calculations. Using the group‐theoretical methods, the symmetry of the calculated unstable phonons is determined. The symmetries of the unstable phonons observed at wave vectors 2πa[12,12,0](N) and 2πa[23,23,23](Λ) are the same as the symmetries of the N4− and Λ1 irreducible representations, respectively. Transformations of the β‐phase due to the atomic motion of unstable phonons and the subsequent structure relaxation are discussed. One possible way to explain the transformation of the β‐phase to the hexagonal close‐packed α‐phase (P63/mmc) is through an orthorhombic structure (either Cmcm or Pnnm). The atomic motion of an unstable N4− phonon results in the orthorhombic structure and following structure relaxation transforms the orthorhombic structure to the α‐phase. Similarly, the transformation of the β‐phase to another hexagonal close‐packed ω‐phase (P6/mmm) can be considered to be happening through a trigonal structure (either P3¯m1 or P3m1). The atomic motion of an unstable Λ1 phonon forms the trigonal structure and subsequent structure relaxation transforms the trigonal structure to the ω‐phase. The space group of the intermediate phase is a common subgroup of the space groups of the initial β‐phase and the final α/ω‐phase. Therefore, the β–α/ω transformation can be described as an unstable phonon‐induced reconstructive transformation of the second type. There is no activation energy barrier along each of the four energy‐minimizing paths, and the transformation strains are accommodated. [ABSTRACT FROM AUTHOR]

A comparative analysis of Raman spectra of barium titanate in various forms is performed over a broad range of temperatures (from 80 to 400 K). In particular, the evolution of A1(2TO) and E(1TO) soft‐mode components, sensitive to strain, is monitored for such materials as single crystal, ceramic, and polycrystalline film. Based on the temperature dependence of these components, a sequence of phase transitions is established for each of the aforementioned forms. [ABSTRACT FROM AUTHOR]

The lattice misfit between the substrate and an epitaxial film leads in general to static forces, which define the interface stress, and dynamic responses that modify the thin‐film lattice dynamics. Although these are both fundamental concepts that are important for film growth and thin‐film properties, they have not been investigated in a combined way so far. Therefore, herein, surface stress experiments in combination with surface phonon studies for three different, cubic oxide ultrathin film systems are reviewed. Within the class of binary oxides, NiO(001) grown on Ag(001) is chosen, which exhibits a −2.2% lattice mismatch, and BaO(001) on Pt(001), a system with a negligible lattice mismatch. For the ternary oxides, perovskite thin films of BaTiO3 grown epitaxially on Pt(001) with a lattice mismatch of −2.3% are focused upon. The surface stress experiments are conducted with an optical two‐beam curvature technique under in situ growth conditions. Surface and thin‐film phonons are determined by high‐resolution electron energy loss spectroscopy. Surface stress and lattice dynamics are discussed in the range from the oxide monolayer to thin films of about 20 unit cell in thickness. [ABSTRACT FROM AUTHOR]

The lattice dynamics of orthorhombic Sb2Se3 is studied by a combination of inelastic neutron and 121Sb nuclear inelastic scattering giving access to the total and Sb partial density of phonon states (DPS). The Se partial DPS is determined from the difference between the total and Sb partial DPS. The total DPS is determined at 39, 150, and 300 K, and an analysis of the temperature‐induced mode shifts in combination with low‐temperature powder diffraction data is provided. Using an earlier reported theoretical approach, the corresponding total and partial DPS of Sb2Se3 are calculated by first‐principles calculations. Herein, a detailed analysis of the Grüneisen parameter, element‐specific and bulk Debye temperatures, and the mean force constants as derived from the experimental data and discrete Fourier transform calculations is provided. In general, the calculations underestimate the strength of the covalent SbSe bonds. [ABSTRACT FROM AUTHOR]

Phonon spectra of two novel nonlinear optical crystals, PbGa2GeSe6 and PbGa2GeS6, are studied experimentally by Raman and infrared reflection spectroscopy and theoretically by density functional theory (DFT) lattice dynamics calculations. The experimental peak frequencies are compared with calculated vibration frequencies, and a very reasonable agreement is established. A comparative analysis of results for PbGa2GeSe6 and PbGa2GeS6 with earlier reported data on relevant binary and ternary metal chalcogenides clearly indicates that the similarity of their phonon spectra stems from the presence of GeS4 and GeSe4 tetrahedra as structural building blocks. [ABSTRACT FROM AUTHOR]

