THE CRYSTAL-STRUCTURE OF ALPHA-NA2CUP2O7 AND BETA-NA2CUP2O7

The crystal structures of both alpha- and beta-modifications of copper disodium diphosphate (Na2CuP2O7) were solved at 295 K. The low-temperature phase alpha-Na2CuP2O7 has been found to undergo an irreversible phase transition to beta-Na2CuP2O7, at 843 K. The alpha-allomorph crystallizes with a faint blue color and the space group P2(1)/n (Z = 4) with unit cell parameters a = 8.823(3), b = 13.494(3), c = 5.108(2) Angstrom, and beta = 92.77(3)degrees. Single crystals of the high-temperature form beta-Na2CuP2O7, recognized by their dark blue color, have also a monoclinic symmetry with the space group C2/c (Z = 4) and unit cell parameters a = 14.728(3), b = 5.698(1), c = 8.067(1) Angstrom, and beta = 115.15(1)degrees. The structure parameters were refined to a final R of 0.020 (wR = 0.031) for 1550 independent reflections [I > 3 sigma(1)] in the case of alpha-Na2CuP2O7 (alpha-NCP) and a final R of 0.024 (wR = 0.043) for 1234 independent reflections [I > 3 sigma(I)] in the case of beta-Na2CuP2O7 (beta-NCP). Both alpha- and beta-NCP structures are built up of infinite ribbons [CuP2O7](2n-)(n) made of alternating diphosphate groups (P2O7)(4-) and Cu2+ cations. Each copper cation is connected to two adjacent (P2O7)(4-) groups via two oxygens belonging to two different [PO4] tetrahedra. The main structural differences between the two networks reside in the shape of the ribbons (corrugated or flat) and the way the sodium cations are set out between appropriate ribbons. In beta-Na2CuP2O7 all the ribbons are corrugated and stacked in layers connected to each other by sheets of Na+ parallel to (100). The lattice may then be regarded as having a lamellar character. The low-temperature form can be seen as made of infinite pairs of [CuP2O7](2n-)(n) flat ribbons embedded in a matrix of Na+. In the latter phase, the ribbons, running parallel to [001], are symmetrically related by a center of inversion and a glide plane n belonging to the space group P2(1)/n. The mean values of the bond lengths in beta-NCP are all smaller than equivalent ones in the low-temperature modification. Furthermore all diphosphate groups adopt an almost eclipsed configuration in both allomorphs. The P-O-P angle, equal to 118.66 degrees in alpha-Na2CuP2O7, represents the smallest value found for all diphosphates so far known. The copper coordination is square-pyramid-like in the alpha-NCP phase, while it is of planar square type in the beta-NCP variety. (C) 1995 Academic Press Inc.