  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.
