Barocaloric Cooling: A Combined Experimental and Computational Study.

Refrigeration accounts for 17% of the global electricity usage. The current vapour-compression technology releases greenhouse gases which has led to the demand for solid-state replacements.¹Refrigerant effects can be induced in materials named barocaloric solids through the application of hydrostatic pressure which causes a change in isothermal entropy and adiabatic temperature of the system.<br/>Organic ionic plastic crystals (OIPCs) are a class of materials which exhibit at least one solid-solid phase transition upon varying the temperature of the system. Upon cooling these structures, the mobility of the ions becomes limited, increasing the order within the structures. It is thought that OIPCs have the ability to undergo disorder-order phase transitions which would result in large entropy changes.² In this work we investigate the phase behaviour of the OIPC (cyanomethyl)trimethylammonium hexafluorophosphate, via experiment and simulation. Variable temperature and high-pressure crystallography and thermoanalytical methods were performed to understand if it has the desired characteristics of a barocaloric material. At room temperature, (cyanomethyl)trimethylammonium hexafluorophosphate is in a high symmetry, disordered, trigonal phase, upon cooling to 253 K the structure transitions to an ordered, monoclinic phase. The isothermal entropy change associated with this solid-solid phase transition has been experimentally determined to be 138 J K^-1 kg^-1 and molecular dynamic simulations will be used in order to understand the configurational, rotational and vibrational contributions towards this value.<br/><br/>[1]X. Moya and N. D. Mathur, <i>Science</i>, <b>2020</b>, 370, 797–803. [2] J. M. Pringle, P. C. Howlett, D. R. MacFarlane and M. Forsyth, <i>Journal of Materials Chemistry</i>, <b>2010</b>, 20, 2056.