Jumping crystals of the spinels NiCr2O4 and CuCr2O4

The crystal structures of cubic and tetragonal NiCr2O4 and of tetragonal CuCr2O4 have been refined and their cell parameters have been measured, using single-crystal and powder X-ray diffraction. It has been observed that the differences in the Jahn-Teller distortions in both compounds are reflected in their M(2+)-O tetrahedral environments and in their cell parameters. A calculation of the atomic shifts during the tetragonal-cubic phase transition showed that the average shifts for the oxygen atoms are 0.10(1) and 0.04(1) Angstrom for the nickel and chromium atoms, respectively. Crystals of both compounds jump when they go through the phase transition. This behaviour is especially spectacular for NiCr2O4 since the phase transition takes place at 320 K, this temperature being reached when the crystals are illuminated. A comparison with organic jumping crystals is presented, and characteristics of the chromite crystals are discussed.