Q1(a). Quartz-type aluminophosphate

Answer in brief

AlPO₄ is structurally and electronically analogous to two SiO₂ units: alternating AlO₄ and PO₄ tetrahedra form a neutral, corner-sharing framework.

Key points

• Replacing two Si⁴⁺ centres by Al³⁺ and P⁵⁺ preserves the total charge: 2(4) = 3 + 5.
• Both Al and P can have tetrahedral oxygen coordination in this framework. The tetrahedra share corners through Al-O-P bridges.
• Sharing each oxygen between two tetrahedral centres gives AlPO₄, analogous to (SiO₂)₂; no charge-balancing extra-framework cation is required.
• The quartz-like polymorph is berlinite. AlPO₄ also has other polymorphs.