Heat capacity CP(T) (2–300 K) and unit cell parameters a(T), b(T), and c(T) (5–300 K) of type‐VII clathrates SrNi2P4 and BaNi2P4 have been investigated experimentally. The studied compounds are prepared by a two‐stage ampoule synthesis. Diffuse maxima of CP(T) near 5 K along with anomalies in temperature dependencies of unit cell parameters below 10 K are observed and related to the influence of two‐level systems in guest‐atom substructures. Joint analysis of temperature dependencies of heat capacity and unit cell volume of both clathrates within the frames of the Debye–Einstein model allowed determining characteristic temperatures of the clathrate frameworks and guest atoms as well as parameters of the two‐level systems. The relationships of the obtained characteristics and the peculiarities of the crystal structures of SrNi2P4 and BaNi2P4 are discussed. [ABSTRACT FROM AUTHOR]

First‐principles calculations are performed to study the effects of biaxial strains and high pressure on the structural, electronic, and lattice dynamical properties of defect chalcopyrite HgM2Te4 (M = Al, In) semiconductors. The evolutions of optimized structural parameters, band‐gap energy, crystal‐field splitting energy, and Γ‐point phonon frequencies with the biaxial strain and pressure have been analyzed. The optimized lattice parameters are reasonable compared with the existing experimental results. Both compounds undergo an indirect to direct band gap transition under tensile ϵ x x, however, the semiconductors retain their indirect band gap character under pressure. The increasing pressure pushes the crystal‐field‐splitting hole (CH) band downwards; in contrast, the increasing biaxial strain makes the CH band shift upwards, which results in that the crystal‐field splitting energies Δ c f decrease from positive to negative. The pressure‐induced softness of low‐frequency modes around the X point suggests that both compounds become dynamically instable. Atomic displacement patterns show that the high frequency modes are mainly determined by the vibrations of group‐III cations. [ABSTRACT FROM AUTHOR]

Experimental Raman scattering study of mixed AgxGaxGe1−xSe2 (0.167 ≤ x ≤ 0.333) single crystals is reported. The spectra transformation upon composition variation exhibits the two‐mode type of behavior, with lines of the end‐member compounds of the solid solution (AgGaSe2 and GeSe2) dominating in the spectra, whose relative intensities vary with composition. [ABSTRACT FROM AUTHOR]

The Mössbauer spectra of 119Sn in PbZr1-xSnxO3 compound were studied from 30 to 300°C with particular emphasis on the regions of phase transitions. Some changes of the Mössbauer spectrum near the phase transition temperatures were observed, as expected from the change in the crystal structure. The measurements also showed that the Mössbauer fraction considerably drops near the temperature of the phase displacive transition between two antiferroelectric phases (AFE1–AFE2), whereas this drop is considerably smaller at Curie point. Little difference was found in the center shift of the spectrum, indicating that the bonding character is not affected by the phase transitions. The obtained results were interpreted according to Dvorak’s suggestion that in antiferroelectrics the frequency over the entire optical branch may soften at particular phase transitions. According to the theory, the mode softening should manifest as a decrease in the Mössbauer–Lamb recoil-free fraction (f-factor) due to the increasing vibrational amplitude of the soft mode. Such softening was already observed in Raman scattering measurements in the PbZr1-xSnxO3 single crystals. [ABSTRACT FROM AUTHOR]

Na3OBr is expected to be a promising superionic conductor for use as solid state electrolyte in large scale energy storage systems, but its lattice dynamic and thermal conductivity properties have not been well studied till now. In this work, we performed a detailed study on these properties as well as its electronic and elastic properties using first principles method. The results indicate that Na3OBr is a direct band gap crystal. It is mechanically stable but elastically anisotropic. Its phonons at the Brillouin zone center were classified using group theory analysis and its Born effective charges, LO-TO splitting, and dielectric constants were calculated and discussed. Its phonon dispersion curves were obtained and their origins were revealed. Based on the phonon dispersion curves, its thermal conductivity as a function of temperature was predicted, which give a value of 7.30 Wm−1 K−1 at 300 K. Further study reveals that, for Na3OBr, the simple Slack's model can give almost the same thermal conductivity curve as that obtained from the phonon dispersion curves. [ABSTRACT FROM AUTHOR]

We report first-principles phonon frequencies and anharmonic thermodynamic properties of h.c.p. Re and Tc calculated within the quasi-harmonic approximation, including Grüneisen parameters, temperature-dependent lattice parameters, thermal expansion, and isobaric heat capacity. We discuss the differences between a full treatment of anisotropy and a simplified approach with a constant [ABSTRACT FROM AUTHOR]

