From the ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ NMR study of the compounds ${\mathrm{LiM}}_2$(${\mathrm{PO}}_4$${)}_3$ (${\mathrm{M}}^{\mathrm{IV}}$=Ge, Ti, Sn, and Hf), structural distortions of the NASICON (acronym for Na superionic conductors) network and the occupation of structural sites by lithium have been analyzed. In compounds with larger cations (${\mathrm{M}}^{\mathrm{IV}}$=Sn and Hf), structural distortions reduce the ideal symmetry of the NASICON framework (rhombohedral R3-barc), producing the differentiation of three phosphorous sites. In these low-symmetry phases, ${\mathrm{PO}}_4$ tetrahedra distortions, deduced from the analysis of the ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ NMR chemical shift tensor, are higher than those of rhombohedral phases. These additional distortions are eliminated by sample heating above their corresponding transition temperature. On the other hand, from analysis of the dipole and quadrupole interactions of ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ NMR spectra, recorded at low temperatures, it has been deduced that lithium atoms are located at ${\mathrm{M}}_1$ sites in the rhombohedral ${\mathrm{LiGe}}_2$(${\mathrm{PO}}_4$${)}_3$ and ${\mathrm{LiTi}}_2$(${\mathrm{PO}}_4$${)}_3$ and at ${\mathrm{M}}_2$ sites in the low-symmetry phases of ${\mathrm{LiSn}}_2$(${\mathrm{PO}}_4$${)}_3$ and ${\mathrm{LiHf}}_2$(${\mathrm{PO}}_4$${)}_3$ compounds.

DOI:https://doi.org/10.1103/PhysRevB.55.14270