Unraveling Atomic Positions in an Oxide Spinel with Two Jahn−Teller Ions: Local Structure Investigation of CuMn2O4

Abstract

At first sight, the quenched tetragonal spinel CuMn₂O₄ can be formulated with Cu²⁺ and Mn³⁺, implying that the tetrahedral site is Jahn−Teller (JT)-active Cu²⁺ and the octahedral site is JT-active Mn³⁺. High-resolution, high-momentum-transfer neutron scattering analysis suggests that the sample has ∼30% inversion: Mn on the tetrahedral Cu site with compensating Cu on the octahedral site. Reverse Monte Carlo (RMC) analysis of the pair distribution function allows details of metal−oxygen connectivity to be probed in a manner that is significantly on the local rather than the average scale. Bond valence analysis of the RMC supercell reveals that both JT ions disproportionate to higher and lower valence states as a means of avoiding their JT tendency, particularly on the tetrahedral site. The occurrence of Cu³⁺ in particular is suggested for the first time and is supported by X-ray photoelectron spectroscopy data. The bimodal distribution of O−Cu−O bond angles at the tetrahedral site (distinct from what is seen for O−Mn−O bond angles) further reveals a hidden distinction between sites previously considered to be equivalent. Application of total scattering techniques originally developed for highly disordered materials permits the examination of nanoscale crystalline structure with elemental specificity that is not available in traditional reciprocal-space analysis.