SYNTHESIS AND CHARACTERIZATIONS OF A POLYMORPHIC SODIUM COBALT PHOSPHATE WITH EDGE-SHARING CO2+ OCTAHEDRAL CHAINS

A new sodium cobalt phosphate with a three-dimensional framework struc ture has been prepared by the hydrothermal method and characterized by thermal analysis, susceptibility measurements, and single crystal X-r ay diffraction. The oxide framework is built from octahedral and tetra hedral atoms (Co2+ and P5+) and consists of cross-linked infinite Co-O -Co chains that propagate along the crystallographic b-axis. The frame work connectivity can be correlated to that of potassium titanyl phosp hate (KTP) type structure. The structure of NaCoPO4 is also compared t o other divalent metal phosphates and a summary of structures isotypic to NaCoPO4 is given. Magnetic susceptibility measurements show that t he compound displays Curie-Weiss behavior down to a temperature of 13K at which an antiferromagnetic phase transition occurs. The thermal an alysis shows that the octahedral polymorph is a product of a phase tra nsformation of the trigonal bipyramidal polymorph at about 710 degrees C. The octahedral polymorph transforms into the tetrahedral polymorph at about 920 degrees C. Crystal data for NaCoPO4: M=176.89, space gro up Pnma (No. 62), a = 8.8957(12), b = 6.8007(9), c = 5.0341(7), V = 30 4.55(7)Angstrom(3), Z=4, D-c = 3.858 g cm(-3) pink thin plate, MoK alp ha, lambda=0.71073 Angstrom, mu = 6.128 mm(-1), 2 theta(max) = 56.38 d egrees, R(F)= 3.68% for 41 parameters and 328 reflections with I>2 sig ma(I). (C) 1997 Academic Press.