Large barocaloric effect in metal-organic frameworks driven by geometrical frustration

Solid-state refrigeration based on caloric effects presents a sustainable alternative to conventional vapor-compression systems which rely on environmentally harmful refrigerants. Among these, the barocaloric effect (BCE) is particularly promising due to its ability to induce large entropy change through hydrostatic pressure. However, most BCE materials suffer from narrow refrigeration temperature range near their phase transitions and the trade-off between pressure sensitivity ($\mathrm{d}{T}_{t}/\text{d}P$) and entropy change (\ensuremath{\Delta}$S$), which largely limits their practical application. Here, we report that metal-organic frameworks (MOFs) doped by the C4 group (MOF-5-C4) overcome these challenges by exhibiting colossal BCE (\ensuremath{\Delta}$S$ = $274\phantom{\rule{0.16em}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) with record high pressure sensitivity ($\mathrm{d}{T}_{t}/\text{d}P=284\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}{\mathrm{kbar}}^{\ensuremath{-}1}$), driven by a frustrated amorphous-crystalline transition that is stabilized by dispersive interactions with adjacent C4 side groups. It achieves a remarkable 30% volume change under moderate pressure, with volume entropy being the dominant contributor, while maintaining efficient cooling across a broad temperature range (290--400 K). Through thermodynamic analysis, we demonstrate that the colossal barocaloric response originates from substantial structural deformations; both molecular vibration and rotation positively contribute to the total entropy change. In our work, we provide atomic-scale insights on the structural and thermodynamic of MOFs with BCE, which is helpful to achieve superior caloric materials for application by molecular designing in the near future.