Lattice Dynamics in the NASICON NaZr₂(PO₄)₃ Electrolyte Revealed from Temperature-Dependent Neutron, NMR and Ab Initio Computational Studies

NASICON compounds form a rich and highly chemically-tunable family of crystalline materials that are of widespread interest because they include exemplars with high ionic conductivity, low thermal expansion, and redox tunability, making them suitable candidates for applications ranging from solid-state batteries to nuclear waste storage materials. The key to an understanding of these properties, including the origins of effective cation transport and low, anisotropic (and sometimes negative) thermal expansion, lies in the dynamics associated with specific details of the crystal structure. Here, we closely examine the prototypical NASICON compound, NaZr₂(PO₄)₃, and obtain detailed insights into such anharmonic behavior via neutron diffraction and variable-temperature ²³Na and ³¹P solid-state NMR studies, coupled with comprehensive density functional theory-based calculations of NMR parameters. Temperature-dependent NMR studies yield some surprising trends in the chemical shifts and the quadrupolar coupling constants that are not captured by computation unless the underlying vibrational modes of the crystal are explicitly taken into account. The work presented here widens the utility of NMR crystallography to include thermal effects as a unique probe of interesting lattice dynamics in functional materials.