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 Cs₃FeCl₅, Cs₃FeBr₅, Cs₃CoCl₅, Cs₃CoBr₅, CsNiCl₃, and CsNiBr₃ NPs. CsMX NPs with Fe and Co exhibited Cs₃M₁X₅ and Cs₂M₁X₄ structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX₃ structure. The differences in structural stability by central metal ions were explained using spin-polarized DFT calculations. The analysis revealed tetragonal Cs₃M₁X₅ and orthorhombic Cs₂M₁X₄ structures to have similar thermodynamic stabilities in the case of Fe and Co, whereas the hexagonal CsMX₃ 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 Cs₃CoBr₅ NPs easily developed into Cs₂CoCl₄ nanorods with an increase in temperature.