HIGH-TEMPERATURE TRANSFORMATIONS IN CALCIUM ORTHOVANADATE STUDIED WITH RAMAN-SCATTERING

Raman spectra of Ca-3(VO4)(2) calcium orthovanadate (R3c space group), a high temperature ferroelectric with T-c = 1383 K, are analyzed up t o 1483 K with a group theory approach and discussed in relation to Ram an spectra of other compounds with the palmierite structure (R (3) ove r bar m space group). The ferroelectric-paraelectric phase transition at T-c = 1383 K is interpreted as due to the R3c --> R (3) over bar m structural transformation At the 500-800 K temperature range, there oc curs a transition to an intermediate between the R (3) over bar m and R3c phases, with all the VO43- ions at sites which 3-fold symmetry. Th ere is no soft-mode behavior observed up to 1483 K, suggesting an orde r-disorder character of the transitions.