We investigate lattice ordering phenomena for the heterovalent ternaries that are based on the wurtzite lattice, under the constraint that the octet rule be preserved. We show that, with the single ...exception of a highly symmetric twinned structure, all allowed lattice orderings can be described by a pseudospin model corresponding to the two different stackings ABAB of rows of atoms in the basal plane that occur in the Pna2sub 1 and Pmc2sub 1 crystal structures. The Raman and photo-luminescence spectra of ZnSnNsub 2 are interpreted in light of the ordering model developed here. The observation that ZnSnNsub 2 orders in the Pna2sub 1 structure under appropriate growth conditions is consistent with the larger difference in the energies of formation of Pna2sub 1 the and Pmc2sub 1 structures for this material. The ordering model presented here has important implications for the optical, electronic, and lattice properties of all wurtzite-based heterovalent ternaries.
Higher order topological tnsulators are d-spatial dimensional systems featuring topologically protected gapless states at their (d − n) -dimensional boundaries. With the help of ab initio ...calculations and tight-binding models along with symmetry considerations, we show that monolayer buckled honeycomb structures of group-V elements (Sb, As), which have already been synthesized, belong in this category and have a spinless charge fractionalization of e/2 at the corner states as well as weak topological edge states, protected by the S6 symmetry operation, which classify this system as a quadrupole topological insulator. The robustness of these edge and corner states to perturbations is explicitly demonstrated.
The trends in electronic band structure are studied in the cubic ABX3 halide perovskites for A = Cs; B = Pb, Sn, Ge, Si; and X = I, Br, Cl. The gaps are found to decrease from Pb to Sn and from Ge to ...Si, but increase from Sn to Ge. The trend is explained in terms of the atom s levels of the group-IV element and the atomic sizes which changes the amount of hybridization with X-p and hence the valence bandwidth. Both effects compensate each other to a certain degree. The trend with halogens is to reduce the gap from Cl to I, i.e., with decreasing electronegativity. The role of the tolerance factor in avoiding octahedron rotations and octahedron edge sharing is discussed. We find the rhombohedral phase to have lower total energy and slightly larger gaps but to present a less significant distortion of the band structure than the edge-sharing octahedral phases, such as the yellow phase in CsSnI3.
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