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

Abstract

Structural changes in the cubic spinels CdCr2−xGaxSe4 have been studied bymeans of single-crystal x-ray diffraction at low temperature and energy-dispersivediffraction in a diamond-anvil cell at high pressure. In stoichiometric samples (x = 0), aspontaneous magnetostriction reduces the thermal expansion coefficient from 6.7 ×10−6 K−1 in the paramagnetic phase to 2.2 × 10−6 K−1 in the ferromagnetic phase(TC = 130K). In the samples with Ga3+ admixtures (x = 0.06 and0.12), a slight structural distortion causes an order–disorder-type phase transition at Td ≈ 285K connected with changes in the electronic configuration of the Jahn–Teller-activeCr cations. The magnetostriction is apparently not very sensitive tothe Ga3+ admixtures in the present concentration range. At highpressure 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-sharingmetal coordination polyhedra, altering their shape. The transitionpressure is about 10 GPa. The bulk modulus of the cubic phase isB0 = 103GPa.