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

Highlights: • Huge refrigerant capacity in [Fe(pzt)₆](PF₆)₂. • Giant barocaloric effect in spin crossover materials. • Entropic caloric effect triggered from (LS)-(HS) phase transition. • Solid refrigerant materials. 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 (γ_HS) that describes the molar fraction of high spin molecules (Fe⁺²N₆)-($t_{2g}^4{e}_g^2$) 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 γ_HS data. Besides, the determination of molecular volume change between high and low spin states through caloric measurements was discussed.