Driving Barocaloric Effects in a Molecular Spin-Crossover Complex at Low Pressures

Abstract

Barocaloric effectsthermal changes in a material induced by applied hydrostatic pressureoffer promise for creating solid-state refrigerants as alternatives to conventional volatile refrigerants. To enable efficient and scalable barocaloric cooling, materials that undergo high-entropy, reversible phase transitions in the solid state in response to a small change in pressure are needed. Here, we report that pressure-induced spin-crossover (SCO) transitions in the molecular iron­(II) complex Fe­[HB­(tz)3]2 (HB­(tz)3 – = bis­[hydrotris­(1,2,4-triazol-1-yl)­borate]) drive giant and reversible barocaloric effects at easily accessible pressures. Specifically, high-pressure calorimetry and powder X-ray diffraction studies reveal that pressure shifts as low as 10 bar reversibly induce nonzero isothermal entropy changes, and a pressure shift of 150 bar reversibly induces a large isothermal entropy change (>90 J kg–1 K–1) and adiabatic temperature change (>2 K). Moreover, we demonstrate that the thermodynamics of the SCO transition can be fine-tuned through systematic deuteration of the tris­(triazolyl)­borate ligand. These results provide new insights into pressure-induced SCO transitions and further establish SCO materials as promising barocaloric materials.

A colossal barocaloric effect induced by the creation of a high-pressure phase

“…Unlike other counterparts, BCEs are not system-selective because the quantities of pressure and volume are always formulated in the free energy of a system. So far, BCEs have been observed in a diversity of materials including hybrid organic–inorganic compounds, 12,14,15 ferroelectrics, 16 ferroelastics, 17 thermoelectrics, 18 plastic crystals, 10,11 frustrated antiferromagnets, 19 materials with metal-to-insulator transitions, 20 charge-transfer oxides, 21 spin-crossover complexes, 22,23 and so forth. Based on these materials, the conventional wisdom for finding a barocaloric material can be concluded.…”

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

“…This phenomenon is possible in the presence of a suitable ligand field and is typically induced with temperature, pressure, or light irradiation . The change in the magnetic, optical, structural, and electronic properties of SCO compounds is relevant for possible applications in sensors, displays, spintronics, and molecular electronics. − The most common metal ion to display SCO is Fe­(II), − and significant advances have been made in the practical deployment of Fe­(II) SCO complexes. ,− Iron­(III) SCO compounds, though relatively less explored than Fe­(II), have been used in molecular junctions, forming nanowires and self-assembled materials, and have shown interesting hysteretic and light-induced properties . Typically, an N 4 O 2 coordination sphere gives rise to Fe­(III) SCO. ,, The development of SCO complexes based on ions other than Fe­(II) is important to expand functionality and properties.…”

Host Spin‐Crossover Thermodynamics Indicate Guest Fit

“…Spin‐crossover (SCO) between these two states may be triggered by external stimuli, such as temperature or light. [ 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 ] The presence of SCO in cages,[ 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 ] enriches their potential applications, for example as molecular sensors and switches. [ 66 , 67 , 68 ]…”