The structure of the lithium-rich garnets li6la2m2o12 and li6.4sr1.4la1.6m2o12 (m = sb, ta)

Lithium garnets of the composition Li6SrLa2M2O12 and Li6.4Sr1.4La1.6M2O12 (M = Sb, Ta) have been prepared by solid state methods. Stuctural refinement against neutron powder diffraction data shows that these compounds crystallise in the space group Ia (3) over bard with lattice parameters in the range a = 12.85750(14) angstrom for Li6SrLa2Ta2O12 to 12.91371(13) angstrom for Li6.4Sr1.4La1.6Sb2O12. The observed increase in lattice parameter between M = Ta and Sb in both the Li6SrLa2M2O12 and Li6.4Sr1.4La1.6M2O12 compounds arises as a result of the different polarisabilities of these two cations. These phases contain a disordered arrangement of Sr2+/La3+ in eightfold coordination and Ta/Sb in fully-occupied oxide octahedra. All compounds contain lithium in a mixture of partially occupied tetrahedral and octahedral interstices with considerable positional disorder observed in the latter. Increasing the lithium stoichiometry causes a reduction in the occupation of the tetrahedrally-coordinated sites and a simultaneous increase in the concentration of lithium found within the oxide octahedra