Dimorphic lanthanum orthovanadate

Abstract

LaVO 4 has been found to be dimorphic, the crystalline form depending upon the mode of preparation. Precipitation from solution yields a tetragonal form, isomorphous to the other rare earth vanadates with a 0 = 7·44 1 and c 0 = 6·50 3. Heating the lanthanum precipitate above approximately 300°C converts tetragonal LaVO 4 to its well known monoclinic form. The other rare earth vanadates have also been prepared by the precipitation method and their tetragonal lattice parameters are about the same as those obtained by firing. Comparison of the dimorphic forms of the rare earth series of chromates, arsenates, phosphates and vanadates yields a linear correlation between the cationic radius and the pentavalent ▪ ionic radius of the ▪ tetrahedral oxyanion rather than with the ▪ bond length or the tetrahedral volume of the oxyanion. On the basis of this correlation it is predicted that PmAsO 4 exhibits dimorphism.