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Solid State Sciences 34 (2014) 63e68

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Solid State Sciences

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Crystal structures and magnetism of DyAlxGa3�x (where x ¼ 0.33and x ¼ 0.85)

Pavlo Lyutyy a, c, *, Oliver Niehaus b, Volodymyr Svitlyk b, d, Rainer P€ottgen b,Iryna Porodko e, Anatolii Fedorchuk f

a G.V. Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Naukova Street 5, UA-79601 Lviv, Ukraineb Institut für Anorganische und Analytische Chemie, Universit€at Münster, Corrensstrasse 30, Münster D-48149, Germanyc Institute of Physics, J. Dlugosz University Częstochowa, Armii Krajowej 13/15, Częstochowa, Polandd ID27 High Pressure Beamline, ESRF, BP220, 38043 Grenoble, Francee Lviv Academy of Commerce, Samchuka Str., 9, UA-79011 Lviv, Ukrainef S.Z. Gzhytskyj Lviv National University of Veterinary Medicine and Biotechnologies, Pekarska Street 50, UA-79010 Lviv, Ukraine

a r t i c l e i n f o

Article history:Received 17 January 2014Received in revised form28 April 2014Accepted 17 May 2014Available online 2 June 2014

Keywords:Ternary compoundGalliumDysprosiumAluminiumCrystal structure

* Corresponding author. G.V. Karpenko Physico-Meof Ukraine, Naukova Street 5, UA-79601 Lviv, Ukraine

E-mail address: pavlo_lyutyy@ukr.net (P. Lyutyy).

http://dx.doi.org/10.1016/j.solidstatesciences.2014.05.01293-2558/© 2014 Elsevier Masson SAS. All rights res

a b s t r a c t

New ternary compounds have been obtained from arc-melting of the elements. The crystal structureshave been investigated by powder X-ray diffraction analysis. DyAl0.33Ga2.67 crystallizes in space groupP63/mmc (a ¼ 6.1649(1) Å, c ¼ 23.0792(1) Å) with Ta(Rh0.33Pd0.67)3-type structure. The structure ofDyAl0.85Ga2.15 has been refined with rhombohedral symmetry (space group R-3m, a ¼ 6.1702(1) Å,c ¼ 20.7797(1) Å), BaPb3-type structure. The structures of the compounds have been analyzed crystal-lographically and the structural relationship has been established. Heat capacity measurements proveantiferromagnetic ordering at N�eel temperatures of 12.6(1) K for DyAl0.33Ga2.67 and 10.0(1) K forDyAl0.85Ga2.15.

© 2014 Elsevier Masson SAS. All rights reserved.

1. Introduction

Rare-earth based compounds are an interesting subject ofresearch due to their widespread potential applications such aspermanent magnets [1], magnetocaloric systems [2], hydrogenstorage materials [3] and other applications.

A literature review of binary rare-earth-metal systems attractedour attention to the DyeGa and AleDy systems, namely to the bi-nary compounds DyGa3 and DyAl3 which crystallize in numerouspolymorphic modifications with closely related structures. Ac-cording to the Massalski compilation of binary phase diagrams [4],DyGa3 undergoes two temperature-induced polymorphic modifi-cations. The ht2-DyGa3 phase crystallize in the AuCu3 structuretype (1045 �Ce550 �C), the ht1-DyGa3 modification adopts theTa(Rh0.33Pd0.67)3 structure type (550 �Ce350 �C) and the rt-DyGa3

chanical Institute of the NAS. Tel./fax: þ380 322 633 088.

04erved.

phase crystallizes in the Mg3In structure type (below 350 �C).Dysprosium trialuminide is characterized by the existence of twomodifications: the TiNi3-type onewhich is stable up to 1005 �C, anda HoAl3-type (1005e1090 �C) one [4]. A high pressure AuCu3-typemodification is also formed [5]. Metastable PuGa3 and BaPb3polymorphs obtained by rapid solidification, were reported inRef. [6].

