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Yb(Cu,T)5 and Yb(Cu,T)4.5 solid solutions (T = Ag, Au, Pd)
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Intermetallics 16 (2008) 399e405 www.elsevier.com/locate/intermet
Yb(Cu,T)5 and Yb(Cu,T)4.5 solid solutions (T ¼ Ag, Au, Pd) M. Giovannini a,b,*, R. Pasero a,b, S. De Negri a, A. Saccone a a
Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, Via Dodecaneso 31, I-16146 Genova, Italy b LAMIA-INFM-CNR, Corso Perrone 24, I-16152 Genova, Italy Received 30 July 2007; received in revised form 28 November 2007; accepted 30 November 2007 Available online 22 January 2008
Abstract The binary YbeCu phase diagram was re-investigated in the Cu-rich part. Starting from this region, solid solutions formed by substitution of Cu by Ag, Au, and Pd have been investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A competition in compound formation between Yb(Cu,T)5 (T ¼ Ag, Au, Pd), cubic AuBe5-type and its monoclinic superstructure Yb(Cu,T)4.5 is generally evident. Whereas for Cu/Au and Cu/Ag substitutions deeply-extended Yb(Cu,T)5 solid solutions are formed, in the case of substitution of Cu by Pd the cubic and monoclinic phases coexist. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Crystal chemistry of intermetallics; F. Diffraction; F. Electron microscopy, scanning
1. Introduction Several aspects make Yb compounds attractive for research studies, such as valence fluctuations and other low-temperature anomalies. Recently a strong emphasis was given to the investigation of the solid solutions YbCu5xMx (M ¼ Ag, Au, In) crystallizing in the cubic AuBe5-type structure, where the substitution of Cu by M offers the possibility to study the different evolution of ground state properties depending on the M element, starting from dense Kondo behaviour for low xvalues. In fact, whereas YbCu5xAgx exhibits Kondo lattice behaviour in the whole substitution range 0 < x 1, Cu/In and Cu/Au substitutions drive the system through a crossover to a valence fluctuation system [1] and to long-range magnetic order [2,3], respectively. In particular, for the YbCu5xAux system a quantum phase transition seems to occur, although an agreement on the precise location of the quantum critical point (QCP) has still to be found (xc ¼ 0.2e0.4 [3], xc ¼ 0.5 [2]). The common starting point (at x ¼ 0) of these solid * Corresponding author. Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, Via Dodecaneso 31, I-16146 Genova, Italy. Tel.: þ39 010 3536648; fax: þ39 010 3625051. E-mail address: [email protected] (M. Giovannini). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.11.010
solutions, YbCu5, has been recently synthesized in the cubic AuBe5-type only by melt-spinning technique [4] or under high pressure [5]. Furthermore, the current assessment of the binary YbeCu system [6] reports the existence of YbCu5 crystallizing in the CaCu5 structure, although it is now well established that YbCu5 does not form at equilibrium under ambient pressure [5]. Concerning the formation of the solid solutions Yb(Cu,T)5 with the transition metals T ¼ Au and Ag, the interest in this topic was triggered by the investigation on the heavy fermions YbCu4T (T ¼ Ag, Au and Pd) done by Rossel and co-workers [7]. These compounds crystallize in the MgCu4Sn structure (an ordered derivative of the AuBe5-type). It was found that the compounds with T ¼ Au and Pd order magnetically below 1 K, whereas YbCu4Ag has a nonmagnetic ground state. In another investigation carried out on these three compounds by Severing and co-workers [8], from X-ray and neutrondiffraction measurements it was observed that, whereas YbCu4Au and YbCu4Ag are cubic AuBe5-type single phase, YbCu4Pd contains other extra-peaks and, in addition, the Bragg lines are strongly broadened. Later on, it was recognized that YbCu4Au and YbCu4Ag are points of crystallographic order of YbCu5xTx solid solutions, whereas, up to the present time, no detailed information has been given on
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
3. Results and discussion 3.1. Cu-rich region of the YbeCu phase diagram In the assessed phase diagram of the YbeCu system [6], mainly based on the phase relations determined by Iandelli and Palenzona [11], the following intermediate phases are reported: YbCu5, Yb2Cu9, Yb2Cu7 (the last two compounds, in the following indicated as YbCu4.5 and YbCu3.5, are reported with unknown crystal structure), YbCu2 and YbCu. All the compounds have peritectic formation except YbCu4.5 which forms congruently at 937 C. According to this assessment, YbCu5, crystallizing in the CaCu5 structure, is the Cu-richest compound, although Hornstra and Buschow [12] previously observed the formation of a compound of stoichiometry YbCu6.5 instead of YbCu5, crystallizing in a structure closely related to that of CaCu5. These authors did not report the temperature range of stability of YbCu6.5 and, in addition, phases with this stoichiometry had not been found for any of the other lanthanides. Probably due to these reasons, the compound was not reported in the assessed YbeCu phase diagram [6]. Fig. 1 shows the partial YbeCu phase diagram determined in our investigation on the basis of differential thermal analysis (DTA), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Table 1 reports the crystal data obtained
40
30
50
60
1100 1084.8
1000 935 900
870
850
825 800 760
700
YbCu2
YbCu3.5
The metals used were Yb of 99.9 mass% purity, and Cu, Ag, Au, Pd of 99.99 mass% purity. Ingot of pure elements for a total weight of about 1.0 g were enclosed in small tantalum crucibles, sealed by arc welding under pure argon, in order to prevent mass loss due to the high vapour pressure of Yb. The samples were then melted and remelted (in order to attain proper homogenization) in an induction furnace under continuous shaking of the crucibles in a stream of pure argon. Subsequently, for ternary samples a heat treatment at T ¼ 700 C for 10 days in argon atmosphere was carried out. X-ray diffraction (XRD) analysis (Cu Ka radiation), optical and scanning electronic microscopy (SEM), electron probe microanalysis (EPMA) and, in some cases, differential thermal analysis (DTA) were used to characterize the samples. The details of the measurement techniques were fully described in previous papers [9,10].
20
YbCu4.5
2. Experimental details
mass % Yb 10
YbCu6.5
the crystal structure of YbCu4Pd and the possible formation of an analogous solid solution with T ¼ Pd. Moreover, a careful investigation on the minimum amount of Au necessary to stabilize the cubic YbCu5xAux AuBe5-type solid solution is still missing. This is an important issue to be addressed before coming to any conclusion about the precise location of the QCP in YbCu5xAux. The present paper reports our investigation on the Cu-rich solid solutions in the ternary systems YbeCueT (T ¼ Ag, Au, Pd). An update of the Cu-rich region of the YbeCu phase diagram is also reported, including the fact that YbCu5 does not exist at equilibrium under ambient pressure.
