Electrical-resistance behavior of Ca at high pressures and low temperatures

The electrical-resistance behavior of Ca is studied from 2 to 300 K at various pressures up to 440 kbar using a clamp-type sintered diamond-tipped opposed-anvil apparatus. It is found that the electrical resistance of Ca at room temperature increases monotonically as a function of pressure and reaches a maximum at about 180 kbar. With additional increase in pressure, the resistance of Ca decreases to a minimum around 250-300 kbar and then increases again sharply with a Ba-like up-jump at about 360 kbar and a second maximum around 420 kbar. Apparently, many first-order phase transformations in Ca have occurred at these high pressures. There is also a possibility for Ca to become superconducting at about 2 K and 440 kbar. No resistance anomalies at the low-temperature end were observed which might reflect the formation of an excitonic phase. The possibility of the nonexistence of the excitonic phase for Ca and the prediction of metal-semimetal-metal transition for Ca from the band calculations are discussed in light of the experimental findings.