The structure of the lithium-rich garnets Li6SrLa2M2O12 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. Structural refinement against neutron powder diffraction data shows that these compounds crystallise in the space group Ia3¯d with lattice parameters in the range a = 12.85750(14) Å for Li6SrLa2Ta2O12 to 12.91371(13) Å 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.

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