Two forms of aluminium phosphate tridymite from X‐ray powder data

Monoclinic and triclinic (pseudo‐orthorhombic) AlPO₄ tridymites have been refined from X‐ray powder diffraction data using the silica analogues as starting models. The framework structures of both forms of tridymite are made up of six‐membered rings of tetrahedra which differ in the distortion patterns of the ring shapes. Ordered occupation of alternate tetrahedra by Al and P leads to a doubling of the a lattice parameter for monoclinic AlPO₄ tridymite (space group Pc) and loss of the C‐centring with respect to the isotypic silica tridymite (space group Cc). Triclinic AlPO₄ tridymite was refined in the same space group (F1) as the SiO₂ analogue.