Temperature (°C)
400
600
500 Cu
10
20
30
40
at. % Yb Fig. 1. YbeCu system in the Cu-rich region. Phase diagram under ambient pressure revised in the composition range between 10 and 24 at.% Yb.
in this work for the YbeCu intermediate compounds, together with those from the literature. Some differences emerge by the comparison of this phase diagram with the current assessment of the YbeCu system. In our investigation the Cu-richest compound resulted to be YbCu6.5 rather than YbCu5. It presents peritectic formation at 870 C and it forms with Cu an eutectic at 850 C and 10 at.% Yb. The micrograph of an alloy at 10 at.% Yb, showing primary crystallization of YbCu6.5 in presence of eutectic structure, is reported in Fig. 2. YbCu6.5 does not show any evidence of decomposition, as proved by DTA and microscopy examinations of samples subject to DTA and annealing. In particular, an alloy with 17 at.% Yb observed after annealing for 20 days at T ¼ 600 C resulted to be constituted by the two phases YbCu6.5 and YbCu4.5 (see the appearance of the microphotograph in Fig. 3). From our investigation it was confirmed that YbCu5 does not exist and YbCu6.5 is the Cu-richest stable compound. The X-ray pattern of YbCu6.5 was indexed on the basis of a hexagonal ˚ c ¼ 4.112 A ˚ . These lattice parameunit cell with a ¼ 5.006 A ter values are in a good agreement with the values ˚ , c ¼ 4.118 A ˚ ) obtained by Hornstra and Buschow (a ¼ 5.004 A [12]. The crystal structure of YbCu6.5 has been determined by these authors by difference Fourier synthesis of powder data. It is related to the CaCu5-structure with Yb on the 1a site and Cu atoms on the 2c and 3g sites of the space group P6/mmm (N. 191). The excess of Cu is constituted by pairs of Cu atoms located on the 2e site at (0,0,0.31) in replacement to 18% Yb atoms at the origin. Furthermore, a relaxation of Cu atoms at z ¼ 0 close to a Yb vacancy was introduced by the authors in order to improve the reliability factor R [12]. It is worth noting that for hyperstoichiometric AB5þx-type compounds dumbbell-substitution models in CaCu5-type structures have been
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401
Table 1 YbeCu intermetallics: crystal structures and lattice parameter data ˚) Lattice parameters (A
References
YbCu6.5 Hexagonal related to the hP6-CaCu5
a ¼ 5.004, c ¼ 4.118 a ¼ 5.006, c ¼ 4.112
[12] This work
YbCu4.5 Monoclinic superstructure derived from AuBe5-type
as ¼ 48.961, bs ¼ 48.994, [14] cs ¼ 45.643, b ¼ 91.24 as ¼ 49.021, bs ¼ 48.992, This work cs ¼ 45.597, b ¼ 91.14
Phase
Symmetry/ crystal structure
YbCu3.5 Monoclinic superstructure derived from AuBe5-type?
[16]
YbCu2
Orthorhombic oI12-CeCu2 a ¼ 4.291, b ¼ 6.899, c ¼ 7.386
[11]
YbCu
Orthorhombic oP8-FeB
a ¼ 7.568, b ¼ 4.260, c ¼ 5.771
[11]
proposed by several authors (see e.g. the paper of Joubert et al. on LaNi5þx [13]). The YbCu4.5 compound melts congruently at 935 C; SEM and EPMA analyses carried out on samples close to YbCu4.5 indicate for this phase the existence of a maximum homogeneity range of about 2 at.% Yb, extending from the 1:4.5 stoichiometry towards the Yb-side (from 18.2 to 20 at.% Yb at T ¼ 825 C). The crystal structure of YbCu4.5 was described ´ and co-workers as a very complicated monoclinic by Cerny superstructure of 7448 atoms per unit cell derived from the cubic AuBe5-type structure via the introduction of planar defects parallel to {hhh} [14]. The agreement found for this compound between the X-ray powder pattern obtained in our ´ and reported by Yoshimura work and that observed by Cerny et al. [5] is very good. Moreover, we calculated the lattice ˚, parameters of the monoclinic supercell (as ¼ 49.021 A ˚ , cs ¼ 45.597 A ˚ , b ¼ 91.14 ) from the lattice bs ¼ 48.992 A
Fig. 3. Scanning electron micrograph of a 17 at.% Yb alloy annealed at 600 C for 20 days. EPMA determined the following phases: YbCu4.5 (grey phase) and YbCu6.5 (black phase).
parameters of the average monoclinic cell by multiplication with 7, 7, 6.5, as suggested by AlamieYadri and co-workers in the case of GdCu4.5 and DyCu4.5 [15]. The values obtained are in line with literature values (see in Table 1). Starting from the structural model of YbCu4.5, it was shown [16] that it is possible to construct a similar model also for RCu3.5 (R ¼ Yb, Dy) built up by AuBe5-type and MgCu2type blocks with a 1:1 ratio, that correspond to structural fragments of RCu5 and RCu2, respectively. Following this model, for the YbeCu system another monoclinic superstructure should form with an overall composition of YbCu5 þ YbCu2 ¼ Yb2Cu7. Similarly to YbCu4.5, this superstructure can be described as derived from the cubic AuBe5 type by introducing planar defect parallel to {hhh} with a supercell of ˚. 3 3 2.5 times an average monoclinic subcell of w7 A Nevertheless, because of easier synthesis conditions, a crystallographic investigation was done only on DyCu3.5 [16], and, to date, no structural data is available for YbCu3.5. In our work, likely due to the poor quality of powder XRD data, the diffraction pattern of this compound could not be indexed with the above-mentioned monoclinic subcell. Finally, the peritectic formation of YbCu3.5 at 825 C, and of YbCu2 at 760 C were confirmed by DTA and EPMA. 3.2. Cu-rich solid solutions in YbeCueT (T ¼ Ag, Au, Pd) ternary systems
Fig. 2. Scanning electron micrograph of a 10 at.% Yb alloy melted and cooled in DTA equipment at 10 C/min. EPMA determined the following phases: primary crystals of YbCu6.5 and eutectic structure [(Cu) þ YbCu6.5].
