Giant adiabatic temperature change and its direct measurement of a barocaloric effect in a charge-transfer solid.

Solid refrigerants exhibiting a caloric effect upon applying external stimuli are receiving attention as one of the next-generation refrigeration technologies. Herein, we report a new inorganic refrigerant, rubidium cyano-bridged manganese-iron-cobalt ternary metal assembly (cyano-RbMnFeCo). Cyano-RbMnFeCo shows a reversible barocaloric effect with large reversible adiabatic temperature changes of 74 K (from 57 °C to -17 °C) at 340 MPa, and 85 K (from 88 °C to 3 °C) at 560 MPa. Such large reversible adiabatic temperature changes have yet to be reported among caloric effects in solid-solid phase transition refrigerants. The reversible refrigerant capacity is 26000 J kg-1 and the temperature window is 142 K. Additionally, cyano-RbMnFeCo shows barocaloric effects even at low pressures, e.g., reversible adiabatic temperature change is 21 K at 90 MPa. Furthermore, direct measurement of the temperature change using a thermocouple shows +44 K by applying pressure. The temperature increase and decrease upon pressure application and release are repeated over 100 cycles without any degradation of the performance. This material series also possesses a high thermal conductivity value of 20.4 W m-1 K-1. The present barocaloric material may realize a high-efficiency solid refrigerant.

Evidence of proton-coupled mixed-valency by electrochemical behavior on transition metal complex dimers bridged by two Ag+ ions.

H-Bonded metal complex dimers with reversible redox behaviour, which are connected by a low-barrier hydrogen bond (LBHB) with a very low energy barrier for proton transfer, can provide a unique mixed-valency state stabilized by the proton-coupled electron transfer (PCET) phenomenon. Using cyclic voltammetry measurements, newly prepared [ReIIICl2(PnPr3)2(Hbim)]2 (2) and [OsIIICl2(PnPr3)2(Hbim)]2 (3) existing as H-bonded dimers in a CH2Cl2 solution showed a four-step and four-electron transfer containing two mixed-valency states of ReIIReIII and ReIIIReIV, and OsIIOsIII and OsIIIOsVI, respectively. Furthermore, [ReIIICl2(PnPr3)2(Agbim)]2 (4) and [OsIIICl2(PnPr3)2(Agbim)]2 (5), bridged by two Ag+ ions instead of two H-bonding protons, were prepared, and their electrochemical behaviours changed to a two-step and four-electron transfer. It is clear that the H-bonded complex dimers 2 and 3, connected by an LBHB, can be electrochemically stabilized into unique pairs of mixed-valency states by PCET, and the H-bonding proton transfer also controls the electrochemical redox behaviour.