Search

Owing to their very large piezoelectric coefficients and spontaneous polarizations, (Sc,Y)xAl1−xN alloys have emerged as a new class of III-nitride semiconductor materials with great potential for high-frequency electronic and acoustic devices. The thermal conductivity of constituent materials is a key parameter for design, optimization, and thermal management of such devices. In this study, transient thermoreflectance technique is applied to measure the thermal conductivity of ScxAl1−xN and YxAl1−xN (0 ≤x ≤0.22) layers grown by magnetron sputter epitaxy in the temperature range of 100–400 K. The room-temperature thermal conductivity of both alloys is found to decrease significantly with increasing Sc(Y) composition compared to that of AlN. We also found that the thermal conductivity of YxAl1−xN is lower than that of ScxAl1−xN for all studied compositions. In both alloys, the thermal conductivity increases with the temperature up to 250 K and then saturates. The experimental data are analyzed using a model based on the solution of the phonon Boltzmann transport equation within the relaxation time approximation. The contributions of different phonon-scattering mechanisms to the lattice thermal conductivity of (Sc,Y)xAl1−xN alloys are identified and discussed.

Data-driven approaches are becoming increasingly valuable for modern science, and they are making their way into industrial research and development (R&D). Supervised machine learning of statistical models can utilize databases of materials parameters to speed up the exploration of candidate materials for experimental synthesis and characterization. In this paper we introduce the HADB database, which contains properties of industrially relevant chemically disordered hard-coating alloys, focusing on their thermodynamic, elastic and mechanical properties. We present the technical implementations of the database infrastructure including support for browse, query, retrieval, and API access through the OPTIMADE API to make this data findable, accessible, interoperable, and reusable (FAIR). Finally, we demonstrate the usefulness of the database by training a graph -based machine learning (ML) model to predict elastic properties of hard-coating alloys. The ML model is shown to predict bulk and shear moduli for out out-of-sample alloys with less than 6 GPa mean average error. Funding Agencies|Competence Center Functional Nanoscale Materials (FunMat-II) , Sweden (Vinnova) [2016-05156]; Knut and Alice Wallenberg Foundation, Sweden (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government [2020-05402]; Swedish e-Science Research Centre (SeRC) , Sweden; Swedish Research Council (VR) , Sweden [VR-2021-04426]; VR, Sweden [2018-05973]; Swedish Research Council, Sweden; [2009 00971]

Nature 605(7909), 274 - 278 (2022). doi:10.1038/s41586-022-04550-2, Theoretical modelling predicts very unusual structures and properties of materials at extreme pressure and temperature conditions1,2. Hitherto, their synthesis and investigation above 200 gigapascals have been hindered both by the technical complexity of ultrahigh-pressure experiments and by the absence of relevant in situ methods of materials analysis. Here we report on a methodology developed to enable experiments at static compression in the terapascal regime with laser heating. We apply this method to realize pressures of about 600 and 900 gigapascals in a laser-heated double-stage diamond anvil cell3, producing a rhenium–nitrogen alloy and achieving the synthesis of rhenium nitride Re$_7$N$_3$—which, as our theoretical analysis shows, is only stable under extreme compression. Full chemical and structural characterization of the materials, realized using synchrotron single-crystal X-ray diffraction on microcrystals in situ, demonstrates the capabilities of the methodology to extend high-pressure crystallography to the terapascal regime., Published by Nature Publ. Group, London [u.a.]

Crystalline black nitrogen (BN) is an allotrope of nitrogen with the black phosphorus (BP) structure recently synthesized at high pressure by two independent research groups [Ji et al., Sci. Adv. 6, eaba9206 (2020); Laniel et al., Phys. Rev. Lett. 124, 216001 (2020)]. Here, we present a systematic study of the electronic and optical properties of BN focusing on its comparison with BP. To this end, we use the state-of-the-art quasiparticle self-consistent $GW$ approach with vertex corrections in both the electronic and optical channels. Despite many similarities, the properties of BN are found to be considerably different. Unlike BP, BN exhibits a larger optical gap (2.5 vs 0.26 eV), making BN transparent in the visible spectral region with a highly anisotropic optical response. This difference can be primarily attributed to a considerably reduced dielectric screening in BN, leading to enhancement of the effective Coulomb interaction. Despite relatively strong Coulomb interaction, exciton formation is largely suppressed in both materials. Our analysis of the elastic properties shows exceptionally high stiffness of BN, comparable to that of diamond., Comment: Final version; 10 pages, 9 figures, 6 tables

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf4N20 center dot ...

