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

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, , 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) ų, Z = 1, T = 230 K; Li₇RuO₆ : triclinic, , 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) ų, Z = 1, T = 295 K; Li₇OsO₆ : triclinic, , 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) ų, 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.