Synthesis and electrical conductivity of La ₁₀Si _5.5B _0.5O _{27 + δ} (B = In, Si, Sn, Nb) ceramics

Apatite-type lanthanum silicates doped with different multivalent cations at the Si site, La ₁₀Si _5.5B _0.5O _{27 + δ} (B = In, Si, Sn, Nb) ceramics, are synthesized via the high temperature solid state reaction process. X-ray diffraction and complex impedance analysis are used to analyze crystal structure and electrical conductivity of La ₁₀Si _5.5B _0.5O _{27 + δ} ceramics. Both La ₁₀Si _5.5In _0.5O _26.75 and La ₁₀Si _5.5Nb _0.5O _27.25 ceramics consist only of a hexagonal apatite structure with a space group P63/m, however, La ₁₀Si ₆O ₂₇ and La ₁₀Si _5.5Sn _0.5O ₂₇ ceramics are composed of a hexagonal apatite structure and a small amount of second phase La ₂SiO ₅. The measured total conductivity of each composition gradually increases with increasing temperature from 673 K to 1173 K, which follows the Arrhenius behavior. Lanthanum silicates doped with trivalent In ³⁺ or pentavalent Nb ⁵⁺ cations at the Si ⁴⁺ site have a higher total conductivity than undoped lanthanum silicate, however, La ₁₀Si _5.5Sn _0.5O ₂₇ ceramic doped with tetravalent Sn ⁴⁺ cations at the Si ⁴⁺ site exhibits a comparable total conductivity to undoped La ₁₀Si ₆O ₂₇ ceramic. La ₁₀Si _5.5In _0.5O _26.75 and La ₁₀Si _5.5Nb _0.5O _27.25 ceramics have a total conductivity of 1.75 × 10 ^{- 2} S·cm ^{- 1} and 1.23 × 10 ^{- 2} S·cm ^{- 1} at 1073 K, respectively, which are considerably higher than that of undoped La ₁₀Si ₆O ₂₇. At temperature of 773 K, La ₁₀Si _5.5In _0.5O _26.75 ceramic has the highest total conductivity of 8.92 × 10 ^{- 4} S·cm ^{- 1}.