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

Abstract

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.