A Comparison between the Raman Spectra of Ce1-xCaxVO4-0.5x (0 ≤ x ≤ 0.41) and Ce1-xBixVO4 (0 ≤ x ≤ 0.68)

The Raman spectra of Ce1-xCaxVO4-0.5x (0 less than or equal to x less than or equal to 0.41) and Ce1-xBixVO4 (0 less than or equal to x less than or equal to0.68) solid solutions, where aliovalent Ca2+ or isovalent Bi3+ cations are substituted for Ce3+ in the tetragonal cerium vanadate CeVO4, are presented as a function of x to compare with each other. There was a remarkable difference in that the three broad features appeared in the Raman spectra of Ce1-xCaxVO4-0.5x whereas they were absent for Ce1-xBixVO4. The appearance of the broad features could be attributed to the charge transfer effect on the aliovalent Ca2+ substitution, which is responsible for a substantial enhancement in electrical conductivity in Ce1-xCaxVO4-0.5x relative to Ce1-xBixVO4 with x. From the changes in the modes' frequency with unit-cell volume, which exhibit a different decrease with x between Ce1-xCaxVO4-0.5x and Ce1-xBixVO4, we could estimate mode Griineisen parameters that vary in magnitude and/or sign as well as between Ce1-xCaxVO4-0.5x and Ce1-xBixVO4; this reflects the fact that different lattice distortions are induced when aliovalent or isovalent cations with similar/different ionic radii are substituted to form both the solid solutions. (C) 2001 Academic Press.