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Intermetallics 40 (2013) 60e64

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Intermetallics

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On the crystal structure of the CeZn1.35Ga2.65 and CeZnGa4 ternary phases

Yuriy Verbovytskyy a,*, Mathieu Pasturel b, António Pereira Gonçalves a

aCampus Tecnológico e Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, Estrada Nacional 10, 2686-953, Sacavém, Portugalb Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes, France

a r t i c l e i n f o

Article history:Received 18 March 2013Received in revised form10 April 2013Accepted 11 April 2013Available online

Keywords:A. Rare-earth intermetallicsB. Crystal chemistry of intermetallicsC. Crystal growthF. Diffraction

* Corresponding author. Tel.: þ351 219946184; fax:E-mail address: yuryvv@bigmir.net (Y. Verbovytsk

0966-9795/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2013.04.008

a b s t r a c t

Two ternary compounds, CeZn1.35Ga2.65 and CeZnGa4, were prepared using the self-flux method.Their crystal structures were derived by direct methods from single crystal X-ray diffractiondata. CeZn1.35Ga2.65 crystallizes in the BaAl4 structure type and CeZnGa4 in a structure related to theCe2NiGa10-type, both with the space group I4/mmm. The refined structural parameters for CeZn1.35Ga2.65and CeZnGa4 are a ¼ 4.2717(2)�A, c ¼ 10.6975(4)�A, V ¼ 195.202(15)�A3 and a ¼ 4.2944(5)�A,c ¼ 26.147(3)�A, V ¼ 482.21(10)�A3, respectively.

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1. Introduction

Ternary rare-earth (R) e d-metal gallides are being extensivelystudied among the intermetallic compounds. They exhibit a widevariety of crystal structures aswell as diversityof interesting physicalproperties, as highmagnetic anisotropy orheavy-fermion behaviour.Information about the crystal structure of such compounds and onsome ReMeGa phase diagrams can be found in Refs. [1e3]. Criticalanalysis on the interaction among the components in the Ga-basedternary systems has been presented in Ref. [4]. Among the ReMe

Ga systems, the ReZneGa ones were poorly studied. The phase di-agram of the {Ce, Eu, Yb}eZneGa systems at 400 �C have beenconstructedonly in the regionwith0e33.3 at.% R [5,6]. Studies on theReZneGa intermetallic compounds with (or related to) BaAl4structure types are published in Refs. [7e14]. Investigations on thephases with BaHg11 structure type are given in Refs. [10,15]. Identi-fication of the new intermediate phaseswith the CeCu2, CaIn2, CeCd2and AlB2 structure types from the RGa2eRZn2 cross-section arepublished in Refs. [16,17]. Single crystal studies of theYZn0.2Ga1.8 andthe La4.76Zn1.5Ga1.5 phases are shown in Refs. [18] and [19], respec-tively. In the course of our systematic studies on the ternary ReZneGa systems two ternary gallides, namely CeZn1.35Ga2.65 andCeZnGa4,were identified. The synthesis and crystal structure refinements ofthese compounds are reported herein.

þ351 219941455.yy).

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2. Experimental details

Metals with nominal purities above 99.95 wt. % (cerium ingots,Zn spheres and Ga pieces) were used as starting materials. Thepieces of the elemental components, with a 1.0e1.5 g total weight,weremixed, sealed into quartz ampoules under Ar-cleaned vacuum(10�4 Torr) and reacted at 950 �C.

Plate-like crystals (I), with dimensions up to 2 mm3, wereextracted from an alloy with initial composition 13Ce:17Zn:70Gawhich was cooled down from 950� to 400 �C within 24 h. Blockcrystals (II), up to 5 mm3 size, were isolated from a 8Ce:11Zn:81Gaalloy, after lowering the temperature of the reactants at 6 �C/h ratefrom 950 to 150 �C. The crystals I and IIwere analysed by EDX usinga Jeol JSM 6400 scanning electron microscope.

Small single crystals were mechanically separated from I and IImaterials inside a hot concentrated aqueous solution of potassiumhydroxide (w5 M). Single crystal X-ray diffraction measurementswere performed on a 4-circle Nonius Kappa CCD diffractometer(Mo Ka, l ¼ 0.71073 �A) at room temperature. An empirical ab-sorption correction was applied using the program SADABS [20].The structures were solved by direct methods and refined usingSHELXL-97 [21] available in the WinGX package [22].

3. Results and discussion

The average compositions of the I and II crystals have beenfound to be Ce20Zn27Ga53 and Ce16Zn17Ga67, respectively, by EDX

Table 1Crystal data and structure refinement for CeZn1.35Ga2.65 (I) and CeZnGa4 (II).

