Pressure induced phase transformations in NaZr2(PO4)3 studied by X-ray diffraction and Raman spectroscopy

•NaZr2(PO4)3 undergoes two reversible phase transitions at 5 and 6.7 GPa.•The transition at 5 is from rhombohedral R-3c to R3 structure.•Bulk modulus of NaZr2(PO4)3 is lower than that for the isostructural RbTi2(PO4)3.•Compressibility increases with substitution of a smaller cation (Na).•Zr-translational and PO4 librational modes contribute to phase transition.

Raman spectroscopic and x-ray diffraction measurements on NaZr2(PO4)3 were carried out up to 30 GPa at close intervals of pressure, revealing two structural phase transformations around 5 and 6.6 GPa. The second phase at 5.4 GPa is indexed to R3 space group similar to that of RbTi2(PO4)3. Bulk modulus decreases abruptly from 53 GPa (B′=4) to 36 GPa (B′=4) in the second phase above 5 GPa. The structure of the phase III at 8.2 GPa is indexed as orthorhombic similar to the case of high temperature phase of monoclinic LiZr2(PO4)3. Bulk modulus of this phase III is found to be 65 GPa (B′=4), which is higher than that of the ambient phase. In high pressure Raman studies, modes corresponding to 72 and 112 cm−1 soften in the ambient phase whereas around 5 GPa, the ones at 60, 105, 125 and 190 cm−1 soften with pressure contributing negatively to overall thermal expansion.

Graphical abstractHigh pressure study of NaZr2(PO4)3 shows a reversible phase transition from R-3c to R3 structure at 5 GPa accompanied by an increase in compressibility signaling a polyhedral tilt transition.Figure optionsDownload full-size imageDownload as PowerPoint slide