Fabrication of Colloidal Cesium Metal Halide (CsMX: M = Fe, Co, and Ni) Nanoparticles and Assessment of Their Thermodynamic Stability by DFT Calculations.

We synthesized colloidal cesium metal halide CsMX (M = Fe, Co, Ni; X = Cl, Br) nanoparticles (NPs) and assessed their crystal stability by density functional theory (DFT) calculations. We successfully synthesized Cs 3 FeCl 5 , Cs 3 FeBr 5 , Cs 3 CoCl 5 , Cs 3 CoBr 5 , CsNiCl 3 , and CsNiBr 3 NPs. CsMX NPs with Fe and Co exhibited Cs 3 M 1 X 5 and Cs 2 M 1 X 4 structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX 3 structure. The differences in structural stability by central metal ions were explained using spin-polarized DFT calculations. The analysis revealed tetragonal Cs 3 M 1 X 5 and orthorhombic Cs 2 M 1 X 4 structures to have similar thermodynamic stabilities in the case of Fe and Co, whereas the hexagonal CsMX 3 structure in the case of Ni was the most stable. Moreover, the calculation results were the same as the experimental results. In particular, cobalt-related Cs 3 CoBr 5 NPs easily developed into Cs 2 CoCl 4 nanorods with an increase in temperature.