Structure and magnetic properties of rare-earth chromium germanides RECr_xGe₂ (RE=Sm, Gd–Er)

Abstract

The ternary rare-earth chromium germanides RECr_xGe₂ (RE=Sm, Gd–Er) have been obtained by reactions of the elements, either in the presence of tin or indium flux, or through arc-melting followed by annealing at 800 °C. The homogeneity range is limited to 0.25⩽x⩽0.50 for DyCr_xGe₂. Single-crystal and powder X-ray diffraction studies on the RECr_0.3Ge₂ members revealed that they adopt the CeNiSi₂-type structure (space group Cmcm, Z=4, a=4.1939(5)–4.016(2) Å, b=16.291(2)–15.6579(6) Å, c=4.0598(5)–3.9876(2) Å in the progression for RE=Sm to Er), which can be considered to be built up by stuffing transition-metal atoms into the square pyramidal sites of a “REGe₂” host with the ZrSi₂-type structure. (The existence of YbCr_0.3Ge₂ is also implicated.) Only the average structure was determined here, because unusually short Cr–Ge distances imply the development of a superstructure involving distortions of the square Ge net. Magnetic measurements on RECr_0.3Ge₂ (RE=Gd–Er) indicated that antiferromagnetic ordering sets in below T_N (ranging from 3 to 17 K), with additional transitions observed at lower temperatures for the Tb and Dy members.

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], Sc₇Cr_4+xGe_10–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, REM_xTt₂ (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 TbFe_0.25Ge₂ [14]. Much of the interest on these REM_xTt₂ 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 RECr_xGe₂, 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.