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.
@article{arxiv.1302.5746,
title = {Crystal structures of spin-Jahn-Teller ordered MgCr_2O_4 and ZnCr_2O_4},
author = {Moureen C. Kemei and Phillip T. Barton and Stephanie L. Moffitt and Michael W. Gaultois and Joshua A. Kurzman and Ram Seshadri and Matthew R. Suchomel and Young-II Kim},
journal= {arXiv preprint arXiv:1302.5746},
year = {2013}
}