Structural changes at the triclinic-rhombohedral transition and their influence on the Li mobility of the fast-ion conductor LiHf2(PO4)(3)

Structural changes produced at the triclinic-rhombohedral transition of LiH f2(PO4)(3) have been analyzed by NMR spectroscopy: the reduction of symmetr y produces differentiation of three tetrahedra PO4, whose distortions corre late well with chemical shift anisotropies determined by P-31 NMR. At the p hase transition, P-31 chemical shift values change according to the modific ation of the P-O-Hf angles determined by neutron diffraction data. On the o ther hand, in the triclinic (low-temperature) phase, the location of lithiu m along the conduction channels, at midway positions between M-1 and M-2 si tes, was confirmed by Li-7 NMR spectroscopy. In these positions, a distorte d almost planar fourfold coordination, with Li-O distances near 2.1 Angstro m, explains the observed Li-7 quadrupole coupling constants. In the rhomboh edral (high-temperature) phase, the structural changes destabilize lithium coordination. In this cast, Li-7 NMR spectra show important lithium mobilit y and spectral features are intermediate between those of M-1, M-2, and mid way sites. The important delocalization of lithium explains the strong incr ease on conductivity detected at the phase transition that males this compo und one of the best reported lithium ion conductors.