Temperature–induced phase transition of AlPO<sub>4</sub>–moganite studied by in–situ Raman spectroscopy

The moganite–form of AlPO4 has recently been discovered from our high–pressure study. Similar to SiO2–moganite, a temperature–induced displacive phase transition is expected. In order to confirm the phase transition, high–temperature in–situ Raman spectroscopy study was conducted at ambient pressure up to 600 °C. One of the low–frequency Raman modes (74 cm−1 at room temperature) significantly softened with temperature, and disappeared at 420 °C. Its frequency versus temperature relation can be well fitted with an order parameter equation, and the mode is interpreted as a soft mode with a critical exponent of 0.232(8). According to this fitting, the transition temperature is determined as 415 °C. Some hard modes also revealed slight softening or hardening with temperature up to ~ 420 °C and reached nearly constant frequency at higher temperature. Vibrational mode calculations by the first–principles density functional theory (DFT) method showed that the soft mode corresponds to tetrahedral rotations, representing the pathway of the transformation.