The reversible α→β phase transition of Cu2As2O7

Single crystals of copper(II) pyroarsenate(V), Cu2As2O7, were prepared by chemical transport reactions in sealed and evacuated silica ampules starting from stoichiometric mixtures of the component oxides and chlorine as transport agent (temperature gradient 880 --> 800 degreesC, 5 days). Cu2As2O7 is dimorphous and shows a reversible a (low-temperature) H P (high-temperature) phase transition at 356(2) C detected by differential scanning calorimetry (DSC) and high-temperature X-ray powder diffraction (XRPD) measurements. The crystal structure of alpha-Cu2As2O7 (C2/c, Z = 4, a = 7.237(3), b = 8.2557(17), c = 9.780(3) Angstrom, beta = 111.03(2)degrees, R[F-2 > 2sigma(F-2)] = 0.028) was determined from single-crystal data at room temperature. It crystallizes isotypically with alpha-Cu2P2O7 and beta-Cu2V2O7. The thortveitite-type crystal structure of beta-Cu2As2O7 (C2/m, Z = 2, a = 7.0987(3), b = 8.2777(4), c = 4.8666(2) Angstrom, beta = 110.206(4)degrees, R(Bragg) = 0.1056) was determined by means of high-temperature XRPD recorded at 400 C. The crystal structures of both polymorphs are closely related and consist of infinite sheets of [CuOx] polyhedra (a: x = 5; : x = 6) and interjacent As2O7 anions that occur either in a bent (As-O-As) configuration (alpha-Cu2As2O7, bridging angle 145.9(2)degrees) or in a linear (As-O-As) configuration (beta-Cu2As2O7). alpha-Cu2As2O7 was further characterized by vibrational spectroscopy. Its IR and Raman spectra are discussed on the basis of a factor group analysis.