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 ](PF 6 ) 2 (pzt = 1-propyltetrazole) spin-crossover material. The refrigerant capacity in [Fe(pzt) 6 ](PF 6 ) 2 is RC = 1380 J kg −1 , 5 times higher than the big value reported in (NH 4 ) 2 SO 4 , 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 (γ HS ) that describes the molar fraction of high spin molecules (Fe +2 N 6 )-( t 2 g 4 e g 2 ) in [Fe(pzt) 6 ](PF 6 ) 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 γ HS data. Besides, the determination of molecular volume change between high and low spin states through caloric measurements was discussed. [ABSTRACT FROM AUTHOR]