Effect of Bi Substitution on Cs3Sb2Cl9: Structural Phase Transition and Band Gap Engineering

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–xBixCl9 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–xBixCl9 series.