Shifting of phase transition temperatures and/or the stabiliza-tion of metastable phases with corresponding changes in physicalproperties is interesting both from theoretical and practical pointsof view. Such possibilities were demonstrated in Refs. [7e9] and asignificant influence on the magnetic properties was achieved bysubstitution of a small amount of a nonmagnetic element.

Based on previous results and a literature review, a hypothesisthat an isoelectronic Al/Ga atoms replacement (r(Al)at. ¼ 1.431,r(Al)at. ¼ 1.221, difference in atomic radii ~15% [10]) may have astabilizing effect was made.

In this work we present results of investigations of the pseudo-binary system DyGa3eDyAl3, including crystallographic peculiar-ities of new phases and their magnetic susceptibilities.

Table 1Structural data and crystallographic data recording/refinement conditions forDyAl0.33(2)Ga2.67(2) and DyAl0.85(2)Ga2.15(2).

Refinement composition DyAl0.32(2)Ga2.68(2) DyAl0.85(2)Ga2.15(2)

Structure type Ta(Rh0.33Pd0.67)3 BaPb3Space group P63/mmc R-3mPearson symbol hP40 hR36Cell par.a, Å 6.1649(1) 6.1702(1)c, Å 23.0792(1) 20.7797(1)

Cell volume (Å3) 759.63(3) 685.13(3)Z 10 9Calculated density (g/cm3) 7.82 7.32X-ray machine Guinier Huber G 670Wavelength CuKa1Two-theta range 5.0e100.79 5.0e100.31Refinement, program WINCSDU, V, W 0.05199; �0.05882;

0.035320.13887; �0.10022;0.05159

RI 0.0782 0.0970RP 0.1106 0.1167RwP 0.9204 0.7903

RI ¼PjIo � Icj/

PjIoj, Rp ¼Pjyoi � ycij/

Pjyoij, Io e obs. intensity; Ic e calc. intensity;

P. Lyutyy et al. / Solid State Sciences 34 (2014) 63e6864

2. Experimental details

2.1. Synthesis and structural analysis

For the investigation of the pseudo-binary system DyGa3eDyAl3nine samples were prepared from elemental Dy (99.95%), Al(99.999%) and Ga (99.99%). During the arc-melting procedure, ti-taniumwas heated prior to the melting of the reactant mixtures tofurther purify the argon atmosphere. The buttons were flipped overseveral times and remelted to achieve high homogeneity, thenweighed back in order to check for possible mass losses. No sig-nificant mass loss (more than 1%) was found. The samples werethen wrapped in Ta foil, sealed in evacuated fused silica capsules,and annealed at 870 K for 10 days and subsequently quenched inwater. The crystal structures of the ternary compounds wererefined by the Rietveld method using X-ray powder diffraction datacollected on a Guinier Huber G 670 (CuKa1 radiation) machine. Allprocedures including indexing, refinement of the profile andstructural parameters as well as calculations of interatomic dis-tances were performed with the WINCSD [11] program package.

yoi e exp. intensity point; yci e calc. intensity point.

2.2. Magnetic measurements

Magnetic and heat capacity investigations have been performedfor the two samples Dy25Al10Ga65 (Ta(Rh0.33Pd0.67-type structure))and Dy25Al20Ga55 (BaPb3-type structure). The measurements werecarried out on a Quantum Design Physical Property MeasurementSystem using the Vibrating Sample Magnetometer and the HeatCapacity option, respectively. For the magnetic (heat capacity)measurements 15.033 (4.278) mg of Dy25Al10Ga65 and 17.061(7.987) mg of Dy25Al20Ga55 were used. These small pieces obtainedfrom bulk materials were attached to the sample holder rod bykapton foil. For the heat capacity measurements, the pieces werefixed to a pre-calibrated heat capacity puck using Apiezon N grease.Magnetic investigations were performed in the temperature rangeof 2.5e300 K with a magnetic flux density up to 80 kOe and heatcapacity measurements in the range of 2e300 K.

