NEW CRYSTALLINE AND GLASSY PHASES IN THE PSEUDO-BINARY SYSTEM K4P2O7-PB2P2O7

The pseudo-binary system K4P2O7-Pb2P2O7 has been investigated by x-ray diffraction, vibrational spectroscopy and differential thermal analysis. Two new compounds, K2PbP2O7 and K2Pb3(P2O7)2 have been evidenced. Two polymorphs of K2PbP2O7 are observed, with a transition temperature of 571-degrees-C. The transformation is reversible, but the high-temperature (HT) polymorph can be isolated by rapid quenching from 600-degrees-C. The parameters of its monoclinic unit cell are: a = 9.542(1), b = 5.887(1), c = 6.979(1) angstrom, beta = 108.07(2)-degrees. Its vibrational spectrum (Raman and infrared) exhibits broad bands, suggesting the existence of some structural disorder. The low-temperature (LT) polymorph exhibits a complex x-ray powder diagram, unindexed so far, but is vibrational spectrum is very sharp, thus suggesting that the LT --> HT transformation is of the order-disorder type. The corresponding Rb compound Rb2PbP2O7 is isotypic with HT K2PbP2O7. K2Pb3(P2O7)2 is orthorhombic with a = 17.758(3), b = 9.280(2), c = 8.596(2) angstrom; it is isotypic with Rb2Pb3(P2O7)2. Broad bands in the vibrational spectra suggest some structural disorder. Na2PbP2O7 is not isotypic with K2PbP2O7. It is triclinic with a = 6.881(1), b = 9.363(2), c = 5.482 (1) angstrom, alpha = 96.95(2), beta = 108.22(2), gamma = 105.63(2)-degrees. The vibrational spectrum is very sharp (no structural disorder). The system (K4P2O7)1-x(Pb2P2O7)x gives glasses only in a rather limited range of compositions (0.73 less-than-or-equal-to x less-than-or-equal-to 1). This range is much smaller than that observed for the corresponding Na or Li glasses (0.4 less-than-or-equal-to x less-than-or-equal-to 1). The P2O7 group is preserved in the structure of these glasses.