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Register a new userBond-bending modes and stability of tetrahedral semiconductors under high pressure: a puzzle of AlN
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Lattice vibrations of the wurtzite-type AlN have been studied by Raman spectroscopy under high pressure up to the structural phase transition at 20 GPa. We have shown that the widely debated bond-bending E_2^1 mode of w-AlN has an abnormal positive pressure shift up to the threshold of the phase transition, whereas in many tetrahedral semiconductors the bond-bending modes soften on compression. This finding disagrees with the results of ab initio calculations, which give a normal negative pressure shift. Combination of high dynamical and low thermodynamical stability of AlN breaks the correlation between the mode Gruneisen parameters for the bond-bending modes and the transition pressure, which holds for CdS, InP, ZnO, ZnTe, ZnSe, ZnS, Ge, Si, GaP, GaN, SiC and BeO.
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A comparative computational study of stability of candidate structures for an as yet unknown silver dichloride AgCl2 is presented. It is found that all considered candidates have a negative enthalpy of formation, but are unstable towards charge transfer and decomposition into silver(I) chloride and chlorine within the DFT and hybrid DFT approaches in the entire studied pressure range. Within SCAN approach, several of the true AgIICl2 polymorphs (i.e. containing Ag(II) species) exhibit a region of stability below ca. 20 GPa. However, their stability with respect to aforementioned decomposition decreases with pressure by account of all three DFT methods, which suggests a limited possibility of high pressure synthesis of AgCl2. Some common patterns in pressure induced structural transitions observed in the studied systems also emerge, which further testify to an instability of hypothetical AgCl2 towards charge transfer and phase separation.
The Kitaev model of spin-1/2 on a honeycomb lattice supports degenerate topological ground states and may be useful in topological quantum computation. Na$_{2}$IrO$_{3}$ with honeycomb lattice of Ir ions have been extensively studied as candidates for the realization of the this model, due to the effective $J_{text{eff}}=1/2$ low-energy excitations produced by spin-orbit and crystal-field effect. As the eventual realization of Kitaev model has remained evasive, it is highly desirable and challenging to tune the candidate materials toward such end. It is well known external pressure often leads to dramatic changes to the geometric and electronic structure of materials. In this work, the high pressure phase diagram of Na$_{2}$IrO$_{3}$ is examined by first-principles calculations. It is found that Na$_{2}$IrO$_{3}$ undergoes a sequence of structural and magnetic phase transitions, from the magnetically ordered phase with space group $C2/m$ to two bond-ordered non-magnetic phases. The low-energy excitations in these high-pressure phases can be well described by the $J_{text{eff}}=1/2$ states.
Nitrogen oxides are textbook class of molecular compounds, with extensive industrial applications. Nitrogen and oxygen are also among the most abundant elements in the universe. We explore the N-O system at 0 K and up to 500 GPa though ab initio evolutionary simulations. Results show that two phase transformations of stable molecular NO2 exist at 7 and 64 GPa, and followed by decomposition of NO2 at 91 GPa. All of the NO+NO3- structures are found to be metastable at T=0 K, so experimentally reported ionic NO+NO3- is either metastable or stabilized by temperature. Upon increasing pressure, N2O5 transforms from P-1 to C2/c structure at 51 GPa. NO becomes thermodynamically stable at 198 GPa. This polymeric phase is superconducting (Tc = 2.0 K) and contains a -N-N- backbone.
Concurrent molecular dynamics simulations and ab initio calculations show that densification of silica under pressure follows a ubiquitous two-stage mechanism. First, anions form a close-packed sub-lattice, governed by the strong repulsion between them. Next, cations redistribute onto the interstices. In cristobalite silica, the first stage is manifest by the formation of a metastable phase, which was observed experimentally a decade ago, but never indexed due to ambiguous diffraction patterns. Our simulations conclusively reveal its structure and its role in the densification of silica.
Melting of orthorhombic boron silicide B6Si has been studied at pressures up to 8 GPa using in situ electrical resistivity measurements and quenching. It has been found that in the 2.6-7.7 GPa range B6Si melts congruently, and the melting curve exhibits negative slope of -31(2) K/GPa that points to a higher density of the melt as compared to the solid phase. At very high temperatures B6Si melt appears to be unstable and undergoes disproportionation into silicon and boron-rich silicides. The onset temperature of disproportionation strongly depends on pressure, and the corresponding low-temperature boundary exhibits negative slope of -92(3) K/GPa which is indicative of significant volume decrease in the course of B6Si melt decomposition.
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