High pressure synthesis of rhenium nitride pernitride ReN$_2$ with crystal structure unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the P2$_1$/c phase of ReN$_2$ and compare them with two other polytypes, C2/m and P4/mbm phases suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to pressure p=170 GPa, followed by the stabilization of the P4/mbm phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the P4/mbm ReN$_2$ at $\approx$175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P2$_1$/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y$_2$ point of the Brillouin zone while the disruption of a neck takes place slightly off from the $\Gamma$-A line., Comment: 15 pages, 7 figures, 3 tables

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Using highly accurate ab initio molecular dynamic simulations we calculate elastic constants of Ti0.5Al0.5N as a function of temperature up to 1500 K and compare the results with those obtained for TiN. We analyze the variation of the materials elastic anisotropy with temperature by calculating directional Youngs moduli and Poisson ratios on the (100), (110) and (111) crystallographic planes. We show that though the elastic moduli of Ti0.5Al0.5N strongly decrease upon heating, the elastic anisotropy increases with temperature unlike in TiN. Since several approximate approaches have recently been utilized to predict elastic constants of Ti0.5Al0.5N at elevated temperature we compare our results with published data and benchmark the different approximate schemes. Giving the fact that Ti(1-x)AlxN is a prototypical system for hard coating applications, we conclude that the recently developed symmetry imposed force constants approach combined with the temperature dependent effective potential method is accurate and computationally cost-effective for this material class. Funding Agencies|Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkodping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRCAgency for Science Technology & Research (ASTAR); Swedish Research Council (VR)Swedish Research Council [2019-05600]; VINN Excellence Center Functional Nanoscale Materials (FunMat-2) [2016-05156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]

An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).

High-temperature thermal stability, elastic moduli and anisotropy are among the key properties, which are used in selecting materials for cutting and machining applications. The high computational demand of ab initio molecular dynamics (AIMD) simulations in calculating elastic constants of alloys promotes the development of alternative approaches. Machine learning concept grasped as hybride classical molecular dynamics and static first principles calculations have several orders less computational costs. Here we prove the applicability of the concept considering the recently developed moment tensor potentials (MTP), where moment tensors are used as materials descriptors which can be trained to predict the elastic constants of the prototypical hard coating alloy, Ti0.5Al0.5N at 900 K. We demonstrate excellent agreement between classical molecular dynamics simulations with MTPs and AIMD simulations. Moreover, we show that using MTPs one overcomes the inaccuracy issues present in approximate AIMD simulations of elastic constants of alloys. Funding Agencies|Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University [2009 00971]; SeRCAgency for Science Technology & Research (ASTAR); Swedish Research Council (VR)Swedish Research Council [2019-05600]; VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [2016-05156]; RFBRRussian Foundation for Basic Research (RFBR) [20-53-12012]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]

ACS nano 15(8), 13539 - 13546 (2021). doi:10.1021/acsnano.1c04325, Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN$_2$ which possesses atomic-thick layers comprised of Ni$_2$N$_3$ pentagons forming Cairo-type tessellation. The layers of NiN$_2$ are weakly bonded with the calculated exfoliation energy of 0.72 J/m$^2$, which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN$_2$ is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN$_2$ to be a precursor of a pentagonal 2D material with a tunable direct band gap., Published by Soc., Washington, DC

