Crystal structures of spin-Jahn-Teller-ordered MgCr2O4 and ZnCr2O4

Magnetic ordering in the geometrically frustrated magnetic oxide spinels MgCr₂O₄ and ZnCr₂O₄ is accompanied by a structural change that helps to 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 I4₁/amd and orthorhombic Fddd symmetries. The Cr₄ tetrahedra of the pyrochlore lattice are distorted at these low-temperatures, with the Fddd phase displaying larger distortions than the I4₁/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 NiCr₂O₄ and CuCr₂O₄. In analogy with NiCr₂O₄ and CuCr₂O₄, we further suggest that the precise nature of magnetic ordering can itself provide a second driving force for structural change.