Temperature effects on the vibrational properties of the Cs2SnX6 ‘defect’ perovskites (X = I, Br, Cl)

Air-stable, vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) have been attracting particular interest due their potential applications in solar cells and thermoelectrics based on their favorable structural, optical and transport properties. In this work, a comprehensive Raman investigation of Cs2SnX6 perovskites is performed over an extended temperature range of 83–433 K in order to provide insight to their lattice dynamics. Substitution of the lighter and more electronegative Cl halogens with the heavier bromide and iodide ones resulted in the decrease of the frequencies for all Raman modes comprising the internal vibrations in the SnX6 octahedra and the νL(F2g) mode due to Cs vibrations against SnX6 in the rigid lattice. Blue shift and narrowing of the SnX6 stretching and bending bands was invariably observed for all halogen analogues with temperature decrease, evidencing the presence of lattice anharmonicity. More importantly, the elusive νL(F2g) phonon was only observed for Cs2SnCl6 for all temperatures, complying with the predictions of Hirshfeld surface analysis for the enhanced Cs–SnCl6 interactions. The latter mode emerged for Cs2SnI6 only above room temperature, indicating that combination of anharmonic lattice dynamics with the increased Cs–SnI6 oscillation energy upon increasing temperature may activate this specific “lattice mode”, which in the limit of very weak Cs–I interaction approaches the “rattling’’ Cs vibrations in the cuboctahedral cage. Strong lattice anharmonicity for the chemically stable Cs2SnX6 compounds can be accordingly inferred, which is a key aspect for their performance in future optoelectronic and thermoelectric applications.