Phase transition of Mg3(PO4)2 polymorphs at high-temperature: In-situ synchrotron X-ray diffraction and Raman spectroscopic study

Abstract

In-situ synchrotron X-ray diffraction and Raman spectroscopy were used to study the stabilities, thermal expansion and vibrational modes of synthetic Mg3(PO4)2. The polymorphs (Mg3(PO4)2-I, II, III) were investigated in the temperature range of 299 ∼ 1173 K at ambient pressure. An irreversible phase transition was observed for both Mg3(PO4)2-II and Mg3(PO4)2-III, whereas Mg3(PO4)2-I is stable in the present study. Based on the in-situ synchrotron X-ray diffraction and Raman spectroscopic measurements, Mg3(PO4)2-II and Mg3(PO4)2-III transform to Mg3(PO4)2-I at 1073 K and 1023 K, respectively. The volumetric thermal expansion coefficients of Mg3(PO4)2-I, II and III were determined as 3.31(4) × 10-5 K−1, 3.91(4) × 10-5 K−1, and 3.25(5) × 10-5 K−1, respectively. All three Mg3(PO4)2 polymorphs show axial thermal expansive anisotropy since the thermal expansion coefficients along different axes are inconsistent. The effect of temperature on the Raman vibrations of the three Mg3(PO4)2 polymorphs was quantitatively analyzed. And the isobaric mode Grüneisen parameters of three Mg3(PO4)2 polymorphs are calculated, which are in the range of 0.07 ∼ 3.54.