  At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are
tetragonally distorted and crystallize in the I4_1/amd space group due to
cooperative Jahn-Teller ordering driven by the orbital degeneracy of
tetrahedral Ni$^{2+}$ ($t_2^4$) and Cu$^{2+}$ ($t_2^5$). Upon cooling, these
compounds undergo magnetic ordering transitions; interactions being somewhat
frustrated for NiCr_2O_4 but not for CuCr_2O_4. We employ variable-temperature
high-resolution synchrotron X-ray powder diffraction to establish that at the
magnetic ordering temperatures there are further structural changes, which
result in both compounds distorting to an orthorhombic structure consistent
with the Fddd space group. NiCr_2O_4 exhibits additional distortion, likely
within the same space group, at a yet-lower transition temperature of $T$ = 30
K. The tetragonal to orthorhombic structural transition in these compounds
appears to primarily involve changes in NiO_4 and CuO_4 tetrahedra.
