Ge(HPO 4 ) 2 'H 2 O has been hydrothermally prepared and its thermal behavior has been studied by DTA and thermodi4ractometry. Three pyrophosphates can be obtained on heating at high temperature. -GeP 2 O 7 is protocrystalline with a layered structure related to that of the pristine material. -GeP 2 O 7 transforms to -GeP 2 O 7 above 9503C and this yields -GeP 2 O 7 above 10003C. However, -GeP 2 O 7 could not be prepared as single phase at high temperature and ambient pressure because P 2 O 5 is partly released leading to a contamination with Ge 5 O(PO 4 ) 6 which is the 5nal thermal decomposition product.
-GeP 2 O 7 has been synthesized as a single phase by heating -GeP 2 O 7 at 20 kbar and 10003C. -GeP 2 O 7 , previously reported as cubic, is monoclinic with a ؍ 22.8647(4) A > , b ؍ 22.8783(4) A > , c ؍ 22.9429(4) A > , ؍ 90.328(1)3, V ؍ 12001.3(4) A > 3 and symmetry P2 1 /c or lower. -GeP 2 O 7 , with a simple stoichiometry, crystallizes in a very large unit cell with 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 31 P MAS-NMR pro5les of -GeP 2 O 7 , -GeP 2 O 7 , and Ge 5 O(PO 4 ) 6 are reported and discussed. The thermal expansion of -GeP 2 O 7 is also described.