Subsolidus solution and ionic conductivity of rock-salt structured Li_3+5xTa_1−xO₄ electroceramics

Lithium tantalate solid solution, Li_3+5xTa_1−xO₄ was prepared by conventional solid-state reaction at 925 °C for 48 h. The XRD analysis confirmed that these materials crystallized in a monoclinic symmetry, space group C2/C and Z = 8, which was similar to the reported International Crystal Database (ICDD), No. 98-006-7675. The host structure, β-Li₃TaO₄ had a rock-salt structure with a cationic order of Li⁺:Ta⁵⁺ = 3:1 over the octahedral sites. A rather narrow subsolidus solution range, i.e. Li_3+5xTa_1−xO₄ (0 ⩽ x ⩽ 0.059) was determined and the formation mechanism was proposed as a replacement of Ta⁵⁺ by excessive Li⁺, i.e. Ta⁵⁺ ↔ 5Li⁺. Both Scherrer and Williamson-Hall (W-H) methods indicated the average crystallite sizes in the range of 31 nm to 51 nm. Two secondary phases, Li₄TaO_4:5 and LiTaO₃ were observed at x = 0.070 and x = −0:013, respectively. These materials were moderate lithium ionic conductors with the highest conductivity of ~2.5 × 10⁻³ Ω ⁻¹ ˙ cm−¹ at x = 0, at 0 °C and 850 °C; the activation energies were found in the range of 0.63 eV to 0.68 eV.