Anomalous Thermal Expansion Of Rare-Earth Transition-metal Compounds As 2:17 Type

By means of X-Ray diffraction and magnetization measurements, this paper studied the crystal structure, magnetic properties and thermal expansion behaviors of R2Fe17(R=Nd, Tm) compounds. The effect of substitution of Cr and Si on the crystal structure, Curie temperature, thermal expansion and magnetostriction of R2Fe17(R=Tm, Gd) compounds are analyzed and discussed in detail.The research results of Nd2Fe17-xCrx(x=0,0.5,1.0,1.5,20) compounds prepared by the electric arc furnace indicate that Nd2Fe17-xCrx(x=0,0.5,1.0,1.5,2.0) compounds has Th2Zn17-type structure at room temperature (-300K). With the increase of substituting amounts of Cr, the unit-cell parameters of compounds changed nonlinearly, which is related to atomic volume and magnetic interaction. The investigations of magnetic indicate that the Curie temperature of Nd2Fe17-xCrx compounds is raised by the substitution of Si, and the maximum is 390K when x=1.0. It is found that strong spontaneous magnetostriction phenomenon existed in Nd2Fer17-xCrx compounds. Spontaneous magnetostriction and thermal expansion caused the anomalous thermal expansion of Nd2Fe17-xCrx compounds in a certain temperature range. Through variable temperature X-ray diffraction temperature dependence of the unit-cell parameters of compounds is obtained. And through further study based on the Debye theory and the Gruneisen relation, rules of change with the temperature of body magnetostrictive coefficient ωs, line magnetostrictive coefficient λc and line magnetostrictive coefficient λa of Nd2Fe17-x-Crx compounds are figured out. It showed that λc is much larger than λa at the lower temperature. It indicates that the spontaneous magnetostriction is anisotropic. Compared λc, λa and ωs, we could know that contribution of body magnetostricton occurs mainly in c-axis. By means of differential thermal/thermogravimetric analyzer, the result showed that Nd2Fe17-xCrl-compounds was in two phase structure. There was a more stable phase structure of Nds(Fe,Ti)29 and phase structure of Th2Zn17 coexist, when the compound annealed at 950℃ and the substitution of Cr is x=2.5. At temperature of 1150℃, compound was in a single phase of Nd3(Fe,Ti)29 structure.The research results of Tm2CrFe16-xSix(x=0.5,1.0,1.5,2.0,2.5,3.0) compounds prepared by the electric arc furnace indicate that Tm2CrFe16-xSix(x=0.5,1.0,1.5,2.0,2.5, 3.0) compounds has Th2Zn17-type structure. Curie temperature of compounds substituted by Si is much higher than that of Tm2CrFe16 compound and Tm2Fe17 compound (about 290K). The max Curie temperature is 450K when x=1.5. Based on the temperature dependence of the unit-cell parameters of compounds, the Debye theory and the Gruneisen relation, we calculated the rules of change with the temperature of body magnetostrictive coefficient ωs, line magnetostrictive coefficient λc and line magnetostrictive coefficient λa ofcompounds. We also found that thermal expansion combined results in anomalous thermal expansion of Tm2Fe17-xCrx:compounds in a certain temperature range. It indicate that spontaneous magnetostriction of compound was strong and it was anisotropic. Compared λc, λa and ωs, we could know that contribution of body magnetostricton occurs mainly in c-axis.