METAL-INSULATOR-TRANSITION AND THERMOELECTRIC PROPERTIES IN THE SYSTEM (R1-XCAX)MNO3-DELTA(R-TB, HO, Y)

(R1-xCax)MnO3-δ (R: Tb, Ho, Y) solid solutions with the orthorhombic perovskite-type structure were synthesized and their electrical resistivity and thermoelectric power were measured in the temperature range 80 to 1000 K. At low temperature, the electrical properties of (R1-xCax)MnO3-δ are explained by the variable range hopping of electrons model. At high temperature, hopping conduction occurs in the composition range 0.4 ≦ x ≦ 0.6, while metal-insulator transition occurs in the range 0.7 ≦ x ≦ 0.9. Transition temperature increases with increasing average MnO distance. It is concluded that the increase in MnO distance enhances the magnitude of the electrostatic field, and consequently the transition temperature is raised. (R1-xCax)MnO3-δ (R: Tb, Ho, Y, 0.7 ≦ x ≦ 0.9) are candidates as high temperature thermoelectric conversion materials. © 1991.