Tetragonal Cs1.17In0.81Cl3: A Charge-Ordered Indium Halide Perovskite Derivative

Polycrystalline samples of Cs_1.17In_0.81Cl₃ were prepared by annealing a mixture of CsCl, InCl, and InCl₃, stoichiometric for the targeted CsInCl₃. Synchrotron powder X-ray diffraction refinement and chemical analysis by energy dispersive X-ray indicated that Cs_1.17In_0.81Cl₃, a tetragonal distorted perovskite derivative (I4/m), is the thermodynamically stable product. The refined unit cell parameters and space group were confirmed by electron diffraction. In the tetragonal structure, In⁺ and In³⁺ are located in four different crystallographic sites, consistent with their corresponding bond lengths. In1, In2, and In3 are octahedrally coordinated, whereas In4 is at the center of a pentagonal bipyramid of Cl because of the noncooperative octahedral tilting of In4Cl₆. The charged-ordered In+ and In³⁺ were also confirmed by X-ray absorption and Raman spectroscopy. Cs_1.17In_0.81Cl₃ is the first example of an inorganic halide double perovskite derivative with charged-ordered In⁺ and In³⁺. Band structure and optical conductivity calculations were carried out with both generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) approach; the GGA calculations estimated the band gap and optical band gap to be 2.27 eV and 2.4 eV, respectively. Finally, the large and indirect band gap suggests that Cs_1.17In_0.81Cl₃ is not a good candidate for photovoltaic application.