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

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.