Transport and magnetic properties of Pr_1-xCa_xCrO₃ (x = 0.0–0.5): effect of t_2g orbital degeneracy on the thermoelectric power

Abstract.

We report on the resistivity (ρ), thermoelectric power (S for Seebeck) and magnetic measurements of the series Pr_1-xCa_xCrO₃ (x=0.0–0.5). These orthochromites exhibit a T independent, large and positive S, from 100 K to 700 K which follows the Cr⁴⁺ concentration. Upon Ca²⁺ for Pr³⁺ substitution, a concomitant decrease of the resistivity and S values is found. The evolution of S as a function of hole concentration (Cr⁴⁺) can be nicely fitted by the modified Heikes formula which takes into account the orbital degeneracy associated to Cr³⁺/Cr⁴⁺. This is in good agreement with the model previously calculated by Marsh and Parris, in the case of weak magnetic coupling [Phys. Rev. B 54, 7720 (1996)]. The magnetic susceptibility measurements support the assumption of a weak magnetic coupling since the antiferromagnetic ordering T_N values are found to be lower than 250 K. For Pr_0.7Ca_0.3CrO₃, the power factor PF= S²/ρ measured at 700 K is equal to 1.9×10^-4 W m^-1K^-2. The present system, chemically stable in air up to T > 1000 °C, is promising for thermoelectric application at very high temperature.