Experimental study of the CeNi5–CeCu5 system

Intermetallics 14 (2006) 695–701 www.elsevier.com/locate/intermet

Experimental study of the CeNi5–CeCu5 system J. Wang a,b,*, J.L. Jorda a, A. Pisch c, R. Flu¨kiger b a LAIMAN-ESIA Universite´ de Savoie, BP806, F-74016 Annecy Cedex, France Department of Condensed Matter Physics, University of Geneva, 24 quai E. Ansermet, Geneva CH-1211, Switzerland c Laboratoire de Thermodynamique et Physico-Chimie Me´tallurgiques (UMR 5614 CNRS/INPG/UJF), E.N.S.E.E.G., BP.75, 38402 Saint Martin d’He`res, France b

Received 23 May 2005; received in revised form 30 October 2005; accepted 4 November 2005 Available online 4 January 2006

Abstract The Ce(Ni1KxCux)5 system has been studied by X-ray diffraction, DTA, aluminium drop solution calorimetry, optical and electron microscopy coupled to an EDX analysis system. The existence of a continuous, ideal solid solution Ce(Ni1KxCux)5 has been confirmed and a temperaturecomposition section is given. In addition, it was found that a Ni solubility exists in the solid solution shifting the composition to the Ce-poor direction. No extended Cu solubility has been observed. q 2005 Elsevier Ltd. All rights reserved. Keywords: B. Crystallography; B. Solid-solution; B. Phase diagrams; F. Calorimetry

1. Introduction The Ce(Ni1KxCux)5 section of the ternary Ce–Ni–Cu system has been studied in the past to investigate magnetic properties which are expected to follow the valence fluctuations of the cerium as copper substitutes nickel and also because alloys in this section exhibit a high rate of hydrogen absorption. A first investigation of the thermal dependence of the susceptibility of Ce(Ni,Cu)5 by Gignoux et al. [1] suggested a valence change of cerium in a defined Cu composition range. By combining susceptibility measurements and cell size analysis, Pedziwiatr et al. [2] separated a Curie–Weiss contribution associated with the Ce3C ions to the paramagnetic behavior of samples in which more than 40% of copper substitute Ni. From a thermodynamic point of view, the enthalpies of formation of compounds over the range with xZ0.2, 0.4, 0.6 and 0.8 have been studied by Meyer-Liautaud et al. [3] for hydrogen storage ability. These studies agree with the existence of a continuous solid solution covering all the compositional range. There is however no systematic study on the phase relations in the system CeNi5–CeCu5, * Corresponding author. Address: LAIMAN-ESIA Universite´ de Savoie, BP806, F-74016 Annecy Cedex, France. Tel.: C33 4 5009 6518; fax: C33 4 5009 6649. E-mail address: [email protected] (J. Wang).

0966-9795/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2005.11.007

especially concerning the high temperature solid–liquid equilibrium. We thus decided to reinvestigate this solid solution. 2. Experimental The samples were prepared by arc melting under a pure argon atmosphere. The processing was repeated a number of times (at least four times) to ensure good mixing. The purities of the starting elements were 99.9 wt% for Cu (CERAC) and Ce (ALDRICH) and 99.95 wt% for Ni (ALFA AESAR). After melting all samples were annealed in the solid state for two weeks at two different temperatures (750 and 800 8C). Specimens to be annealed were encapsulated in quartz tubes with a residual argon pressure less than 300 mbar at room temperature. The tubes were then placed in an Al2O3 crucible located at the center of a horizontal tube furnace. With this arrangement a uniform temperature zone of G3 8C existed over the length of the samples. Differential thermal analysis accompanied with thermogravimetric measurement (DTA/TG) was carried out in argon atmosphere and alumina crucibles with a heating/cooling rate of 10 K/min. No oxidation or other reaction layer has been observed on the samples after a DTA run, even when a liquid phase appeared. The TG curves ensured that the samples did not oxidize during the thermal cycles. All samples in equilibrium were characterized by X-ray Diffraction (XRD). Indexing of the diffraction peaks, refinement of the cell

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J. Wang et al. / Intermetallics 14 (2006) 695–701

Table 1 Sample composition in Ce(Ni1KxCux)5 system Sample analysis (at.%)