Available information concerning the elastic moduli of refractory carbides at temperatures (T) of relevance for practical applications is sparse and/or inconsistent. Ab initio molecular dynamics (AIMD) simulations at T = 300, 600, 900, and 1200 K are carried out to determine the temperature-dependences of the elastic constants of rocksalt-structure (B1) TiC, ZrC, HfC, VC, TaC compounds, as well as high-entropy (Ti,Zr,Hf,Ta,W)C and (V,Nb, Ta,Mo,W)C. The second-order elastic constants are calculated by least-square fitting of the analytical expressions of stress/strain relationships to simulation results obtained from three tensile and three shear deformation modes. Sound-velocity measurements are employed to validate AIMD values of bulk, shear, and elastic moduli of single-phase B1 (Ti,Zr,Hf,Ta,W)C and (V,Nb,Ta,Mo,W)C at ambient conditions. In comparison with the predictions of previous ab initio calculations - where the extrapolation of finite-temperature elastic properties accounted for thermal expansion while neglecting intrinsic vibrational effects - AIMD simulations produce a softening of shear elastic moduli with T in closer agreement with experiments. The results show that TaC is the system which exhibits the highest elastic resistances to tensile and shear deformation up to 1200 K, and indicate the (V,Nb,Ta,Mo,W)C system as candidate for applications that require superior toughness at room as well as elevated temperatures. (C) 2021 The Author(s). Published by Elsevier Ltd. Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR-2015-04630]; Center for High Performance Computing (PDC) in Stockholm; High Performance Computing Center North (HPC2N) in Umea, Sweden; Competence Center Functional NanoscaleMaterials (FunMat-II) (Vinnova Grant) [2016-05156]; Swedish Research Council (VR)Swedish Research Council [2019-05600]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Knut and Alice Wallenberg Foundation through Wallenberg Scholar Project [2018.0194]; Olle Engkvist Foundation; Office of Naval Research ONR-MURIMURIOffice of Naval Research [N00014-15-1-2863]; Ministry of Science and Higher Education of the Russian Federation of NUSTMISIS [K2-2020-026, 211]

Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature ( T-c) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electron-phonon coupling. It is found that increasing pressure suppresses T-c in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by co-deposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions. Funding Agencies|Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Research Council (VR) through International Career GrantSwedish Research Council [2014-6336, 2019-05403]; Marie Sklodowska Curie Actions [INCA 600398]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Office of the Permanent Secretary, Ministry of Higher Education, Science, Research and Innovation: Research Grant for New Scholar [RGNS 63-013]; Swedish National Infrastructure for Computing (SNIC); National Supercomputer Centre (NSC)

Ab initio-calculated ideal strength and toughness describe the upper limits for mechanical properties attainable in real systems and can, therefore, be used in selection criteria for materials design. We employ density-functional ab initio molecular dynamics (AIMD) to investigate the mechanical properties of defect-free rocksalt-structure (B1) TiN and $\text{B}1\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{A}{\mathrm{l}}_{x}\mathrm{N}\phantom{\rule{4pt}{0ex}}(x=0.25$, 0.5, 0.75) solid solutions subject to [001], [110], and [111] tensile deformation at room temperature. We determine the alloys' ideal strength and toughness, elastic responses, and ability to plastically deform up to fracture as a function of the Al content. Overall, TiN exhibits greater ideal moduli of resilience and tensile strengths than (Ti,Al)N solid solutions. Nevertheless, AIMD modeling shows that, irrespective of the strain direction, the binary compound systematically fractures by brittle cleavage at its yield point. The simulations also indicate that $\mathrm{T}{\mathrm{i}}_{0.5}\mathrm{A}{\mathrm{l}}_{0.5}\mathrm{N}$ and $\mathrm{T}{\mathrm{i}}_{0.25}\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{N}$ solid solutions are inherently more resistant to fracture and possess much greater toughness than TiN due to the activation of local structural transformations (primarily of B1 \ensuremath{\rightarrow} wurtzite type) beyond the elastic-response regime. In sharp contrast, (Ti,Al)N alloys with 25% Al exhibit similar brittleness as TiN. The results of this work are examples of the limitations of elasticity-based criteria for prediction of strength, brittleness, ductility, and toughness in materials able to undergo phase transitions with loading. Comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions that are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms.

