Cationic environment in silicate glasses studied by neutron diffraction with isotopic substitution

The method of neutron diffraction coupled with isotopic substitution is pre sented and recent investigations on the environment around cations in silic ate (Ti in K2O . TiO2 . 2SiO(2), Ca and Ni in 2CaO . NiO . 3SiO(2)) and alu minosilicate (Li in Li2O . Al2O3 . 2SiO(2)) glasses are reviewed. The exami nation of the cation-centered pairs obtained from the first difference func tion presents striking similarities for all investigated cations. These fun ctions indicate a well-defined short-and medium-range environment around ca tions. The local site generally presents a lower coordination number than t hat found in the crystals of similar composition. The environment around Ti in vitreous K2O . TiO2 . 2SiO(2) corresponds to a square-based pyramid and direct TiO5-TiO5 linkages were observed experimentally in the second diffe rence function, contrary to crystals. A detailed description of the cation site distortion for Li and Ca may be given by this method. The distribution of cations at medium range, which can be extracted by the double differenc e method, reveals the presence of cation-rich regions in silicate glasses. The cation-cation distances often indicate a two-dimensional character in t he cationic organization. On the contrary, Li-aluminosilicate glass shows a more homogeneous cation distribution, in relation with the charge-compensa ting role of Li in this glass. This non-homogeneous distribution of cations may be related to the nano-inhomogeneities proposed in the models of super cooled liquids. (C) 2001 Elsevier Science B.V. All rights reserved.