Sodium ion conducting solid electrolytes
Compound formation and phase relations in the ternary system, Na2O-Zro2-P2O5, were investigated. A phase diagram for the orthophosphate join, NaZr2 (PO4)3-Na3PO4 is presented; three new phases were prepared and their crystal chemistry, polymorphism and electrical conductivities determined. One of the new phases is a new Na+ ion conducting solid electrolyte material, of formula: Na9-4yZry (PO4)3: 0 < y < 0.57. This new material, Zr-doped Na3PO4, exhibits a maximum conductivity of 2.5 x 10-2 ohm-1 cm-1 at 300°C and it may have applications as a new practical solid electrolyte. The other new phases are, Na5Zr(PO4)3 and a related solid solution series, S, of formula: Na5-4xZr1+x(PO4)3; 0.04 < x < 0.15. The conductivities of these latter phases at 300° C are ~1 x 10-5 and 1 x 10-3 ohm-1 cm-1, respectively. Zr-doped Na3PO4 exists in various polymorphic forms. The high temperature solid solutions have the cubic Na3PO4 structure. Below ~980° C an ordering reaction occurs, for most compositions, to give the ord phase which has a supercell essentially eight times the volume of the subcell. The order-disorder reaction, ord, was followed by using X-ray powder diffractometry. Disordering occurs by a gradual decrease in the size of the ordered domains within the ord, structure. A model for the structure of the ordered domains is discussed. For some compositions, other polymorphs occur at lower temperatures. The unit cell, lattice parameters of Na5Zr (PO4)3 and S, Na5-4xZr1+x-(PO4)3, were determined by single crystal X-ray diffraction and selected area electron diffraction (SAED). Na5Zr (PO4)3 has trigonal symmetry and rhombohedral lattice type; aHEX= 9.168, cHEX = 21.80. SAED patterns of S reveal an incommensurate supercell with a subcell similar to that of Na5Zr (PO4)3. SAED patterns and X-ray powder data of S were successfully indexed using a system of integral and fractional indices. The origin of the incommensurate superstructure is discussed. Finally, a preliminary investigation of the various proposed formulae of the Nasicon Na+ ion solid electrolyte, is presented.