Using ab initio calculations, we have analyzed the influence of anharmonic effects on the electronic structure and the phonon-dispersion relations of body-centered-cubic (bcc) niobium (Nb) and investigated the temperature dependence of the Kohn anomaly in this metal. A comparison of the results obtained in the framework of the temperature-dependent effective potential method with those derived within the quasiharmonic approximation demonstrates the importance of the explicit treatment of the finite-temperature effects upon the theoretical description of bcc Nb lattice dynamics. In agreement with experimental results, the inclusion of anharmonic vibrations in our calculations leads to the disappearance of the Kohn anomaly for the acoustic mode in a vicinity of the Gamma point with increasing temperature. Moreover, the calculated phonon self-energy indicates that the origin of the temperature dependence is related to the change of the electronic structure. We have calculated the temperature dependence of the electronic spectral function and analyzed the Fermi surface of Nb. A significant temperature-induced smearing of the electronic states has been identified as the origin of the disappearance of the Kohn anomaly in Nb at elevated temperature. Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2015-04391, 2019-05551]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]

Using first principles calculations and experimental methods we show that B1 structured solid solution TixNbyAlzN can be grown. The mixing free energy surface indicates that the alloys should decompose. Theoretical analysis of the thermodynamic driving force towards the spinodal decomposition shows that the force can be different in alloys with equally low thermodynamic stability but different Nb content, indicating that the detailed picture of the decomposition should also be different. Electron microscopy and nanoindentation underlines different age hardening of the samples. We demonstrate that an alloy with the optimized composition, Ti0.42Nb0.17Al0.41N combines high thermal stability and age hardening behavior. Keywords: Hard coating films, NbN, Cathodic arc deposition, Thermodynamic stability, Hardness

Angewandte Chemie / International edition 57(29), 9048 - 9053 (2018). doi:10.1002/anie.201805152, A nitrogen‐rich compound, $ReN_{8}·xN_{2}$, was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser‐heated diamond anvil cell. Single‐crystal X‐ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular‐shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, $ReN_{8}·xN_{2}$ is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [−N=N−]$_∞$ that constitute the framework have not been previously observed in any compound. Ab initio calculations on $ReN_{8}·xN_{2}$ provide strong support for the experimental results and conclusions., Published by Wiley-VCH, Weinheim

We have explored the high entropy alloy (HEA) concept in the AlTiVNbCr-nitride material system. (AlTiVNbCr)N coatings synthesized by reactive cathodic arc deposition are close to an ideal cubic solid solution with a positive mean-field enthalpy of mixing of 0.06 eV/atom. First principle calculations show a higher thermodynamic stability for the solid solution relative to their binaries thereby indicating a possible entropy stabilization at a temperature above 727 °C. However, the elevated temperature annealing experiments show that the solid solution decomposes to w-AlN and c-(TiVNbCr)N. The limited thermal stability of the solid solution is investigated in relation to several thermodynamic parameters. We suggest that the HEA designed multiprincipal element (AlTiVNbCr)N solid solutions are in a metastable state.

We give a comprehensive overview of the elastic properties of cubic quaternary transition metal nitride alloys and coherent nitride multilayers for design of wear resistant hard coatings. The elast ...

In this study, we present a computational technique to investigate non-equilibrium vacancies in metastable alloys. In contrast to equilibrium materials, calculation of the energy of removing an alloying atom requires a finite size configurational correction. We explain that in metastable alloys the vacancy formation energy is defined up to an arbitrary constant. We argue that in non-equilibrium alloys no statistical considerations are required and there is a distribution of the vacancy formation energy with large variation. We show for the example of Ti0.5Al0.5N that the configuration correction is significant. Using cluster expansion, we demonstrate the major importance of the first two metallic coordination shells in Ti0.5Al0.5N. We introduce a three-dimensional representation of the energies of removing Ti and Al atoms as a function of number of Al atoms in the neighboring shells and we predict the metal atom vacancy formation energy in arbitrary local chemical environments. Neglecting the interactions between the vacant sites and assuming their simultaneous occurrence, we show how to extract information about the energy cost of a vacancy concentration in metastable alloys. We conclude that in metastable disordered alloys vacancies should occur in local environments that correspond to the lowest formation energies rather than distributed statistically. JEL classification: 61.72.Bb, 61.72.jd, 61.66.Dk, Keywords: Metastable materials, Non-equilibrium, Vacancy formation energy, Hard coatings, Supercell approach

