Giant barocaloric effects in sodium hexafluorophosphate and hexafluoroarsenate.

Solid-state refrigeration using barocaloric materials is environmentally friendly and highly efficient, making it a subject of global interest over the past decade. Here, we report giant barocaloric effects in sodium hexafluorophosphate (NaPF₆) and sodium hexafluoroarsenate (NaAsF₆) that both undergo a cubic-to-rhombohedral phase transition near room temperature. We have determined that the low-temperature phase structure of NaPF₆ is a rhombohedral structure with space group R 3 ¯ by neutron powder diffraction. There are three Raman active vibration modes in NaPF₆ and NaAsF₆, i.e., F_2g, E_g, and A_1g. The phase transition temperature varies with pressure at a rate of dT_t/dP = 250 and 310 K GPa⁻¹ for NaPF₆ and NaAsF₆. The pressure-induced entropy changes of NaPF₆ and NaAsF₆ are determined to be around 45.2 and 35.6 J kg⁻¹ K⁻¹, respectively. The saturation driving pressure is about 40 MPa. The pressure-dependent neutron powder diffraction suggests that the barocaloric effects are related to the pressure-induced cubic-to-rhombohedral phase transitions. [ABSTRACT FROM AUTHOR]