Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join

Refractive index, density and thermal expansion have been measured for the system Al2O3P2O5SiO2 near the AlPO4 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 AlPO4 join, as the AlPO4 content increases, the index and thermal expansion decrease linearly, the mass density increases slowly and the molar density, assuming an ideal mixture of AlPO4SiO2, remains invariant. Previous work has theorized the formation of AlO2PO2 structural units which replace two SiO2 units. The shape of the isofracts supports this view and implied that AlPO4 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.