A combination of quantum mechanics calculations with machine learning (ML) techniques can lead to a paradigm shift in our ability to predict materials properties from first principles. Here we show that on-the-fly training of an interatomic potential described through moment tensors provides the same accuracy as state-of-the-art {\it ab inito} molecular dynamics in predicting high-temperature elastic properties of materials with two orders of magnitude less computational effort. Using the technique, we investigate high-temperature bcc phase of titanium and predict very weak, Elinvar, temperature dependence of its elastic moduli, similar to the behavior of the so-called GUM Ti-based alloys [T. Sato {\ it et al.}, Science {\bf 300}, 464 (2003)]. Given the fact that GUM alloys have complex chemical compositions and operate at room temperature, Elinvar properties of elemental bcc-Ti observed in the wide temperature interval 1100--1700 K is unique., Comment: 15 pages, 4 figures

Density functional theory is a standard model for condensed-matter theory and computational material science. The accuracy of density functional theory is limited by the accuracy of the employed approximation to the exchange-correlation functional. Recently, the so-called strongly constrained appropriately normed (SCAN) [Sun, Ruzsinszky, and Perdew, Phys. Rev. Lett. 115, 036402 (2015)] functional has received a lot of attention due to promising results for covalent, metallic, ionic, as well as hydrogen- and van der Waals-bonded systems alike. In this work, we focus on assessing the performance of the SCAN functional for itinerant magnets by calculating basic structural and magnetic properties of the transition metals Fe, Co, and Ni. We find that although structural properties of bcc-Fe seem to be in good agreement with experiment, SCAN performs worse than standard local and semilocal functionals for fcc-Ni and hcp-Co. In all three cases, the magnetic moment is significantly overestimated by SCAN, and the 3d states are shifted to lower energies, as compared to experiments. Funding Agencies|Swedish e-Science Research Centre (SeRC); Swedish Research Council (VR) through the International Career Grant [20146336]; Marie Sklodowska CurieActions, Cofund, Project [INCA 600398]; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 6 program; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; competence center FunMat-II - Vinnova [201605156]; Russian Science Foundation [18-12-00492]

A nitrogen-rich compound, ReN

The thermal expansion coefficient of technologically relevant multicomponent cubic nitride alloys are predicted using the Debye model with ab initio elastic constants calculated at 0 K and an isotropic approximation for the Gruneisen parameter. Our method is benchmarked against measured thermal expansion of TiN and Ti(1-x)Al x N as well as against results of molecular dynamics simulations. We show that the thermal expansion coefficients of Ti(1-x-y)X y Al x N (X = Zr, Hf, Nb, V, Ta) solid solutions monotonously increase with the amount of alloying element X at all temperatures except for Zr and Hf, for which they instead decrease for [Formula: see text].

Nature Communications 9(1), 2756 (2018). doi:10.1038/s41467-018-05143-2, Poly-nitrogen compounds have been considered as potential high energy density materialsfor a long time due to the large number of energetic N–NorN=N bonds. In most cases highnitrogen content and stability at ambient conditions are mutually exclusive, thereby makingthe synthesis of such materials challenging. One way to stabilize such compounds is theapplication of high pressure. Here, through a direct reaction between Fe and $N_2$ in a laser-heated diamond anvil cell, we synthesize three ironnitrogen compounds $Fe_3N_2$, $FeN_2$ and $FeN_4$. Their crystal structures are revealed by single-crystal synchrotron X-ray diffraction. $Fe_3N_2$, synthesized at 50 GPa, is isostructural to chromium carbide $Cr_3C_2$. $FeN_2$ has a mar-casite structure type and features covalently bonded dinitrogen units in its crystal structure. $FeN_4$, synthesized at 106 GPa, features polymeric nitrogen chains of [$N_4$$^{2−}]_n$ units. Based onresults of structural studies and theoretical analysis, [$N_4$$^{2−}]_n$ units in this compound revealcatena-poly[tetraz-1-ene-1,4-diyl] anions., Published by Nature Publishing Group, London