A series of alloys along section YbCu5xAux have been prepared, and some results on the formation and transport properties of the cubic AuBe5-type phase (which in the following will be indicated as t1) have already been discussed in our previous paper [17]. In the present paper we focus more on the extension of t1, which we suggest being 0.4 x 1.8, and on the phase equilibria just outside this range. Fig. 4 shows the
402
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
Fig. 4. Variation of the lattice parameter a of t1 vs x of section YbCu5xAux.
trend of the lattice parameter a of the phase t1 as a function of x of section YbCu5xAux. As already described elsewhere [17], the slight change of slope at x ¼ 1 is due to the fact that YbCu4Au is at the crossover between two kinds of disordered sublattices. In fact, as the Au concentration x is increased, the Au atoms first replace the Cu atoms on the 4c site of the F43m space group until the site is fully occupied (at x ¼ 1), and then replace the Cu atoms on the 16e site until the structure becomes unstable (at x ¼ 1.8). A comparison with the lattice parameters evaluated by Yoshimura and co-workers [3] is made in the 0.2 x 1 range. The agreement with the literature throughout this range is very good, although the trend in the region of saturation 0.2 x < 0.4 was interpreted by these authors as a positive deviation from Vegard’s law and not as a saturation value. Actually we found that, whereas inside the 0.4 x 1.8 homogeneity range samples exhibit practically single-phase microstructures, in the 0.2 x < 0.4 region the microstructures are generally more complex. A comparison between XRD measurements carried out in the two regions is reported in Fig. 5b and c. Whereas in the region 0.4 x 1.8 XRD patterns are fully indexed on the basis of the cubic AuBe5-type (see Fig. 5c), patterns of samples in the region 0.2 x < 0.4 (see Fig. 5b) display a general broadening of the Bragg peaks related to the cubic structure and, in addition, the presence of some satellite peaks around the main peaks, similarly to the pattern of the binary YbCu4.5 shown in Fig. 5a. These results indicate that in the region 0.2 x < 0.4 the cubic phase and its monoclinic superstructure coexist. Interestingly, the samples prepared above the upper limit of the homogeneity range at values of x ¼ 1.9e2.4 show the same qualitative XRD patterns of the samples prepared below the inferior limit of t1 at values of x ¼ 0.2e0.4 (compare Fig. 5b and d). In order to try to shed some light on this point, we prepared some alloys also along section Yb(Cu,Au)4.5 corresponding to the compositional line at 18.2 at.% Yb. Two samples prepared along this section with an amount of Au of 3 at.% and 6.67 at.% Au resulted to be mostly single phase with X-ray patterns very similar to that of YbCu4.5. Evaluation of the lattice parameters for the sample at 6.67 at.% Au
Fig. 5. X-ray diffraction (Cu Ka) powder patterns for (a) YbCu4.5, and selected YbCu5xAux samples: (b) at x ¼ 0.2; (c) at x ¼ 0.8; (d) at x ¼ 1.9.
(considering the average monoclinic structure and multiplying ˚ , bs ¼ 49.233 A ˚, for 7, 7, 6.5) yields the values as ¼ 49.100 A s ˚ and b ¼ 90.97 , which are in line with an inc ¼ 45.667 A crease of the unit cell volume of the YbCu4.5-based structure by substituting Cu by Au. It is worth noting that the sample prepared at 6.67 at.% Au and at slightly shifted Yb composition (16.67 instead of 18.2 at.% Yb, corresponding to YbCu4.6Au0.4) crystallizes in the cubic rather than the monoclinic structure. Moreover, XRD patterns of other samples prepared along section Yb(Cu,Au)4.5 with composition of 10, 20 and 25 at.% Au have been fully indexed with the
Fig. 6. A schematic compositional phase diagram showing the homogeneity ranges of Cu-rich solid solutions in the YbeCueAu system. t1 ¼ cubic AuBe5-type phase, t2 ¼ monoclinic YbCu4.5-based phase.
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
cubic AuBe5-type structure. These results indicate that, starting from the YbeCu binary system, Cu/Au substitution along section at 17e18 at.% Yb retains the monoclinic superstructure with the formation of a solid solution Yb(Cu,Au)4.5 (t2) up to about 7e8 at.% Au. At higher concentrations of Au, for the compositional range 0.4 < x < 1.8 of YbCu5xAux, the XRD patterns are fully indexed on the basis of the cubic AuBe5-type, whereas for further Cu/Au substitution (from 31 to 40 at.% Au) the cubic phase becomes unstable and the monoclinic superstructure appears again as the stable phase. In Fig. 6 a schematic view of the homogeneity ranges of the
403
two competing solid solutions is shown, whereas crystallographic data of the ternary alloys prepared are shown in Table 2. The two phases form sequentially increasing the amount of Au, with a small overlapping region at about 7e 8 at.% Au. With respect to the location of a possible QCP in YbCu5xAux, the data obtained in this work at ambient pressure imply that the QCP should be at a critical concentration of xc 0.4. Section YbCu5xPdx has been investigated as well. A few samples along this section were synthesized and characterized (see Table 2). All the samples prepared, including YbCu4Pd,
Table 2 Crystallographic data of the ternary alloys YbeCueT (T ¼ Ag, Au, Pd) prepared in this work Alloy nominal composition (at.%)
Phase analysis
Yb16.67Cu81.67Au1.66
YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu5xAux YbCu5xAux YbCu5xAux
AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 YbCu4.5 YbCu4.5 YbCu4.5 YbCu4.5 YbCu4.5 AuBe5 AuBe5 AuBe5
YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx YbCu4.5xAgx YbCu5xPdx YbCu4.5xPdx (Cu) YbCu5xPdx YbCu4.5xPdx (Cu) YbCu5xPdx YbCu4.5xPdx (Cu)
AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 Unknown AuBe5 Unknown AuBe5 Unknown AuBe5 YbCu4.5 AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu
Yb16.67Cu80Au3.33
Yb16.67Cu79.17Au4.16
Yb16.67Cu78.83Au4.5 Yb16.67Cu78.33Au5 Yb16.67Cu76.66Au6.67 Yb16.67Cu73.33Au10 Yb16.67Cu70Au13.33 Yb16.67Cu51.67Au31.66 Yb16.67Cu46.67Au36.66 Yb16.67Cu43.33Au40 Yb18.2Cu78.8Au3 Yb18.2Cu75.13Au6.67 Yb18.2Cu71.8Au10 Yb18.2Cu61.8Au20 Yb18.2Cu56.8Au25 Yb16.67Cu80.0Ag3.33 Yb16.67Cu76.67Ag6.66 Yb16.67Cu75.0Ag8.33 Yb16.67Cu70.0Ag13.33 Yb16.67Cu66.67Ag16.66 Yb16.67Cu63.35Ag19.98 Yb16.67Cu58.28Ag25.05 Yb16.67Cu53.36Ag29.97 Yb16Cu75.67Ag8.33 Yb19Cu72.67Ag8.33 Yb16.67Cu75Pd8.33
Yb16.67Cu66.67Pd16.66
Yb16.67Cu63.35Pd19.98
(Cu) ¼ solid solution of Au in Cu.
