Structural disorder in the cubic 3x3x3 superstructure of TiP2O7. XRD and NMR study

The cubic 3 x 3 x 3 superstructure of TiP2O7 has been studied by XRD and MAS NMR techniques. The refinement of the superstructure has been carried out by the Rietveld method, using DLS analysis to derive starting models. In agreement with space group <Pa(3) over bar>, the P-31 NMR spectrum is composed by 11 components associated with eight P atoms in 24d and three in 8c crystallographic sites. From analysis of XRD and NMR data, P-O-P angles have been determined; deduced values for six independent pyrophosphate groups were between 139 and 145 degrees. As temperature decreases, a small change in the P-O-P angles of those pyrophosphate groups lying on the 3-fold axes was detected by NMR spectroscopy. The large values of the thermal parameters obtained by XRD for O(5) and O(6) bridging oxygens are explained solely in terms of atomic positional disorder.