Cs3M2X9 (M = Sb, Bi; X = Cl, Br) perovskites known as two-dimensional (2D) materials crystallize in two structure types, trigonal P321 and orthorhombic Pnma space groups. These compounds containing less toxic elements have generated enormous research interest due to their inherent photovoltaic properties. In this article, we have studied the structure stability of Cs3Sb2Cl9 and the effect of Bi substitution on the structure and optical properties of the above phase. The pure trigonal Cs3Sb2Cl9 phase was obtained by reacting metal chlorides at ≤85 °C, while the pure orthorhombic phase was obtained at ≥130 °C. Bismuth substitution in Cs3Sb2–x Bi x Cl9 gives a mixture of trigonal and orthorhombic phases until x < 0.1, and further substitution (x ≥ 0.1) gives a pure orthorhombic phase. The phase transitions are unequivocally characterized by both powder X-ray diffraction and Raman studies. Single crystal study confirms the orthorhombic Cs3Sb2Cl9, Cs3Sb1.94Bi0.06Cl9, and Cs3Bi2Cl9 phases. From both single crystal and Rietveld refinement studies on Cs3Sb1.94Bi0.06Cl9 and Cs3Sb1.9Bi0.1Cl9, respectively, it is observed that Bi preferably substitutes at the Sb(1) crystallographic site. A theoretical study using the Vienna Ab initio Simulation Package (VASP) shows that both the trigonal and orthorhombic Cs3Sb2Cl9 phases are indirect band gap semiconductors, and their band gap is smaller than orthorhombic Cs3Bi2Cl9. From an optical study, it is observed that the bandgap of the pure orthorhombic (2.86 eV) and trigonal phase (2.89 eV) of Cs3Sb2Cl9 is in a similar range, while the Bi analogue compound, orthorhombic Cs3Bi2Cl9, shows a higher band gap of 3.0 eV. Further, the band gap of the trigonal Cs3Sb2Cl9 phase is successfully reduced by Bi substitution, and it goes through the lowest value of 2.6 eV for x = 0.1 in the Cs3Sb2–x Bi x Cl9 series.

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Crystal structure and properties of a new member of oxy-bismuth-sulfide family SmO 0.5 F 0.

Na(6)Cd(16)Au(7) has been synthesized via typical high-temperature reactions, and its structure refined by single crystal X-ray diffraction as cubic, Fm ̅3m, a = 13.589(1) Å, Z = 4. The structure consists of Cd(8) tetrahedral star (TS) building blocks that are face capped by six shared gold (Au2) vertexes and further diagonally bridged via Au1 to generate an orthogonal, three-dimensional framework [Cd(8)(Au2)(6/2)(Au1)(4/8)], an ordered ternary derivative of Mn(6)Th(23). Linear muffin-tin-orbital (LMTO)-atomic sphere approximation (ASA) electronic structure calculations indicate that Na(6)Cd(16)Au(7) is metallic and that ∼76% of the total crystal orbital Hamilton populations (-ICOHP) originate from polar Cd-Au bonding with 18% more from fewer Cd-Cd contacts. Na(6)Cd(16)Au(7) (45 valence electron count (vec)) is isotypic with the older electron-richer Mg(6)Cu(16)Si(7) (56 vec) in which the atom types are switched and bonding characteristics among the network elements are altered considerably (Si for Au, Cu for Cd, Mg for Na). The earlier and more electronegative element Au now occupies the Si site, in accord with the larger relativistic bonding contributions from polar Cd-Au versus Cu-Si bonds with the neighboring Cd in the former Cu positions. Substantial electronic differences in partial densities-of-states (PDOS) and COHP data for all atoms emphasize these. Strong contributions of nearby Au 5d(10) to bonding states without altering the formal vec are the likely origin of these effects.