Ce

Ni

Cu

Ce

Ni

Cu

16.67 16.67 16.67 16.67 16.67 16.67 16.67 16.67

83.33 66.67 50.00 41.67 33.34 16.67 8.33 0

0 16.67 33.33 41.66 50.00 66.66 75.0 83.33

16.7 16.9 16.5 16.9 16.7 16.6 16.4 16.5

83.3 67.4 50.9 41.8 34.1 17.0 8.3 0

0 15.7 32.6 41.3 49.2 66.4 75.3 83.5

3. Results and discussions The nominal and analysed compositions of all annealed samples studied in this report are given in Table 1. The samples were annealed for 2 weeks at 750 8C for Cu-rich compositions and at 800 8C for Ni-rich compositions. The analytical results are in good agreement with the nominal composition (difference less than 1 at.%). 3.1. Crystal structure analysis The XRD patterns of the annealed samples are presented in Fig. 1. All samples have the CaCu5-type structure indicating that, at least in the high temperature range 500–750 8C for which diffusion is efficient, Cu may completely substitute Ni in the Ce(Ni1KxCux)5 section. For the nickel-free compound, however, CeCu6 was detected as an impurity phase, persisting during the peritectic formation of CeCu5: LCCeCu6ZCeCu5 at 798 8C [6]. The crystal structure has been refined and the a and c lattice parameters have been extracted. They are given in Fig. 2. Our

*

+ Ce(Ni,Cu)6 (201)

*

3.0

* Ce(Ni,Cu)5

* (002)

*

(220)

3.5

(111)

4.0 (110)

parameters and quantitative analysis were obtained with the ‘Fullprof’ computer program. Optical microscopy and scanning electron microscopy (SEM) complemented the XRD analysis. The chemical compositions of the samples have been determined with an accuracy of 1 at.% by EDX using the Ka radiation line for Ni and Cu and the La radiation for Ce. Drop solution calorimetry using a liquid aluminium bath has been used to determine the enthalpies of formation of the Ce(Ni1KxCux)5 solid solution. The equipment used for the measurement is a double cell high-temperature Calvet type microcalorimeter (SETARAM HT1000) [4]. The calorimeter cells consist of silica tubes in which high density alumina crucibles are inserted. The alumina crucibles have been heated at 1000 8C for 24 h prior to the measurement in order to evaporate all traces of water and organic solvents. The calorimetric experiments have been performed under a slight Ar overpressure to prevent any air contamination due to leakage of the system. Additionally, titanium getters were placed just above the alumina crucibles inside the silica tubes. The heat effects inside the calorimeter are measured as a change of emf given by the detector, integrated with respect to time. The emf was recorded every 0.5 s using a KEITHLEY high sensitive voltmeter connected to a PC by an IEEE-488 interface. The area underneath the thermogram was integrated using own software which takes into account a possible baseline drift during the measurement. For the dissolution experiment, pure aluminum (99.999% Pechiney) has been used as a solvent. Calibration of the calorimeter has been performed by dropping small pieces of pure aluminum (30–40 mg each) from room temperature into the aluminum bath, which was held at 722 8C. The total amount of the aluminum bath was around 11 g (0.4 mol). The heat content of aluminum between 25 and 722 8C (30,793.4 J/mol) has been taken from the SGTE pure element database [5]. The measurement is considered as good, when the standard deviation of the calibration constant is below 1%. The bulk Ce(Ni1KxCux)5 alloy has been cut into small pieces and samples have been taken from the center to avoid any oxide contaminated material. The weight of the dropped samples ranged from 15 to 20 mg. For each composition, a minimum of seven samples have been dissolved.

(101)

0.0 0.2 0.4 0.5 0.6 0.8 0.9 1.0

Nominal composition (at.%)

Relative intensity

Sample (x)

*

*

x=0.0

2.5

x=0.2

2.0

x=0.5

1.5

x=0.8

1.0

x=0.9

0.5 +

++

+ +

+

+ +

+

+

+ x=1.0

0.0 25

30

35

40

45

50

55

2θ (°) ˚ ), of annealed Fig. 1. X-ray patterns using the Cu(Ka) radiation (lZ1.5406 A Ce(Ni1KxCux)5 samples. The annealing temperatures are 800 8C for x%0.8 and 750 8C for xR0.9.

