Barocaloric effect in ferroelastic fluorides and oxyfluorides

Abstract

Barocaloric effect in ferroelastic fluorides and oxyfluoridesUsing the data about the temperature -pressure phase diagram and the phase transition entropy at ambient and high pressure, intensive and extensive barocaloric effects were analyzed in the region of single and successive order-disorder phase transitions in fluorides and oxyfluorides with the elpasolites-cryolite structure. It was found that hydrostatic as well as chemical pressure are the effective tools to change the entropy of compounds with disordered ionic groups or atoms in structure and to realize the significant intensive and extensive barocaloric effects comparable with the values of baro-and magnetocaloric effects, characteristic for materials, considered as promising solid refrigerants.

Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol

“…These values of dT/dp are amongst the largest observed for BC materials (Table S1) are absent in the range ~314-342 K, evidencing reversibility. Our largest reversible isothermal entropy change |ΔS| ~ 510 J K -1 kg -1 arises at ~320 K for |p| ~ 0.52 GPa, and substantially exceeds the BC effects of |ΔS| ≤ 70 J K -1 kg -1 that were achieved using similar values of |p| in a range of materials near room temperature [Figure 4(a)], namely magnetic alloys [7][8][9][10][11][12]34 , ferroelectric 13,35,36 and ferrielectric 15 materials, fluorides and oxifluorides 14,[37][38][39][40] , hybrid perovskites 16 , and superionic conductors 17,41,42 . Moreover, our largest value of |ΔS| substantially exceeds the values recorded for magnetocaloric 30,[43][44][45][46] , electrocaloric 30,47,48 , and elastocaloric 30,49 materials, and is comparable to the values observed in the standard commercial hydrofluorocarbon refrigerant fluid R134a 18 , for which |ΔS| = 520 J K -1 kg -1 at ~310 K for much smaller operating pressures of ~ 0.001 GPa [Figure 4(a)].…”

Conventional and inverse barocaloric effects around triple points in ferroelastics (NH 4 ) 3 NbOF 6 and (NH 4 ) 3 TiOF 5

“…That is why for a long time BCE in solids has been studied to a much lesser extent than magneto(MCE)-and electro(ECE)-caloric effects in ferromagnets and ferroelectrics some of which are considered as potential solid refrigerants competitive in comparison with gaseous coolants. [1,2,3,4,5] However, near phase transitions where many properties of solids exhibit strongly anomalous behaviour, a value of the (∂V /∂T ) p derivative can often vary by several orders of magnitude. In such a case both barocaloric characteristics, ∆S BCE and ∆T AD , can reach very large magnitudes comparable with the highest values of extensive and intensive MCE and ECE.…”

Giant mechanocaloric materials for solid-state cooling*

“…These oxyfluorides have four types, namely (NH 4 ) 2 NbOF 5 , (NH 4 ) 2 WOF 4 , (NH 4 ) 2 MoO 2 F 4 , and Rb 2 KTiOF 5 . [79][80][81][82][83] The ammonia compound (NH 4 ) 2 NbOF 5 undergoes two phase transitions accompanied by entropy changes. For (NH 4 ) 2 NbOF 5 , the disordered phase I transforms into the partially ordered phase II on cooling at 258 K and then transforms to a fully ordered phase III at 220 K. [84] The strengths of the BCE and ECE are almost equal.…”

“…In 2016, a review of the MCE in ferroelastic fluorides and oxyfluorides was published. [83] This paper reviewed the effects of uniaxial stress and hydrostatic pressure on phase transitions, the MCE, and the experiment methods used to obtain the temperaturepressure phase diagram and entropy under high pressure. The thermodynamic characteristics of selected materials are compared in Table 2.…”