15 - Proton containing NASICON phases

Summary

Triphosphate phases

Tetravalent metals readily form triphosphates of the type M(I)M(IV)₂(PO₄)₃, where M(I) may be any alkali metal and M(IV) is most commonly Zr. A large number of such compounds have been synthesized by high temperature solid state methods. These compounds are of interest because they are low expansion materials and form the basis for an extensive family of solid electrolytes. Single crystals of NaZr₂(PO₄)₃ have been grown from melts and the structure determined. A schematic drawing of a portion of the structure is shown in Fig. 15.1. The crystals are rhombohedral a = 8.8043(2) Å, c = 22.7585(9) Å, space group. The structure consists of zirconium octahedra and phosphate tetrahedra which are linked by corners. A basic unit consists of two octahedra and three tetrahedra separated by Na⁺ ions which form columns parallel to the c axis. Thus, the structure consists of [Zr₂(PO₄)₃]^– anions and Na⁺ cations. The anions are linked together through bridging phosphate groups to form a rigid framework which creates cavities. The cavity labelled A in Fig. 15.1 contains the Na⁺ which is coordinated by phosphate oxygens in a trigonal anti-prism. Only half the structure in the a-direction is shown in the figure. On adding the second half, three more cavities are created. Six such cavities, three above and three below, termed type II, surround the A type (type I) and are connected through narrow passageways.