ISODISTROTLaMTeO6 (M = Ga3+ and Mn3+): the critical role of electronic configuration and cation ordering in crystal structures

Abstract

In this work, PbSb₂O₆-type oxides LaMTeO₆ (M = Ga³⁺ and Mn³⁺) were synthesized and structurally characterized by Rietveld refinements against high-resolution X-ray powder diffraction data. The Ga³⁺/Te⁶⁺ partial ordering within the honeycomb-like two-dimensional [GaTeO₆]³⁻ anionic layer leads to the loss of the inversion center between Ga³⁺ and Te⁶⁺; however the inversion center on the -roto-inversion axis is preserved, thereby resulting in a 2-fold PbSb₂O₆-type superstructure by doubling the c-axis associated with a structural symmetry descending from the original P1m to P1c symmetry. In contrast, LaMnTeO₆ (P2₁/c) adopts a monoclinically distorted 4-fold superstructure with lattice dimensions of a ≈ a_H, b ≈ √3a_H, c ≈ 2c_H, where a_H and c_H represent the lattice parameters of trigonal PbSb₂O₆. The formation of this P2₁/c-superstructure is attributed to the combination of complete Mn³⁺/Te⁶⁺ ordering and the first-order Jahn–Teller distortion of Mn³⁺ with the electronic configuration of d⁴. Such a monoclinic distortion can effectively lift the Mn³⁺ spin moments arranged on the triangular sublattice, resulting in a sharp peak for antiferromagnetic transition, which is in stark contrast to subtle magnetic transitions for PbSb₂O₆-type tellurates AMn(VI)TeO₆ (A = alkaline earth and Pb²⁺) and LnCrTeO₆ (Ln = rare earth) with higher structural symmetry. Our findings highlight that the electronic configuration effects of M-cations play a critical role in controlling the structure symmetry of LaMTeO₆, providing a strategy to fine-tune the crystal structures and physical properties.