CATIONS IN GLASSES UNDER AMBIENT AND NON-AMBIENT CONDITIONS

Structural data on the short-and medium-range structure of multicomponent oxide glasses may be obtained by taking advantage of the chemical selectivity of X-ray Absorption Spectroscopy (XAS). Ni-K edge XAS in silicate glasses shows that this element occurs in 4- and 5-coordination, the ratio between the two coordination states being composition dependent. Ni-[4] is bound to the silicate network, with large alkalis completing charge-compensation. Coordination changes from Ni-[6] to Ni-[5] and Ni-[4] are also observed in berate glasses with increasing potassium content. Low alkali glasses show a medium-range structure consistent with the grafting of nickel octahedra onto boroxol rings. Zirconium is another element which shows a coordination change from Zr-[6] to Zr-[8] as a function of glass composition. The structural environment of glass components may be modeled using the interatomic distances derived from EXAFS and a bond valence model derived from the Pauling rules. XAS experiments may also be performed in situ at high pressure or high temperature using synchrotron radiation. High pressure EXAFS spectra on germanate glasses show a coordination change of germanium from 4 to 6 with increasing pressure. These modifications are reversible in glasses, which illustrates the need for in situ measurements. High temperature EXAFS and XANES show also a coordination change of Ni during the melting of sodium silicate glass, with an increase of the Ni-[4] content in the melt.