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.