J. Wang et al. / Intermetallics 14 (2006) 695–701

50 Present data Pedziwiatr data Meyer-Liautaud Data

0.510

x=0.0

40

x=0.2

30 normalized Heat flow

Lattice parameter a (nm)

0.515

0.505 0.500 0.495 0.490 0.485

x=0.4

20

x=0.5 x=0.6

10 x=0.8

0

x=0.9

–10

x=1.0

–20 –30

795°C

–40 0.0

0.2

0.4

0.6

0.8

1.0

X value of Ce(Ni1-xCux)5

–50 700

800

900

1000 1100 1200 Temperature (°C)

1300

1400

Fig. 3. DTA of Ce(Ni1KxCux)5 samples.

0.412

Lattice parameter c (nm)

697

0.410 Present data Pedziwiatr data Mey-Liautaud data

0.408 0.406 0.404 0.402 0.400 0.398 0.0

0.2

0.4

0.6

0.8

1.0

X value of Ce(Ni1-xCux)5 Fig. 2. Lattice parameters a and c as a function of x in the Ce(Ni1KxCux)5 samples.

results are in good agreement with the data of Pedziwiatr et al. [2] and Meyer-Liautaud et al. [3]. As expected from Vegard’s law, it is seen that the a and c cell parameters increase as Ni is substituted by Cu. However, a pronounced discontinuity for xZw0.55 is observed in the c(x) relation.. According to Gignoux et al. [1], there is no ordering on the 2 M sites of the RM5 type-structure (RZRare Earth) when Cu substitutes Ni in the Ce(Ni1KxCux)5 solid solution indicating that the irregular variation of the c-parameter with x can be only ascribed to the same change of the Ce radius. Such a variation might imply that the valence change of Ce happens in a defined Cu composition range in which the two valence states of Ce could co-exist. This assumption was confirmed by Pedziwiatr et al. [2]. 3.2. Isopletic section The high temperature behavior of the Ce(Ni1KxCux)5 alloys is displayed in the thermograms shown in Fig. 3 which represent the DTA of samples with xZ0.0, 0.2, 0.4, 0.5, 0.6,

0.8, 0.9 and 1. The solidus and liquidus lines may be clearly separated. Up to a composition with xZ0.8, these lines are representative of the existence of a lens-type two-phase field, liquidCCe(Ni1KxCux)5. For xO0.8 the high temperature equilibrium includes the CeCu6 phase as suggested by the presence of four endothermic reactions at 918, 928, 937 and 980 8C for Ce(Ni0.1Cu0.9)5 and also by the SEM micrograph in Fig. 4 of the same sample. In the cast state, the two phases, Ce(Ni,Cu)5 and Ce(Ni,Cu)6 have been unambiguously identified (Fig. 4a) whereas after annealing at 750 8C, only the Ce(Ni,Cu)5 phase can be observed (Fig. 4b). It has to be noticed that for Ni-rich compounds (x!0.8), cast alloys appear to be single phase with the composition similar to the nominal one. For xZ1.0 the invariant reaction at 795 8C and a liquidus temperature of 940 8C confirms the peritectic formation of CeCu5 from the liquid and CeCu6 [6]. This analysis leads to the temperature– composition section represented in Fig. 5a. In this figure, the copper-rich field contains a three-phase field equilibrium: liquidCCe(Ni,Cu)5CCe(Ni,Cu)6 with extrapolated limits from experimental data and in agreement with Palatnik and Landau rules [7]. For better understanding, isothermic sections at 1000 (Fig. 5b) and 900 8C (Fig. 5c) have been schematically drawn. 3.3. Standard enthalpy of formation The mean value of the measured heats of dissolution of Ce(Ni1KxCux)5 dropped from 25 8C as well as the partial enthalpy of mixing for pure Ce, Ni and Cu are presented in Table 2 together with the calculated standard enthalpies of formation as compared to the literature. The values of the elements have been corrected by adding the heat content which corresponds to the temperature difference of the original measuring temperature to 722 8C by using the data from [5]. The error given in the table takes into account the error of the

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J. Wang et al. / Intermetallics 14 (2006) 695–701

Fig. 4. SEM images of Ce(Ni0.1Cu0.9)5 sample: (a) as-cast and (b) annealed at 750 8C for 2 weeks. The white spots are polishing material residues on the sample surface.

calorimeter calibration constant as well as the statistical standard deviation of the calculated mean value. The measured value for CeNi5 agrees well with the data from Guo et al. [8] and Yamaguchi et al. [9]. The data from Prabhakara Reddy et al. [10] and Colinet et al. [11] and a value calculated in a recent assessment of the Ni–Cu binary system by Palumbo et al. [12] are more exothermic. Within the Ce(Ni1KxCux)5 solid solution, the linear dependency of the standard enthalpy of formation, as determined by MeyerLiautaud et al. [3] has been confirmed (Fig. 6).

