Li mobility in triclinic and rhombohedral phases of the Nasicon-type compound LiZr2(PO4)3 as deduced from NMR spectroscopy

The Nasicon-type LiZr2(PO4)3 compound exhibits, around 310 K, a first order transition from the triclinic C to the rhombohedral Rc form, which has been followed by DSC, XRD and NMR (31P and 7Li) techniques. During heating–cooling treatments a hysteresis cycle was detected in which triclinic and rhombohedral phases coexist. In the low-temperature triclinic form, the four-fold coordination of Li deduced from neutron diffraction experiments, at midway positions between M1 and M2 sites, was confirmed. From relaxation NMR data, a microscopic activation energy of Em = 0.43 eV was measured in the triclinic phase. Above the phase transition a strong delocalisation of Li over structural sites disposed along conduction paths was detected. The high mobility of Li detected in the high-temperature form explains the outstanding conductivity reported for the rhombohedral phase.