Phase I II

Refinementcomposition

CeZn1.35Ga2.65 CeZn1.05Ga3.89

Relative mass 413.13 496.33Crystal system Tetragonal TetragonalSpace group I4/mmm (N� 139) I4/mmm (N� 139)Pearson’s symbol tI10 tI23.76

Unit cell dimensionsa 4.2717(2)�A 4.2944(5)�Ac 10.6975(4)�A 26.147(3)�AV 195.202(15)�A3 482.21(10)�A3

Formula units per cell 2 4Calculated density 7.029 g/cm3 6.837 g/cm3

Crystal shape andcolour

Plate, silvery Block, silvery

Diffractometer Nonius Kappa CCD Nonius Kappa CCDWavelength Mo-Ka

(l ¼ 0.71073 �A)Mo-Ka (l ¼ 0.71073 �A)

Monochromator Graphite GraphiteTemperature 293(2) K 293(2) KAbsorption

coefficient37.398 mm�1 36.892 mm�1

F(000) 361 869q range for data collection 3.81e41.98� 4.68e41.94�

Range in hkl �8/7, �7/8,�20/16

�7/8, �8/8,�48/43

Total no. ofreflections

2444 6239

Independentreflection

233 568

Reflections with I > 2s(I) 211 397Req, Rs 0.0393, 0.0207 0.0611, 0.0309Structure

refinementSHELXL-97(Sheldrick-1997)

SHELXL-97(Sheldrick-1997)

Refinementmethod

Full-matrixleast-squares on F2

Full-matrixleast-squares on F2

Data/restraints/parameters 233/0/9 568/3/33Goodness-of-fit

on F21.224 1.028

Final R indices (I > 2s(I))a R1 ¼ 0.0256,wR2 ¼ 0.0556

R1 ¼ 0.0402,wR2 ¼ 0.0935

R indices (all data) R1 ¼ 0.0308,wR2 ¼ 0.0580

R1 ¼ 0.0651,wR2 ¼ 0.1082

Weighting schemeb a ¼ 0.0266,b ¼ 0.8806

a ¼ 0.0567,b ¼ 8.4696

Extinctioncoefficient

0.0126(16) 0.0014(4)

Largest diff.peak/hole

3.585/�3.121 e/�A3 4.386/�4.700 e/�A3

a R1 ¼ P(jjFoj � jFcjj)/

PjFoj, wR2 ¼ {P

w[(Fo2� Fc2)2]/

Pw[(Fo2)2]}1/2.

b w ¼ 1/[s2(Fo2)þ(aP)2 þ bP], in which P¼ (Fo2þ 2Fc2)/3].

Fig. 1. Projection onto the XZ plane and coordination polyhedra of the atoms in thestructure of CeZn1.35Ga2.65.

Table 3Interatomic distances in the CeZn1.35Ga2.65 structure within the first coordina-tion spheres.

Atoms d, �A

Ce 8M2 3.264(1)8M1 3.423(1)2M2 4.112(1)4Ce 4.272(1)

M1 4M2 2.574(1)4M1 3.021(1)4Ce 3.423(1)

M2 1M2 2.474(1)4M1 2.574(1)4Ce 3.264(1)

Y. Verbovytskyy et al. / Intermetallics 40 (2013) 60e64 61

analyses, without any impurity elements in the 1 at.% limit ofdetection of the apparatus.

Recent investigations on the phase diagram of the CeeZneGasystem at 400 �C [6] evidenced the formation of a ternary solidsolution with BaAl4 structure type along the 20 at.% Ce with a wide

Table 2Atom coordinates and isotropic displacement parameters for CeZn1.35Ga2.65.

Atom Site x y

Ce 2a 0 0M1 4d 0 ½M2 4e 0 0

Anisotropic displacement parameters for CeZn1.35Ga2.65

Atom U11 U22 U33

Ce 0.00786(17) 0.00786(17) 0.0100(2)M1 0.0128(2) 0.0128(2) 0.0103(3)M2 0.0113(2) 0.0113(2) 0.0103(3)

homogeneity range CeZnxGa4�x (xw1e2), including the early re-ported CeZn2Ga2 [8] intermetallic. Crystal structure investigationsof CeZnxGa4�x were performed from powder X-ray diffraction datafor x ¼ 1.5 and 2 [6,8]. The EDX analysis of crystal I points to acompound belonging to this homogeneity range, with awCeZn1.35Ga2.65 composition.

