Anomalous lattice parameter increase in alkali earth aluminium substituted tungsten defect pyrochlores

The structures of the defect pyrochlores AAl0.33W1.67O6 where A=K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP. The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state 27Al MAS NMR studies of this pyrochlore system reveal shifts in the δ∼21–22 ppm range that are indicative of pseudo-5 coordinate Al environments and emanate from distorted Al octahedral with one abnormally long Al–O bond. Solid state 39K, 85Rb, 87Rb and 133Cs MAS and static NMR studies reflect the local cation disorder demonstrated in the structural studies.