The special quasirandom structure (SQS) approach is a successful technique for modelling of alloys, however it breaks inherently the point symmetry of the underlying crystal lattice. We demonstrate that monocrystalline and polycrystalline elastic moduli can scatter significantly depending on the chosen SQS model and even on the supercell orientation in space. Also, we demonstrate that local disturbances, such as vacancies or interfaces change the SQS configuration in a way, that significantly affects the values of the calculated physical properties. Moreover, the diversity of local environments in random alloys results in a large variation of the calculated local properties. We underline that improperly chosen, generated or handled SQS may result in erroneous theoretical findings. The challenges of the SQS method are discussed using bulk Ti0.5Al0.5N alloy and TiN/Ti0.5Al0.5N multilayer as model systems. We present methodological corrections for the mindful application of this approach in studies of advanced properties of alloys.

In this work, we discuss the mixing thermodynamics of cubic (B1) Ti1-xAlxN/TiN(001) multilayers. We show that interfacial effects suppress the mixing enthalpy compared to bulk Ti1-xAlxN. The strong ...

The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)(1-x)(B13C2)(x), as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)(1-x)(B13C2)(x) is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young's and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)(1-x)(B13C2)(x) system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)(1-x)(B13C2)(x). The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Ti-ad and N-ad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Ti-ad and N-ad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that N-ad species are considerably more mobile than Ti-ad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, E-a = 1.03 for Ti-ad and 0.61 eV for N-ad, compared to 0 K values E-aOK = 1.55 (Ti-ad) and 0.79 eV (N-ad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111). (C) 2016 Elsevier B.V. All rights reserved. Funding Agencies|Knut and Alice Wallenberg Foundation [2011.0094]; Swedish Research Council (VR) [2014-5790]; Linkoping Linnaeus Initiative LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials through Swedens innovation agency VINNOVA [MatLiU 2009-00971]

By combining theoretical prediction and experimental verification we investigate the piezoelectric properties of yttrium indium nitride (YxIn1-xN). Ab initio calculations show that the YxIn1-xN wurtzite phase is lowest in energy among relevant alloy structures for 0

The potential of quarternary wurtzite TMx/2Mx/2Al1-xN (TM = Ti, Zr, Hf; M = Mg, Ca, Zn) alloys for piezoelectric applications is investigated using first-principles calculations. All considered alloys show increased piezoelectric response compared to pure AlN, and competing with the best ternary system proven to date: ScAlN. (Zr, Hf)(x/2)(Mg, Ca)(x/2)Al1-xN alloys are particularly promising. Calculations reveal positive mixing enthalpies indicative for phase separating systems; their values are smaller compared to related nitride alloys, which still can be grown as metastable thin films. The wurtzite phase of the alloys is lowest in energy at least up to x = 0.5 and for Tix/2Znx/2Al1-xN in the full composition range. Moreover, calculations reveal that wurtzite TM0.5Zn0.5N (TM = Ti, Zr, Hf) are piezoelectric alloys with d(33,f) = 19.95, 29.89, and 24.65 pC/N respectively, up to six times that of AlN. Finally, we discuss the physical origin behind the increased piezoelectric response and show that the energy difference between tetrahedrally coordinated zinc-blende (B3) and the layered hexagonal (B-k) phases of the TM0.5M0.5N alloy can be used as a descriptor in a high-throughput search for complex wurtzite alloys with high piezoelectric response. Funding Agencies|Knut and Alice Wallenberg Foundation Scholar Grant; Swedish Research Council (VR) Linkoping Linnaeus Initiative LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials [MatLiU 2009-00971]; Swedish Research Council [621-2011-4417, 330-2014-6336]; Swedish Foundation for Strategic Research (SSF) Programme SRL [10-0026]; Swedish Research Council (VR) [621-2011-4426]

