Possible pitfalls in theoretical determination of ground-state crystal structures: The case of platinum nitride (original) (raw)
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We report an ab initio study of the mechanical stability of platinum nitride (PtN), in four different crystal structures, the rock salt (rs-PtN), zinc blende (zb-PtN), cooperite and a face centered orthorhombic phase. Only the rs-PtN phase is found to be stable and has the highest bulk modulus B = 284 GPa. Its electronic density of states shows no band gap making it metallic. The zb-PtN phase does not stabilize or harden by the nitrogen vacancies investigated in this study. Therefore, the experimental observation of super hardness in PtN remains a puzzle.
Chinese Journal of Physics, 2017
The structural phase transformations of the PtN compound with a 1:1 stoichiometric ratio of Pt:N were investigated using the framework of density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient (PBE-GGA) and the Engel-Vosko generalized gradient (EV-GGA) approximations were used. A comparative study of the experimental and theoretical results is provided on the structural properties of zinc-blende (ZB), rock-salt (RS), cesium chloride (CsCl), wurtzite (WZ), nickel arsenide (NiAs), lead monoxide (PbO), and tungsten carbide (WC) phases. The calculated band structure using the modified version of the Becke and Johnson (mBJ) exchange potential reveals the metallic character of the PtN compound. The present study also shows that the PtN compound crystallizes in the WZ phase under ambient conditions. The theoretical transition pressures from WZ to RS, NiAs, PbO, and CsCl transformations are found to be 9.441 GPa, 7.705 GPa, 18.345 GPa and 31.9 GPa, respectively, using the PBE-GGA method.
Structures and electronic properties of platinum nitride by density functional theory
Physical Review B, 2005
We present the results of ground state electronic structure calculations of the zinc-blende and rocksalt phases of binary platinum nitride ͑PtN͒ using density functional theory. Several exchange-correlation functionals including the local spin density approximations, generalized gradient approximations ͑GGA͒, a nonempirical meta-GGA, and a screened Coulomb hybrid functional have been employed. We use Gaussian type orbitals within the framework of periodic boundary conditions. Our results confirm earlier findings, in that the zincblende structure of PtN is energetically more stable than the rocksalt structure. The predicted energy difference between the two phases is rather small with the more elaborate functionals. Both phases are predicted to be metallic and extended Pt d-N p hybridizations are found in both of them. We have also calculated the phase transition pressure between both phases. The bulk modulus of the zinc-blend phase of PtN is significantly higher than that of bulk platinum.
Crystal structure, electronic structure and magnetism of transition metal nitrides
physica status solidi (c), 2005
A systematic computational study of the relative stability of the zinblende (ZB) versus rocksalt (NaCl or RS) structure of the transition metal nitrides (TMN) is presented. The early TMN prefer NaCl, the later ones ZB with the crossing occuring at MnN. The minimum energy lattice constant of the ZB phase is always significantly larger than that of RS. The TMN are shown to have a stronger tendency to be magnetic in the RS than in the ZB phase.
Mechanical stability of possible structures of PtN investigated using first-principles calculations
Physical Review B, 2006
We report an ab initio study of the mechanical stability of platinum nitride ͑PtN͒, in four different crystal structures, the rock salt ͑rs-PtN͒, zinc-blende ͑zb-PtN͒, cooperite, and a face-centered orthorhombic phase. Of these phases only the rs-PtN phase is found to be stable and has the highest bulk modulus B = 284 GPa. Its electronic density of states shows no band gap making it metallic. The zb-PtN phase does not stabilize or harden by the nitrogen vacancies investigated in this study. Therefore, the experimental observation of super hardness in PtN remains a puzzle.
VLSI Design, 2001
A set of software tools for the determination of the band structure of zinc-blende, wurtzite, 4H, and 6H semiconductors is presented. A state of the art implementation of the nonlocal empirical pseudopotential method has been coupled with a robust simplex algorithm for the optimization of the adjustable parameters of the model potentials. This computational core has been integrated with an array of Matlab functions, providing interactive functionalities for defining the initial guess of the atomic pseudopotentials, checking the convergence of the optimization process, plotting the resulting band structure, and computing detailed information about any local minimum. The results obtained for wurtzite-phase III-nitrides (ALN, GaN, InN) are presented as a relevant case study.
Global space-group optimization problem: Finding the stablest crystal structure without constraints
Finding the most stable structure of a solid is one of the central problems in condensed matter physics. This entails finding both the lattice type ͑e.g., fcc, bcc, and orthorhombic͒ and ͑for compounds͒ the decoration of the lattice sites by atoms of types A, B, etc. ͑"configuration"͒. Most approaches to this problem either assumed that both lattice type and configuration are known, optimizing instead the cell volume and performing local relaxation. Other approaches assumed that the lattice type is known, searching for the minimum-energy decoration. We present here an approach to the global space-group optimization ͑GSGO͒ problem, i.e., the problem of predicting both the lattice structure and the atomic configuration of a crystalline solid. This search method is based on an evolutionary algorithm within which a population of crystal structures is evolved through mating and mutation operations, improving the population by substituting the highest total-energy structures with new ones. The crystal structures are not represented by bit strings as in conventional genetic algorithms. Instead, the evolutionary search is performed directly on the atomic positions and the unit-cell vectors after a similarity transformation is applied to bring structures of different unit-cell shapes to a common basis. Following this transformation, we can define a crossover operation that treats, on the same footing, structures with different unit-cell shapes. Once a new structure has been generated by mating or mutation, it is fully relaxed to the closest local total-energy minimum. We applied our procedure for the GSGO in the context of pseudopotential total-energy calculations to the semiconductor systems Si, SiC, and GaAs and to the metallic alloy AuPd with composition Au 8 Pd 4 . Starting from random unit-cell vectors and random atomic positions, the present search procedure found for all semiconductor systems studied the correct lattice structure and configuration. In the case of Au 8 Pd 4 , the search retrieved the correct underlying fcc lattice, but energetically closely spaced ͑ϳ2 meV/ at.͒ alloy configurations were not resolved. This approach to GSGO opens the way to predicting unsuspected structures by direct optimization using, in the cases noted above, an order of 100 total-energy ab initio calculations.
Physical Review B, 2001
A number of diverse bulk properties of the zincblende and wurtzite III-V nitrides AlN, GaN, and InN, are predicted from first principles within density functional theory using the plane-wave ultrasoft pseudopotential method, within both the LDA (local density) and GGA (generalized gradient) approximations to the exchange-correlation functional. Besides structure and cohesion, we study formation enthalpies (a key ingredient in predicting defect solubilities and surface stability), spontaneous polarizations and piezoelectric constants (central parameters for nanostructure modeling), and elastic constants. Our study bears out the relative merits of the two density functional approaches in describing diverse properties of the III-V nitrides (and of the parent species N2, Al, Ga, and In), and leads us to conclude that the GGA approximation, associated with high-accuracy techniques such as multiprojector ultrasoft pseudopotentials or modern all-electron methods, is to be preferred in the study of III-V nitrides.