Anomalous electronic state in CaCrO$_3$ and SrCrO$_3$

Measurements of thermal conductivity, thermoelectric power, electrical conductivity, magnetization and the equation of state have been carried out on ceramic samples of CaCrO$_{3}$ and SrCrO$_{3}$ that were synthesized under high pressure. Contrary to earlier reports, both compounds have been found to be a spin-glass insulator. While the magnetic susceptibility $\chi $(T) of SrCrO$_{3}$ becomes completely incompatible with the Curie-Weiss law, the $\mu _{eff}$=3.4 $\mu _{B}$ obtained in CaCrO$_{3}$ is close to the spin-only moment of a localized electronic state. Suppression of the thermal conductivity in both compounds indicates that orbital fluctuations are present, which confirms further the ``localized'' electronic state. Factors such as a higher $\kappa $(T) and weaker temperature dependence of $\chi $(T) for SrCrO$_{3}$ than CaCrO$_{3}$ suggest that SrCrO$_{3}$ is close to the crossover from the localized to the itinerant electronic state. More importantly, the Cr-O bond length in SrCrO$_{3}$ is much smaller than that calculated from the ionic radii. An anomalous small bulk modulus found for SrCrO$_{3}$ at P $>$ 40 kbar confirms unambiguously that the electronic state transition is induced under high pressure. The bulk modulus of SrCrO$_{3}$ below 40 kbar and CaCrO$_{3}$ falls in line with other perovskite oxides.