Crystal Structures, Dimorphism and Lithium Mobility of Li7MO6 (M = Bi, Ru, Os)

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Abstract

Li7MO6 (M = Bi, Ru, Os) have been synthesized by solid state reaction of Li2O with Bi2O3, or MO2 (M = Ru, Os) and characterized using powder X-ray diffraction, differential scanning calorimetry, magnetic susceptibility (for M = Ru, Os), ionic conductivity and 6Li solid state NMR (for M = Bi) measurements. All three compounds exhibit a temperature induced triclinic – rhombohedral phase transition. Structures of the new low temperature triclinic phases have been refined by the Rietveld method from powder X-ray data using atomic parameters of Li7TaO6 as a starting model (Li7BiO6: triclinic, equation image, a = 5.5071(1), b = 6.0425(1), c = 5.5231(1) Å, α = 116.912(1), β = 120.867(1), γ = 62.234(1)°, V = 133.96(1) Å3, Z = 1, T = 230 K; Li7RuO6: triclinic, equation image, a = 5.3654(1), b = 5.8584(1), c = 5.3496(1) Å, α = 117.182(1), β = 119.117(1), γ = 62.632(1)°, V = 124.43(1) Å3, Z = 1, T = 295 K; Li7OsO6: triclinic, equation image, a = 5.3786(1), b = 5.8725(1), c = 5.3591(1) Å, α = 117.193(1), β = 119.277(1), γ = 62.700(1)°, V = 125.15(1) Å3, Z = 1, T = 295 K). Upon cooling, Li7RuO6 and Li7OsO6 undergo a magnetic transition at 12 and 13 K, respectively, from the paramagnetic to the antiferromagnetic state. The higher ionic conductivity of Li7BiO6 at T < 300 °C, as compared to Li7RuO6 and Li7OsO6, can be ascribed to the undergoing of the triclinic – rhombohedral transition at a much lower temperature. At T > 300 °C, the ionic conductivity of all three compounds increases sharply due to the melting of the lithium sublattice; for Li7RuO6 and Li7OsO6 the latter effect is superimposed by the phase transitions to the rhombohedral modifications.

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