The structure of gamma-Bi2Sn2O7 at 725 degrees C by high-resolution neutron diffraction: implications for bismuth(III)-containing pyrochlores

High-resolution neutron powder diffraction has been used to show that at 725 degrees C gamma-Bi2Sn2O7 does not adopt an ideal pyrochlore structure as was previously supposed. The bismuth(III) ion does not reside at a centre of symmetry (<(3)over bar m>) but is disordered on a site of lower symmetry. This results in a distortion of the co-ordination sphere of the Bi-III. in accord with the steric activity of the 6s(2) lone pair. These results are compared with those for other known bismuth(III)-containing pyrochlores.