A linear regression of the enthalpy values as a function of composition results in DfH[Ce(Ni1KxCux)5]ZK172.3C 109.5x (kJ/mol). The calculated value for pure CeCu5 of K62.8 kJ/mol is in agreement with the value obtained by Calphad modeling (K68.85) by Zhuang et al. [16]. The discrepancy between Yamaguchi’s data [9] and Colinet’s value [11] has been claimed to be due to sample impurity [10], because the samples measured by Yamaguchi et al. [9] with a composition CeNi5.13 should contain two phases on the basis of the phase equilibrium diagram [6]. This

(a) 1300

1

Temperature (°C)

1200

1100

2 1000 1L 2 L+Ce(Ni,Cu)5 3 L+Ce(Ni,Cu)6

900

4 L+Ce(Ni,Cu)5+Ce(Ni,Cu)6

4 3

6

5 L+Ce(Ni,Cu)5

5

6 Ce(Ni,Cu)5

800 0.0

0.2

0.4 0.6 0.8 x values ofCe(Ni1-xCux)5 compounds

(b)

(c)

liquid (L)

liquid

6

Ce(Ni,Cu)5 Ce(Ni,Cu)5

+

(Ni,Cu)

1

2

Ce(Ni,Cu)

1.0

CeCu5 L + Ce(Ni,Cu)5

5

5

6

+

4

liquid CeCu6

Ce(Ni,Cu)5 + (Ni,Cu)

3

L + Ce(Ni,Cu)6 (Ni,Cu) + Ce(Ni,Cu)6

Fig. 5. Temperature–composition section (a) for the Ce(Ni1KxCux)5 solid solution based on DTA measurements and metallographic observations. Schematic isothermal sections at 1000 8C (b) and 900 8C (c) have been included for better understanding.

J. Wang et al. / Intermetallics 14 (2006) 695–701

699

Table 2 Measured heat effects for pure Ce, Ni and Cu and calculated enthalpies of formation of Ce(Ni,Cu)5 alloys as compared with the literature Sample

Tdiss (K)

Number of dissolutions

Q (kJ/mol)

Ce Ni Cu CeNi5

966 948 984 995

9 17 9 11

K158.8G0.3 (K157.7) K119.75G2.0 (K118.3) K4.2G0.1 (K3.9) K574.4G8.8

Ce(Ni0.8Cu0.2)5 Ce(Ni0.806Cu0.194)5 Ce(Ni0.610Cu0.390)5 Ce(Ni0.5Cu0.5)5 Ce(Ni0.394Cu0.606)5 Ce(Ni0.2Cu0.8)5 Ce(Ni0.188Cu0.812)5

995

7

K485.9G8.6

995

10

K349.3G6.9

995

8

K204.2G6.1

DfH (kJ/mol)

Reference

K174.8G13.3 K199.0 K201.9G11.9 K166.0G2.0 K168.6G7.8 K189 K148.9G11.7 K179.0 K151.0 K113.9G8.5 K129.0 K87.4G6.4 K100.0

[13] [14] [15] This work [11] [10] [9] [8] [12] This work [3] [3] This work [3] This work [3]

The enthalpies of dissolution for the elements have been corrected to the temperature of the bath using [5] (values in brackets).

has not been confirmed in the present work. Indeed we prepared a CeNi5.2 sample which was found to be single phase, as may be seen in the X-ray diffraction pattern given in Fig. 7 in which all diffraction lines could be indexed with the CaCu5 type structure with aZ0.4916 nm and cZ0.4010 nm. It may be then concluded that the quality of the sample is not the right argument for the discrepancy between the formation enthalpy previously measured. In addition, the fact that CeNi5.2 is single phase indicates that CeNi5 is not a pure stoichiometric compound as reported in the Ce–Ni phase diagram [6]. The phase exists with a homogeneity range in the Ni-rich direction. This behavior is supposed to persist in the Ce(Ni1KxCux)5 solid solution. In order to confirm this assumption, four samples with a Ni-rich composition ranging between Ce(Ni,Cu)5 and CeCu6 were prepared and annealed at 800 8C for 1 week (see Table 3). All peaks in the X-ray diffraction patterns (Fig. 8) were indexed as the CaCu5 type structure.

