SYNTHESIS OF A NEW LAYERED SODIUM COPPER(II) PYROPHOSPHATE, NA2CUP2O7, VIA AN EUTECTIC HALIDE FLUX

Crystals of Na2CuP2O7 have been grown in a low-temperature eutectic fl ux of 23% NaCl and 77% CuCl (mp = 315 degrees C). The X-ray structural analysis shows that this sodium copper(II) pyrophosphate crystallizes in a monoclinic lattice with a = 14.715(1) Angstrom, b = 5.704(2) Ang strom, c = 8.066(1) Angstrom, beta = 115.14(1)degrees, and V = 612.9(2 ) Angstrom(3); C2/c (No. 15); Z = 4. The structure has been refined by the least-squares method to R = 0.020, R(w) = 0.034, and GOF = 1.93. The framework of the title compound can be described as slabs of [CuP2 O7], which are composed of undulating [CuP2O7](infinity) ribbons, cros s-linked by Na cations. The Cu2+ cation adopts a CuO4 square planar co ordination geometry which shares four corner oxygens with two P2O7 pyr ophosphate (diphosphate) groups in a trans orientation and the Na+ cat ion appears in a distorted NaO6 octahedral coordination geometry. The extended [CuP2O7](infinity) ribbon structure is constructed by alterna ting CuO4 and P2O7 units and is characterized by Cu-O-P-O-Cu linkages. In this paper, the synthesis, structure, bonding, thermal property, a nd infrared spectrum of the title compound are presented. Structural c omparison with the Li2CuP2O7 and Sr2CuSi2O7 phases allows a possible c onclusion concerning the role of the A-site cation in the framework fo rmation to be drawn. The relative A-O bond strength and its relationsh ip to the thermal stability, with respect to melting, of the A(2)CuP(2 )O(7) (A = Li, Na, K) compound family are illustrated; the unknown str ucture of the tetragonal K2CuP2O7 phase is briefly discussed.