Syntheses, structures, and thermal expansion of germanium pyrophosphates

alpha -Ge(HPO4)(2).H2O has been hydrothermally prepared and its thermal beh avior has been studied by DTA and thermodiffractometry, Three pyrophosphate s can be obtained on heating at high temperature. alpha -GeP2O7 is protocry stalline with a layered structure related to that of the pristine material, alpha -GeP2O7 transforms to beta -GeP2O7 above 950 degreesC and this yield s gamma -GeP2O7 above 1000 degreesC, However, gamma -GeP2O7 could not be pr epared as single phase at high temperature and ambient pressure because P2O 5 is partly released leading to a contamination with Ge5O(PO4)(6) which is the final thermal decomposition product. gamma -GeP2O7 has been synthesized as a single phase by heating GL-GeP207 at 20 kbar and 1000 degreesC, gamma -GeP2O7, previously reported as cubic, is monoclinic with a = 22.8647(4) A ngstrom, b = 22.8783(4) Angstrom, c = 229429(4) Angstrom, beta = 90.328(1)d egrees, V = 120013(4) Angstrom (3) and symmetry P2(1)/c or lower, gamma -Ge P2O7, with a simple stoichiometry, crystallizes in a very large unit cell w ith at least 26 Ge, 54 P, and 190 O crystallographically independent atoms. This complex superstructure is caused by the ordered pattern of the P-O-P bent groups. The P-31 MAS-NMR profiles of gamma -GeP2O7, beta -GeP2O7, and Ge5O(PO4)(6) are reported and discussed. The thermal expansion of gamma -Ge P2O7 is also described. (C) 2001 Academic Press.