System Ca3(VO4)2-Na3VO4-LaVO4

The phase equilibria of the Ca-3(VO4)(2)-LaVO4-Na3VO4 system were studied. A extensive solid solution in calcium orthovanadate bounded by the compositions Ca-3(VO4)(2)-Ca9La(VO4)(7)-Ca10Na(VO4)(7)-Ca8.5Na1.75La0.75(VO4)(7) was found. The unit cell parameters of whitlockite vanadates increase with a rise in the sodium or lanthanum concentration. Ferroelectric phase transitions in Ca-3 La-- 3x(2x)(VO4)(2) vanadates were studied using thermal analysis, electrical conductivity measurements, and dielectric constant measurements. The nonmonotonic decrease in the beta <----> beta' transition temperature with lanthanum doping is due to lanthanum being inhomogeneously distributed over cation positions M(1), M(2), and M(3). The weakness of thermal events due to beta <----> beta transition is associated with the disorder of the low-temperature beta-phases of Ca-3 La-- 3x(2x)(VO4)(2) vanadates.