We demonstrate how supercell implementations of conventional lattice dynamical calculations can be used to determine the extent and nature of disorder-induced broadening in the phonon dispersion spectrum of disordered crystalline materials. The approach taken relies on band unfolding, and is first benchmarked against virtual crystal approximation phonon calculations. The different effects of mass and interaction disorder on the phonon broadening are then presented, focussing on the example of a simple cubic binary alloy. For the mass disorder example, the effect of introducing correlated disorder is also explored by varying the fraction of homoatomic and heteroatomic neighbors. Systematic progression in the degree of phonon broadening, on the one hand, and the form of the phonon dispersion curves from primitive to face-centered cubic type, on the other hand, is observed as homoatomic neighbors are disfavored. The implications for rationalizing selection rule violations in disordered materials and for using inelastic neutron scattering measurements as a means of characterizing disorder are discussed. [ABSTRACT FROM AUTHOR]

An updated thermodynamic description of pure Co was obtained by applying new models for the third generation of Calphad databases. In these models, different contributions to the heat capacity, especially the vibrational part, were treated separately, each with a clear physical meaning. More importantly, the phase stabilities of the various allotropes are now physically well defined. Thus, the derived thermodynamic properties vary more reasonably and smoothly from 0 K and up. Calculated thermodynamic properties were compared with experimental data and good agreement was obtained. [ABSTRACT FROM AUTHOR]

Orbital ordering has recently emerged as another important state in iron-based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO1−x Fx with a focus on resistivity, Nernst effect, thermal expansion, and [ABSTRACT FROM AUTHOR]

Orbital ordering has recently emerged as another important state in iron-based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO1−x Fx with a focus on resistivity, Nernst effect, thermal expansion, and [ABSTRACT FROM AUTHOR]

The phonon structure of the silver sulfide Ag2S was investigated, experimentally using Raman spectroscopy, and theoretically using the density-functional perturbation theory for the first time. Seven Raman-active modes were observed and identified at 23, 39, 42, 44, 62, 65, and 243 cm−1. Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. The phonon band structure and the relative Raman intensities were also investigated by ab initio calculations and compared with the experimental data. The temperature, laser power, and illumination time dependencies of frequency, linewidth, and intensity of the Raman-active modes are discussed. In the Raman spectra at higher frequencies 1300-1700 cm−1, additional broad Raman modes observed in all samples at higher laser powers 8-10 mW were ascribed to luminescence from β-Ag2S. The phonon and Raman spectra of the β-Ag2S provide a useful insight into the β-Ag2S → α-Ag2S phase transition. Finally, calculated infrared vibrational mode frequencies were compared with measured infrared mode frequencies. [ABSTRACT FROM AUTHOR]

A domain of locations of a proton in the perovskite structure is determined by studying the local structure and a hard sphere point charge model. Two intra- and one inter-octahedron processes with different energy barriers of ⟨ E1⟩, ⟨ E2⟩, and ⟨ E3⟩ have been studied, respectively. The order of the energy barriers is predicted as ⟨ E3⟩ > ⟨ E1⟩ ≥ ⟨ E2⟩ by studying both the changes of bonding in these processes and the charge imbalance between the acceptor dopant ion and the B type ion. The inter-octahedron hopping, ⟨ E3⟩, is shown to be a necessary and rate limiting step by using percolation and charge imbalance argument. This result differs with earlier calculations, however, it agrees with the result of quasi-elastic neutron and other experimental results. Through studying the lattice dynamics of a 1D O-H chain and its monomer, we have developed new formulae for a hydrogen bond system and applied them to explain the experimentally observed red-shift and broadening of the O-H vibration peak. These phenomena are revealed as indications of the quasi-free state of a proton instead of the formation of hydrogen bond. [ABSTRACT FROM AUTHOR]

The crystal structure of Cu [ABSTRACT FROM AUTHOR]

We present calculations of the Raman intensities scattered by the phonons of the mixed crystals Ga1- xAl xAs for five small concentrations up to x = 0.14, away from resonance conditions. Each mixed crystal is approached by a primitive cell, the volume of which is 64 times the volume of the bulk zincblende primitive cell. The phonons have been calculated using a version of a three-dimensional Valence Overlap Shell Model (VOSM) and the Raman spectra with the bond-polarizability model (BPM). The values of the bond-polarizability (BP) parameters of Ga-As and Al-As bonds necessary for the calculation of the Raman intensities are, for the first time, not estimated arbitrarily, but we have evaluated them using certain relations involving directly measurable quantities. We also show that the frequencies of the AlAs LO(Γ)-like modes and their Raman profiles and intensities as well, depend, for a given Al concentration, strongly on the way the cations are distributed over the mixed-crystal cation planes. Concerning the GaAs LO(Γ)-like modes, differences in the cation distribution affect only a little their frequencies and their intensities, but may result in pronounced left asymmetries of the profiles of the corresponding strong Raman peaks. The comparison between the calculated and the experimental Raman spectrum for the Ga0.86Al0.14As is very encouraging, especially when the mixed crystal is described by a disorder configuration where the Al ions are gathered to neighboring cation planes. [ABSTRACT FROM AUTHOR]

