Cationic ordering in oxide glasses: the example of transition elements

Structural data have been obtained on the cation surroundings in multi-comp onent silicate and borosilicate glasses using chemically selective spectros copic and scattering methods, such as extended X-ray absorption and neutron scattering with isotope substitution (NSIS). Transition elements such as N i or Ti may occur in unusual 5-coordinated sites which coexist with other c oordination numbers, depending on glass composition. Distribution of cation ic sites in the glassy structure is responsible for unusual spectroscopic p roperties, as shown by Fe2+ Mossbauer spectroscopy. The environment of cati ons such as Zn, Zr or Mo, has been determined by EXAFS and discussed using the bond valence theory, which predicts the way to charge compensate the ox ygen neighbours and which indicates the linkage of cationic sites with the silicate framework. Cation-cation correlations are given by NSIS up to simi lar to 8 Angstrom, indicating an extensive Medium Range Ordering (MRO) with corner- and edge-linked cationic polyhedra, for Ti and Ni-bearing glasses, respectively. This heterogeneous cationic distribution in glasses is consi stent with the presence of two-dimensional domains in which cation mixing m ay occur, as shown in a Ca-Ni metasilicate grass. Three-dimensional domains have also been found by Ni-K edge EXAFS in the case of low alkali berate g lasses, with a local structure which mimics some aspects of crystalline NiO . The presence of ordered cationic domains, clearly illustrated by Reverse Monte Carlo simulations helps to rationalize the physical properties of mul ti-component silicate glasses.