Study On Structural And Magnetocaloric Effect Of A-sites Size-disorder La_2/3-xR_ExCa_1/3-ySr_yMnO₃ Compounds

In this paper, the development of magnetocaloric effect, principle and applianceof magnetic refrigeration, studying course of magnetic refrigeration and manganeseoxides refrigeration materials, and theory base of thermodynamics were reviewedsimply; crystal and electronic structure of manganites, the interaction betweendouble-exchange and super-exchange and Jahn-Teller effect were showed slightly; theRietveld refinement method and skill of manganese oxides is told, and so on.According to keeping A-site mean ionic radius and Mn₃₊/Mn₄₊ ratio unchanged,chemical contents of element La, Y, Eu, Ca, Sr, Mn, O can be calculated accurately;two series of manganese oxides La_2/3-xRE_xCa_1/3-ySr_yMnO₃ (R_E as rare earth Y and Eu)were prepared by solid phase reaction method; the structure and phase of sampleswere carried out by X-ray diffraction instrument Rigaku Dmax-rB 12KW, and latticeparameters and Mn-O-Mn band angle were obtained from refinement resultscalculated by Rietveld method; magnetization curve and dependence ofmagnetization M on temperature T were measured by VSM Lakeshore7300; thefirst-order magnetic phase transition or the second-order magnetic phase transitioncan be estimated by Arrot curve.From the results of X-ray diffraction patterns and Rietveld refinement, it isshowed that lattice constants of two series of samples do not change scarcely andperovskite orthorhombic structure with single phase are kept as the dopedconcentration x increase; as doped disorder disgreeσ² increase, lattice distortionincrease and Curie temperature Tc decrease; furthermore, Curie temperature ofsamples La_2/3-xEu_xCa_1/3-ySr_yMnO₃ decrease monotonously, however, Curietemperature of sample La_2/3-xY_xCa_1/3-ySr_yMnO₃ (x=0.2) increase reversely becauseMn-O-Mn band angle increasing sharply contribute to enhancement ofdouble-exchange interplay and ferromagnetism exchange interaction, it is showed thatthe competition between doped disorder and double-exchange interaction result inthese phenomena; as the doped disorder degree increase, it is exhibited, from magnetic entropy change and Arrot curve, the first-order magnetic phase transitionleading to large magnetocaloric effect is shifted to the second-order magnetic phasetransition, furthermore, magnetic entropy change of samples La_2/3-x Eu_xCa_1/3-ySr_yMnO₃ almost decrease monotonously; it is showed by analyzing that, as thedoped disorder degree increase, sharp change of M-T curve became soft near Curietemperature owing to electron-lattice coupling effect decreasing. In addition,magnetic phase transition takes place from the first-order to second-order presentedby Arrot curve due to the rivalship between doped disorder and electron-latticecoupling effect. Furthermore, samples with large magnetocaloric effect may be usedas a candidate of magnetic refrigeration materials at a special temperature scope.