Thermopower of calcium at high pressures

Calcium at megabar pressures undergoes numerous structural transitions and has a complex phase diagram. At the same time, according to the recent theoretical investigations, an anomalous behavior of many physical properties, including a transition to the state of a narrow-gap semiconductor, can be expected even in the region of stability of the normal-pressure phase of calcium with the fcc structure at moderate pressures$P\sim(5$–$15)\,$GPa. Data on the thermopower of calcium in the pressure range up to $9$ GPa have been reported. The thermopower in this pressure range is positive, has a smooth maximum at $5$–$6$ GPa, and decreases quite rapidly at higher pressures. The absolute values of the thermopower ($5$–$12$ мV/K) indicate that calcium in this pressure range is a metal. The difference between the thermopowers in the direct and inverse passages in the range of $5$–$7.5$ GPa is fairly noticeable (${\approx}\,10\%$). The possible reasons for such an anomalous behavior, as well as new calculations of the band structure of calcium, have been discussed.