Crystal Structures, Dimorphism and Lithium Mobility of Li 7 MO 6 (M = Bi, Ru, Os)

Abstract Li 7 MO 6 (M = Bi, Ru, Os) have been synthesized by solid state reaction of Li 2 O with Bi 2 O 3 , or MO 2 (M = Ru, Os) and characterized using powder X‐ray diffraction, differential scanning calorimetry, magnetic susceptibility (for M = Ru, Os), ionic conductivity and 6 Li 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 Li 7 TaO 6 as a starting model ( Li 7 BiO 6 : triclinic, $P{\bar 1}$ , 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) Å 3 , Z = 1, T = 230 K; Li 7 RuO 6 : triclinic, $P{\bar 1}$ , 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) Å 3 , Z = 1, T = 295 K; Li 7 OsO 6 : triclinic, $P{\bar 1}$ , 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) Å 3 , Z = 1, T = 295 K). Upon cooling, Li 7 RuO 6 and Li 7 OsO 6 undergo a magnetic transition at 12 and 13 K, respectively, from the paramagnetic to the antiferromagnetic state. The higher ionic conductivity of Li 7 BiO 6 at T < 300 °C, as compared to Li 7 RuO 6 and Li 7 OsO 6 , 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 Li 7 RuO 6 and Li 7 OsO 6 the latter effect is superimposed by the phase transitions to the rhombohedral modifications.