Network-forming oxides with non-centrosymmetric structural groups – diffraction results on molybdate and tellurite glasses

The structural units of two types of conditional glass-forming oxides are irregular polyhedra whose distorted shapes result from intrinsic electronic causes. First example: The d0-transition metal cations Mo6+ form an amount of MoO6 in Ag molybdate glasses with the Mo6+ cation displaced towards an octahedral edge. Reverse Monte-Carlo results and comparisons with crystal structures corroborate the earlier diffraction interpretations of three distinct Mo−O distances in the MoO6 units. The post-transitional main group cation Te4+ forms high fractions of TeO5E units in phosphate glasses, where E denotes the lone-pair corner of the structural group. Instead of the regular TeO5E pyramids known for some related crystals, only irregular units are found in the glasses. The bond valences of their longest Te−O bonds are small, requiring the occurrence of three-fold coordinated oxygens.