Diffraction and Solid State NMR Studies of Inorganic Framework Materials.

Abstract

Framework materials with the formula AM2O7 and AM2O8 are of great interest and have been shown to exhibit negative thermal expansion (NTE) and interesting oxygen dynamics. Solid-state NMR was used to probe the local environment of the nuclei, which often complements information obtained from diffraction based techniques. Ab initio calculations were also used and compared to experimental spectra.
Cubic-ZrMo2O8 is of particular interest due as it has been shown to exhibit NTE over a wide temperature range (0-1020 K). In this report we have characterised various phases of ZrMo2O8 using variable temperature powder X-ray diffraction, SEM and 17O NMR. We have shown that the particle size of the precursor determines whether the cubic phase can be made purely.
RbNbOP2O7 exhibits 2 reversible phase transitions occur at 346 K and 276 K. The data suggests the first phase transition on warming changes from low to high symmetry, followed by a second phase transition from high to low symmetry to a structure similar to the low temperature structure. However, the intermediate phase has not yet been characterised and NMR has been employed to help complement other diffraction methods. 31P 1D NMR and 97Nb echo experiments showed little difference in the NMR spectra obtained at different temperatures. However, much larger changes have been observed in 87Rb echo NMR performed on a 400 MHz spectrometer and the 850 MHz National Facility spectrometer. The data shows the intermediate phase structure is related to the low temperature phase.
SnMo2O8 differs in its behaviour compared to other members of AM2O8 family, in that it exhibits positive thermal expansion with a phase transition at approximately 300 K. In this case, broad features that do not vary significantly with temperature, suggesting static disorder.