ISSN 1063-7834, Physics of the Solid State, 2011, Vol. 53, No. 3, pp. 510–517. © Pleiades Publishing, Ltd., 2011. Original Russian Text © E.A. Mikhaleva, I.N. Flerov, V.S. Bondarev, M.V. Gorev, A.D. Vasiliev, T.N. Davydova, 2011, published in Fizika Tverdogo Tela, 2011, Vol. 53, No. 3, pp. 478–484. 510 1. INTRODUCTION In recent years, the electrocaloric, magnetocaloric, and barocaloric effects have attracted increased atten- tion of researchers due to the fundamental and applied aspects of these phenomena. As regards the applied aspects, the case in point is the possibility of using materials having caloric effects in solid-state cooling devices [1, 2]. The cooling technique based on any caloric effect lies in a reversible change in the temper- ature or entropy of a thermodynamic system (ferro- electric, ferromagnetic, and ferroelastic) in response to external fields (electric, magnetic, and mechanical stresses) under adiabatic or isothermal conditions, respectively, which makes it possible to organize a classical inverse Carnot cycle. For a long time, the magnetocaloric and electrocaloric effects have not attracted special attention, because their magnitudes in materials known by that time were too small in order to provide a high efficiency of refrigeration cycles. However, gradual progress in theoretical and experimental methods of investigation of magneto- thermal and electrothermal properties of materials and discovery of new compounds (ferroics), which undergo phase transitions accompanied by significant changes in the entropy, favored the renewal of scien- tific and practical interest in the study of the magneto- caloric and electrocaloric effects. At present, there exist a wide range of magnets and a rather limited number of ferroelectrics that can be recommended for the use as solid refrigerants [1–3]. Among magnetic materials, large magnitudes of the intensive (ΔT ad ) and extensive (ΔS ce ) effects are characteristic of perovs- kite-like manganites, alloys, intermetallic com- pounds, etc. In the family of ferroelectrics, order–dis- order phase transitions, i.e., transitions associated with the large change in the entropy ΔS/R > ln2, occur more rarely. The largest electrocaloric effect was observed in lead zirconate titanate thin films, and, only in this form, the material appeared to be compet- itive in the caloric efficiency with the most promising magnetic solid-state refrigerants [1, 4]. Experimental investigations of the barocaloric effect (BCE), which has long been used in vapor–gas refrigeration cycles, have been started rather recently, and, correspondingly, this effect has been studied to a considerably lesser extent as compared to the magne- tocaloric and electrocaloric effects. The technique of adiabatic cooling in the vicinity of the structural phase transition induced by the external pressure in com- pounds containing rare-earth ions was proposed in 1998 [5]. More recently, investigations of the baroca- loric effect in ferroelastics revealed that, at relatively low hydrostatic pressures, fluoroxygen compounds that undergo phase transitions associated with the ordering of cations and anions can be characterized by values of ΔS ce and ΔT ad comparable to the best param- eters of the magnetocaloric effect in magnets [6, 7]. It was demonstrated that one of the factors responsible FERROELECTRICITY Phase Transitions and Caloric Effects in Ferroelectric Solid Solutions of Ammonium and Rubidium Hydrosulfates E. A. Mikhaleva a, b, *, I. N. Flerov a, b, **, V. S. Bondarev a, b , M. V. Gorev a, b , A. D. Vasiliev a, b , and T. N. Davydova b a Institute of Engineering Physics and Radioelectronics, Siberian Federal University, pr. Svobodny 79, Krasnoyarsk, 660041 Russia * e-mail: katerina@iph.krasn.ru ** e-mail: flerov@iph.krasn.ru b Kirensky Institute of Physics, Siberian Branch, Russian Academy of Sciences, Akademgorodok 50, Krasnoyarsk, 660036 Russia Received June 24, 2010 Abstract—Structural, calorimetric, dielectric, and electrocaloric investigations of Rb x (NH 4 ) 1– x HSO 4 ferro- electric solid solutions have been performed. It has been found that rubidium atoms inhomogeneously occupy nonequivalent crystallographic positions in the structure P2 1 /c. The influence of the rubidium con- centration on the sequence of phase transitions in the NH 4 HSO 4 compound has been established. It has been revealed that the consequences of the Landau theory can be applied to the description of the temperature dependences of the anomalous heat capacity and the electrocaloric effect over a wide range of temperatures. Comparative evaluations of the electrocaloric and barocaloric effects in hydrosulfate and triglycine sulfate crystals have been carried out. DOI: 10.1134/S1063783411030188