Table 2Atomic coordinates and isotropic displacements parameters of DyAl0.33(2)Ga2.67(2).

Atom Site x/a y/b z/c Biso/egb, Å2

Dy1 2b 0 0 1/4 0.97(4)Dy2 4f 1/3 2/3 0.0430(1) 1.12(4)Dy3 4f 2/3 1/3 0.3504(1) 1.07(4)X1a 6h 0.5186(2) 0.0372(2) 1/4 1.36(7)X2a 12k 0.1728(2) 2x 0.1494(1) 1.20(6)X3a 12k 0.1433(1) 2x 0.5521(1) 0.95(5)

a Mixed occupation. X1 ¼ 0.98(1)Ga þ 0.02(1)Al; X2 ¼ 0.82(1)Ga þ 0.18(1)Al;X3 ¼ 0.92(1)Ga þ 0.08(1)Al.

b Biso=eg ¼ ð8p2=3ÞPi

P

j

P

ijaiajaiaj:

Table 3Atomic coordinates and isotropic displacements parameters of DyAl0.85(2)Ga2.15(2).

Atom Site x/a y/b z/c Biso/egb, Å2

Dy1 3a 0 0 0 1.50(3)Dy2 6c 0 0 0.2166(1) 1.02(2)X1a 9e 1/2 0 0 1.27(4)X2a 18h 0.4796(1) 0.5203(1) 0.2237(1) 1.43(3)

a Mixed occupation. X1 ¼ 0.43(1)Al þ 0.57(1)Ga; X2 ¼ 0.21(1)Al þ 0.79(1)Ga.b Biso=eg ¼ ð8p2=3ÞPPP

aiajaiaj:

3. Results and discussion

3.1. Structure determination and refinement

After the substitution of 5 atomic per cent of aluminium bygallium atoms in DyAl3, the diffraction pattern showed the pres-ence of the DyAl3 phase (TiNi3-type) together with a new second-ary phase. During further substitution (10%) no TiNi3-type phasewas observed.

After powder data indexing of the Dy25Al65Ga10 sample, theBaPb3 structure type was taken as an initial model, no additionalun-indexed peaks could be observed. The phase with this typeremains stable up to 55 at. % Ga. Since aluminium and gallium arep1-elements and they have related chemical nature, Ga/Al (X) sta-tistical mixtures were set in the initial model and the structure wassuccessfully refined. The main crystallographic data were stan-dardized by the STRUCTURE TIDY program [12] and the results ofthe Rietveld refinement of the Dy25Al20Ga55 (DyAl0.8Ga2.2) sampleare summarized in Tables 1, 2 and 4. Graphical representation isshown on Fig. 1a. Other tested derivative models did not yield asuccessful refinement. No significant changes of the unit cell vol-umes in the solid solution (BaPb3-type) were observed, lattice pa-rameters are represented in Table 5. Therefore the packingefficiencies (p.e.) (p.e. ¼ (volume of atoms/volume of unitcell) � 100%) for the extreme solid solution composition werecalculated. It was found that the p.e. for the Dy25Al10Ga65 phase is

77% and 66% for the Dy25Al20Ga55 phase which explains the con-stant unit cell volume during the Ga/Al substitution.

Further substitution causes the next phase transition. Accordingto the X-ray phase analyses, the sample with the nominal compo-sition Dy25Al10Ga65 (DyAl0.4Ga2.6) does not belong to the TiNi3-,BaPb3- or AuCu3-type. Crystal structure of this phase was suc-cessfully refined in Ta(Rh0.33Pd0.67)3 type, standardized crystallo-graphic are represented in Tables 1, 3 and 4. Fig. 1b shows thegraphical representation of the Rietveld refinement.

