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Monoclinic AlPO4 tridymite at 473 and 463 K from X-ray powder data.

Upon cooling from its hexagonal high-temperature modification, AlPO4 (aluminium phosphate) tridymite successively transforms to several displacively distorted forms, including a normal structure-incommensurate-lock-in phase transition sequence. The space-group symmetries in this series are P112(1), P112(1)(alphabeta0) and P2(1)2(1)2(1), respectively. The distortion pattern of the intermediate P112(1) phase can be described as alternate shifts of adjacent layers of tetrahedra coupled with tilting of the tetrahedra. The symmetry and direction of the shifts are different from the analogous SiO2 tridymite modification. The atomic displacement parameters of the O atoms are strongly anisotropic due to thermal motions of the rigid tetrahedra. Condensation of a lattice vibration mode results in the formation of an incommensurate structural modulation below 473 K. The 3+1 superspace-group symmetry of the modulated phase is P112(1)(alphabeta0).