Pseudomorphic stabilization in wurtzite Sc x Al 1− x N/AlN and Sc x Al 1− x N/In y Al 1− y N superlattices ( x = 0.2, 0.3, and 0.4; y = 0.2–0.72), grown by reactive magnetron sputter epitaxy was investigated. X-ray diffraction and transmission electron microscopy show that in Sc x Al 1− x N/AlN superlattices the compressive biaxial stresses due to positive lattice mismatch in Sc 0.3 Al 0.7 N and Sc 0.4 Al 0.6 N lead to the loss of epitaxy, although the structure remains layered. For the negative lattice mismatched In-rich Sc x Al 1− x N/In y Al 1− y N superlattices, a tensile biaxial stress promotes the stabilization of wurtzite Sc x Al 1− x N even for the highest investigated concentration x = 0.4. Ab initio calculations with fixed in-plane lattice parameters show a reduction in mixing energy for wurtzite Sc x Al 1− x N under tensile stress when x ⩾ 0.375 and corroborate the experimental results.

Ti1−xAlxN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C11 decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy. Funding agencies: Swedish Research Council (VR) [621-2011-4426, 621-2012-4401, 637-2013-7296]; Swedish Foundation for Strategic Research (SSF) [RMA08-0069, SRL10-0026]; VINNOVA [2013-02355(MC2)]; Erasmus Mundus Joint European Doctoral Program DocMASE; Ministry of EducationVid tidpunkten för disputation förelåg publikation som manuskript

Substituting Al for Ti in TiN(001), TiN(011), and N- and Ti-terminated TiN(111) surfaces has significant effects on adatom surface energetics which vary strongly with the adatom species and surface orientation. Here, we investigate Ti, Al, and N adatom surface dynamics using density functional methods. We calculate adatom binding and diffusion energies with both a nudged elastic band and grid-probing techniques. The adatom diffusivities are analyzed within a transition-state theory approximation. We determine the stable and metastable Ti, Al, and N binding sites on all three surfaces as well as the lowest energy migration paths. In general, adatom mobilities are fastest on TiN(001), slower on TiN(111), and slowest on TiN(011). The introduction of Al has two major effects on the surface diffusivity of Ti and Al adatoms. First, Ti adatom diffusivity on TiN(001) is significantly reduced near substituted Al surface atoms; we observe a 200% increase in Ti adatom diffusion barriers out of fourfold hollow sites adjacent to Al surface atoms, while Al adatom diffusivity between bulk sites is largely unaffected. Secondly, on TiN(111), the effect is opposite; Al adatoms are slowed near the substituted Al surface atom, while Ti adatom diffusivity is largely unaffected. In addition, we note the importance of magnetic spin polarization on Ti adatom binding energies and diffusion path. These results are of relevance for the atomistic understanding of Ti1 − xAlxN alloy and Ti1 − xAlxN/TiN multilayer thin-film growth processes.

The phase diagrams of the Ti-Zn-N, Zr-Zn-N, and Hf-Zn-N systems are determined using large-scale high-throughput density functional calculations. Thermodynamically stable ordered phases of TiZnN2, ZrZnN2, and HfZnN2 have been found to be promising candidates in piezoelectric devices/applications for energy harvesting. The identified stable phase of TiZnN2 is an ordered wurtzite superstructure, and the stable phases of ZrZnN2 and HfZnN2 have a layered structure with alternating tetrahedral ZnN and octahedral (Zr, Hf)N layers. All of the TMZnN2 (TM = Ti, Zn, Hf) structures exhibit electronic bandgaps and large piezoelectric constants, d33TiZnN2=14.21, d24ZrZnN2=−26.15, and d24HfZnN2=−21.99 pC/N. The strong piezoelectric responses and their thermodynamical stability make materials with these phases promising candidates for piezoelectric applications.

Recently, ScAlN alloys attracted attention for their giant piezoelectric moduli. In this study the piezoelectric response of the wurtzite group-III nitrides AlN, GaN, and InN mixed with 50 mol% of ...

Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications. Funding Agencies|Swedish Research Council|621-2008-5535621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs|10-0026|project Designed Multicomponent Coatings (MultiFilms)||Erasmus Mundus doctoral program DocMase||Ministry of Education and Science of the Russian Federation within the framework of Program Research and Pedagogical Personnel for Innovative Russia|14.B37.21.089010.09.2012