Structure type
Lattice parameters ˚) a (A
˚) b (A
˚) c (A
b ( )
49.233
45.667
90.97
7.00
7.00
7.000 7.000 7.003 7.018 7.028
49.100 7.019 7.064 7.118 6.998 7.022 7.028 7.051 7.055
404
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
exhibit the same qualitative microstructures and XRD patterns. In particular, the general broadening of the main peaks, the presence of satellite peaks (see Fig. 7) and the microstructure resemble the features of the samples in the 0.2 x < 0.4 region of YbCu5xAux. In fact, for section YbCu5xPdx the cubic AuBe5-type was always found to coexist with its monoclinic superstructure. The cubic phase seems not to be stable even in the case of YbCu4Pd, as it was previously found by Severing et al. in the literature [8]. This different feature of the sample synthesized at YbCu4Pd, compared with YbCu4Au and YbCu4Ag, is well illustrated in Fig. 7 where XRD patterns of YbCu4T (T ¼ Ag, Au, Pd) are shown. Samples were synthesised also along section YbCu5xAgx (see Table 2) and the existence of YbCu4Ag and, more generally, of the cubic YbCu5xAgx AuBe5-type, has been confirmed. Making a comparison with the case of Au, differences in the homogeneity range seem to be evident. In fact, samples prepared in the range 1 < x < 1.8 were not single phased and, indeed, YbCu4Ag is located at the upper limit of the cubic solid solution. Furthermore, a sample prepared at x ¼ 0.2 had practically a single phase of cubic AuBe5type structure. This is in line with the literature data of the YbCu5xAgx solid solution [5], reporting a homogeneity range 0.125 x 1 through a complete Cu/Ag substitution occurring on the 4c site of the space group F43m (N. 216). Differently from the case of Au, no further replacement of Cu by Ag occurs on the 16e site where, compared to the 4c site, a smaller space is generally available [17]. This may be due to differences in volume and electronic structures of YbCu4Au and YbCu4Ag, which exhibit different ground states in spite of the isoelectronic behaviour of Au and Ag [18,19]. Finally, similar to the case of Cu/Au, in the case of Cu/Ag substitution
the two phases Yb(Cu,Ag)5 cubic AuBe5-type and its monoclinic superstructure form sequentially with a small overlapping region (from 2 to 8 at.% Au) slightly shifted in Yb composition. In fact, XRD measurements of two samples at 8 at.% Ag, with 16 at.% and 19 at.% Yb reported in Fig. 8, show cubic AuBe5-type and YbCu4.5-based structures, respectively.
The YbeCu phase diagram was re-investigated in the Curich region. The YbCu6.5 compound (instead of YbCu5 reported in the current assessment) has been found. The two other Cu-rich phases YbCu4.5 and YbCu3.5 were confirmed. Starting from the Cu-rich region of the YbeCu phase diagram, we investigated the solid solutions into the ternary field formed by Cu/T substitutions (T ¼ Ag, Au, Pd). A strong competition on compound formation between the cubic Yb(Cu,T)5 AuBe5-type and its monoclinic superstructure Yb(Cu,T)4.5 is generally evident. In the case of Cu/Au and Cu/Ag substitutions, a Yb(Cu,T)4.5 solid solution is formed up to Au (or Ag) content of about 8 at.%. Further substitutions stabilize the YbCu5xTx (T ¼ Au, Ag), cubic AuBe5-type up to x ¼ 1 and x ¼ 1.8 in case of Ag and Au, respectively, although small regions of overlapping between the two phases, slightly shifted in Yb composition, are present for low amounts of Au and Ag. At higher concentrations of Au (from 31 to 40 at.% Au) another homogeneity region of Yb(Cu,Au)4.5 is formed. Finally, in case of Cu/Pd substitution, our results indicate the presence of a mixture of both the cubic and monoclinic phases. The investigation of homogeneity range of solid solutions is an important prerequisite in order to perform accurate research
Fig. 7. X-ray diffraction (Cu Ka) powder patterns of YbCu4T (T ¼ Ag, Au, Pd).
Fig. 8. X-ray diffraction (Cu Ka) powder patterns of (a) Yb16Cu76Ag8 and (b) Yb19Cu73Ag8.
4. Conclusion
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
of ground state properties. Therefore, a careful investigation on the minimum amount of Au necessary to stabilize, at ambient pressure, the cubic YbCu5xAux AuBe5-type was carried out, implying that a possible QCP in this system should be located at critical concentrations xc 0.4. Acknowledgements This work was done within the framework of the thematic subject ‘‘Emergent Behaviour in Correlated Matter’’ (ECOM) of the European Research Project COST P16, which is acknowledged by M.G. for financial support. References [1] He J, Tsujii N, Yoshimura K, Kosuge K, Goto T. J Phys Soc Jpn 1997;66:2481. [2] Galli M, Bauer E, Berger St, Dusek Ch, Della Mea M, Michor H, et al. Physica B 2002;312/313:489. [3] Yoshimura K, Kawabata T, Sato N, Tsujii N, Terashima T, Terakura C, et al. J Alloys Comp 2001;317e318:465. [4] Reiffers M, Idzikowski B, Sebek J, Santava E, Ilkovic S, Pristas G. Physica B 2006;378e380:738.
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[5] Yoshimura K, Tsujii N, He J, Kato M, Kosuge K, Michor H, et al. J Alloys Comp 1997;262e263:118. [6] Subramanian PR, Laughlin DE. The YbeCu system. In: Subramanian PR, Chakrabarti DJ, Laughling DE, editors. Phase diagrams of binary Cu alloys. Ohio: ASM International Metals Park; 1994. p. 482. [7] Rossel C, Yang KN, Maple MB, Fisk Z, Zirngiebl E, Thompson JD. Phys Rev B 1987;35:1914. [8] Severing A, Murani AP, Thompson JD, Fisk Z, Loong CK. Phys Rev B 1990;41:1739. [9] Saccone A, Cardinale AM, Delfino S, Ferro R. Z Metallkd 2001;92:959. [10] Giovannini M, Saccone A, Rogl P, Ferro R. Intermetallics 2003;11:197. [11] Iandelli A, Palenzona A. J Less-Common Met 1971;25:333. [12] Hornstra J, Buschow KHJ. J Less-Common Met 1972;27:123. ´ R, Latroche M, Leroy E, Gue´ne´e L, Percheron[13] Joubert JM, Cerny Gue´gan A, et al. J Solid State Chem 2002;166:1. ´ R, Franc¸ois M, Yvon K, Jaccard D, Walker E, Petrˇ´ıcek V, et al. J [14] Cerny Phys: Condens Matter 1996;8:4485. [15] Alami-Yadri K, Jaccard D, Link P. Phys Lett A 1996;212:227. ´ R, Gue´ne´e L, Wessicken R. Solid State Chem 2003;174:125. [16] Cerny [17] Giovannini M, Saccone A, Mu¨ller St, Michor H, Bauer E. J Phys: Condens Matter 2005;17:S877. [18] Indiger A, Bauer E, Gratz E, Hauser R, Hilscher G, Holubar T. Physica B 1995;206e207:349. [19] Monachesi P, Continenza A. Phys Rev B 1996;54:13558.