By comparisonwith the literature data we can conclude that thecompound DyAl2.6e0.8Ga0.4e2.2 can be considered as a metastablepseudo-binary compound stabilized by the Al/Ga substitution,similar to rapid solidification as reported in Ref. [6].DyAl0.33(2)Ga2.67(2) (Ta(Rh0.33Pd0.67)3-type) can be considered as astabilization of the ht1-DyGa3 phase, at a temperature 50 K higherthan the temperature of its existence.

Since the valence electron concentration remains the sameduring the isoelectronic Al/Ga substitution we suppose that thestabilizations are induced by the changes of the internal lattice

i j ij

Table 4Interatomic distances in DyAl0.33(2)Ga2.67(2) and DyAl0.85(2)Ga2.15(2).

DyAl0.33(2)Ga2.67(2) DyAl0.85(2)Ga2.15(2)

Atoms Distance Atoms Distance

Dy1 6-Ga2 2.966(1) Dy1 6-Ga2 3.030(1)6-Ga1 3.089(1) 6-Ga1 3.085(1)

Dy2 3-Ga3 2.991(1) Dy2 3-Ga2 2.990(1)3-Ga2 2.994(2) 3-Ga1 3.009(1)6-Ga3 3.099(1) 6-Ga2 3.096(1)

Dy3 3-Ga3 3.029(2) Ga1 4-Ga2 2.831(1)3-Ga1 3.047(1) 2-Dy2 3.009(1)6-Ga2 3.083(1) 2-Dy1 3.085(1)

Ga1 2-Ga1 2.739(2) 4-Ga1 3.085(1)4-Ga2 2.849(2) Ga2 2-Ga2 2.707(1)2-Dy3 3.047(1) 2-Ga1 2.831(1)2-Dy1 3.089(1) 2-Ga2 2.840(1)2-Ga1 3.426(2) 1-Dy2 2.990(1)

Ga2 2-Ga3 2.821(2) 1-Dy1 3.030(1)2-Ga1 2.849(2) 2-Dy2 3.096(1)1-Dy1 2.966(1) 2-Ga2 3.462(1)2-Ga2 2.968(1)1-Dy2 2.994(2)2-Dy3 3.083(1)2-Ga2 3.197(1)

Ga3 2-Ga3 2.650(1)2-Ga2 2.821(2)2-Ga3 2.852(2)1-Dy2 2.991(1)1-Dy3 3.029(2)2-Dy2 3.099(1)2-Ga3 3.515(1)

Table 5Lattice parameter versus composition in the BaPb3-type solid solution.

Composition a, Å c, Å V, Å3

Dy25Ga10Al65 6.1616(1) 20.881(1) 686.56(3)Dy25Ga15Al60 6.1543(1) 21.029(1) 689.80(3)Dy25Ga25Al50 6.1589(1) 20.967(1) 688.81(3)Dy25Ga35Al40 6.1535(1) 21.059(1) 690.59(3)Dy25Ga45Al30 6.1685(1) 20.826(1) 686.30(3)Dy25Ga55Al20 6.1702(1) 20.779(1) 685.13(3)

P. Lyutyy et al. / Solid State Sciences 34 (2014) 63e68 65

pressure by replacing the atoms of smaller size (Ga) by bigger ones(Al) and vice versa. Similar critical shifting points through theapplication of external pressure were reported in Ref. [5].

3.2. Structural discussion

DyAl0.33(2)Ga2.67(2) and DyAl0.85(2)Ga2.15(2) were analyzed withrespect to their crystal chemistry. Comparison of the DyeDy, XeX

Fig. 1. Graphical representation of the Rietveld refinement of the powder data of a)DyAl0.33(2)Ga2.67(2); b) DyAl0.85(2)Ga2.15(2) (observed X-ray powder diffraction pattern isrepresented by the dotted line, calculated and difference (bottom) by the solid lines,vertical bars indicate the Bragg positions).

and DyeX interatomic distances shows good correlation with thecorresponding sum of the atomic radii r(Dy) ¼ 1.773 Å,r(Al) ¼ 1.431 Å and r(Ga) ¼ 1.221 Å [10].

