Local structure and disorder in crystalline Pb9Al8O21

Crystalline Pb9Al8O21 is a model compound for the structure of non-linear optical glasses containing lone-pair ions, and its structure has been investigated by neutron powder diffraction and total scattering, and 27Al magic angle spinning NMR. Rietveld analysis (space group image (No. 205), a=13.25221(4) Å) shows that some of the Pb and O sites have partial occupancies, due to lead volatilisation during sample preparation, and the non-stoichiometric sample composition is Pb9−δAl8O21−δ with δ=0.54. The NMR measurements show evidence for a correlation between the chemical shift and the variance of the bond angles at the aluminium sites. The neutron total correlation function shows that the true average Al–O bond length is 0.8% longer than the apparent bond length determined by Rietveld refinement. The thermal variation in bond length is much smaller than the thermal variation in longer interatomic distances determined by Rietveld refinement. The total correlation function is consistent with an interpretation in which AlO3 groups with an Al–O bond length of 1.651 Å occur as a result of the oxygen vacancies in the structure. The width of the tetrahedral Al–O peak in the correlation function for the crystal is very similar to that for lead aluminate glass, indicating that the extent of static disorder is very similar in the two phases.