Prediction of large barocaloric effects in thermoelectric superionic materials

We predict the existence of large barocaloric effects above room temperature in the thermoelectric fast-ion conductor Cu$_{2}$Se by using classical molecular dynamics simulations and first-principles computational methods. A hydrostatic pressure of $1$ GPa induces large isothermal entropy changes of $|\Delta S| \sim 15$-$45$ Jkg$^{-1}$K$^{-1}$ and adiabatic temperature shifts of $|\Delta T| \sim 10$ K in the temperature interval $400 \le T \le 700$ K. Structural phase transitions are absent in the analysed thermodynamic range. The causes of such large barocaloric effects are significant $P$-induced variations on the ionic conductivity of Cu$_{2}$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, $|\Delta S| \sim 1$ Jkg$^{-1}$K$^{-1}$ and $|\Delta T| \sim 0.1$ K, due to practically null influence on the ionic diffusivity of Cu$_{2}$Se. 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.