组合结构化合物Ho2Ni7-xFex(x=0-3.0)的晶体结构、结构转变和磁性

在金属间化合物的结构演变中,原子尺寸因素起着重要的作用.由于密堆积效应,不同原子半径比的元素往往形成不同的结构.而自由电子填充于原子构成的晶体结构的间隙中,它对化合物的结构也有影响.基于组合结构化合物Ho2Ni7-xFex,结合原子尺寸与自由电子对晶体结构的不同影响,文章探讨一种单位体积内自由电子浓度的经验方法来判断Ho2Ni7-xFex化合物中两种异构体间的转变.随着Fe含量的增加,Ho2Ni7-xFex化合物先结晶成Gd2Co7型三方结构,然后结晶成Ce2Ni7型六方结构.利用Rietveld精修技术和磁测量,获得了化合物的晶体结构参数和饱和磁化强度.化合物晶胞常数随Fe含量增加而增加,饱和磁化强度则随之减少(dMs/dx=-2).分析结果表明,单位体积内自由电子浓度更高,化合物形成三方结构,反之则形成六方结构.

Structure transformation and magnetisms in Ho2Ni7-xFex compounds

Based on the composite compounds Ho₂Ni_7-xFe_x (x=0-3.0), a method of describing the structure transition from rhombohedral to hexagonal is discussed in terms of free electron concentration. The transition is investigated by X-ray powder diffraction and magnetic analysis. The compounds crystallize into the rhombohedral Gd₂Co₇-type structure for x=0-0.5 and into the hexagonal Ce₂Ni₇-type structure for x=0.5-2.5. The values of lattice parameters a and c of the Ho₂Ni_7-xFe_x compounds increase with the addition of Fe, and the saturation magnetization M_s decreases with Fe content increasing at a rate of d M_s/d x=-2, manifesting antiparallel alignments of the Fe and Ho moment. The higher the free electron concentration, the stabler the rhombohedral structure is, otherwise the hexagonal structure is stabler, which provides a meaningful parameter to distinguish the two allotropies in composite structure intermetallics.