Metal-insulator transition in Na-doped post-spinel CdRh₂O ₄

Na doping of post-spinel CdRh₂O₄ was achieved over the full composition range to NaRh₂O₄ using high-pressure and high-temperature synthesis conditions. X-ray diffraction analysis confirmed that Na occupies the Cd site and that the host structure is retained, in sharp contrast to what was observed for spinel CdRh₂O₄, where Na was not accommodated at all. As the Na content was increased, the electrically insulating state changed to a conducting state at a critical composition close to x = 0.75 in Cd_1-xNa_xRh₂O ₄. The specific heat data confirm that the density of electronic states at the Fermi level was enhanced around this composition, suggesting that the metal-insulator transition is likely induced by band filling. In addition, unusual antiferromagnetism was observed in several compounds with intermediate Na concentrations, and this phenomenon was seemingly independent of the metal-insulator transition.