Large barocaloric effects in thermoelectric superionic materials

Abstract

We predict the existence of large barocaloric effects above room temperature in the thermoelectric fast-ion conductor ${\mathrm{Cu}}_{2}\mathrm{Se}$ by using classical molecular dynamics simulations and first-principles computational methods. A hydrostatic pressure of 1 GPa induces large isothermal entropy changes of $|\mathrm{\ensuremath{\Delta}}S|\ensuremath{\sim}15--45\phantom{\rule{0.28em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and adiabatic temperature shifts of $|\mathrm{\ensuremath{\Delta}}T|\ensuremath{\sim}10\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in the temperature interval $400\ensuremath{\le}T\ensuremath{\le}700\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Structural phase transitions are absent in the analyzed thermodynamic range. The causes of such large barocaloric effects are significant $P$-induced variations on the ionic conductivity of ${\mathrm{Cu}}_{2}\mathrm{Se}$ and the inherently high anharmonicity of the material. Uniaxial stresses of the same magnitude, either compressive or tensile, produce comparatively much smaller caloric effects, namely, $|\mathrm{\ensuremath{\Delta}}S|\ensuremath{\sim}1\phantom{\rule{0.28em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and $|\mathrm{\ensuremath{\Delta}}T|\ensuremath{\sim}0.1\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, due to practically null influence on the ionic diffusivity of the material. Our simulation work shows that thermoelectric compounds presenting high ionic disorder, like copper and silver-based chalcogenides, may render large mechanocaloric effects and thus are promising materials for engineering solid-state cooling applications that do not require the application of electric fields.