The crystal structure of DyAl0.33(2)Ga2.67(2) is shown in Fig. 2. Thedysprosium atoms are coordinated only by the Al/Ga (X) (statisticalmixture) atoms, while the X atoms are coordinated by X and Dyatoms. The coordination polyhedra of Dy3 and X2 are cuboctahedra(CN12) and Dy1, Dy2, X1 and X3 are coordinated by an anti-cuboctahedron (CN12).

A unit cell projection as well as the coordination polyhedra ofthe Dy and X atoms in the DyAl0.85(2)Ga2.15(2) structure are repre-sented in Fig. 3. Similar to DyAl0.33(2)Ga2.67(2), the atoms are coor-dinated by cuboctahedra and anti-cuboctahedra for Dy1, X1 andDy2, X2, respectively.

The crystal structure relationships between the binary andternary compounds in the DyGa3DyAl3 pseudo-binary system areshown on Fig. 4.

The structures of DyAl0.33(2)Ga2.67(2) and DyAl0.85(2)Ga2.15(2) canbe represented as a packing of cuboctahedra and anti-cuboctahedra. The cuboctahedra can be considered as buildingblocks characteristic for rt-DyGa3 (AuCu3-type) and the anti-cuboctahedra are characteristic blocs for the MgCd3 structure type.According to Refs. [13,14], the crystal structures of AuCu3 andMgCd3 are substructures to the Cu and Mg types. It means that thatabove mentioned new ternary compounds are derivatives fromclosest packed structures (fcc and hcp).

Fig. 2. Unit cell projection and the coordination polyhedra in the structure ofDyAl0.33(2)Ga2.67(2).

Fig. 3. Unit cell projection and the coordination polyhedra in the structure ofDyAl0.85(2)Ga2.15(2).

Fig. 5. Magnetic properties of Dy25Al20Ga55: (top) temperature dependence of themagnetic susceptibility c and its reciprocal c�1 measured with a magnetic fieldstrength of 10 kOe. The inset shows the magnetic susceptibility in zero-field- (ZFC) andfield-cooled (FC) mode at 500 Oe; (bottom) magnetization isotherms at 3, 10 and 50 K.

P. Lyutyy et al. / Solid State Sciences 34 (2014) 63e6866

3.3. Physical properties

The superior panels of Figs. 5 and 6 show the temperaturedependence of the magnetic and inverse magnetic susceptibility (cand c�1 data) of Dy25Al10Ga65 and Dy25Al20Ga55 measured at10 kOe. A fit of the c�1 data with the CurieeWeiss law could berealized for both compounds in the region above 50 K. This revealedeffectivemagnetic moments of meff¼ 10.90(1) mB and meff¼ 10.54(1)mB/Dy atom for Dy25Al10Ga65 and Dy25Al20Ga55, respectively.Especially, the first value differs slightly from the theoretical valueof 10.65 mB for a free Dy3þ-ion. This is most likely due to smallamounts of impurity phases, but the trivalency of dysprosium isclearly confirmed. Both Weiss constants exhibit negative values(qp ¼ �30(2) K for Dy25Al10Ga65 and qp ¼ �26(2) K for

Fig. 4. Structural relationship between the binary and ternary compounds in theDyGa3DyAl3 pseudo-binary system represented as a packing of cuboctahedra (greencolour) and anti-cuboctahedra (marine colour). (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

Dy25Al20Ga55) that point towards antiferromagnetic interactions inthe paramagnetic range. Besides an antiferromagnetic ordering canbe approved obviously for Dy25Al20Ga55 at around 10 K.Dy25Al10Ga65 exhibits as well a non CurieeWeiss behaviour below15 K, but so far no ordering phenomenon can be determined un-equivocally from the ZFC measurement.

