Simulation of crystal and electronic structures of octahedral molybdenum cluster complex compound Cs₂[Mo₆Cl₁₄] using various DFT functionals

Abstract

The electronic and structural characteristics of the octahedral molybdenum cluster-based ternary compound, Cs₂[Mo₆Cl₁₄], were investigated based on density functional theory (DFT) and subsequent comparisons with experimentally observed results. The geometry optimization and band structure calculations of Cs₂[Mo₆Cl₁₄] were performed using three standard functionals: local density approximation, Perdew-Burke-Ernzerhof (PBE) as a generalized gradient approximation, and PBE revised for solid compounds (PBEsol). The validity of the calculated results was experimentally examined via X-ray powder diffraction, ultraviolet–visible (UV–vis) diffuse reflection, and X-ray photoemission spectra (XPS) measurements. PBEsol was found to show the best performance in terms of reproducing the experimentally refined lattice structure of the compound. The calculated band gap energy (E_g) was consistent with the value evaluated from the UV–vis measurement. Furthermore, the XPS valence spectrum of the compound was well reproduced by the calculated projected density of state weighted with the photoionization probabilities of Al K_α. Although the spectral shapes simulated using the three functionals were similar, PBEsol reproduced the energy levels of the electronic states of both [Mo₆Cl₁₄]²⁻ and Cs⁺ ion with greater consistency. Therefore, it was concluded that PBEsol is the most appropriate functional for DFT calculations of the metal cluster-based lattice system.