Structure and Properties of Silica Containing Aluminum and Phosphorus near the A1PO sub 4 Join

Refractive index and density have been measured for Al sub 2 O sub 3 -P sub 2 O sub 5 -SiO sub 2 near the A1PO sub 4 join down to 78% silica with ratios of A1 to P from 0.5 to 2.0. As the join is approached at constant silica content, both properties decrease. Along the join, as A1PO sub 4 content increases, the index decreases below silica, the density increases and the molar density remains invariant. Previous work has theorized A1O sub 2 PO sub 2 structural units which replace two SiO sub 2 units. The shape of the isofracts supports this view and implies that A1PO sub 4 units are formed with near complete efficiency. The glass thus consists of A1PO sub 4 -P sub 2 O sub 5 -SiO sub 2, if phosphorus rich, or A1PO sub 4 -Al sub 2 O sub 3 -SiO sub 2, if aluminum rich. The thermal expansion coefficient along the join is qualitatively similar to silica while viscosity is greatly diminished. The glass is silica-like but requires lower processing temperatures.