Configurational order-disorder induced metal-nonmetal transition in B13_{13}C2_{2} studied with first-principles superatom-special quasirandom structure method

Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B$_{13}$C$_{2}$ has been a controversial topic in the field of icosahedral boron-rich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while ordered ground state B$_{13}$C$_{2}$ is metallic, configurationally disordered B$_{13}$C$_{2}$, modeled with a superatom-special quasirandom structure method, goes through a metal to non-metal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chain-end carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a B$_{12}$ icosahedron bonded to it, giving rise to a B$_{11}$C$^{e}$(BBC) unit. The atomic configuration of the substitutionally disordered B$_{13}$C$_{2}$ thus tends to be dominated by a mixture between B$_{12}$(CBC) and B$_{11}$C$^{e}$(BBC). Due to splitting of valence states in B$_{11}$C$^{e}$(BBC), the electron deficiency in B$_{12}$(CBC) is gradually compensated

Thermodynamic stability and properties of boron subnitrides from first principles

We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different alpha-rhombohedral-boron-like boron subnitrides, with the compositions of B6N, B13N2, and B38N6, proposed in the literature. We find that, out of these subnitrides with the structural units of B-12(N-N), B-12(NBN), and [B-12(N-N)](0.33)[B-12(NBN)](0.67), respectively, only B38N6, represented by [B-12(N-N)](0.33)[B-12(NBN)](0.67), is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and a-tetragonalboron- like boron subnitride B50N2. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B-12(N-N)](0.33)[B-12(NBN)](0.67) is bothmechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B-12(N-N)](0.33)[B-12(NBN)](0.67) are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of a-rhombohedral-boron-like boron subnitride

Carbon-rich icosahedral boron carbides beyond B4 C and their thermodynamic stabilities at high temperature and pressure from first principles

We investigate the thermodynamic stability of carbon-rich icosahedral boron carbide at different compositions, ranging from B4C to B2C, using first-principles calculations. Apart from B4C, generally addressed in the literature, B2.5C, represented by B10C2p(C-C), where Cp and (C-C) denote a carbon atom occupying the polar site of the icosahedral cluster and a diatomic carbon chain, respectively, is predicted to be thermodynamically stable under high pressures with respect to B4C as well as pure boron and carbon phases. The thermodynamic stability of B2.5C is determined by the Gibbs free energy G as a function of pressure p and temperature T, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. The stability range of B2.5C is then illustrated through the p-T phase diagrams. Depending on the temperatures, the stability range of B2.5C is predicted to be within the range between 40 and 67 GPa. At T 500 K, the icosahedral Cp atoms in B2.5C configurationally disorder at the polar sites. By investigating the properties of B2.5C, e.g., elastic constants and phonon and electronic density of states, we demonstrate that B2.5C is both mechanically and dynamically stable at zero pressure, and is an electrical semiconductor. Furthermore, based on the sketched phase diagrams, a possible route for experimental synthesis of B2.5C as well as a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested. © 2016 American Physical Society

Structural models of increasing complexity for icosahedral boron carbide with compositions throughout the single-phase region from first principles

We perform first-principles calculations to investigate the phase stability of boron carbide, concentrating on the recently proposed alternative structural models composed not only of the regularly studied B11Cp(CBC) and B12(CBC), but also of B12(CBCB) and B12(B4). We find that a combination of the four structural motifs can result in low-energy electron precise configurations of boron carbide. Among several considered configurations within the composition range of B10.5C and B4C, we identify in addition to the regularly studied B11Cp(CBC) at the composition of B4C two low-energy configurations, resulting in a new view of the B-C convex hull. Those are [B12(CBC)]0.67[B12(B4)]0.33 and [B12(CBC)]0.67[B12(CBCB)]0.33, corresponding to compositions of B10.5C and B6.67C, respectively. As a consequence, B12(CBC) at the composition of B6.5C, previously suggested in the literature as a stable configuration of boron carbide, is no longer part of the B-C convex hull. By inspecting the electronic density of states as well as the elastic moduli, we find that the alternative models of boron carbide can provide a reasonably good description for electronic and elastic properties of the material in comparison with the experiments, highlighting the importance of considering B12(CBCB) and B12(B4), together with the previously proposed B11Cp(CBC) and B12(CBC), as the crucial ingredients for modeling boron carbide with compositions throughout the single-phase region. © 2018 American Physical Society

Role of spin-orbit coupling in the alloying behavior of multilayer Bi1-xSbx solid solutions revealed by a first-principles cluster expansion

We employ a first-principles cluster-expansion method in combination with canonical Monte Carlo simulations to study the effect of spin-orbit coupling on the alloying behavior of multilayer Bi1-xSbx. Our simulations reveal that spin-orbit coupling plays an essential role in determining the configurational thermodynamics of Bi and Sb atoms. Without the presence of spin-orbit coupling, Bi1-xSbx is predicted to exhibit at low-temperature chemical ordering of Bi and Sb atoms, leading to formation of an ordered structure at x approximate to 0.5. Interestingly, the spin-orbit-coupling effect intrinsically induced by the existence of Bi and Sb results in the disappearance of chemical ordering of the constituent elements within an immiscible region existing at T &amp;lt; 370 K, and consequently Bi1-xSbx displays merely a tendency toward local segregation of Bi and Sb atoms, resulting in coexistence of Bi-rich and Sb-rich Bi1-xSbx solid solutions without the formation of any ordered structure of Bi1-xSbx as predicted in the absence of spin-orbit coupling. These findings distinctly highlight an influence of spin-orbit coupling on the alloying behavior of Bi1-xSbx and probably other alloys composed of heavy elements, where the spin-orbit-coupling effect is supposed to be robust.Funding Agencies|Chulalongkorn UniversityChulalongkorn University [CUGR_62_66_23_26]; Thailand Toray Science Foundation (TTSF); Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Research Council (VR)Swedish Research Council [2019-05403]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]</p

