Structural, Magnetic, and Electronic Properties of the Cr_1-TiO₂ Solid

Cr_1-TiO₂ (0 ≤ x ≤ 1) solid solution was synthesized by a high-pressure and high-temperature method, whereafter systematic experimental and computational studies were conducted on the Cr_1-TiO₂ system. The crystal structure of the samples where 0 ≤ x ≤ 0.4 and x = 1 was of a rutile structure (P4₂/mnm), while samples where 0.5 ≤ x ≤ 0.9 crystallized in a CaCl₂ structure (Pnnm). The structural transformation from rutile-type to CaCl₂-type structure should be due to the combined action of positive chemical pressure and physical pressure. The saturation magnetization of the Cr_1-TiO₂ samples decreased linearly with the increase of x because Ti⁴⁺ is nonmagnetic. In addition, the Curie temperature of the Cr_1-TiO₂ samples also decreased noticeably with the increase of x. When nonmagnetic Ti⁴⁺ randomly replaced Cr⁴⁺ and occupied its position, the net exchange coupling in Cr_1-TiO₂ would decrease. When Ti⁴⁺ occupied the majority in the system, Cr⁴⁺ ions would be separated by nonmagnetic Ti⁴⁺ ions far enough for the long-range ferromagnetic order to weaken or even disappear with the increase of x, causing the Cr_1-TiO₂ system to finally approach a paramagnetic state. Density functional theory calculations were performed for the Cr_1-TiO₂ system, and the predicted trends of the magnetic properties agreed well with the experimental results. These calculations also showed that Cr_1-TiO₂ was still half-metallic until x reached 0.6.