Structural transformations in R3Cu4Sn4 (R = Ho, Er, Tm) intermetallic compounds
Introduction
The R3T4X4 family (R = rare earth; T = Cu, Ag, Au, Pd, Mn; X = Si, Ge, Sn) is an extensive series of isostructural intermetallic compounds that exhibit a rich variety of magnetic ordering. The intrinsic magnetic behaviour of this family of compounds has been the subject of numerous studies by neutron diffraction, magnetometry and Mössbauer spectroscopy. The R atoms occupy two distinct crystallographic sites in the orthorhombic unit cell and, in general, the R moments order antiferromagnetically, with distinct magnetic structures being adopted by the two R sublattices. In many cases, the two R sites have quite different ordering temperatures and we refer the reader to our recent review of the magnetism of the R3T4X4 compounds [1] for a detailed discussion of this system. A useful summary of the magnetic properties of the R3T4X4 compounds has also been written by Wawrzyńska and Szytuła [2].
At room temperature (RT), the R3T4X4 compounds have a generic orthorhombic Gd3Cu4Ge4-type structure (space group Immm, #71) [3], [4] with two formula units per orthorhombic cell. The R atoms occupy two crystallographic sites (2a and 4f), the transition metal (T) occupies the 8m site and the X atoms occupy the 4e and 4j sites. The exceptions to this orthorhombic structure are Tm3Cu4Sn4 [4] and Lu3Cu4Sn4 [5] which have a monoclinic C2/m structure (or I2/m in a different setting of the Space Group #12) at RT. In a recent paper [6] we showed that Tm3Cu4Sn4 transforms from the monoclinic I2/m structure to the generic orthorhombic Immm structure when heated above RT and we determined the transition temperature to be 458(3) K by X-ray diffraction. In a previous brief report we noted that Er3Cu4Sn4 and Ho3Cu4Sn4 present similar phase transitions at different temperatures [7].
In this paper we will focus on the crystallography of the R3Cu4Sn4 members of this extensive family. Magnetic and crystallographic studies of the R3Cu4Sn4 compounds encompass virtually the entire rare-earth series: R = La [8], Ce [9], [10], Pr [8], [11], [12], Nd [8], [11], [12], [13], Sm [8], Gd [9], [14], [15], [16], [17], [18], Tb [14], [15], [19], [20], Dy [14], [15], [19], [7], Ho [13], [14], [15], [19], [20], [22], [23], [24], [7], Er [14], [15], [19], [25], [26], [7], Tm [4], [6], [21] and Lu [5].
During the course of our neutron diffraction work on the Er3Cu4X4 (X = Si, Ge, Sn) compounds [25], we observed significant broadening of the nuclear peaks when X = Sn at temperatures around 200–250 K, well above the magnetic ordering temperatures of the two Er sublattices, namely 6 K and 3.5 K [19]. Upon cooling below 200 K, many of these broadened peaks split, suggesting a crystallographic distortion to a symmetry lower than the generic Immm orthorhombic symmetry.
Here, we present the results of our high-resolution synchrotron radiation and neutron powder diffraction measurements made on Dy3Cu4Sn4, Ho3Cu4Sn4 and Er3Cu4Sn4 and Tm3Cu4Sn4. The monoclinic to orthorhombic phase transition that occurs when R = Ho, Er and Tm is discussed. The crystallographic transition temperature scales linearly with the ionic radius of the rare-earth. There is no such distortion in Dy3Cu4Sn4 down to 1.7 K, in agreement with the scaling law.
Section snippets
Experimental methods
Stoichiometric amounts of the pure elements (R 99.9 wt.%, Cu and Sn 99.999 wt.%) were arc-melted under high-purity argon. The resulting ingots were then sealed under vacuum in quartz tubes and annealed for 20 days at 873 K, followed by water quenching. Cu-Kα X-ray diffraction and electron microprobe analyses, carried out at RT, confirmed that the majority phase was the orthorhombic Immm phase for R = Dy, Ho and Er, whereas the RT structure of Tm3Cu4Sn4 is monoclinic C2/m (I2/m), in agreement
Results and discussion
In this section we will present the results of our diffraction experiments, placing the emphasis on Er3Cu4Sn4. Following the detailed discussion of Er3Cu4Sn4 we will briefly present our results on Ho3Cu4Sn4 and Tm3Cu4Sn4, obtained using the same procedure as for Er3Cu4Sn4. Finally, we will present our diffraction results on Dy3Cu4Sn4 in which we see no evidence for a distortion, down to 1.7 K. We will show that this behaviour is consistent with the dependence of the crystallographic distortion
Discussion
As discussed in detail previously [6], the most significant difference between the orthorhombic and monoclinic crystal structures of the R3Cu4Sn4 stannides lies with the Cu sites, whose relative atomic displacement parameters are nearly three times those of the other atoms, as obtained from the Rietveld refinements. The distortion may also be seen in the tilting of the Sn–Sn pairs relative to the R–R axes. In Fig. 9 we compare the monoclinic and orthorhombic cells of the R3Cu4Sn4 compounds.
Conclusions
We have investigated the orthorhombic to monoclinic structural transformation in the intermetallic compounds R3Cu4Sn4 (R = Ho, Er, Tm). This crystallographic transformation from the Immm to the C2/m (I2/m) space group occurs at 62(2) K for R = Ho, 262(2) K for R = Er and 458(3) K for R = Tm [6]. No such distortion was observed for R = Dy, down to 1.7 K, consistent with the observed linear scaling between the transformation temperature and the ionic radius of the rare-earth.
Acknowledgements
JMC is grateful to the University of New South Wales (Grant SPF01) for its financial support. Initial parts of this work were carried out while JMC was on the faculty of the University of Manitoba, supported by the Canada Research Chairs programme (Grant 203392) and the Canadian funding agencies NSERC (Grants 355414-2008, 42310-2013), CFI (Grants 23406, 23285) and MRIF. DHR acknowledges support from Fonds Québécois de la Recherche sur la Nature et les Technologies. We are grateful to the staff
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