Intermetallics 16 (2008) 399e405 www.elsevier.com/locate/intermet
Yb(Cu,T)5 and Yb(Cu,T)4.5 solid solutions (T ¼ Ag, Au, Pd) M. Giovannini a,b,*, R. Pasero a,b, S. De Negri a, A. Saccone a a
Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, Via Dodecaneso 31, I-16146 Genova, Italy b LAMIA-INFM-CNR, Corso Perrone 24, I-16152 Genova, Italy Received 30 July 2007; received in revised form 28 November 2007; accepted 30 November 2007 Available online 22 January 2008
Abstract The binary YbeCu phase diagram was re-investigated in the Cu-rich part. Starting from this region, solid solutions formed by substitution of Cu by Ag, Au, and Pd have been investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A competition in compound formation between Yb(Cu,T)5 (T ¼ Ag, Au, Pd), cubic AuBe5-type and its monoclinic superstructure Yb(Cu,T)4.5 is generally evident. Whereas for Cu/Au and Cu/Ag substitutions deeply-extended Yb(Cu,T)5 solid solutions are formed, in the case of substitution of Cu by Pd the cubic and monoclinic phases coexist. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Rare-earth intermetallics; B. Crystal chemistry of intermetallics; F. Diffraction; F. Electron microscopy, scanning
1. Introduction Several aspects make Yb compounds attractive for research studies, such as valence fluctuations and other low-temperature anomalies. Recently a strong emphasis was given to the investigation of the solid solutions YbCu5xMx (M ¼ Ag, Au, In) crystallizing in the cubic AuBe5-type structure, where the substitution of Cu by M offers the possibility to study the different evolution of ground state properties depending on the M element, starting from dense Kondo behaviour for low xvalues. In fact, whereas YbCu5xAgx exhibits Kondo lattice behaviour in the whole substitution range 0 < x 1, Cu/In and Cu/Au substitutions drive the system through a crossover to a valence fluctuation system [1] and to long-range magnetic order [2,3], respectively. In particular, for the YbCu5xAux system a quantum phase transition seems to occur, although an agreement on the precise location of the quantum critical point (QCP) has still to be found (xc ¼ 0.2e0.4 [3], xc ¼ 0.5 [2]). The common starting point (at x ¼ 0) of these solid * Corresponding author. Dipartimento di Chimica e Chimica Industriale, Universita` di Genova, Via Dodecaneso 31, I-16146 Genova, Italy. Tel.: þ39 010 3536648; fax: þ39 010 3625051. E-mail address: [email protected] (M. Giovannini). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.11.010
solutions, YbCu5, has been recently synthesized in the cubic AuBe5-type only by melt-spinning technique [4] or under high pressure [5]. Furthermore, the current assessment of the binary YbeCu system [6] reports the existence of YbCu5 crystallizing in the CaCu5 structure, although it is now well established that YbCu5 does not form at equilibrium under ambient pressure [5]. Concerning the formation of the solid solutions Yb(Cu,T)5 with the transition metals T ¼ Au and Ag, the interest in this topic was triggered by the investigation on the heavy fermions YbCu4T (T ¼ Ag, Au and Pd) done by Rossel and co-workers [7]. These compounds crystallize in the MgCu4Sn structure (an ordered derivative of the AuBe5-type). It was found that the compounds with T ¼ Au and Pd order magnetically below 1 K, whereas YbCu4Ag has a nonmagnetic ground state. In another investigation carried out on these three compounds by Severing and co-workers [8], from X-ray and neutrondiffraction measurements it was observed that, whereas YbCu4Au and YbCu4Ag are cubic AuBe5-type single phase, YbCu4Pd contains other extra-peaks and, in addition, the Bragg lines are strongly broadened. Later on, it was recognized that YbCu4Au and YbCu4Ag are points of crystallographic order of YbCu5xTx solid solutions, whereas, up to the present time, no detailed information has been given on
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
3. Results and discussion 3.1. Cu-rich region of the YbeCu phase diagram In the assessed phase diagram of the YbeCu system [6], mainly based on the phase relations determined by Iandelli and Palenzona [11], the following intermediate phases are reported: YbCu5, Yb2Cu9, Yb2Cu7 (the last two compounds, in the following indicated as YbCu4.5 and YbCu3.5, are reported with unknown crystal structure), YbCu2 and YbCu. All the compounds have peritectic formation except YbCu4.5 which forms congruently at 937 C. According to this assessment, YbCu5, crystallizing in the CaCu5 structure, is the Cu-richest compound, although Hornstra and Buschow [12] previously observed the formation of a compound of stoichiometry YbCu6.5 instead of YbCu5, crystallizing in a structure closely related to that of CaCu5. These authors did not report the temperature range of stability of YbCu6.5 and, in addition, phases with this stoichiometry had not been found for any of the other lanthanides. Probably due to these reasons, the compound was not reported in the assessed YbeCu phase diagram [6]. Fig. 1 shows the partial YbeCu phase diagram determined in our investigation on the basis of differential thermal analysis (DTA), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Table 1 reports the crystal data obtained
40
30
50
60
1100 1084.8
1000 935 900
870
850
825 800 760
700
YbCu2
YbCu3.5
The metals used were Yb of 99.9 mass% purity, and Cu, Ag, Au, Pd of 99.99 mass% purity. Ingot of pure elements for a total weight of about 1.0 g were enclosed in small tantalum crucibles, sealed by arc welding under pure argon, in order to prevent mass loss due to the high vapour pressure of Yb. The samples were then melted and remelted (in order to attain proper homogenization) in an induction furnace under continuous shaking of the crucibles in a stream of pure argon. Subsequently, for ternary samples a heat treatment at T ¼ 700 C for 10 days in argon atmosphere was carried out. X-ray diffraction (XRD) analysis (Cu Ka radiation), optical and scanning electronic microscopy (SEM), electron probe microanalysis (EPMA) and, in some cases, differential thermal analysis (DTA) were used to characterize the samples. The details of the measurement techniques were fully described in previous papers [9,10].
