Colossal barocaloric effect of the spin-crossover compound {Fe(pz)2(BH3CN)2} near room temperature.

As one of the most likely alternatives to traditional vapor compression refrigeration technology, solid refrigeration technology based on the barocaloric effect (BCE) has attracted extensive attention in recent years. Spin-crossover (SCO) compounds are considered suitable for working at low driving pressures due to high-pressure sensitivity and small hysteresis width. However, the entropy change (ΔS_SCO) of the SCO compound is smaller than that of other excellent barocaloric materials (plastic crystals and two-dimensional perovskites). Here, we report the BCE of the SCO compound {Fe(pz)₂(BH₃CN)₂} (pz = pyrazine) with a smaller molar mass and a third source of entropy change besides electron and vibrational entropy changes. Compound {Fe(pz)₂(BH₃CN)₂} exhibits high pressure sensitivity ( d T 1 / 2 d P = 20.2 K kbar⁻¹) as well as entropy change (Δ S SCO = 202 J kg⁻¹ K⁻¹). The maximum values of reversible isothermal entropy change (ΔS_it,rev,max) and adiabatic temperature change (ΔT_ad,rev,max) at 1 kbar are only 103 J kg⁻¹ K⁻¹ and ∼0 K, respectively, due to the hysteresis behavior. However, at sufficiently high driving pressures, ΔS_it,rev,max exceeds 200 J kg⁻¹ K⁻¹, and ΔT_ad,rev,max can reach ∼47 K, which exceeds all SCO compounds reported in BCE studies and is comparable to some plastic-crystalline and two-dimensional perovskite barocaloric materials. The excellent BCE of the SCO compound {Fe(pz)₂(BH₃CN)₂} is mainly due to its small molar mass, which makes the unit mass compound exhibit higher ΔS_SCO, while the introduction of the third source of entropy change—the reorientation entropy change (ΔS_reo), only plays a small role. This is expected to promote the practical application of SCO compounds as barocaloric refrigerants. [ABSTRACT FROM AUTHOR]