Structures and Phase Transitions in (MoO₂)₂P₂O₇

We report structural investigations into (MoO₂)₂P₂O₇ using a combination of X-ray, neutron and electron diffraction, and solid-state NMR supported by first principles quantum chemical calculations. These reveal a series of phase transitions on cooling at temperatures of 377 and 325 K. The high temperature γ-phase has connectivity consistent with that proposed by Kierkegaard at room temperature (but with improved bond length distribution), and contains 13 unique atoms in space group Pnma with lattice parameters a = 12.6577(1) Å, b = 6.3095(1) Å, c = 10.4161(1) Å, and volume 831.87(1) ų from synchrotron data at 423 K. The low temperature α-structure was indexed from electron diffraction data and contains 60 unique atoms in space group P2₁/c with cell parameters a = 17.8161(3) Å, b = 10.3672(1) Å, c = 17.8089(3) Å, β = 90.2009(2)°, and volume 3289.34(7) ų at 250 K. First principles calculations of ³¹P chemical shift and J couplings were used to establish correlation between local structure and observed NMR parameters, and 1D and 2D ³¹P solid-state NMR used to validate the proposed crystal structures. The intermediate β-phase is believed to adopt an incommensurately modulated structure; ³¹P NMR suggests a smooth structural evolution in this region.