Temperature dependent lattice instability in single crystals of ferromagnetic CdCr2Se4 diluted with In and Sb

In ferromagnetic CdCr2Se4 diluted with Me-x(3+) = In and Sb, deviations from cubic symmetry Fd (3) over barm appear in the paramagnetic phase just below room temperature, and they increase with decreasing temperature. For Sb admixture, the unit-cell anomalies indicate a structural phase transition to occur at the same temperature as the magnetic transition, T-c = 130 K, which also is the same T-c as for the parent crystal CdCr2Se4. The low temperature phase has been described in orthorhombic space group Fddd. For In admixture, a structural transition occurs in the paramagnetic state at about T-a approximate to 200 K ( which is higher than T-c = 125 K), to a tetragonal structure with space group I4(1)/amd. This behaviour is attributed to macroscopic spontaneous strain due to chemical heterogeneities, and to spin frustrations due to mixed valencies of Cr. The paramagnetic Curie-Weiss temperature theta(C-W) decreases for both admixtures, indicating changes in competing ferromagnetic and antiferromagnetic interactions. The magnetization at 2.1 K exhibits saturation for H > 0.6 T. The magnetic moments are mu(sat) = 6.26 and 5.47 mu(B) mol(-1) for Sb and In admixtures, respectively. These values are consistent with the Cr3+ and Cr2+ mixed valencies, and with the proposed cation distribution model. A spin-phonon coupling of the transitions in the crystal with Sb admixture is suggested, while such a correspondence is not clear for the In admixture.