SYNTHESIS, CRYSTAL-STRUCTURE, AND MAGNETIC-PROPERTIES OF A NEW POLYMORPHIC SODIUM COBALT PHOSPHATE WITH TRIGONAL BIPYRAMIDAL CO2+ AND A TUNNEL STRUCTURE

A new sodium cobalt phosphate with a three-dimensional framework struc ture has been prepared by hydrothermal methods and characterized by si ngle crystal X-ray diffraction and magnetic susceptibility measurement s. The framework can be described as interconnected identical layers s tacked along the crystallographic a-axis. The layer is built from half of the Co2+ trigonal bipyramids and half of the P5+ tetrahedra and ca n be classified as a distorted (4.8(2)) net commonly found in zeolite frameworks. The structure has one-dimensional channels where charge-ba lancing sodium cations are situated. The Co-O-Co linkages give rise to a tetramer of Co2+ bipyramids which may be related to the low-tempera ture magnetic behavior. The magnetic susceptibility measurements show that the compound displays Curie-Weiss behavior down to a temperature of 15 K at which an antiferromagnetic phase transition occurs. The str uctural features and magnetic properties are compared with two other p olymorphs of sodium cobalt phosphates which are built from tetrahedral and octahedral Co2+, respectively. Crystal data for NaCoPO4: M=176.89 , space group P2(1)/c (No. 14), a=5.7856(1), b=11.0954(2), c=9.9228(3) , beta=92.668(2), V=636.29(2) Angstrom(3), Z=8, D-c=3.693 g cm(-3), da rk red needle, MoK alpha, lambda=0.71073 Angstrom, mu=5.867 mm(-1), 2 theta(max)=56.54 degrees, R(F)=2.07% for 128 parameters and 1471 refle ctions with I>2 sigma(I). (C) 1997 Academic Press.