Stability of Jahn-Teller distortion ordering inLaMn1−xScxO3

Abstract

We have investigated the role of $\mathrm{M}{\mathrm{n}}^{3+}$ Jahn-Teller distortion in driving ferromagnetism in the $\mathrm{LaM}{\mathrm{n}}_{1\text{\ensuremath{-}}x}\mathrm{S}{\mathrm{c}}_{x}{\mathrm{O}}_{3}$ series. The replacement of Mn by Sc in $\mathrm{LaMn}{\mathrm{O}}_{3}$ decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the $\mathrm{LaSc}{\mathrm{O}}_{3}$ sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in $\mathrm{LaMn}{\mathrm{O}}_{3}$ into a nearly regular one in $\mathrm{LaSc}{\mathrm{O}}_{3}$. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc $K$ edges reveal the stability of both Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedron and nearly regular $\mathrm{Sc}{\mathrm{O}}_{6}$ octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra spatially ordered as in $\mathrm{LaMn}{\mathrm{O}}_{3}$ and nearly regular $\mathrm{Sc}{\mathrm{O}}_{6}$ octahedra. This result contrasts with the pseudocubic phase and the appearance of regular $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra in $\mathrm{LaM}{\mathrm{n}}_{1\text{\ensuremath{-}}x}\mathrm{G}{\mathrm{a}}_{x}{\mathrm{O}}_{3}$ for $xg0.5$. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the $\mathrm{M}{\mathrm{n}}^{3+}$ atom is allocated. Besides, a ferromagnetic ground state is observed for $xg0.5$ in both series independently of the presence (or not) of Jahn-Teller distortions around $\mathrm{M}{\mathrm{n}}^{3+}$, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in $\mathrm{LaM}{\mathrm{n}}_{1\text{\ensuremath{-}}x}{B}_{x}{\mathrm{O}}_{3}$ $(B=\text{Sc or Ga})$.