First-principles prediction of stabilities and instabilities of compounds and alloys in the ternary B-As-P system

We examine the thermodynamic stability of compounds and alloys in the ternary B-As-P system theoretically using first-principles calculations. We demonstrate that the icosahedral B12As2 is the only stable compound in the binary B-As system, while the zinc-blende BAs is thermodynamically unstable with respect to B12As2 and the pure arsenic phase at 0 K, and increasingly so at higher temperature, suggesting that BAs may merely exist as a metastable phase. On the contrary, in the binary B-P system, both zinc-blende BP and icosahedral B12P2 are predicted to be stable. As for the binary As-P system, As1-xPx disordered alloys are predicted at elevated temperature-for example, a disordered solid solution of up to similar to 75 at.% As in black phosphorus as well as a small solubility of similar to 1 at.% P in gray arsenic at T = 750 K, together with the presence of miscibility gaps. The calculated large solubility of As in black phosphorus explains the experimental syntheses of black-phosphorus-type As1-xPx alloys with tunable compositions, recently reported in the literature. We investigate the phase stabilities in the ternary B-As-P system and demonstrate a high tendency for a formation of alloys in the icosahedral B-12(As1-xPx)(2) structure by intermixing of As and P atoms at the diatomic chain sites. The phase diagram displays noticeable mutual solubility of the icosahedral subpnictides in each other even at room temperature as well as a closure of a pseudobinary miscibility gap around 900 K. As for pseudobinary BAs1-xPx alloys, only a tiny amount of BAs is predicted to be able to dissolve in BP to form the BAs1-xPx disordered alloys at elevated temperature. For example, less than 5% of BAs can dissolve in BP at T = 1000 K. The small solubility limit of BAs in BP is attributed to the thermodynamic instability of BAs with respect to B12As2 and As

A comparison of the mixing thermodynamics of the antifluorite-structured Mg2Si1-xGex, Mg2Sn1-xGex and Mg2Si1-xSnx alloys from first principles

The mixing thermodynamics of the antifluorite-structured Mg2Si1-xGex is investigated using the first-principles calculations. We find that Mg2Si and Mg2Ge readily mix with each other leading to formation of a single-phase random solid solutions of Mg2Si1-xGex across the entire composition range from the temperature of about 50 K and above. At 0 K, Mg2Si1-xGex exhibits a weak energy preference toward local phase segregation into Mg2Si and Mg2Ge without forming any ordered patterns of Si and Ge atoms. Through a comparison with the mixing thermodynamics of Mg2Sn with Mg2Si or Mg2Ge, a small lattice misfit between Mg2Si and Mg2Ge of less than 1 % is responsible for the formation of stable Mg2Si1-xGex random solid solutions at such a low temperature. Besides their thermodynamic stability, our prediction reveals that the random solid solutions of Mg2Si1-xGex are dynamically and mechanically stable. These findings justify the uses of structural models of Mg2Si1-xGex, assuming a random distribution of Si and Ge atoms in the previous theoretical studies, and also provide an insight into the complete solubility of Mg2Ge in Mg2Si and vice versa at all temperature where the atomic diffusion is activated.Funding Agencies|Thailand Toray Science Foundation (TTSF); Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University, Faculty Grant SFOMatLiU [2009 00971]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Research Council (VR)Swedish Research Council [2019-05403]; Knut and Alice Wallenberg Foundation, Sweden (Wallenberg Scholar Grant) [KAW-2018.0194]</p

Boron vacancy-driven thermodynamic stabilization and improved mechanical properties of AlB2-type tantalum diborides as revealed by first-principles calculations

Thermodynamic stability as well as structural, electronic, and elastic properties of boron-deficient AlB _2 -type tantalum diborides, which is designated as $\alpha-$ TaB $_{2-x}$ , due to the presence of vacancies at its boron sublattice are studied via first-principles calculations. The results reveal that $\alpha-$ TaB $_{2-x}$ , where 0.167 $\lesssim\,x\,\lesssim$ 0.25, is thermodynamically stable even at absolute zero. On the other hand, the shear and Young’s moduli as well as the hardness of stable $\alpha-$ TaB $_{2-x}$ are predicted to be superior as compared to those of $\alpha-$ TaB _2 . The changes in the relative stability and also the elastic properties of $\alpha-$ TaB $_{2-x}$ with respect to those of $\alpha-$ TaB _2 can be explained by the competitive effect between the decrease in the number of electrons filling in the antibonding states of $\alpha-$ TaB _2 and the increase in the number of broken bonds around the vacancies, both induced by the increase in the concentration of boron vacancies. A good agreement between our calculated lattice parameters, elastic moduli and hardness of $\alpha-$ TaB $_{2-x}$ and the experimentally measured data of as-synthesized AlB _2 -type tantalum diborides with the claimed composition of TaB $_{\thicksim2}$ , available in the literature, suggests that, instead of being a line compound with a stoichiometric composition of TaB _2 , AlB _2 -type tantalum diboride is readily boron-deficient, and its stable composition in equilibrium may be ranging at least from TaB $_{\thicksim1.833}$ to TaB $_{\thicksim1.75}$ . Furthermore, the substitution of vacancies for boron atoms in $\alpha-$ TaB _2 is responsible for destabilization of WB _2 -type tantalum diboride and orthorhombic Ta _2 B _3 , predicted in the previous theoretical studies to be thermodynamically stable in the Ta−B system, and it thus enables the interpretation of why the two compounds have never been realized in actual experiments