High-temperature phase transitions and intermediate incommensurate modulation of the tridymite form of AlPO4

Abstract

Similar to silica tridymite, the corresponding AlPO4 analogue shows a sequence of displacive phase transitions at elevated temperatures. The transformation temperatures are lower than for SiO2 tridymite. The incommensurate structure of the tridymite form of AlPO4 between ca. 94 and 167 °C was refined in first harmonic approximation from X-ray powder diffraction data in the 3 + 1 dimensional superspace group P21(αβ0). The major components of the displacive modulation are wavy transversal shifts of the centers of the PO4/2 and AlO4/2 tetrahedra associated with additional tilting of the rigid tetrahedra. The wave vector varies continuously from q 1 = 0.068 a* + 0.006 b* at 165 °C to q 2 = 0.051 a* + 0.003 b* at 96 °C. The amplitudes of the wave-like displacements increase with decreasing temperature to 0.2 Å at 96 °C for the Al and P atoms and up to ca. 0.6 Å for some of the oxygen atoms. The modulation is assumed to arise from the condensation of a rigid unit lattice vibration mode like in isostructural intermediate silica tridymite.