Coordination number of aluminum ions in alkali-free alumino-silicate glasses

Abstract

The oxygen coordination number of aluminum ions in alkali-free alumino-silicate glasses has been investigated by means of Alkα fluorescence spectra. The glasses studied were XCaO·(100- X- Y)Al 2O 3· YSiO 2 wt% (with 50 ⩾ X ⩾ 20, 70 ⩾ Y ⩾ 0) and, 20MgO·(80− Y′)Al 2 O 3· Y′SiO 2 wt% (65 ⩾ Y′ ⩾ 45). The density, thermal expansion coefficient and elastic moduli of the glasses were also measured. In glasses with an Al/(Al+Si) ratio larger than 0.5, the chemical shifts of the Alkα lines for the glasses relative to aluminum metal were equal to that for Na-feldspar, indicating that Al 3+ ions are four-fold coordinated in the glasses. In glasses with an Al/(Al + Si) ratio less than 0.5, the chemical shifts depended on SiO 2 content and increased from the value nearly equal to that for Na-feldspar towards those for α-Al 2O 3, dickite and kaolinite as the Al/(Al+Si) ratio decreased, indicating that the average coordination number of Al 3+ ions in these glasses increases with SiO 2 content. The minimum, maximum oor an abrupt change in the slope of property-composition curves of alkali-free alumino-silicate glasses could be understood by assuming the presence of six-fold coordinated Al 3+ ions.