Rare earth elements in silicate [formula omitted] systems: I. Effects of composition on the coordination environments of La, Gd, and Yb

Abstract

The structural environments of trace levels (≤0.2 wt%) of La 3+, Gd 3+, and Yb 3+ in several silicate glasses were examined as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions were albite, sodium trisilicate, and a peralkaline composition approximately halfway between the two. Yb is 5–6 coordinated [ d(Yb-O) = 2.21 A ̊ ]. Gd coordination ranges from 6 in albite glass [ d(Gd-O) = 2.30 A ̊ ] through a mixture of 6 to 8 in peralkaline glass [ d(Gd-O) = 2.36 A ̊ ]to 8 in sodium trisilicate glass [ (Gd-O) = 2.43 A ̊ ]. La is 7-coordinated in the peralkaline and sodium trisilicate glasses [ d(La-O) = 2.43 A ̊ ], but has a coordination of 8–9 in albite glass [ d(La-O) = 2.59 A ̊ ]. Regularity of the rare earth site (i.e., the range of REE-O distances) appears to decrease as polymerization increases. This is caused, in part, by the decreasing number of non-bridging oxygens in the coordination sphere of the rare earth element with increasing melt polymerization. Regularity of the rare earth site also appears to increase as the size of the rare earth decreases. This is due to the greater ability of the smaller, higher field strength rare earths to compete for non-bridging oxygens. An exception to these trends is the La site in albite glass; it is more regular than in the other glasses because the large La ion is centered in the “cage” formed by interconnected rings of Si-and Al-tetrahedra, which leads to a more regular distribution of La-O bond lengths. As polymerization decreases, the rare earth's local environment is more strongly influenced by its bonding requirements and less influenced by network topology.