Internal friction studies ofLa2−xBaxMo2O9−δoxide-ion conductors

Abstract

Three internal friction peaks (${P}_{L}$, ${P}_{H}$, and ${P}_{C}$) are observed in the Ba substituted ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9\ensuremath{-}\ensuremath{\delta}}$ samples ($x=0$, 0.05, 0.1, 0.15, and 0.2), among which ${P}_{L}$ and ${P}_{C}$ are of a relaxational type while ${P}_{H}$ is associated with a kind of phase transition. The activation energy of ${P}_{L}$ decreases from about $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with $x$ increasing from 0 to 0.15, and the preexponential factor of relaxation time, ${\ensuremath{\tau}}_{0}$, is in the range of ${10}^{\ensuremath{-}15}\char21{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$, which are typical values for the diffusion of oxygen vacancies in this kind of compounds. The activation energy of ${P}_{C}$ increases from about $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at $x=0.05$ to $2.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at $x=0.15$, and the ${\ensuremath{\tau}}_{0}$ is in the range of ${10}^{\ensuremath{-}16}\char21{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. The height of peak ${P}_{C}$ increases with Ba concentration but decreases with the increasing peak temperature. These facts suggest that peak ${P}_{C}$ is associated with the stress-induced reorientation of Ba ions via vacancies (Snoek-type relaxation). For peak ${P}_{H}$, a mechanism of phase transition from the dynamic disordered state to static disordered state in oxygen ion/vacancy distribution was suggested. It is found that more than 2.5% $(x=0.05)$ Ba doping at La site can completely stabilize the cubic ${\mathrm{La}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$ phase at room temperature and 10% Ba concentration already exceeds the solid solubility of Ba in ${\mathrm{La}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{9}$.