Giant barocaloric effects in formamidinium iodide

Barocaloric effect is the thermal response of materials to the external pressures, which provides a promising alternative to the current vapor-compression refrigeration. The effect has been widely observed in a variety of materials. Here, we report giant barocaloric effects in formamidinium iodide [CH(NH2)2I] that was known as the main precursor to synthesize the hybrid perovskite photovoltaic materials. The compound exhibits two successive phase transitions at 345 and 386 K, where isothermal entropy changes of about 50 J kg−1 K−1 are found under applied pressures below 60 MPa. As for the low-temperature transition with the thermal hysteresis of about 6 K, we obtain a refrigerant capacity of 179 J kg−1 under a pressure change of 100 MPa and an estimated adiabatic temperature change of about 24 K. Temperature- and pressure-dependent Raman scattering measurements indicate that the phase transitions and the barocaloric effects are attributed to influences on hydrogen bonds.