Compound formation and conductivity in the system Na2O-ZrO2-P2O5: Sodium zirconium orthophosphates
Two new orthophosphate phases have been synthesised. One is the stoichiometric phase Na5ZrP3O12 which melts incongruently at 1160°C. It has a hexagonal unit cell, a=9.17, c=21.80 A with a rhombohedral lattice type. The other is a solid solution series, S, of formula Na5−4xZr1+xP3O12:0.04<x<0.11 at 1000°C but 0.05<x<0.15 at 1100°C. Below ∼930°C, S undergoes a slow transformation to a low temperature polymorph. The previously reported phase Na2ZrP2O8 was prepared as an equilibrium phase below ∼930°C. A partial subsolidus phase diagram for the orthophosphate join is presented. The a.c. conductivities of Na5ZrP3O12 and one S composition have been measured; for Na5ZrP3O12, σ300°C∼2×10−5Ω−1cm−1 and for S, x=0.075, σ300°C∼1×10−3Ω−1cm−1; these are net conductivities.