Raman spectroscopic and X-ray diffraction study of α- and β-Mg₂P₂O₇ at various temperatures

Raman spectra and X-ray diffraction patterns of Mg₂P₂O₇ polymorphs (α- and β-phase) were investigated at various temperatures up to 1073 K at ambient pressure. Typical Raman spectra and X-ray diffraction patterns were observed for the reversible phase transition between low-temperature α-Mg₂P₂O₇ and high- temperature β-Mg₂P₂O₇ during heating and cooling. The effect of temperature on the Raman vibrations for the two Mg₂P₂O₇ polymorphs was quantitatively analyzed. All the observed Raman active bands of the two Mg₂P₂O₇ polymorphs showed linear temperature dependence with different slopes. The quantitative temperature dependences of the Raman bands are -4.01 × 10^-2 ∼ 1.94 × 10^-2 and -2.31 × 10^-2 ∼ -0.44 × 10^-2 cm^-1 K^-1 for α- and β-Mg₂P₂O₇, respectively. The force constant evolution of [P₂O₇]^4- stretching vibrations and the temperature derivatives for both α- and β-Mg₂P₂O₇ were also determined. The thermal expansion coefficient of β-Mg₂P₂O₇ was estimated at 2.97(8) × 10^-5 K^-1. Hence the isobaric mode Grüneisen parameters of β-Mg₂P₂O₇ were calculated.