20
YbCu4.5
2. Experimental details
mass % Yb 10
YbCu6.5
the crystal structure of YbCu4Pd and the possible formation of an analogous solid solution with T ¼ Pd. Moreover, a careful investigation on the minimum amount of Au necessary to stabilize the cubic YbCu5xAux AuBe5-type solid solution is still missing. This is an important issue to be addressed before coming to any conclusion about the precise location of the QCP in YbCu5xAux. The present paper reports our investigation on the Cu-rich solid solutions in the ternary systems YbeCueT (T ¼ Ag, Au, Pd). An update of the Cu-rich region of the YbeCu phase diagram is also reported, including the fact that YbCu5 does not exist at equilibrium under ambient pressure.
Temperature (°C)
400
600
500 Cu
10
20
30
40
at. % Yb Fig. 1. YbeCu system in the Cu-rich region. Phase diagram under ambient pressure revised in the composition range between 10 and 24 at.% Yb.
in this work for the YbeCu intermediate compounds, together with those from the literature. Some differences emerge by the comparison of this phase diagram with the current assessment of the YbeCu system. In our investigation the Cu-richest compound resulted to be YbCu6.5 rather than YbCu5. It presents peritectic formation at 870 C and it forms with Cu an eutectic at 850 C and 10 at.% Yb. The micrograph of an alloy at 10 at.% Yb, showing primary crystallization of YbCu6.5 in presence of eutectic structure, is reported in Fig. 2. YbCu6.5 does not show any evidence of decomposition, as proved by DTA and microscopy examinations of samples subject to DTA and annealing. In particular, an alloy with 17 at.% Yb observed after annealing for 20 days at T ¼ 600 C resulted to be constituted by the two phases YbCu6.5 and YbCu4.5 (see the appearance of the microphotograph in Fig. 3). From our investigation it was confirmed that YbCu5 does not exist and YbCu6.5 is the Cu-richest stable compound. The X-ray pattern of YbCu6.5 was indexed on the basis of a hexagonal ˚ c ¼ 4.112 A ˚ . These lattice parameunit cell with a ¼ 5.006 A ter values are in a good agreement with the values ˚ , c ¼ 4.118 A ˚ ) obtained by Hornstra and Buschow (a ¼ 5.004 A [12]. The crystal structure of YbCu6.5 has been determined by these authors by difference Fourier synthesis of powder data. It is related to the CaCu5-structure with Yb on the 1a site and Cu atoms on the 2c and 3g sites of the space group P6/mmm (N. 191). The excess of Cu is constituted by pairs of Cu atoms located on the 2e site at (0,0,0.31) in replacement to 18% Yb atoms at the origin. Furthermore, a relaxation of Cu atoms at z ¼ 0 close to a Yb vacancy was introduced by the authors in order to improve the reliability factor R [12]. It is worth noting that for hyperstoichiometric AB5þx-type compounds dumbbell-substitution models in CaCu5-type structures have been
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401
Table 1 YbeCu intermetallics: crystal structures and lattice parameter data ˚) Lattice parameters (A
References
YbCu6.5 Hexagonal related to the hP6-CaCu5
a ¼ 5.004, c ¼ 4.118 a ¼ 5.006, c ¼ 4.112
[12] This work
YbCu4.5 Monoclinic superstructure derived from AuBe5-type
as ¼ 48.961, bs ¼ 48.994, [14] cs ¼ 45.643, b ¼ 91.24 as ¼ 49.021, bs ¼ 48.992, This work cs ¼ 45.597, b ¼ 91.14
Phase
Symmetry/ crystal structure
YbCu3.5 Monoclinic superstructure derived from AuBe5-type?
[16]
YbCu2
Orthorhombic oI12-CeCu2 a ¼ 4.291, b ¼ 6.899, c ¼ 7.386
[11]
YbCu
Orthorhombic oP8-FeB
a ¼ 7.568, b ¼ 4.260, c ¼ 5.771
[11]
proposed by several authors (see e.g. the paper of Joubert et al. on LaNi5þx [13]). The YbCu4.5 compound melts congruently at 935 C; SEM and EPMA analyses carried out on samples close to YbCu4.5 indicate for this phase the existence of a maximum homogeneity range of about 2 at.% Yb, extending from the 1:4.5 stoichiometry towards the Yb-side (from 18.2 to 20 at.% Yb at T ¼ 825 C). The crystal structure of YbCu4.5 was described ´ and co-workers as a very complicated monoclinic by Cerny superstructure of 7448 atoms per unit cell derived from the cubic AuBe5-type structure via the introduction of planar defects parallel to {hhh} [14]. The agreement found for this compound between the X-ray powder pattern obtained in our ´ and reported by Yoshimura work and that observed by Cerny et al. [5] is very good. Moreover, we calculated the lattice ˚, parameters of the monoclinic supercell (as ¼ 49.021 A ˚ , cs ¼ 45.597 A ˚ , b ¼ 91.14 ) from the lattice bs ¼ 48.992 A
Fig. 3. Scanning electron micrograph of a 17 at.% Yb alloy annealed at 600 C for 20 days. EPMA determined the following phases: YbCu4.5 (grey phase) and YbCu6.5 (black phase).
parameters of the average monoclinic cell by multiplication with 7, 7, 6.5, as suggested by AlamieYadri and co-workers in the case of GdCu4.5 and DyCu4.5 [15]. The values obtained are in line with literature values (see in Table 1). Starting from the structural model of YbCu4.5, it was shown [16] that it is possible to construct a similar model also for RCu3.5 (R ¼ Yb, Dy) built up by AuBe5-type and MgCu2type blocks with a 1:1 ratio, that correspond to structural fragments of RCu5 and RCu2, respectively. Following this model, for the YbeCu system another monoclinic superstructure should form with an overall composition of YbCu5 þ YbCu2 ¼ Yb2Cu7. Similarly to YbCu4.5, this superstructure can be described as derived from the cubic AuBe5 type by introducing planar defect parallel to {hhh} with a supercell of ˚. 3 3 2.5 times an average monoclinic subcell of w7 A Nevertheless, because of easier synthesis conditions, a crystallographic investigation was done only on DyCu3.5 [16], and, to date, no structural data is available for YbCu3.5. In our work, likely due to the poor quality of powder XRD data, the diffraction pattern of this compound could not be indexed with the above-mentioned monoclinic subcell. Finally, the peritectic formation of YbCu3.5 at 825 C, and of YbCu2 at 760 C were confirmed by DTA and EPMA. 3.2. Cu-rich solid solutions in YbeCueT (T ¼ Ag, Au, Pd) ternary systems
Fig. 2. Scanning electron micrograph of a 10 at.% Yb alloy melted and cooled in DTA equipment at 10 C/min. EPMA determined the following phases: primary crystals of YbCu6.5 and eutectic structure [(Cu) þ YbCu6.5].
