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Abstract: The structures of new phases Li6CaLa2Sb2O12 and Li6.4Ca1.4La2Sb2O12 have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3̄d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li6.4Ca1.4La2Sb2O12 show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li6CaLa2Sb2O12 is several orders of magnitude smaller than related lithium-stuffed garnets with σ=10−7 Scm−1 at 95°C and an activation energy of 0.82(3)eV. The transport properties of the conventional garnets Li3Gd3Te2O12, Li3Tb3Te2O12, Li3Er3Te2O12 and Li3Lu3Te2O12 have been evaluated and consistently show much lower values of conductivity, σ≤4.4×10−6 Scm−1 at 285°C and activation energies in the range 0.77(4)≤E a /eV≤1.21(3). [Copyright &y& Elsevier]

Abstract: The garnets Li3Nd3W2O12 and Li5La3Sb2O12 have been prepared by heating the component oxides and hydroxides in air at temperatures up to 950°C. Neutron powder diffraction has been used to examine the lithium distribution in these phases. Both compounds crystallise in the space group with lattice parameters a=12.46869(9)Å (Li3Nd3W2O12) and a=12.8518(3)Å (Li5La3Sb2O12). Li3Nd3W2O12 contains lithium on a filled, tetrahedrally coordinated 24d site that is occupied in the conventional garnet structure. Li5La3Sb2O12 contains partial occupation of lithium over two crystallographic sites. The conventional tetrahedrally coordinated 24d site is 79.3(8)% occupied. The remaining lithium is found in oxide octahedra which are linked via a shared face to the tetrahedron. This lithium shows positional disorder and is split over two positions within the octahedron and occupies 43.6(4)% of the octahedra. Comparison of these compounds with related d 0 and d 10 phases shows that replacement of a d 0 cation with d 10 cation of the same charge leads to an increase in the lattice parameter due to polarisation effects. [Copyright &y& Elsevier]