Temperature- and pressure-induced lattice distortion in CdCr2-xGaxSe4 (x = 0, 0.06, and 0.12)

Structural changes in the cubic spinels CdCr2-xGaxSe4 have been studied by means of single-crystal x-ray diffraction at low temperature and energy-dispersive diffraction in a diamond-anvil cell at high pressure. In stoichiometric samples (x = 0), a spontaneous magnetostriction reduces the thermal expansion coefficient from 6.7 x 10(-6) K-1 in the paramagnetic phase to 2.2 x 10(-6) K-1 in the ferromagnetic phase (T-C = 130 K). In the samples with Ga3+ admixtures (x = 0.06 and 0.12), a slight structural distortion causes an order-disorder-type phase transition at T-d approximate to 285 K connected with changes in the electronic configuration of the Jahn-Teller-active Cr cations. The magnetostriction is apparently not very sensitive to the Ga3+ admixtures in the present concentration range. At high pressure the cubic unit cell transforms to a tetragonal one with c/a = 0.91. The Jahn-Teller effect is combined with the rocking motions of corner-sharing metal coordination polyhedra, altering their shape. The transition pressure is about 10 GPa. The bulk modulus of the cubic phase is B-0 = 103 GPa.