HIGH-PRESSURE RAMAN STUDIES OF BETA-NAVO3

In an earlier study, the alpha-phase, which is one of the stable forms of NaVO3 under ambient conditions, has been shown to undergo a crysta lline-amorphous phase transition at c. 60 kbar [Shen et al., J. Phys. Chem. Solids 55, 665 (1994)]. In the amorphous phase, the chains consi sting of corner-sharing VO4 tetrahedra break up into individual VO3 py ramids. We now report on the high pressure study of the beta-form, for which three phase transitions have been found. The first transition a t 14 kbar is of first order and involves major structural changes. The second at about 44 kbar is second order and only subtle spectral chan ges are seen. The third is at 80 kbar and the sample may be partially disordered in this phase. Most importantly, despite the fact that both the alpha- and beta-forms are stable phases of NaVO3 under ambient co nditions, their difference persists to very high pressure, and no amor phous phase is observed up to 166 kbar in beta-NaVO3.