Pressure and temperature dependences of electronic transport properties in CaB₆

Using in situ resistance measurements on diamond anvil cell, we have monitored the pressure and temperature dependence of the CaB₆ electronic transport properties. We report that CaB₆ is a semiconductor at room temperature and at ambient pressure, in contrast to previous studies suggesting a semimetal nature. From ambient pressure to 12 GPa, the CaB₆ conductivity smoothly increases, and CaB₆ behaves as a semiconductor, although the CaB₆ conductivity shows a metallic temperature dependence over a temperature interval ranging from 110 to 300 K due to its extrinsic region (where an increase in the temperature produces no effect on the CaB₆ sample carrier densities and the CaB ₆ conductivity is, therefore, solely determined by the carrier mobilities). At around 12.4 GPa, CaB₆ undergoes a metalliclike transition leading to a change in the CaB₆ conductivity by a factor exceeding three orders of magnitude. This large increase in the CaB₆ conductivity has been interpreted as resulting from an overlap between the conduction band and the valence band. From 12.4 to 26 GPa, the CaB₆ conductivity markedly increases and is similar to a metallic one.