Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23

The long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr 11 Mo 4 O 23 have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd 3&cmb.macr; m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr 11 Mo 4 O 23 appears to be isostructural with Ba 11 W 4 O 23 , except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O-O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways. The crystal structure of Sr 11 Mo 4 O 23 and the local correlations between the disordered sites are examined.