Unique Colours of 3d‐Transition‐Metal‐Substituted Lyonsite Molybdates and Their Derivatives: The Role of Multiple Coordination Geometries and Metal‐to‐Metal Charge Transfer

Abstract

We describe the synthesis, crystal structures, and optical absorption spectra/colours of 3d transition‐metal‐substituted lyonsite‐type oxides Li3Al1–xMIIIx(MoO4)3 (0 < x ≤ 1.0; MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (0 < x ≤ 1.0; MII = Co, Ni, Cu). The oxides were readily synthesized by the solid‐state reaction of stoichiometric mixtures of the constituent binaries at around 700 °C in air. The crystal structures determined by Rietveld refinement of PXRD data revealed that in the smaller trivalent‐metal‐substituted lyonsite oxides, the MIII ions occupy the octahedral (8d, 4c) sites and the lithium ions occur exclusively at the trigonal‐prismatic (4c) site in the orthorhombic (Pnma) structure. On the other hand, larger divalent‐metal (CoII/CuII)‐substituted derivatives show occupancy of the CoII/CuII ions at both the octahedral and trigonal‐prismatic sites. We have investigated the colours and optical absorption spectra of Li3Al1–xMIIIx(MoO4)3 (MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (MII = Co, Ni, Cu) and interpreted the results in terms of the average crystal field strengths experienced by MIII/MII ions in multiple coordination geometries. We have also identified the role of metal‐to‐metal charge transfer (MMCT) from the partially filled transition metal 3d orbitals to the empty Mo 4d orbitals in the resulting colours of these oxides.