Barium Sulfide under Pressure: Discovery of Metastable Polymorphs and Investigation of Electronic Properties on ab Initio Level

Abstract

Barium sulfide (BaS) is an important precursor to other barium compounds with applications from ceramics and flame retardants to luminous paints and additives, and recent research shows potential technological applications in electrical and optical devices. Under normal conditions, BaS crystallizes in the NaCl type of structure, and with the increase in pressure BaS undergoes a structural phase transition to a CsCl type modification. This study presents modeling of barium sulfide under pressure with special focus on structural aspects and electronic properties. We predict metastable BaS polymorphs which have not yet been observed in the experiment or in previous calculations, and we investigated their vibrational and thermodynamical properties. Furthermore, we investigate the electronic properties of experimentally known structures as well as novel predicted modifications of BaS on ab initio level using Hartree-Fock, GGA-PBE, and the hybrid B3LYP functional. In this way, we address new possibilities of synthesizing BaS and possible band gap tuning which can have great applications in optoelectrical technologies.

Peculiar bond characters of fivefold coordinated octet compound crystals

“…The band and bond pictures of solids offer complementary perspectives, with the latter conducive to chemical intuition especially in the presence of interfaces and defects. 1,2 Chemical bond characters of a material, not apparent from the extended energy and momentum eigenstates in the band picture, can provide insight into the structure, properties, and synthesis of the material, as we shall show here for a vefold coordinated phase of binary octet compounds A N B 8ÀN consisting of AA 0 stacked planar AB honeycombs [3][4][5] (Fig. 1a and b).…”

“…9 The vefold coordinated structure is an intermediate predicted for some A N B 8ÀN between phases characterized by coordination numbers N c ¼ 4 and 6, along the paths of structural transitions exhibiting a general trend of increasing N c with increasing ionicity upon successive compression. 4,5,[10][11][12][13][14][15][16][17] This structure has the same symmetry (P6 3 /mmc) as h-BN and is referred to as the h-MgO structure (just as NaCl for the rocksalt structure, the compound not necessarily MgO) since it was rst predicted for MgO 4 or as the 5-5 structure for the mutual vefold coordination, 5,[16][17][18][19] or simply as HX standing for hexagonal. 11 As the present work focuses on bonding, we emphasize that the h-MgO structure is distinct from that of h-BN, consisting of threefold coordinated layers held together by a weak van der Waals (vdW) interlayer interaction.…”

“…Previous theoretical studies addressed crystal and electronic structures, [3][4][5] whereas the characters of chemical bonds, especially the unusual bonds between the adjacent (0001) planes, are largely le unexplored, with vefold coordination dened loosely as having a near-unity in-plane to vertical (c-direction) bond length ratio. 4,18 For octet rule obeying A N B 8ÀN , it is intuitive to attribute the threefold symmetry around c to s bonding of sp 2 orbitals in the basal plane.…”

“…Using ab initio calculations (see the ESI † for Computational details), we rst show that their phonon dispersions lack the signatures of vdW layered crystals, clarifying the connotation of vefold coordination beyond the in-plane to vertical nearest neighbor distance ratio criterion used in the literature. 4,5 Simulated core level transition spectra reveal s and p bonding, typical of threefold sp 2 coordination, in isolated monolayer (1L) h-AlN and h-MgO, as in h-BN, but the signatures of this bonding scheme are missing in the bulk crystals. Not only do the bulk band structures bear less resemblance to the corresponding 1L sheets than to the wurtzite counterparts, but also the energy dispersions in the c direction are even stronger in bulk h-AlN and h-MgO than in the wurtzites, manifesting a stronger interlayer interaction in the vefold coordinated phase than the fourfold coordinated phase.…”

Tunability of electronic and optical properties of the Ba–Zr–S system via dimensional reduction

“…Thus we find BaS to be a wide band gap material, consistent with other reports. 30 The implication of the wide gap of BaS is that the rock salt parts of the unit cells of the RP phases can be expected to serve as blocking layers from an electronic point of view, so that the RP phases may be expected to show 2D characteristics in their electronic structure.…”