In situ site-selective transition metal K-edge XAS: a powerful probe of the transformation of mixed-valence compounds

Abstract

We present herein the first in situ site-selective XAS experiment performed on a proof-of-principle transformation of a mixed-valence compound: the calcination of the K_0.1Co^II₄[Co^III(CN)₆]_2.7·20H₂O Prussian Blue analogue (containing Co²⁺ and Co³⁺ ions in two different O_h sites) into Co₃O₄ (containing Co²⁺ ions in a T_d site and Co³⁺ in an O_h site). By recording the Co K-edge X-ray absorption spectra using a spectrometer aligned at the Co Kβ_1,3 emission line, the evolution of each species was singly monitored from 20 °C up to the oxide formation. The experimental spectrum of the Co²⁺(T_d) and Co³⁺ (O_h) species in Co₃O₄ is reported for the first time. Our results demonstrate the possibilities offered by site-selective XAS for the investigation of chemical transformations and the study of materials under working conditions whenever the chemical element of interest is present in several states and/or sites.