Phase Transition and Ionic Mobility in LiHf2(PO4)3 with NASICON Structure

A reversible first-order phase transition has been found around 0 °C in the compound LiHf2(PO4)3, which has been followed by DSC, XRD, NMR (31P and 7Li), and impedance techniques. In the low-temperature phase, a triclinic distortion of the NASICON framework has been detected for the first time in the LiM2(PO4)3 family. In this phase location of lithium ions at low-symmetry sites (probably M2 sites) has been deduced from 7Li NMR data. Above 0 °C, the usual NASICON symmetry (rhombohedral R3̄c) is recovered and an abrupt increase of the ionic mobility is observed: activation energy obtained from bulk dc conductivity decreases from 1.39 to 0.33 eV. From NMR data, a strong delocalization of lithium ions over M1 and M2 sites is detected when going from the triclinic to the rhombohedral phase.