Structural changes at the triclinic-rhombohedral transition and their influence on the Li mobility of the fast-ion conductor ${\mathrm{LiHf}}_2{(\mathrm{P}\mathrm{O}}_4{)}_3$

Structural changes produced at the triclinic-rhombohedral transition of ${\mathrm{LiHf}}_2{(\mathrm{P}\mathrm{O}}_4{)}_3$ have been analyzed by NMR spectroscopy: the reduction of symmetry produces differentiation of three tetrahedra ${\mathrm{PO}}_4,$ whose distortions correlate well with chemical shift anisotropies determined by ${}^{31}\mathrm{P}\mathrm{}\mathrm{NMR}.$ At the phase transition, ${}^{31}\mathrm{P}$ chemical shift values change according to the modification of the P-O-Hf angles determined by neutron diffraction data. On the other hand, in the triclinic (low-temperature) phase, the location of lithium along the conduction channels, at midway positions between $M_1$ and $M_2$ sites, was confirmed by ${}^7\mathrm{Li}\mathrm{}\mathrm{NMR}$ spectroscopy. In these positions, a distorted almost planar fourfold coordination, with Li-O distances near 2.1 Å, explains the observed ${}^7\mathrm{Li}$ quadrupole coupling constants. In the rhombohedral (high-temperature) phase, the structural changes destabilize lithium coordination. In this case, ${}^7\mathrm{Li}\mathrm{}\mathrm{NMR}$ spectra show important lithium mobility and spectral features are intermediate between those of $M_1,$ $M_2,$ and midway sites. The important delocalization of lithium explains the strong increase on conductivity detected at the phase transition that makes this compound one of the best reported lithium ion conductors.