Density-functional calculations of the electronic band structure, optical properties, and lattice dynamics of kesterite CuZnGeSe semiconductor (space group ) are reported. Theoretical optical properties of the material are shown to agree quantitatively with available spectroscopic ellipsometry experimental data that allow detailed interpretation of the main spectral features around the fundamental absorption edge. A good agreement between theoretical and experimental vibrational mode frequencies is also established and the assignment of observed phonon modes to specific lattice eigenmodes is performed. [ABSTRACT FROM AUTHOR]

Lattice dynamics in pyrochlore and defect fluorite type (Eu [ABSTRACT FROM AUTHOR]

Lattice dynamics simulation of NH3CH2COOH [ABSTRACT FROM AUTHOR]

The lattice dynamics of rocksalt CdO is studied as a function of pressure using density-functional linear-response theory. The predicted static phase-transition pressure from rocksalt (B1) to CsCl (B2) structures is 76.12 GPa. Analysis of the phonon calculations suggests that the pressure-induced instabilities at p = 78.26 GPa of the transverse acoustic (TA) modes in a large portion of the Brillouin zone are responsible for the phase transition from B1 → B2 phases of CdO. Moreover, the calculated partial phonon density of states at the precursor phase transition of p = 70 GPa reveals strong reduction of the longitudinal acoustic Cd-like modes and the existence of a large peak related to the longitudinal optic O-like mode. [ABSTRACT FROM AUTHOR]

The structural evolution of Ge-Sb-Te-based phase-change material during molecular beam epitaxy growth is investigated. The in situ and ex situ synchrotron-based X-ray scattering results reveal the formation and evolution of defects-induced inhomogeneities during growth. The scattered intensity in the vicinity of various Bragg reflections indicates a specific scattering vector-dependent anisotropy in the diffuse scattering regime. The average local static distortion fields of the defects-induced inhomogeneities are estimated to be in between 2.5 and 3.0 nm. The defects-induced diffuse scattering anisotropy, observed for the particular Bragg points, arises from the combination of the elastic and crystalline anisotropies that are caused by the large number of vacancies. The experiments show that the large-size defect clusters create inhomogeneity-driven tetragonal fields inserted into the metastable phase of the Ge-Sb-Te host crystal. [ABSTRACT FROM AUTHOR]

Ultrathin Bi films have been theoretically predicted to be an elemental two-dimensional topological insulator recently. Investigation on phonon properties in topological insulators, especially down to a few-layer regime, is an interesting topic. In this paper, the evolution of structures and phonon properties of ultrathin Bi (111) films with thickness is studied from first-principles calculations by including spin-orbit coupling. The calculations of lattice dynamics show that both two Raman modes A1g and Eg in ultrathin Bi (111) films harden compared with those in bulk, which is contrary to other nanomaterials. The possible origin of this anomalous phonon behavior is discussed by considering the peculiar semimetallicity of Bi. With increasing thickness, the frequencies of these two Raman modes decrease gradually toward the values in the bulk. Our calculations also show that the softening and hardening of surface phonon modes coexist in ultrathin Bi films. [ABSTRACT FROM AUTHOR]

The lattice dynamics in the IV-VI compounds GeTe, SnTe and PbTe were studied by 125Te and 119Sn nuclear inelastic scattering and the obtained partial density of phonon states were compared with published theoretical calculations. The phase purity and structure were characterized by high energy X-ray diffraction. The effect of the atomic arrangement, rhombohedral for GeTe and cubic for SnTe and PbTe, is visible in the density of phonon states. Vibrational properties are found to be in good agreement with available calculated data and the softer character of the NaCl-type structures in comparison with the rhombohedral GeTe is confirmed. [ABSTRACT FROM AUTHOR]

We derive expressions for the bond polarizability (BP) parameters involving only directly measurable quantities, such as dielectric and elasto-optic constants of the bulk crystal and apply them to zincblende-type crystals. The vibrational modes are calculated on the basis of an eleven (11) parameter rigid-ion model approximation (RIM). Having estimated the BP parameters for GaSb and AlSb bulk crystals, we calculate the Raman spectra away of resonance conditions for the vibrations of perfect and disordered (GaSb)1/(AlSb)1 (001) superlattices (SL). The disordered SL is approximated with primitive cells much larger than the primitive cell of the perfect 1 × 1 SL. Furthermore, we show that disorder modifies the Raman spectra of the perfect SL by introducing asymmetry as well as by the formation of additional peaks in both the acoustic and optical range of the Raman spectra. Puzzlingly, even a small degree of disorder, results in a blueshift of all modes frequencies, especially the strongest optical ones. [ABSTRACT FROM AUTHOR]