To obtain more precise information about this ordering, low-field measurements were performed in a zero-field- and field-cooled mode (ZFC/FC) with an external field strength of 500 Oe.These are shown in the insets of Figs. 5 and 6. The N�eel temperaturefor Dy25Al20Ga55 could be determined to TN ¼ 10.3(5) K and nobifurcation is visible. A very weak, but sharp anomaly at T¼ 13(1) Kpoints towards an antiferromagnetic ordering for Dy25Al10Ga65,too. Particular features, which need to be mentioned, are furtheranomalies at around 4 K for both compounds. Heat capacity mea-surements will clarify this behaviour and also the situation forDy25Al10Ga65 (vide infra).

The magnetization isotherms measured at 3, 10 and 50 K areshown in the bottom panels of Figs. 5 and 6, respectively. Allisotherms for both compounds exhibit a linear dependence of theexternal field strength, which would be expected for para-magnetic compounds. However, the 3 and 10 K isotherms aresignificantly steeper than the 50 K one, pointing towards veryweak antiferromagnetic orientations of the spins for both sam-ples. Furthermore the 3 K isotherm of Dy25Al20Ga55 is slightly

Fig. 6. Magnetic properties of Dy25Al10Ga65: (top) temperature dependence of themagnetic susceptibility c and its reciprocal c�1 measured with a magnetic fieldstrength of 10 kOe. The inset shows the magnetic susceptibility in zero-field- (ZFC) andfield-cooled (FC) mode at 500 Oe; (bottom) magnetization isotherms at 3, 10 and 50 K.

Fig. 8. Heat capacity of Dy25Al20Ga55 measured in the temperature range of 1.9e300 Kwithout an applied field. The inset shows the magnified low-temperature area tohighlight the antiferromagnetic ordering.

P. Lyutyy et al. / Solid State Sciences 34 (2014) 63e68 67

curved, confirming the assumption of a less distinct antiferro-magnetic orientation. Consequently, the magnetic moments at3 K and 80 kOe are much lower than the expected saturationmagnetization of 10 mB for Dy according to gJ � J. The determined

Fig. 7. Heat capacity of Dy25Al10Ga65 measured in the temperature range of 1.9e300 Kwithout an applied field. The inset shows the magnified low-temperature area tohighlight the antiferromagnetic ordering.

values are 4.4(5) mB for Dy25Al10Ga65 and 4.5(1) mB/Dy atom forDy25Al20Ga55.

Heat capacity measurements were performed without anexternal field in the temperature range of 1.9e300 K for bothsamples. The results are displayed in Figs. 7 and 8. Both compoundsexhibit one l-type anomaly, proving unambiguously the antifer-romagnetic ordering temperatures above 10 K and identifying theanomalies at around 4 K as result of a minor impurity phase. Thisone could not be further clarified. The anomalies at 12.6(1) K forDy25Al10Ga65 and at 10.0(1) K for Dy25Al20Ga55 correspond verywell to the antiferromagnetic ordering temperatures determinedby the ZFC/FC measurements (vide supra).

4. Conclusions

Two new ternary compounds DyAl0.33(2)Ga2.67(2) andDyAl0.85(2)Ga2.15(2) were synthesized from the pure elements andtheir crystal structures were determined by X-ray powder diffrac-tion. It was found that DyAl0.33(2)Ga2.67(2) is a stabilized variantof the binary ht1-DyGa3 type, space group P63/mmc(Ta(Rh0.33Pd0.67)3-structure). Contrary, DyAl0.85(2)Ga2.15(2) crystal-lizes in space group R-3m, BaPb3 structure type which is charac-teristic for REAl3 compounds. The hypothesis that a small amount ofa third component can stabilize the low temperature modificationat higher temperatures was confirmed. Structural relationshipsbetween binary and ternary compounds in the DyGa3DyAl3pseudo-binary system were demonstrated and the magneticbehaviour was elucidated.

Acknowledgements

Volodymyr Svitlyk acknowledges support by the Alexander vonHumboldt Foundation and the Federal Ministry for Education andResearch.

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