Discovery of a Sodium-Ordered Form of Na₃V₂(PO₄)₃ below Ambient Temperature

The crystal structure of the NASICON Na₃V₂(PO₄)₃ phase (NVP) has been investigated as a function of temperature. Combining laboratory and synchrotron X-ray powder diffraction with single crystal X-ray diffraction, we demonstrate the presence of four polymorphs of NVP from 30 to 225 °C. While the high temperature γ-NVP crystallizes in the classical rhombohedral cell (S.G. <i>R</i>3̅<i>c</i>, <i>a</i> = 8.73382(4) Å, <i>c</i> = 21.91438(17) Å), the low temperature α-NVP undergoes a monoclinic distortion (S.G. <i>C</i>2/<i>c</i>, <i>a</i> = 15.1244(6) Å, <i>b</i> = 8.7287(3) Å, <i>c</i> = 21.6143(8) Å, β = 90.163(2)°) together with an ordering of the Na atoms. In the middle temperature range, two incommensurate modulated structures (β- and β′-NVP) are also reported for the first time