These results confirm that the solid solution Ce(Ni1KxCux)5, similarly to the binary phase CeNi5 extends towards Ni-rich compositions with a phase limit close to CeNi5.8. Moze et al. [17], using neutron diffraction claimed that a ternary compound with a composition lying between CeNi3Cu3 and CeNi4Cu2 is formed with the TbCu7 structure (see Fig. 9). This structuretype is a derivative of the CaCu5- type. For compounds such as CeNi5, it offers the possibility for Ce sites to be partially filled by dumb-bell pairs of Ni or Cu or mixed Ni–Cu atoms. Moze et al. [17] found that these sites are preferably occupied by pair of Ni atoms. Our results suggest that CeNi3Cu3 and CeNi4Cu2 could not be true ternary compounds but ordered phases within the Ce(Ni1KxCux)5Cx solid solution. 4. Conclusions The Ce(Ni1KxCux)5 system was found to form a continuous ideal solid solution. The enthalpy of formation is therefore a 111

–100

8000

101

–120 –140

6000

0.0

0.2

0.4

0.6

0.8

200

2000

301 103

–220

112 211 202 300

–200

4000

201

–180

002

This work [Mey85] data [Col83] data [Pra97] data [Yam95] data [Guo98] data [ZHu96] calculated Data

110

–160

intensity

Enthalpy of Formation (kJ/mol)

–80

1.0

X value in Ce(Ni1-xCux)5

30

40

50

60

70

2θ (°) Fig. 6. Measured standard enthalpies of formation of the Ce(Ni1KxCux)5 solid solution compared with literature data.

Fig. 7. X-ray diffraction pattern of single phase CeNi5.2 sample.

700

J. Wang et al. / Intermetallics 14 (2006) 695–701

Table 3 Analyzed Ni-rich samples in Ce(Ni1KxCux)5Cy system Sample composition (at.%)

Ni

Cu

Ce

Ni

Cu

16.13 15.38 15.38 15.45

58.71 59.23 67.69 38.07

25.16 25.38 16.93 6.48

16.1 15.5 15.3 15.5

57.9 59.6 68.2 38.0

26.0 24.9 16.5 16.5

*

*

* Ce(Ni,Cu)5 (201)

*

3.5

*

(002)

4.0

(220)

(111)

Ce

(110)

Ce(Ni0.7Cu0.3)5.2 Ce(Ni0.7Cu0.3)5.5 Ce(Ni0.8Cu0.2)5.5 Ce(Ni0.45Cu0.55)5.5

Nominal composition (at.%)

(101)

Sample

* *

Ce(Ni0.45Cu0.55)5.5

Relative intensity

3.0 2.5 Ce(Ni0.8Cu0.2)5.5

2.0 1.5 Ce(Ni0.7Cu0.3)5.5

1.0 0.5 Ce(Ni0.7Cu0.3)5.2

0.0 25

30

35

40 2θ (°)

45

50

55

Fig. 8. X-ray patterns of annealed Ni-rich samples: (a) Ce(Ni0.7Cu0.3)5.2; (b) Ce(Ni0.7Cu0.3)5.5; (c) Ce(Ni0.8Cu0.2)5.5 and (d) Ce(Ni0.45Cu0.55)5.5 (800 8C/1 week).

linear function of composition with DfH[Ce(Ni1KxCux)5]Z K172.3C109.5x (kJ/mol). The binary compound CeNi5 is not stoichiometric and a limit of CeNi5.8 has been evaluated at 800 8C. The high temperature phase equilibria have been established by DTA and SEM/EDX analysis and a temperature-composition section has been drawn, showing the existence of a three phase field Ce(Ni,Cu)5CCe(Ni,Cu)6C liquid in the copper rich part of the solid solution due to the peritectic formation of CeCu5. Acknowledgements Part of this work has been funded in the frame of the French Rhoˆne-Alpes regional projet ‘SUPERFLEX’ and by an European Brite-Euram project, Acropolis. References Fig. 9. Hexagonal TbCu7-type crystal structure observed for CeNi6KxCux with xZ2 and 3 by Moze et al. [16].

[1] Gignoux D, Givord F, Lemaire R, Tasset F. J Phys (Paris) 1982;43: 257–61.

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