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 CuMn(2)O(4) can be formulated with Cu(2+) and Mn(3+), implying that the tetrahedral site is Jahn-Teller (JT)-active Cu(2+) and the octahedral site is JT-active Mn(3+). High-resolution, high-momentum-transfer neutron scattering analysis suggests that the sample has approximately 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 Cu(3+) 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.

Mixed-valence-driven heavy-fermion behavior and superconductivity in KNi2Se2

“…8(c) Fig.12(a). The single population, centered around BVS = 1.9(3), is comparable to the control simulation (average) with a BVS = 2.0(2), rather than distinct populations from charge disproportionation, as seen in CuMn 2 O 4 , 36 or the value expected from a formal Figure 9. (Color online) (a) Atomic "point clouds" (small spheres), generated by folding the 24000 atom configuration back into the I4/mmm unit-cell, resemble anisotropic Debye-Waller factors (large thermal ellipsoids at 99% probability) on average, consistent with the Rietveld analysis (Table III) and the previously reported crystal structure.…”

“…If a Jahn-Teller distortion were responsible for the distribution of Ni-Ni distances, the configuration would reveal distinct populations of Se-Ni-Se angles, 36 especially when plotted as a function of Ni-Ni distance. There is a correlation between the Ni-Se-Ni angle (ϕ) and the Ni-Ni distance [ Fig.10(b)], as expected from geometric considerations: shorter bonds decrease ϕ.…”

Crystal structure transformation in CeRuSn seen via the atomic pair distribution function

“…The real-space structural details are revealed through the Fourier transformation 13 . The PDF analysis has been proven to be a very successful technique to investigate the local structure changes in many oxide materials [13][14][15] . Among other methods capable to disclose structural changes are for instance x-ray and neutron diffraction and extended X-ray absorption fine structure (EXAFS).…”

Simple synthetic route to manganese-containing nanowires with the spinel crystal structure

“…Interestingly, for tetragonal CuMn 2 O 4 , however, the material is more catalytically-active but has fewer tetravalent manganese ions on the octahedral sites. Reported characterization of this material suggests that half of the tetravalent manganese ions reside on the octahedral sites, along with one third of the trivalent manganese ions and a small number of Cu 2 þ and Cu 3 þ cations [35]. It is possible that having both multivalent copper as well as manganese in CuMn 2 O 4 might provide a synergy which promotes catalytic activity.…”

“…The structure of tetragonal CuMn 2 O 4 , however, is more complicated than that of the cubic phase. Shoemaker et al [35] refined the tetragonal CuMn 2 O 4 structure with c/a ratio of 1.03399 and a 0.27 inversion degree. In this tetragonal spinel, Cu þ , Cu 2 þ , Mn 2 þ , Mn 3 þ ions all take tetrahedral sites, with octahedral sites occupied by Cu 2 þ , Cu 3 þ , Mn 3 þ , Mn 4 þ .…”