High-temperature behavior of vanadyl pyrophosphate (VO)2P2O7(VO)2P2O7

(VO)2P2O7(VO)2P2O7 has been studied at high temperatures by in situ X-ray powder diffraction in an inert atmosphere. Lattice parameters indicate an anisotropic thermal expansion up to Tc=214∘C, followed by an approximately isotropic expansion up to 490∘C. It is proposed that TcTc is the temperature of a second-order phase transition between the known room temperature form with space group Pca21Pca21 and a high-temperature form with space group Pcab  . Above 490∘C a minority phase develops in the sample that was identified as the V3+V3+ containing compound V4(P2O7)3V4(P2O7)3, while the anisotropic expansion along aa suggests that stoichiometric (VO)2P2O7(VO)2P2O7 transforms into (VO)2P2O7+x(VO)2P2O7+x with interstitial oxygen located between the layers. With an onset at 706∘C all of the (VO)2P2O7(VO)2P2O7 transforms into VPO4VPO4.

Temperature-dependent X-ray diffraction of (VO)2P2O7(VO)2P2O7, showing the anisotropic thermal expansion of the lattice parameter aa through the temperature dependence of the (400) reflection.Figure optionsDownload full-size imageDownload as PowerPoint slide