Jumping crystals of the spinels NiCr₂O₄ and CuCr₂O₄

Abstract

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 M²⁺ –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 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.

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