COMPARISON OF THE CRYSTAL-STRUCTURES OF CO2(X2O7).2H2O, X = P AND AS

Crystals of Co2(X2O7).2H2O, X = P/As were synthesized under hydrotherm al conditions. Their crystal structures were determined by single crys tal X-ray diffraction: a = 6.334(1)/6.531(2), b = 13.997(2)/14.206(4), c = 7.637(1)/7.615(2) angstrom, beta = 94.77(2)/94.74(2)-degrees, spa ce group P2(1)/n, R = 0.032/0.046, R(w) = 0.028/0.034 for 2423/2042 re flections and 131/119 variables. Within the two XO4 tetrahedra connect ed via a common comer to an X2O7 group the average [P-O] bond lengths are approximately equal (1.540 and 1.543 angstrom), but [As-O] differs significantly (1.685 and 1.696 angstrom). A comparison with the isoty pic Mn and Mg pyrophosphates shows a correlation between the ratio [Me -O]/[X-O] and the angle O-X-O.