Stability of the tetragonalMn3+O6distortions in theLaMn1−xGaxO3series by x-ray absorption spectroscopy

Abstract

The ${\mathrm{LaMn}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{O}}_{3}$ series has been studied by means of x-ray absorption spectroscopy at both Mn and Ga $K$ edges. A homovalent substitution between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Ga}}^{3+}$ can be deduced from the x-ray absorption near-edge structure (XANES) spectra. Moreover, the evolution of the prepeaks in the Mn $K$-edge XANES spectra seems to indicate a strong mixing of the ${\mathrm{Mn}}^{3+}$ $3d$ states among near neighbors. The local geometric structure around ${\mathrm{Ga}}^{3+}$ barely changes along the series showing that ${\mathrm{GaO}}_{6}$ is a quite rigid octahedron. On the other hand, the local geometric structure around ${\mathrm{Mn}}^{3+}$ (in the ${\mathrm{MnO}}_{6})$ changes along the series evolving from the well-known cooperative tetragonal distortion (Jahn-Teller type) in ${\mathrm{LaMnO}}_{3}$ to a regular ${\mathrm{MnO}}_{6}$ octahedron for $x\ensuremath{\sim}0.6.$ The latter exhibits large Debye-Waller factors indicating a spread of Mn-O distances (non-Jahn-Teller type). This confirms the high instability of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedral local symmetry, the possible distortions being not limited to the tetragonal ones. We also discuss the role of the single-ion Jahn-Teller effect as a mechanism of ${\mathrm{Mn}}^{3+}$ stabilization in solids.