Conductivity and hydration of fluorite-type La_{6 − x} WO_{12 − 1.5x} phases (x = 0.4; 0.6; 0.8; 1)

Abstract

Solid-phase synthesis was used to obtain four phases from the region of cubic fluorite-type La₆WO₁₂-La₁₀W₂O₂₁ (La₅WO_10.5) solid solution. The temperature dependence of conductivity was measured using the impedance spectroscopy technique in the atmosphere of dry $\left( {P_{H_2 O} = 10^{ - 4} atm} \right)$ and humid $\left( {P_{H_2 O} = 3 \times 10^{ - 2} atm} \right)$ air and also when $P_{O_2 }$ was varied from 0.21 to 10⁻²⁰ atm. Ionic transport numbers t (ion) are calculated (0.6 to 0.9). It was shown that the La_5.6WO_11.4 phase had a relatively high stability towards hydrolysis, as the value of its conductivity dropped negligibly (as compared to other studied samples) after long-term sample conditioning in a humid atmosphere. Activation energies of hole (0.85 to 0.98 eV) and proton (0.49 to 0.56 eV) conductivity were calculated. The method of thermogravimetric analysis was used to determine the phase hydration degree $X_{H_2 O}$ in the course of cooling from 980 to 200°C at $P_{H_2 O} = 3 \times 10^{ - 2} atm$. Hydration enthalpies ΔH _hydr (52 to 67 kJ/mol) were calculated.