A series of alloys along section YbCu5xAux have been prepared, and some results on the formation and transport properties of the cubic AuBe5-type phase (which in the following will be indicated as t1) have already been discussed in our previous paper [17]. In the present paper we focus more on the extension of t1, which we suggest being 0.4 x 1.8, and on the phase equilibria just outside this range. Fig. 4 shows the
402
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
Fig. 4. Variation of the lattice parameter a of t1 vs x of section YbCu5xAux.
trend of the lattice parameter a of the phase t1 as a function of x of section YbCu5xAux. As already described elsewhere [17], the slight change of slope at x ¼ 1 is due to the fact that YbCu4Au is at the crossover between two kinds of disordered sublattices. In fact, as the Au concentration x is increased, the Au atoms first replace the Cu atoms on the 4c site of the F43m space group until the site is fully occupied (at x ¼ 1), and then replace the Cu atoms on the 16e site until the structure becomes unstable (at x ¼ 1.8). A comparison with the lattice parameters evaluated by Yoshimura and co-workers [3] is made in the 0.2 x 1 range. The agreement with the literature throughout this range is very good, although the trend in the region of saturation 0.2 x < 0.4 was interpreted by these authors as a positive deviation from Vegard’s law and not as a saturation value. Actually we found that, whereas inside the 0.4 x 1.8 homogeneity range samples exhibit practically single-phase microstructures, in the 0.2 x < 0.4 region the microstructures are generally more complex. A comparison between XRD measurements carried out in the two regions is reported in Fig. 5b and c. Whereas in the region 0.4 x 1.8 XRD patterns are fully indexed on the basis of the cubic AuBe5-type (see Fig. 5c), patterns of samples in the region 0.2 x < 0.4 (see Fig. 5b) display a general broadening of the Bragg peaks related to the cubic structure and, in addition, the presence of some satellite peaks around the main peaks, similarly to the pattern of the binary YbCu4.5 shown in Fig. 5a. These results indicate that in the region 0.2 x < 0.4 the cubic phase and its monoclinic superstructure coexist. Interestingly, the samples prepared above the upper limit of the homogeneity range at values of x ¼ 1.9e2.4 show the same qualitative XRD patterns of the samples prepared below the inferior limit of t1 at values of x ¼ 0.2e0.4 (compare Fig. 5b and d). In order to try to shed some light on this point, we prepared some alloys also along section Yb(Cu,Au)4.5 corresponding to the compositional line at 18.2 at.% Yb. Two samples prepared along this section with an amount of Au of 3 at.% and 6.67 at.% Au resulted to be mostly single phase with X-ray patterns very similar to that of YbCu4.5. Evaluation of the lattice parameters for the sample at 6.67 at.% Au
Fig. 5. X-ray diffraction (Cu Ka) powder patterns for (a) YbCu4.5, and selected YbCu5xAux samples: (b) at x ¼ 0.2; (c) at x ¼ 0.8; (d) at x ¼ 1.9.
(considering the average monoclinic structure and multiplying ˚ , bs ¼ 49.233 A ˚, for 7, 7, 6.5) yields the values as ¼ 49.100 A s ˚ and b ¼ 90.97 , which are in line with an inc ¼ 45.667 A crease of the unit cell volume of the YbCu4.5-based structure by substituting Cu by Au. It is worth noting that the sample prepared at 6.67 at.% Au and at slightly shifted Yb composition (16.67 instead of 18.2 at.% Yb, corresponding to YbCu4.6Au0.4) crystallizes in the cubic rather than the monoclinic structure. Moreover, XRD patterns of other samples prepared along section Yb(Cu,Au)4.5 with composition of 10, 20 and 25 at.% Au have been fully indexed with the
Fig. 6. A schematic compositional phase diagram showing the homogeneity ranges of Cu-rich solid solutions in the YbeCueAu system. t1 ¼ cubic AuBe5-type phase, t2 ¼ monoclinic YbCu4.5-based phase.
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
cubic AuBe5-type structure. These results indicate that, starting from the YbeCu binary system, Cu/Au substitution along section at 17e18 at.% Yb retains the monoclinic superstructure with the formation of a solid solution Yb(Cu,Au)4.5 (t2) up to about 7e8 at.% Au. At higher concentrations of Au, for the compositional range 0.4 < x < 1.8 of YbCu5xAux, the XRD patterns are fully indexed on the basis of the cubic AuBe5-type, whereas for further Cu/Au substitution (from 31 to 40 at.% Au) the cubic phase becomes unstable and the monoclinic superstructure appears again as the stable phase. In Fig. 6 a schematic view of the homogeneity ranges of the
403
two competing solid solutions is shown, whereas crystallographic data of the ternary alloys prepared are shown in Table 2. The two phases form sequentially increasing the amount of Au, with a small overlapping region at about 7e 8 at.% Au. With respect to the location of a possible QCP in YbCu5xAux, the data obtained in this work at ambient pressure imply that the QCP should be at a critical concentration of xc 0.4. Section YbCu5xPdx has been investigated as well. A few samples along this section were synthesized and characterized (see Table 2). All the samples prepared, including YbCu4Pd,
Table 2 Crystallographic data of the ternary alloys YbeCueT (T ¼ Ag, Au, Pd) prepared in this work Alloy nominal composition (at.%)
Phase analysis
Yb16.67Cu81.67Au1.66
YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux (Cu) YbCu5xAux YbCu4.5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu4.5xAux YbCu5xAux YbCu5xAux YbCu5xAux
AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 YbCu4.5 YbCu4.5 YbCu4.5 YbCu4.5 YbCu4.5 AuBe5 AuBe5 AuBe5
YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx Yb19Cu18Ag63 YbCu5xAgx YbCu4.5xAgx YbCu5xPdx YbCu4.5xPdx (Cu) YbCu5xPdx YbCu4.5xPdx (Cu) YbCu5xPdx YbCu4.5xPdx (Cu)
AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 AuBe5 Unknown AuBe5 Unknown AuBe5 Unknown AuBe5 YbCu4.5 AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu AuBe5 YbCu4.5 Cu
Yb16.67Cu80Au3.33
Yb16.67Cu79.17Au4.16
Yb16.67Cu78.83Au4.5 Yb16.67Cu78.33Au5 Yb16.67Cu76.66Au6.67 Yb16.67Cu73.33Au10 Yb16.67Cu70Au13.33 Yb16.67Cu51.67Au31.66 Yb16.67Cu46.67Au36.66 Yb16.67Cu43.33Au40 Yb18.2Cu78.8Au3 Yb18.2Cu75.13Au6.67 Yb18.2Cu71.8Au10 Yb18.2Cu61.8Au20 Yb18.2Cu56.8Au25 Yb16.67Cu80.0Ag3.33 Yb16.67Cu76.67Ag6.66 Yb16.67Cu75.0Ag8.33 Yb16.67Cu70.0Ag13.33 Yb16.67Cu66.67Ag16.66 Yb16.67Cu63.35Ag19.98 Yb16.67Cu58.28Ag25.05 Yb16.67Cu53.36Ag29.97 Yb16Cu75.67Ag8.33 Yb19Cu72.67Ag8.33 Yb16.67Cu75Pd8.33
Yb16.67Cu66.67Pd16.66
Yb16.67Cu63.35Pd19.98
(Cu) ¼ solid solution of Au in Cu.
