Reinvestigation and superstructure of La3.67[Fe(C2)3]

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Abstract

The lanthanum iron carbide La3.67[Fe(C2)3] was prepared from the elements by argon arc-melting followed by annealing. The crystal structure of the ternary phase was reported previously (space group P63/m with a=878.7(2) pm, and c=535.1(1) pm) [A.M. Witte, W. Jeitschko, Z. Naturforsch. 51b (1996) 249–255]. In the present work the compound was reinvestigated by X-ray powder and single crystal diffraction, and was further characterized by metallographic methods and chemical analyses. Our diffraction data clearly reveal a superstructure with weak superstructure reflections in the space group P63/m with a=879.26(8) pm and c=1604.59(15) pm, thus tripling the previously reported subcell. The crystal structure (refinement to R1=0.044 and wR2=0.075 for 1387 unique reflections and 60 variables) contains Fe(C2)3 trigonal planar groups with the C2 ligands bonded end-on to the Fe atoms. The C–C distance is typical for a double bond. La atoms as the least electronegative component surround the complex anions and form a framework of face-sharing tricapped trigonal prisms. The resulting hexagonal channels at 0, 0, z of the partial structure with chemical composition La3FeC6 are occupied by four additional La atoms per unit cell. These La atoms are fully ordered within a linear chain and display a Peierls-like distortion pattern. However, no long-range order in the ab plane has been observed due to the random orientation of the chains. Because of the two different orientations which are possible for each chain the situation is similar to an Ising model on a triangular lattice.

Graphical abstract

Diffraction data reveal a superstructure with weak superstructure reflections for La3.67[Fe(C2)3] in the space group P63/m. The superstructure is caused by an ordered arrangement of La atoms along 0, 0, z with alternating short and long distances.

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Introduction

Recently, the family of ternary carbides RExTyCz (RE=rare-earth metals or actinoids; T=transition metals; C=monoatomic C4− species) has been classified into two groups according to structural characteristics and the ratio (x+y)/z, i.e., carbometalates with (x+y)/z⩽2 containing complex anions and metal-rich carbides with (x+y)/z⩾4 [2]. In the search for novel carbometalates we also explored the systems RE–Fe–C. Compounds of this type have been proposed as potential hard magnet materials [3]. In spite of intensive explorations of the respective phase diagrams only a few compounds with (x+y)/z<2 have been reported: RE2FeC4 (RE=Y, Tb–Lu) [4], Sc3FeC4 [5], REFeC2 [6], ScFeC2 [7] and RE3.67FeC6 (RE=La–Nd, Sm) [1]. All these compounds contain C2-pairs as structural units instead of monoatomic carbon ligands, thus, in a strict sense they do not belong to the class of carboferrates. During our recent work in the system La–Fe–C we reinvestigated the crystal structure of La3.67FeC6, which had been described already in a previous communication by Witte and Jeitschko [1]. In the present work we report on the determination of a superstructure of this compound.

Section snippets

Preparation

All handlings were carried out in a glove box under argon atmosphere (MBraun, p(O2, H2O)<1 ppm). The title compound was prepared by arc-melting of cold pressed pellets of La (Ames, 99.99%), Fe (Alfa Aesar, 99.99%) and graphitic carbon (Chempur, 99.9%) in the molar ratios 3:1:6 and 3.67:1:6. The button specimens of about 0.6 g total mass were remelted several times to ensure compositional homogeneity. The samples were then wrapped in Fe-foils, which in turn were encapsulated in fused silica tubes,

Results and discussion

The crystal structure of La3.67FeC6 was refined in the space group P63/m (176) with residuals R1=0.044 and wR2=0.075 for 1387 unique reflections and 60 variables. The atoms are distributed on 4 La, 2 Fe and 4 C sites in the unit cell. The occupancy factors were refined in separate series of least-squares cycles as a check for partial occupancy. The refinement revealed full occupancy for all sites except those of La3 and La4. In the final cycles of the refinement the site occupancy for La3 and

Conclusion and summary

The ternary carbide La3.67[Fe(C2)3] with maximum possible La content forms a superstructure with a tripled unit cell along the c axis compared to the previously reported crystal structure [1]. The rigid host structure without any occupational disorder and with chemical composition La3FeC6 is identical for the sub- and super-structure (neglecting minimum distortions) and contains Fe(C2)3 trigonal planar groups with C2 ligands bonded end-on to the iron atoms. These complex entities are surrounded

Acknowledgments

We would like to thank Dr. Ulrich Burkhardt, Torsten Vogel, and Monika Eckert (metallographic investigations and EDXS) as well as Steffen Hückmann (X-ray powder diffraction).

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