COPPER-SUBSTITUTED MANGANESE-DENNINGITES, MN(MN1-XCUX)(TE2O5)2 (0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 1) - SYNTHESIS AND CRYSTAL-CHEMISTRY

The crystal structures of copper-substituted manganese-denningites, Mn (Mn1-xCux) (Te2O5)2 (0 less-than-or-equal-to x less-than-or-equal-to 1 ), were refined in space group P4(2)/nbc from single-crystal X-ray dat a. Single crystals with different degree of Cu-substitution suitable f or X-ray investigation were synthesized under hydrothermal conditions, varying the Cu/Mn ratio and the pH-value. One main feature of the cry stal structure is the distribution of Mn and Cu atoms among an eight a nd a six-coordinated site, respectively. Bond strength calculations su pport the site occupancies of the MnO8-polyhedra and MeO6-octahedra (M e = CuxMn1-x). The decrease in length of the four Me[6]-O bonds clearl y correlates with the increase of the Cu-substitution resulting in a d istortion of the octahedra according to the Jahn-Teller effect of diva lent copper. The stronger decrease of the lattice parameter c as compa red to a is probably due to the variations of the bond lengths.