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 degrees C. While the high temperature gamma-NVP crystallizes in the classical rhombohedral cell (S.G. R (3) over barc, a = 8.73382(4) angstrom, c = 21.91438(17) angstrom), the low temperature alpha-NVP undergoes a monoclinic distortion (S.G. C2/c, a = 15.1244(6) angstrom, b = 8.7287(3) angstrom, c = 21.6143(8) angstrom, beta = 90.163(2)degrees) together with an ordering of the Na atoms. In the middle temperature range, two incommensurate modulated structures (beta- and beta '-NVP) are also reported for the first time.