Crystal Structure, Magnetic Properties And Magnetocaloric Effect Of La_1-xPr_xFe_13-ySi_y Compounds

Cubic NaZn₁₃-type compounds La(FexM_1x)₁₃ (M=Al, Si) were promising candidate for magnetic refrigerant materials at room temperature due to their low cost, friendly environment and large magnetocaloric effect. Magnetic properties, magnetocaloric effect, itinerant-electron metamagnetic transition (IEMT) properties of compounds La_1xPr_xFe_13-ySi_y were investigated systematically by substituting other atoms for the rare earth and Fe sites. The main results were as follows:Magnetic properties, magnetocaloric effect, and the itinerant-electron metamagnetic transition properties of the lower silicon compounds La_0.8Pr_0.2Fe_13-xSi_x (x-2.0, 1.8) in the high silicon range (1.6＜x≤2.6) were investigated. The ingots of La_0.8Pr_0.2Fe_13-xSi_x (x=2.0, 1.8) annealed at 1373K in high vacuum for 5 days could be crystallized in cubic NaZn₁₃-type structure. The substitution of silicon for iron led to increase of the Curie temperature T_c and reduction of the unit-cell volume. Increasing Si content drives the order of magnetic transition, near T_c, from first order to second order and weakenes the occurrence of IEMT. The maximum value of magnetic entropy change |ΔS(T, H)_M|_max=10.43 J/kg.K near its curie temperature T_c=210 K in La_0.8Pr_0.2Fe_11.2Si_1.8 was much larger than that of Gd under a magnetic field change of 0-1.5T. And we also found the itinerant-electron metamagnetic transition just above their Curie temperatures induced by extra magnetic field. And this property induced the large isothermal magnetic entropy change, although it was not notable.The crystal structure and component of the annealed ingots in the low silicon range (1.2≤Si≤1.6) compounds La_1-xPr_xFe_11.4Si_1.6(x=0.1, 0.2, 0.3) have been studied. We found that the compounds annealed at 1373K in high vacuum for 5 days have been crystallized in cubic NaZn₁₃-type. That indicated that the substitution of Pr for La in La_1-xPr_xFe_11-4Si_1.6 compounds shortened the annealed time. The crystal lattice aberration and more vacancies were produced after the substitution of Pr for La in these compounds. This would be propitious to the diffusion of vacancy mechanism and accordingly the chemical potency would also increase.The abrupt change of the magnetization of the compounds was observed around the Curie temperature T_c, which attributed to a sharp change of susceptibility in the vicinity of the Curie temperature and exhibited an enhanced coupling effects of magnetic crystals due to the substitution of Pr for La. We explained qualitatively that the ratio of Landau coefficient ac/b² and the critical field B_c(T) above the curie temperature will be enhanced by the substitution of Pr for La in the compounds, which would reduce the energy barrier between the paramagnetic state and the ferromagnetic state, and enhance the IEMT properties. When Pr content x=0.3, there would appear four new phenomena, i. e. the stabilization of crystal structure reducing,α-Fe phase and else phase increasing, curie temperature increasing and the magnetocaloric effect weakening. The maximum values of isothermal entropy change|ΔS(T, H)_M|_max in compounds La_1-xPr_xFe_11.4Si_1.6 is 15.06 J/kg.K, 37.08 J/kg.Kå’Œ15.22 J/kg.K with x=0.1, 0.2 and 0.3, respectively near their corresponding T_c=206 K, 194 K, 206 K, under a field change of 0-1.5T. The large |ΔS(T, H)_M| under a low magnetic field and a low cost due to the 80wt% Fe used in the La_0.8Pr_0.2Fe_11.4Si_1.6 compound, make the compound be promising for use as magnetic refrigerant materials in corresponding temperature range.