A single-crystal neutron diffraction study of the structure of the high-temperature rotor phase of lithium sulphate

The ionic distribution in the high-temperature rotor phase of lithium sulphate, Li2SO4, has been reinvestigated by single-crystal neutron diffraction at 923 K. Thermal vibrational tensor terms up to the fourth rank are refined to give a better description of the space-time-averaged Li+ and SO42- distributions. The picture of Li+ mobility that emerges supports well the earlier proposed notion of rapidly reorienting SO42- ions which effectively gate the movement of Li+ ions, i.e. a 'paddle-wheel mechanism'. The mode of transport for the Li+ ions would appear to be jumps of 3.5-3.7 AA between approximately=90% occupied, tetrahedrally distorted 8c sites (at 1/4, 1/4, 1/4), into a spherical shell of radius 2.8 AA surrounding each of four adjacent tetrahedrally coordinated SO42- ions. Each shell contains, in total, approximately=0.2 Li+ ions. These Li+ ions can then jump further into one of the six adjacent 8c sites. The Li+ transport thus occurs in one of the six directions (110), (110), (101), etc., and gives some support to the notion (corroborated by earlier MD simulation) that the pathways are slightly curved, and hence somewhat in excess of 3.5 AA. This agrees well with the value of 3.7+or-0.4 AA deduced from Raman spectroscopy, and also with diffuse neutron scattering studies which suggest Li+-Li+ pair correlation distances of 3.6-3,7 AA.