A CeZnGa4 compoundwas first identified in the 400 �C annealedalloys of the CeeZneGa system [6]. From the analysis of the powderX-ray diffraction data it was suggested that its structure could berelated to the Ce2NiGa10 structure type [3]. The Ce16Zn17Ga67composition obtained for crystal II indicates that most probably itbelongs to the CeZnGa4 compound.

z Ueq (�A2) G

0 0.00856(14) 1Ce1/4 0.01201(18) 0.67Znþ 0.33Ga0.38437(6) 0.01093(17) 1Ga

U23 U13 U12

0 0 00 0 00 0 0

Table 4Atom coordinates and isotropic displacement parameters for CeZnGa4.

Atom Site x y z Ueq (�A2) G

Ce 4e 0 0 0.14789 (2) 0.00998 (17) 1CeM1 2a 0 0 0 0.044 (3) 0.327(14)ZnM2 4d 0 1/2 1/4 0.0135 (3) 0.33(13)Znþ 0.67(13)GaM3 4e 0 0 0.30477 (5) 0.0108 (3) 0.24(14)Znþ 0.76(14)GaM4 4e 0 0 0.39881 (5) 0.0107 (3) 0.934GaM5 4e 0 0 0.4437 (7) 0.0107 (3) 0.066(5)GaM6 8g 0 1/2 0.0485 (3) 0.0205 (19) 0.56GaM7 16n 0 0.405 (4) 0.0458 (4) 0.020 (2) 0.08Znþ 0.11Ga

Anisotropic displacement parameters for CeZnGa4

Atom U11 U22 U33 U23 U13 U12

Ce 0.00851 (19) 0.00851 (19) 0.0129 (3) 0 0 0M1 0.037 (4) 0.037 (4) 0.057 (7) 0 0 0M2 0.0147 (4) 0.0147 (4) 0.0110 (5) 0 0 0M3 0.0111 (4) 0.0111 (4) 0.0103 (5) 0 0 0M4 0.0108 (4) 0.0108 (4) 0.0103 (5) 0 0 0M6 0.0098 (11) 0.039 (5) 0.0131 (12) 0 0 0M7 0.011 (2) 0.040 (8) 0.0079 (19) 0.010 (3) 0 0

Y. Verbovytskyy et al. / Intermetallics 40 (2013) 60e6462

3.1. CeZn1.35Ga2.65

Crystal structure refinements of crystal I given here are compat-ible with the CeZn1.35Ga2.65 composition (from the best refinement

Fig. 2. Relationship between BaAl4,

after parallel fittings) and clearly confirmed the BaAl4 structure typefor this phase. These results are shown in Tables 1 and 2.

Herein, the Ce atoms occupy the positions of the Ba atoms (2a),pure Ga and its mixture with Zn are located in the positions of the

CaF2 and Ce2NiGa10 structures.

Fig. 3. Comparison between the undistorted [Ga] and distorted [Ga]/[ZnGa] networks in the Ce2NiGa10 and CeZnGa4 structures. Alternative connections in the CeZnGa4 are shownby white filled rods.

Y. Verbovytskyy et al. / Intermetallics 40 (2013) 60e64 63

Al atoms (4e and 4d). The unit cell and the coordination polyhedraof the atoms are shown in Fig. 1. The neighbours of the Ce atomsform 22-vertex [Ce4M18M210] polyhedra. The M1 atoms aresituated inside [Ce4M14M24] heavily distorted cuboctahedra.The coordination polyhedra of the M2 atoms have a form of[Ce4M14M21] one capped square based antiprisms. Analyses of theinteratomic distances between the atoms (see Table 3) indicatethat the strong bonding can be considered only for the M1eM2and M2eM2 atoms. In the other cases, those distances are some-what longer than the sums of the atomic radii of the respectiveatoms [23].

It should be noted that ternary phases with BaAl4 structure typehave been found to exist beside the ReZneGa systems with R ¼ Y,La/Sm, Gd/Er and Yb [5e14] and also in the ReZneAl phasediagrams with R ¼ La/Gd and Yb [24e27], but they were notidentified among the ReZneIn systems [28].

3.2. CeZnGa4

Crystallographic data and results of the refinement performedon the crystal II are presented in Tables 1 and 4, confirming that it isthe CeZnGa4 compound.

