Lithium location in NASICON-type Li⁺ conductors by neutron diffraction. I. Triclinic α′-LiZr₂(PO₄)₃

High-resolution powder neutron diffraction data have been collected at room temperature on the triclinic α phase of LiZr₂(PO₄)₃ (HRPD, ISIS Facility, U.K.). Rietveld refinement analysis (space group C1̄; Z = 4; a = 15.0718(2), b = 8.8556(1), c = 9.1234(1) angstroms, α = 89.660(1), β = 123.912(1), γ = 90.429(1)°) and Fourier difference maps have revealed the presence of monoclinic pseudo-symmetry -x, y, 1/2 - z for all atoms but lithium, which is disordered over two sites Li1 and Li2 with occupancies 0.71(3) and 0.29(3), respectively. The final agreement indexes are R_p = 0.0672, wR_p = 0.0919. R(F²) = 0.0603 for 92 refined parameters. Both Li1 and Li2 are in distorted tetrahedral coordinations, with 〈Li-O〉 = 2.09 and 2.18 angstroms, respectively. Pairs of centrosymmetrical Li1 sites are located within half of the available M′ holes of the NASICON structure, which are split into two independent sets by triclinic breaking of symmetry. This accounts for the lower conductivity of the triclinic phase.