Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li7La3Hf2O12 by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li7La3Hf2O12 has the garnet-related type structure with a space group of I41/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li7La3Hf2O12 has the garnet-type framework structure composed of dodecahedral La(1)O8, La(2)O8 and octahedral HfO6. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li7La3Hf2O12 sample exhibits bulk Li-ion conductivity of σb=9.85×10−7 S cm−1 and grain-boundary Li-ion conductivity of σgb=4.45×10−7 S cm−1 at 300 K. The activation energy is estimated to be Ea=0.53 eV in the temperature range of 300–580 K.