First indirect experimental evidence and theoretical discussion of giant refrigeration capacity through the reversible pressure induced spin-crossover phase transition

We report on the giant barocaloric effect and refrigerant capacity in the [Fe(pzt)6](PF6)2 (pzt = 1-propyltetrazole) spin-crossover material. The refrigerant capacity in [Fe(pzt)6](PF6)2 is RC = 1380 J kg−1, 5 times higher than the big value reported in (NH4)2SO4, upon pressure variation ΔP = 1 kbar. This huge caloric effect is ascribed to the coupling interactions between the crystal lattice (phonons) and the order parameter (γHS) that describes the molar fraction of high spin molecules (Fe+2N6)-(t2g4eg2) in [Fe(pzt)6](PF6)2. Our theoretical entropy includes the lattice, electronic and configurational coupled-contributions and was obtained from a microscopic model. A new methodology to obtain the barocaloric effect potentials is presented using a proper thermodynamic Maxwell relation for spin-crossover systems. The experimental results, for the isothermal entropy change, were calculated from the pressure dependence of γHS data. Besides, the determination of molecular volume change between high and low spin states through caloric measurements was discussed.