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, a-Sr2CrO4 and NaCr2O4 are taken as examples of the compounds with corner- A nd edge-sharing octahedra, respectively. a-Sr2CrO4 has interesting correlation between spin and orbital degrees of freedom, which occurs due to the reverse split of the t2g 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.