Raman study of the structural distortion in the Ni_1-xCo_xTiO₃ solid solution

Raman spectra were collected on Ni_1-xCo_xTiO₃ (0 ≤ x ≤ 1) ilmenite samples as a function of the temperature between 4 and 1200 K. An evident symmetry lowering from the prototype R3 symmetry is observed. The distortion was largest for the x = 0.40 and 0.50 samples and significantly diminished for small and large values of x. The distortion was preserved in the whole temperature range and, except for the x = 0.50 sample, did not show significant changes. Notably, between 25 and 69 K, distortion of the x = 0.40 sample is accompanied by ferromagnetic order. The direct cation-cation and Omediated indirect interactions are discussed as mechanisms behind the distortion and magnetic order. A reversible order-disorder phase transformation, assigned to occur between the ilmenite and corundum phases, took place at 973 K in the x = 0.50 sample. Completion of the transformation took over 1 h. The role of the overlap of Co/Ni and Ti 3d orbitals through O octahedral faces for charge transfer is discussed.