Jumping crystals of the spinels NiCr2O4 and CuCr2O4

The crystal structures of cubic and tetragonal NiCr 2 O 4 and of tetragonal CuCr 2 O 4 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) Å 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 NiCr 2 O 4 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.