Crystal structures of spin-Jahn-Teller ordered MgCr_2O_4 and ZnCr_2O_4

TL;DR

This study reveals the detailed low-temperature crystal structures of MgCr_2O_4 and ZnCr_2O_4 spinels, showing how magnetic ordering induces subtle structural distortions that relieve geometric frustration.

Contribution

It provides high-resolution structural data demonstrating a two-phase model with tetragonal and orthorhombic symmetries and compares the spin-Jahn-Teller distortions to other known spinels.

Findings

Low-temperature structures are described by tetragonal I4_1/amd and orthorhombic Fddd phases.

Cr_4 tetrahedra are distorted at low temperatures, with larger distortions in the Fddd phase.

Spin-Jahn-Teller distortions are smaller than in NiCr_2O_4 and CuCr_2O_4.

Abstract

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr_2O_4 and ZnCr_2O_4 is accompanied by a structural change that helps relieve the frustration. Analysis of high-resolution synchrotron X-ray scattering reveals that the low-temperature structures are well described by a two-phase model of tetragonal I4_1/amd and orthorhombic Fddd symmetries. The Cr_4 tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I4_1/amd phase. The spin-Jahn-Teller distortion is approximately one order of magnitude smaller than is observed in first-order Jahn-Teller spinels such as NiCr_2O_4 and CuCr_2O_4. In analogy with NiCr_2O_4 and CuCr_2O_4, we further suggest that the precise nature of magnetic ordering can itself provide a second driving force for structural change.