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

<jats:title>Abstract</jats:title>
<jats:p>Lithium tantalate solid solution, Li<jats:sub>3+5x</jats:sub>Ta<jats:sub>1−x</jats:sub>O<jats:sub>4</jats:sub> 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-Li<jats:sub>3</jats:sub>TaO<jats:sub>4</jats:sub> had a rock-salt structure with a cationic order of Li<jats:sup>+</jats:sup>:Ta<jats:sup>5+</jats:sup> = 3:1 over the octahedral sites. A rather narrow subsolidus solution range, i.e. Li<jats:sub>3+5x</jats:sub>Ta<jats:sub>1−x</jats:sub>O<jats:sub>4</jats:sub> (0 ⩽ x ⩽ 0.059) was determined and the formation mechanism was proposed as a replacement of Ta<jats:sup>5+</jats:sup> by excessive Li<jats:sup>+</jats:sup>, i.e. Ta<jats:sup>5+</jats:sup> ↔ 5Li<jats:sup>+</jats:sup>. 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, Li<jats:sub>4</jats:sub>TaO<jats:sub>4:5</jats:sub> and LiTaO<jats:sub>3</jats:sub> 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<jats:sup>−3</jats:sup> Ω <jats:sup>−1</jats:sup> ˙ cm−<jats:sup>1</jats:sup> 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.</jats:p>