1,115 research outputs found
Structure and lattice dynamics of the wide band gap semiconductors MgSiN and MgGeN
We have determined the structural and lattice dynamical properties of the
orthorhombic, wide band gap semiconductors MgSiN and MgGeN using
density functional theory. In addition, we present structural properties and
Raman spectra of a MgSiN powder. The structural properties and lattice
dynamics of the orthorhombic systems are compared to wurtzite AlN. We find
clear differences in the lattice dynamics between MgSiN, MgGeN and
AlN, for example we find that the highest phonon frequency in MgSiN is
about 100~cm higher than the highest frequency in AlN and that
MgGeN is much softer. We also provide the Born effective charge tensors
and dielectric tensors of MgSiN, MgGeN and AlN. Phonon related
thermodynamic properties, such as the heat capacity and entropy, are in very
good agreement with available experimental results.Comment: 9 pages, 11 figures, 6 table
Tailoring ferromagnetic chalcopyrites
If magnetic semiconductors are ever to find wide application in real
spintronic devices, their magnetic and electronic properties will require
tailoring in much the same way that band gaps are engineered in conventional
semiconductors. Unfortunately, no systematic understanding yet exists of how,
or even whether, properties such as Curie temperatures and band gaps are
related in magnetic semiconductors. Here we explore theoretically these and
other relationships within 64 members of a single materials class, the Mn-doped
II-IV-V2 chalcopyrites, three of which are already known experimentally to be
ferromagnetic semiconductors. Our first-principles results reveal a variation
of magnetic properties across different materials that cannot be explained by
either of the two dominant models of ferromagnetism in semiconductors. Based on
our results for structural, electronic, and magnetic properties, we identify a
small number of new stable chalcopyrites with excellent prospects for
ferromagnetism.Comment: 6 pages with 4 figures, plus 3 supplementary figures; to appear in
Nature Material
Accessible chemical space for metal nitride perovskites
Building on the extensive exploration of metal oxide and metal halide perovskites, metal nitride perovskites represent a largely unexplored class of materials. We report a multi-tier computational screening of this chemical space. From a pool of 3660 ABN3 compositions covering I–VIII, II–VII, III–VI and IV–V oxidation state combinations, 279 are predicted to be chemically feasible. The ground-state structures of the 25 most promising candidate compositions were explored through enumeration over octahedral tilt systems and global optimisation. We predict 12 dynamically and thermodynamically stable nitride perovskite materials, including YMoN3, YWN3, ZrTaN3, and LaMoN3. These feature significant electric polarisation and low predicted switching electric field, showing similarities with metal oxide perovskites and making them attractive for ferroelectric memory devices
Accessible Chemical Space for Metal Nitride Perovskites
Building on the extensive exploration of metal oxide and metal halide
perovskites, metal nitride perovskites represent a largely unexplored class of
materials. We report a multi-tier computational screening of this chemical
space. From a pool of 3660 ABN compositions covering I-VIII, II-VII, III-VI
and IV-V oxidation state combinations, 279 are predicted to be chemically
feasible. The ground-state structures of the 25 most promising candidate
compositions were explored through enumeration over octahedral tilt systems and
global optimisation. We predict 12 dynamically and thermodynamically stable
nitride perovskite materials, including YMoN, YWN, ZrTaN, and
LaMoN. These feature significant electric polarisation and low predicted
switching electric field, showing similarities with metal oxide perovskites and
making them attractive for ferroelectric memory device
ZONE CENTRE PHONON MODE BEHAVIOUR OF CUBIC AND WURTZITE PHASE OF TERNARY InxGa1-xN
The group-III nitrides have been acknowledged as notable materials for researchers in recent times because of their extra ordinary properties and applications. The fundamental property of these materials is their wide and direct band gap, which can also be tailored by doping. A common characteristic of these device structures is the applicability of their ternary alloys. Despite of the broad range of the ternary alloys of these group-III nitrides only some have been discussed. So in this study, zone centre phonon mode behavior of the ternary alloy InxGa1-xN in both phases (cubic and wurtzite phase) has been studied and with the use of de Launey angular force constant model. The various optical phonon modes at zone centre have been calculated for both phases of InxGa1-xN. The content of Ga and in alloy is in the range 0<x<1. The one mode behavior has been found for both phases of the alloy and it is found that for InxGa1-xN continuous decreases in magnitude of phonon frequency with the increase in the content of In which is due to the fact that frequency varies inversely proportional to the mass, as content of In increases mass of alloy increases
High-throughput screening of the thermoelastic properties of ultrahigh-temperature ceramics
Ultrahigh-temperature ceramics (UHTCs) are a group of materials with high technological interest because of their applications in extreme environments. However, their characterization at high temperatures represents the main obstacle for their fast development. Obstacles are found from an experimental point of view, where only few laboratories around the world have the resources to test these materials under extreme conditions, and also from a theoretical point of view, where actual methods are expensive and difficult to apply to large sets of materials. Here, a new theoretical high-throughput framework for the prediction of the thermoelastic properties of materials is introduced. This approach can be systematically applied to any kind of crystalline material, drastically reducing the computational cost of previous methodologies up to 80% approximately. This new approach combines Taylor expansion and density functional theory calculations to predict the vibrational free energy of any arbitrary strained configuration, which represents the bottleneck in other methods. Using this framework, elastic constants for UHTCs have been calculated in a wide range of temperatures with excellent agreement with experimental values, when available. Using the elastic constants as the starting point, other mechanical properties such a bulk modulus, shear modulus, or Poisson ratio have been also explored, including upper and lower limits for polycrystalline materials. Finally, this work goes beyond the isotropic mechanical properties and represents one of the most comprehensive and exhaustive studies of some of the most important UHTCs, charting their anisotropy and thermal and thermodynamical properties.Ministerio de Ciencia e Innovación PID2019-106871GB-I00European Union 752608Red Española de Supercomputación QS-2019-2-0006, QS-2019-3-0021, QS-2020-2-003
Higher-order Generalized Hydrodynamics: Foundations Within A Nonequilibrium Statistical Ensemble Formalism.
Construction, in the framework of a nonequilibrium statistical ensemble formalism, of a higher-order generalized hydrodynamics, also referred to as mesoscopic hydrothermodynamics, that is, covering phenomena involving motion of fluids displaying variations short in space and fast in time-unrestricted values of Knudsen numbers, is presented. In that way, an approach is provided enabling the coupling and simultaneous treatment of the kinetics and hydrodynamic levels of descriptions. It is based on a complete thermostatistical approach in terms of the densities of matter and energy and their fluxes of all orders covering systems arbitrarily driven away from equilibrium. The set of coupled nonlinear integrodifferential hydrodynamic equations is derived. They are the evolution equations of the Gradlike moments of all orders, derived from a generalized kinetic equation built in the framework of the nonequilibrium statistical ensemble formalism. For illustration, the case of a system of particles embedded in a fluid acting as a thermal bath is fully described. The resulting enormous set of coupled evolution equations is of unmanageable proportions, thus requiring in practice to introduce an appropriate description using the smallest possible number of variables. We have obtained a hierarchy of Maxwell times, associated to the set of all the higher-order fluxes, which have a particular relevance in the process of providing criteria for establishing the contraction of description.9106301
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