Structure and properties of silica containing aluminum and phosphorus near the AlPO 4 join

Abstract

Refractive index, density and thermal expansion have been measured for the system Al 2O 3P 2O 5SiO 2 near the AlPO 4 join. The silica content varies from 100% to 78% with molar ratios of aluminum to phosphorus varying from 0.5 to 2.0. As the join is approached at constant silica content, the three properties decrease. The refractive index drops below that of pure silica while the mass density is slightly higher. Thermal expansion reaches a minimum at the join. Along the AlPO 4 join, as the AlPO 4 content increases, the index and thermal expansion decrease linearly, the mass density increases slowly and the molar density, assuming an ideal mixture of AlPO 4SiO 2, remains invariant. Previous work has theorized the formation of AlO 2PO 2 structural units which replace two SiO 2 units. The shape of the isofracts supports this view and implied that AlPO 4 units are formed with near complete efficiency, containing almost all available aluminum and phosphorus cations. This material is thus much like silica in terms of refractive index, density and thermal expansion but requires a lower processing temperature.