With the growing attention to perovskite materials in the field of magnetic refrigeration. In this work, La 0.8−x Pr x Sr 0.2 MnO 3 (x = 0.05, 0.10, 0.15) (LPSMO) samples were synthesized through the sol–gel method. The effects of Pr 3 ⁺ doping on the structure, morphology, and magnetization of LPSMO were investigated. The XRD analysis revealed that the LPSMO samples have a rhombohedral structure within the R-3c space group. As the amount of Pr 3 ⁺ doping increased, the cell volume decreased. The magnetic properties of LPSMO exhibited a transition from ferromagnetic (FM) to paramagnetic (PM) near the Curie temperature (Tc), with Tc decreasing as the Pr 3 ⁺ doping level increased. The second-order magnetic phase transition of LPSMO near Tc was confirmed by employing the MPMS. The maximum magnetic entropy change ( \(-\Delta {S}_{M}^{max}\) ) of LPSMO is 4.92 J/(kg·K) (x = 0.05), 5.13 J/(kg·K) (x = 0.10) and 5.04 J/(kg·K) (x = 0.15) when the external magnetic field (H) = 5 T, separately. Meanwhile, the relative cooling power ( RCP ) of LPSMO are 283.88, 276.71, and 289.60 J·kg −1 , respectively. The experiments indicate that LPSMO has promising application prospects.
Many solid crystals exhibit a structural phase transition where a subset of its ions or entire molecules become orientationally ordered. As to why such ordering occurs remains mostly unresolved. We consider the extremely weak magnetic elements arising from the reorientations of the molecules experiencing mutual resonance to play the chief role. Two new features are identified in d -camphor: (1) the magnetic susceptibility abruptly changes when crossing the order–disorder phase transition at T II–III = 239.8 K during cooling and at T III–II = 245.2 K during warming and (2) the complex dielectric constant exhibits two successive discontinuities only 0.2 K apart near the critical temperatures when the sweeping rate is only 0.1 K/min. We discuss how the change in entropy associated with order–disorder transitions in plastic crystals represents temporal changes rather than spatial changes in the system. Our findings may be extended to study why many other crystalline solids exhibit orientational ordering and irreversibility.
Tao Wang,Guangming Liu,Guangzhao Zhang,Vincent S. J. Craig
In the present work, we have for the first time systematically investigated the ion specific reentrant behavior of poly(N-isopropylacryamide) (PNIPAM) in water–methanol mixtures. Turbidity measurements demonstrate that SCN– and ClO4– depress the reentrant transition, whereas other anions enhance the transition. As the anion changes from chaotropic to kosmotropic, the minimum critical phase transition temperature (Tmin) decreases and the corresponding volume fraction of methanol (XM) shifts to a larger value. Our results demonstrate that anion specificity is due to the anionic structure making/breaking effect on water/methanol complexes. Cations are found to have a lesser but still significant effect on the reentrant transition, and as Tmin decreases the corresponding XM also shifts to larger values as with the anions. Our studies show that cation specificity is induced by specific interactions between cations and PNIPAM chains. Furthermore, both anion and cation specificities are amplified as XM is increased due to the formation of additional water/methanol complexes. Calorimetry measurements demonstrate that the ion specificity is dominated by changes in entropy.
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