Proton conductivity of fluorite based rare earth titanates (LnxTi1−x)4O8−2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765)

Abstract

Solid solutions of rare earth titanates with high contents of rare earth oxides of up to 50–62% have been synthesized by the co-precipitation method and their structure, microstructure and conductivity in dry and wet air have been studied. Proton conductors have been found for the first time in solid solutions of rare earth titanates with a high content of Ln₂O₃ (>50%) with a nominal formula composition of (Ln_xTi_1−x)₄O_8−2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765). Among (Ln_xTi_1−x)₄O_8−2x (Ln = Yb, Er, Ho, x = 0.684), (Ho_xTi_1−x)₄O_8−2x (x = 0.684) showed the maximum conductivity in wet air. In this context, four additional compositions (Ho_xTi_1−x)₄O_8−2x (x = 0.718, 0.734, 0.75, and 0.765) were synthesized in the holmium series. An increase in the holmium content leads to an increase in the proton transfer coefficients; at the same time, a more complex nature of the dependence of the conductivity under dry and wet atmospheres is observed. For the fluorite-like solid solution (Ho_xTi_1−x)₄O_8−2x (0.701 ≤ x ≤ 0.765), the proton transfer coefficients were found to be ∼0.9 in the range of 200–450 °C. As the temperature continues to rise, the proton conductivity decreases quite sharply and the transfer coefficient becomes as low as 0.3 at 700 °C. The increase in proton conductivity in the Yb–Er–Ho series is associated with an increase in the hydrophilic properties of rare earth cations. In the (Ho_xTi_1−x)₄O_8−2x (x = 0.667 ≤ x ≤ 0.765) series, the conductivity in wet air was ∼1 × 10⁻⁶ S cm⁻¹ at 450 °C for most compositions. The conductivity of ceramics with x = 0.701 and 0.75 is about 2 times higher, which may be due to the optimal size of pyrochlore nanodomains in the fluorite matrix for x = 0.701 and the formation of pure fluorite for x = 0.75, respectively.