Crystal Chemistry of NaSICONs: Ideal Framework, Distortion, and Connection to Properties

The crystal structure of R3̅c NaSICON type AxM2(TO4)3 (M = Al, Ce, Co, Cr, Fe, Ge, Hf, In, Li, Mg, Mn, Mo, Na, Nb, Ni, Sb, Sc, Se, Sn, Ta, Ti, U, V, Y, Yb, Zn, Zr; T = As, Ge, Mo, P, S, Si, V) with its multiple internal degrees of freedom offers considerable flexibility and can accommodate dozens of different ions leading to hundreds of reported compositions. In this work, the ideal R3̅c structure with undistorted [MO6] octahedra and [TO4] tetrahedra is parametrized and used to quantify distortion of about 340 real NaSICON-type oxides consistently across the entire family. It is shown that distortion is primarily driven by the size mismatch between the A cations and M2(TO4)3 framework, that can be used to optimize the geometry of the structure to control properties, such as A-ionic conductivity or thermal expansion.