Structure type
Lattice parameters ˚) a (A
˚) b (A
˚) c (A
b ( )
49.233
45.667
90.97
7.00
7.00
7.000 7.000 7.003 7.018 7.028
49.100 7.019 7.064 7.118 6.998 7.022 7.028 7.051 7.055
404
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
exhibit the same qualitative microstructures and XRD patterns. In particular, the general broadening of the main peaks, the presence of satellite peaks (see Fig. 7) and the microstructure resemble the features of the samples in the 0.2 x < 0.4 region of YbCu5xAux. In fact, for section YbCu5xPdx the cubic AuBe5-type was always found to coexist with its monoclinic superstructure. The cubic phase seems not to be stable even in the case of YbCu4Pd, as it was previously found by Severing et al. in the literature [8]. This different feature of the sample synthesized at YbCu4Pd, compared with YbCu4Au and YbCu4Ag, is well illustrated in Fig. 7 where XRD patterns of YbCu4T (T ¼ Ag, Au, Pd) are shown. Samples were synthesised also along section YbCu5xAgx (see Table 2) and the existence of YbCu4Ag and, more generally, of the cubic YbCu5xAgx AuBe5-type, has been confirmed. Making a comparison with the case of Au, differences in the homogeneity range seem to be evident. In fact, samples prepared in the range 1 < x < 1.8 were not single phased and, indeed, YbCu4Ag is located at the upper limit of the cubic solid solution. Furthermore, a sample prepared at x ¼ 0.2 had practically a single phase of cubic AuBe5type structure. This is in line with the literature data of the YbCu5xAgx solid solution [5], reporting a homogeneity range 0.125 x 1 through a complete Cu/Ag substitution occurring on the 4c site of the space group F43m (N. 216). Differently from the case of Au, no further replacement of Cu by Ag occurs on the 16e site where, compared to the 4c site, a smaller space is generally available [17]. This may be due to differences in volume and electronic structures of YbCu4Au and YbCu4Ag, which exhibit different ground states in spite of the isoelectronic behaviour of Au and Ag [18,19]. Finally, similar to the case of Cu/Au, in the case of Cu/Ag substitution
the two phases Yb(Cu,Ag)5 cubic AuBe5-type and its monoclinic superstructure form sequentially with a small overlapping region (from 2 to 8 at.% Au) slightly shifted in Yb composition. In fact, XRD measurements of two samples at 8 at.% Ag, with 16 at.% and 19 at.% Yb reported in Fig. 8, show cubic AuBe5-type and YbCu4.5-based structures, respectively.
The YbeCu phase diagram was re-investigated in the Curich region. The YbCu6.5 compound (instead of YbCu5 reported in the current assessment) has been found. The two other Cu-rich phases YbCu4.5 and YbCu3.5 were confirmed. Starting from the Cu-rich region of the YbeCu phase diagram, we investigated the solid solutions into the ternary field formed by Cu/T substitutions (T ¼ Ag, Au, Pd). A strong competition on compound formation between the cubic Yb(Cu,T)5 AuBe5-type and its monoclinic superstructure Yb(Cu,T)4.5 is generally evident. In the case of Cu/Au and Cu/Ag substitutions, a Yb(Cu,T)4.5 solid solution is formed up to Au (or Ag) content of about 8 at.%. Further substitutions stabilize the YbCu5xTx (T ¼ Au, Ag), cubic AuBe5-type up to x ¼ 1 and x ¼ 1.8 in case of Ag and Au, respectively, although small regions of overlapping between the two phases, slightly shifted in Yb composition, are present for low amounts of Au and Ag. At higher concentrations of Au (from 31 to 40 at.% Au) another homogeneity region of Yb(Cu,Au)4.5 is formed. Finally, in case of Cu/Pd substitution, our results indicate the presence of a mixture of both the cubic and monoclinic phases. The investigation of homogeneity range of solid solutions is an important prerequisite in order to perform accurate research
Fig. 7. X-ray diffraction (Cu Ka) powder patterns of YbCu4T (T ¼ Ag, Au, Pd).
Fig. 8. X-ray diffraction (Cu Ka) powder patterns of (a) Yb16Cu76Ag8 and (b) Yb19Cu73Ag8.
4. Conclusion
M. Giovannini et al. / Intermetallics 16 (2008) 399e405
of ground state properties. Therefore, a careful investigation on the minimum amount of Au necessary to stabilize, at ambient pressure, the cubic YbCu5xAux AuBe5-type was carried out, implying that a possible QCP in this system should be located at critical concentrations xc 0.4. Acknowledgements This work was done within the framework of the thematic subject ‘‘Emergent Behaviour in Correlated Matter’’ (ECOM) of the European Research Project COST P16, which is acknowledged by M.G. for financial support. References [1] He J, Tsujii N, Yoshimura K, Kosuge K, Goto T. J Phys Soc Jpn 1997;66:2481. [2] Galli M, Bauer E, Berger St, Dusek Ch, Della Mea M, Michor H, et al. Physica B 2002;312/313:489. [3] Yoshimura K, Kawabata T, Sato N, Tsujii N, Terashima T, Terakura C, et al. J Alloys Comp 2001;317e318:465. [4] Reiffers M, Idzikowski B, Sebek J, Santava E, Ilkovic S, Pristas G. Physica B 2006;378e380:738.
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