The Crystal Structure of α- and β-Na2CuP2O7

Abstract

The crystal structures of both α- and β-modifications of copper disodium diphosphate (Na2CuP2O7) were solved at 295 K. The low-temperature phase α-Na2CuP2O7 has been found to undergo an irreversible phase transition to β-Na2CuP2O7, at 843 K. The α-allomorph crystallizes with a faint blue color and the space group P 21/n (Z = 4) with unit cell parameters a = 8.823(3), b = 13.494(3), c = 5.108(2) Å, and β = 92.77(3)°. Single crystals of the high-temperature form β-Na2CuP2O7, recognized by their dark blue color, have also a monoclinic symmetry with the space group C 2/c (Z = 4) and unit cell parameters a = 14.728(3), b = 5.698(1), c = 8.067(1) Å, and β = 115.15(1)°. The structure parameters were refined to a final R of 0.020 (wR = 0.031) for 1550 independent reflections [I > 3 σ(I)] in the case of α-Na2CuP2O7 (α-NCP) and a final R of 0.024 (wR = 0.043) for 1234 independent reflections [I > 3 σ (I)] in the case of β-Na2CuP2O7 (β-NCP). Both α- and β-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 β-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 [CuP2O72n-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 P 21/n. The mean values of the bond lengths in β-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° in αNa2CuP2O7, represents the smallest value found for all diphosphates so far known. The copper coordination is square-pyramid-like in the α-NCP phase, while it is of planar square type in the β-NCP variety.