Structure and magnetic properties of rare-earth chromium germanides RECrxGe2 (RE=Sm, Gd–Er)
Graphical abstract
The average structure of RECrxGe2 corresponds to the CeNiSi2-type, with Cr atoms entering square pyramidal sites. The Cr atoms provide no contribution to the effective magnetic moment in these compounds, which undergo antiferromagnetic ordering below 20 K.
Introduction
Ternary rare-earth transition-metal germanides RE–M–Ge represent a well-investigated class of intermetallics exhibiting a wide range of structures and physical properties [1], [2]. However, those systems containing an early transition metal are still not as well understood as those containing a late transition metal. In particular, reports in the RE–Cr–Ge systems have been restricted to isolated compounds (Sc2Cr4Ge5 [3], Sc7Cr4+xGe10–x [3], [4], ScCrGe2 [3], [5], La5CrGe3 [6], Nd2Cr9Ge8 [7], and Sm117Cr52Ge112 [8]) and only two well-defined series (RECr6Ge6 (RE=Sc, Tb–Er) [3], [9], [10] and RECrGe3 (RE=La–Nd, Sm) [11]), the latter having been identified recently by us. One series that has been missing to date is the set of CeNiSi2-type phases that are adopted by a large number of silicides, germanides, and stannides, REMxTt2 (M=Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Ir, Pt; Tt=Si, Ge, Sn (collectively, the “tetrels”)) [12], [13]. Most of these phases are nonstoichiometric, with the transition-metal content x generally increasing on proceeding to a heavier transition metal or to a lighter tetrel. Surprisingly, detailed single-crystal structure studies have been scarce, and only recently has a modulated superstructure been implicated for TbFe0.25Ge2 [14]. Much of the interest on these REMxTt2 phases has focused on their varied magnetic properties, which depend systematically on the component elements [15].
Continuing our studies of the RE–Cr–Ge systems, we report here the new nonstoichiometric ternary germanides RECrxGe2, which form for the later RE elements (RE=Sm, Gd–Er). Some members could be successfully grown as single crystals through flux methods to permit structural investigations to determine the level of Cr deficiency in the CeNiSi2-type structure adopted. Magnetic measurements were also performed for those members that could be prepared as phase-pure samples.
Section snippets
Synthesis
Starting materials were RE pieces (99.9%, Hefa), Cr powder (99.8%, Alfa-Aesar), and Ge powder (99.999%, Cerac). Products were characterized by powder X-ray diffraction (on an Inel powder diffractometer equipped with a CPS 120 detector) and energy-dispersive X-ray (EDX) analysis (on a Hitachi S-2700 electron microscope).
Single crystals of SmCrxGe2 were first identified as byproducts in the synthesis of SmCrGe3. The elements were loaded in the ratio Sm:Cr:Ge=1:1:3 (total weight: 0.3 g) in the
Structure
The germanides REMxGe2 were previously known for first-row transition metals being restricted to M=Mn, Fe, Co, Ni, Cu [2], and have now been extended to include an earlier transition metal, M=Cr, in the new series RECrxGe2. The range of RE substitution is narrower in RECrxGe2 (RE=Sm, Gd–Er, Yb), and gradually widens on progressing to REMnxGe2 (RE=Nd, Sm, Gd–Tm, Lu) and REMxGe2 (M=Fe, Co, Ni, Cu; RE=Y, La–Sm, Gd–Lu) [12], [13]. These compounds adopt the CeNiSi2-type structure, which has been
Acknowledgments
The Natural Sciences and Engineering Research Council of Canada and the University of Alberta supported this work. We thank Dr. Robert McDonald and Dr. Michael J. Ferguson (X-ray Crystallography Laboratory) for the X-ray data collection and Ms. Christina Barker (Department of Chemical and Materials Engineering) for assistance with the EDX analysis.
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