High-Pressure Synthesis and Electronic Properties of Cr(IV) Oxides

Abstract

Synthesis and electronic properties of Cr(IV) oxides with CrO₆ octrahedra are overviewed focusing on the way of the connection of the octahedra. High-pressure synthesis under 6–8 GPa is effective for the preparation of them. The reaction to make some Cr(IV) oxides is found to be promoted by higher pressures even in case that it is less promoted by elevation of reaction temperatures. Magnetic and electric properties are closely related with the deep energy level of the 3d orbitals of the Cr⁴⁺ ions. For more specific description, α-Sr₂CrO₄ and NaCr₂O₄ are taken as examples of the compounds with corner- and edge-sharing octahedra, respectively. α-Sr₂CrO₄ has interesting correlation between spin and orbital degrees of freedom, which occurs due to the reverse split of the t_2g orbitals to the expectation of the conventional crystal-field theory. NaCr₂O₄ shows a new type of double exchange interaction between Cr spins owing to ligand holes. Electrically, these two compounds are insulating with small energy gaps whereas some related compounds are metallic. Thus, the Cr(IV) oxides appear to be located near the border of Mott transition.