Temperature-induced phase transition of AlPO₄-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.