STRUCTURAL ORDER IN COMPLEXES OF THE A(II)B(II)P(2)O(7) SERIES - BAPBP2O7, PBCOP2O7, PBMNP2O7 AND PBCUP2O7

Four new diphosphate compounds of the type A(II)B(II)P(2)O(7) have bee n synthesized and characterized by single-crystal X-ray diffraction. W hile all four have similar unit cell dimensions, BaPbP2O7 is isotypica l with alpha-Sr2P2O7 (space group Pmnb), whereas PbCoP2O7, PbMnP2O7 an d PbCuP2O7 are similar to alpha-Ca2P2O7 (space group P2(1)/n). P2O74- groups show average O-P ... P-O angles which range from 0.0 degrees to 49.56 degrees and are intermediate between the 0.0 degrees expected o f dichromate-type structures and the 60.0 degrees expected of thorveit ies. The magnitude of the O-P ... P-O angle is correlated with the ion ic radius of the smaller cation in each compound. Furthermore, the cha nge in space group for BaPbP2O7 may be seen to be a consequence of the ionic radii of the B atom. Metal coordination numbers in the four com plexes range from 5 to 9.