The systematic extinctions of the data set are compatible withthe space group I4/mmm. At the beginning five atomic positions (Cein 4e, M2 in 4d, M3 in 4e, M4 in 4e and M6 in 8g) were obtainedfrom the direct method. The M1 (2a) and M7 (16n) positions werelocated from the difference electron-density map after first step ofthe refinement. Due to the elevated value of the isotropicdisplacement parameters for the M1, M6 and M7 positions, partialoccupancies for these sites were refined separately. The commonvolume of occupation for the closest M6 (8g) and M7 (16n) atomswas found with GM6 þ 2GM7 < 1. The difference Fourier synthesis ofthe following refinement evidenced a small peak at the 4e site,which was very close to the M4 position. In the separate cycleswe have included this position (M5) to the refinement and

constraining the occupancy with that of the M4 position. Closelocation of the M4 atoms was also noticed with M6 and M7 ones;thus in the final cycles, occupancy parameters for the M6 and M7atoms were fixed with respect to the occupancy of the M5 atoms.Preference of the occupation of the zinc and gallium atoms in theM1eM7 positions was performed in the parallel fittings. It wassuggested that pure zinc occupied the M1 site; while the statisticalmixture of the zinc and gallium atomswere found to distribute overthe M2, M3 and M7 sites.

From the structural point of view the CeZnGa4 structure can beconsidered as a defect and disordered variant of the Ce2NiGa10structure type. The latter structure can be described in terms of anintergrowth of BaAl4 (truncated square based prisms with ceriuminside) and CaF2 (body centred by nickel deformed cubes) slabs(Fig. 2). In the CeZnGa4 structure the CaF2-type segments arestrongly disordered around the M1 site, which is only partiallyfilled by Zn atoms. The M6 and M7 positions represent the verticesof the deformed cube and they are both partially occupied. Addi-tional disorder in this structure is related to the closest M4 and M5positions where atoms can occupy only one of two adjacent sites. Adifference between the [Ga] or [Ga]/[ZnGa] networks in the Ce2N-iGa10 and CeZnGa4 structures is shown in Fig. 3. Interatomic dis-tances and possible near-neighbours of the atoms are given inTable 5. The shortest CeeM distances, which are lower than thesums of the atomic radii, were found only for CeeM7. Significantlyshorter MeM distances can be observed for the following neigh-bours: M1eM6, M2eM3, M3eM4, M4eM6, M4eM7, M6eM6,M6eM7 and M7eM7.

To conclude, many examples of the gallium-rich intermetallicphases with a distorted gallium network have been established toexist in the binary ReGa and ternary ReMeGa systems [3,4]. Thedisorder in CaF2-type gallium segment of the parent Ce2NiGa10structure type was also recently discovered in the YbGa5 [29],R2Ag1�xGa10�y (R ¼ La and Ce) [30] and RNi1�xGa4 (R ¼ Tb, Dy,Ho and Er) [31] compounds.

Table 5Interatomic distances in the CeZnGa4 structure.

Atoms d, �A

Ce 4M7 3.186(13)4M4 3.273(1)4M3 3.279(1)4M6 3.371(6)4M2 3.426(1)4M7 3.695(14)4M5 3.867(11)1M1 3.867(1)1M3 4.102(1)4Ce 4.294(1)

M1 8M6 2.494(4)8M7 2.822(16)8M5 3.375(8)2Ce 3.867(1)

M2 4M3 2.581(1)4M2 3.037(1)4Ce 3.426(1)

M3 1M4 2.459(2)4M2 2.581(1)4Ce 3.279(1)1M5 3.633(18)1Ce 4.102(1)

M4 1M3 2.459(2)4M6 2.551(4)8M7 2.622(6)4Ce 3.273(1)

M5 1M5 2.944(26)4M1 3.375(8)4Ce 3.867(11)8M7 3.450(16)4M6 3.481(16)1M3 3.633(18)

M6 2M1 2.494(4)2M7 2.499(13)1M6 2.536(11)2M4 2.551(4)4M7 2.764(11)4M6 3.037(1)4M7 3.338(13)2Ce 3.371(6)2M5 3.481(16)4M7 3.703(12)2M7 3.887(17)4M6 3.956(7)

M7 1M7 2.395(15)2M7 2.460(17)1M6 2.499(13)1M7 2.530(16)2M4 2.622(6)2M6 2.764(11)1M1 2.822(16)4M7 3.091(17)1Ce 3.186(13)2M6 3.338(13)2M7 3.433(16)2M5 3.450(16)1M7 3.478(24)2M7 3.614(17)1Ce 3.695(14)2M6 3.703(12)1M6 3.887(17)4M7 3.910(16)

Y. Verbovytskyy et al. / Intermetallics 40 (2013) 60e6464

Acknowledgements

The FCT Grant No. SFRH/BPD/34840/2007 for the research workof Y.V. at ITN, Sacavém, Portugal is highly appreciated.

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