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Register a new userStability of hypothetical AgIICl2 polymorphs under high pressure, revisited : a computational study
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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.
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We performed a series of high-pressure synchrotron X-ray diffraction (XRD) and resistance measurements on the Weyl semimetal NbAs. The crystal structure remains stable up to 26 GPa according to the powder XRD data. The resistance of NbAs single crystal increases monotonically with pressure at low temperature. Up to 20 GPa, no superconducting transition is observed down to 0.3 K. These results show that the Weyl semimetal phase is robust in NbAs, and applying pressure is not a good way to get a topological superconductor from a Weyl semimetal.
This article reports the study of SnO by using the first-principles pseudopotential plane-wave method within the generalized gradient approximation (GGA). We have calculated the structural, elastic, electronic and optical of SnO under high pressure. The elastic properties such as the elastic constants Cij bulk modulus, shear modulus, Young modulus, anisotropic factor, Pugh ratio, Poisson ratio are calculated and analyzed. Mechanical stability of SnO at all pressure are confirmed by using Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. It is also found that SnO exhibits very high anisotropy. The energy band structure and density of states are also calculated and analyzed. The results show the semiconducting and metallic properties at 0 (zero) and high pressure, respectively. Furthermore, the optical properties such as dielectric function, refractive index, photoconductivity, absorption coefficients, loss function and reflectivity are also calculated. All the results are compared with those of the SnO where available but most of the results at high pressure are not compared due to unavailability of the results.
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.
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.
By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from the monoclinic I2/m symmetry stable at ambient conditions to cubic Im-3 symmetry. Considering that the same structural transition occurs at 653 K upon heating at ambient pressure, we propose a rare scenario of reentrant-type phase transition. In the high-pressure Im-3 phase, the Jahn-Teller distortion of the MnO6 octahedra and the orbital order present in the I2/m phase are suppressed, which is promising to investigate the possibility of pressure-induced Mott insulator-metal transition in the ideal situation of no structural distortions. The observation of a progressive line broadening of almost all Raman modes with pressure suggests that this transition may be incipient above 20 GPa.
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