Ca3Ce3-xMxTi7Nb2O26.5 (M=Y, Sm or Gd; x=0, 1 or 2) -: pyrochlore-type ceramic oxide semiconductors

Pyrochlore-type Ca3Y3Ti7Nb2O26.5 is colorless and insulating, but when Y3+ is completely replaced by Ce4+ the compound retains the pyrochlore structure, but it is black and semiconducting. Ca3Ce3Ti7Nb2O26.5 compound was prepared by the conventional solid state reaction technique. X-ray powder diffraction of the calcined compound shows the formation of cubic pyrochlore-type structure and the microstructure of the sintered pellet indicates well-sintered grains in the range 1-4 mum. Resistivity measurements show that this compound exhibits semiconducting behavior. The resistivity at 30degreesC is 3.16 x 10(7) Omega cm and at 450degreesC is 1.02 x 10(3) Omega cm. The activation energy of the carriers is found to be 0.45 eV based on the Arrhenius equation for conductivity. Electrical conductivity in this compound is probably due to the presence of Ce3+, which remains in the reduced state without being oxidized to Ce4+ by structural stabilization. In this compound, when Ce4+ is partially substituted by Y3+, Sm3+ or Gd-,(3+) the structure remains the same but, electrical conductivity decreases and activation energy of the current carriers increases. (C) 2004 Elsevier B.V. All rights reserved.