The α ⇆ β phase transitions of Zn2P2O7 revisited: existence of an additional intermediate phase with an incommensurately modulated structure

Zn2P2O7 crystallizes in a thortveitite-like structure and features temperature-dependent polymorphism. At high temperatures (T > 500 K), the aristotype phase beta-Zn2P2O7 (C2/m, Z = 2, a similar or equal to 6.60, b similar or equal to 8.28, c similar or equal to 4.53 angstrom, beta similar or equal to 105.4 degrees) is stable. At lower temperatures the lock-in phase alpha(1)-Zn2P2O7 [at 350 K: I2/c, Z = 12, a = 20.1131 (13), b = 8.2769 (6), c = 9.109 (3) angstrom, beta = 106.338 (16)degrees], a sixfold superstructure with commensurate modulation vector q = (1/3, 0, 1/2), is stable. Between the stability ranges of the alpha(1)- and beta-phases exists the intermediate, incommensurately modulated alpha(2)-Zn2P2O7 phase with modulation wavevector q similar or equal to (0.33, 0, 0.40) and C2/m(alpha, 0, gamma)0s superspace group symmetry. The alpha(1) -> alpha(2) lock-in phase transition at T-L = 408 K is of first-order and features virtually no hysteresis. It is immediately followed by the second-order alpha(2) -> beta transition to the non-modulated phase at T-I similar or equal to 430 K. This transformation is sluggish and even at T = 500 K very weak satellite reflections of the alpha(2)-phase were observed. Both phase transitions were analyzed with differential scanning calorimetry and high-temperature powder and single-crystal X-ray diffraction. The crystal structures of the alpha(1)- and alpha(2)-phases were refined from single crystal data collected at T = 350, 400, 405, 410, 415, 420, 425, 430, 450 and 500 K. Different models describing the slow transition from the incommensurately modulated alpha(2)- to the non-modulated beta-phase were tested. In the model resulting in the best residuals, the bridging O atom of the [P2O7] group, which is located on a 2/m position in the basic structure, is described as an overlap of an atom ordered in internal space and one atom disordered around the mirror plane. The occupancy of the ordered atom decreases with temperature until at T = 500 K virtually only the disordered atom remains. Simultaneously, the amplitude of the modulation functions of the remaining atoms decreases, so that the T = 500 K structure can be considered as the C2/m aristotype structure, although the diffraction pattern still features satellite reflections of first order with very low intensities.