Cite this: CrystEngComm, 2013, 15,8006

Crystal structure and physical properties of indium fluxgrown RE2AuSi3 (RE = Eu, Yb)3

Received 21st June 2013,Accepted 1st August 2013

DOI: 10.1039/c3ce41198b

www.rsc.org/crystengcomm

Sumanta Sarkar,a Matthias J. Gutmannb and Sebastian C. Peter*a

Two new intermetallic compounds Eu2AuSi3 and Yb2AuSi3 having prismatic and rod like shapes,

respectively, were grown using indium as inert metal flux. The crystal structure of both compounds was

refined using X-ray diffraction data on selected single crystals. Both compounds are ordered variants of the

AlB2 structure type and crystallize in the orthorhombic system. Eu2AuSi3 adopts the Ca2AgSi3 structure

type, Fmmm space group and lattice parameters a = 8.3060(17) Å, b = 9.0369(18) Å, c = 14.377(3) Å.

Yb2AuSi3, on the other hand, crystallizes in the Ba2LiSi3 structure type, Fddd space group and lattice

parameters a = 8.2003(16) Å, b = 14.187(3) Å, c = 16.869(3) Å. Both Eu2AuSi3 and Yb2AuSi3 consist of two

dimensional hexagonal units built up of [Au3Si3] mixed sites and the europium atoms are sandwiched

between two adjacent hexagonal layers. Magnetic measurements suggest both compounds have divalent

rare earth atoms. The temperature dependent magnetic susceptibility data of Eu2AuSi3 suggests the

Curie–Weiss behavior above 30 K and an antiferromagnetic ordering around 26 K. Yb2AuSi3 is

diamagnetic in the higher temperature range (.175 K) and is weakly paramagnetic below 100 K

indicating a probable valence fluctuation in Yb.

1. Introduction

Synthesis and growth of single crystals are central to the studyof complex compounds. The metal flux method1–3 hasemerged as a strong tool for the synthesis of single crystalsfor a large number of intermetallic compounds. The majorityof the compounds were synthesized using Ga, In, Al, Sn, Pb, aseither active or inert flux and notably indium is beingconsidered as the emerging flux because of its ability to existas flux over a wide range of temperature (429–2345 K). A fewexamples of recently reported compounds from our group are:Eu2AuGe3,4 Yb2AuGe3,5 YbMn0.17Si1.88,6 RE4TGe8 (RE = Gd, Yb;T = Cr–Ni, Ag),7 Yb5T4Ge10 (T = Co, Ni),8 YbTxSi22x (T = Cr, Fe,Co)8,9 and YbGe3

10 synthesized using In as an inactive flux,while Yb3AuGe2In3,11 YbCu6In6,12 SmCu6In6, REAu2In4 (RE =Eu, Yb),13 Yb2Au3In5,13 EuAuIn4 and EuInGe14 were synthe-sized using In as an active metal flux.

Recently, we have discovered a few compounds withchemical formula RE2TGe3 (RE = Alkaline or rare earth metals,T = transition metal,). In order to continue our constant searchfor new phases in RE2TX3 (X = p-block elements) series, we

have extended our work to silicon instead of germanium whichis its next neighbor in the same group. The motivation of thiswork was due to the fact that a few silicides were studied forstructural and physical properties, which can be extended withthe discovery of new compounds. Notable examples ofreported silicides are: Ce2CoSi3,15 Ce2CuSi3,16,17 Ce2RhSi3

18

and Ce2PtSi319 show Kondo behavior, Ce2PdSi3 is an anti-

ferromagnetic Kondo lattice, large negative magnetoresistanceis observed in Eu2CuSi3

20 and RE2PdSi3 (RE = Gd, Tb, Dy),21,22

complex spin glass behavior is seen in Eu2PdSi323 and

U2RhSi3,24 novel magnetic properties in Er2PdSi3,25 andmagnetic anisotropy are found in Ho2PdSi3.26 The details ofthe synthesis and properties of these compounds have beensummarized by Pan et al. in their recent review.27 Thoughthere are a handful of reports on the synthesis of silicon basedcompounds using high temperature methods like Bridgman,28

Czockralsky29 and floating-zone30–32 methods, metal fluxtechnique has come in salvation in many cases where theapplication of the above methods is difficult due to the heavycost.33,34

Here, we report the synthesis of two new compoundsEu2AuSi3 and Yb2AuSi3 using the metal flux technique usingindium as inactive flux. The crystal structure of bothcompounds was studied using X-ray diffraction from theselected single crystals. Eu2AuSi3 adopts the Ca2AgSi3 structuretype35 with Fmmm space group whereas Yb2AuSi3 crystallizesin the Ba2LiSi3 structure type36 with Fddd space group. Bothstructures are ordered superstructures of the AlB2 structuretype. A majority of the compounds with general formula

aNew Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,

Jakkur, Bangalore, 560064, India. E-mail: sebastiancp@jncasr.ac.in; Fax: +91

080-22082627; Tel: +91 080-22082298bISIS Facility, STFC-Rutherford Appleton Laboratory, Didcot, OX11 OQX, United

Kingdom

3 Electronic supplementary information (ESI) available: CCDC 945226 and945227 for Eu2AuSi3 and Yb2AuSi3, respectively. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c3ce41198b

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RE2TX3 are derived from the AlB2 prototype through varioussymmetry reduction methods like klassengleiche (k) andtranslationengleiche (t).37 The magnetic susceptibilities of boththe compounds suggest that Eu2AuSi3 and Yb2AuSi3 are havingdivalent rare earth atoms. Eu2AuSi3 is a paramagnet above 30K and orders antiferromagnetically at 26 K and Yb2AuSi3 is adiamagnet without any ordering down to 2 K.

2. Experimental

2.1. Synthesis

Europium (ingots 99.99%, ESPI metals), ytterbium (ingots99.9%, Alfa Aesar), gold (ingots, 99.99%, Alfa Aesar), silicon(shots, 99.999%, Alfa Aesar) and indium (tear drops, 99.99%,Alfa Aesar) were used as purchased without any furtherpurification.

2.2.1. Metal flux method. 3 mmol of rare earths (Eu/Yb), 2mmol of gold, 6 mmol of silicon and 30 mmol of indium werecombined in an alumina crucible under an inert (argon)atmosphere inside a glove box. The excess indium acts as aninactive metal flux. The crucible was placed in a 13 mm quartztube and was flame-sealed under vacuum of 1026 Torr toprevent oxidation during the progress of the reaction. Theevacuated tube was placed in a vertical tube furnace and washeated to 1273 K in 10 h, kept at that temperature for 6 h. Thetemperature was then lowered down to 1123 K in 2 h andannealed at this temperature for 72 h. Finally, the system wasallowed to cool slowly to room temperature in 48 h. Thereaction products were isolated from the excess indium flux byheating at 623 K and subsequent centrifugation through acoarse frit. The remaining flux was removed by immersion andsonication in glacial acetic acid for 24 h. The final crystallineproduct was rinsed with water and dried with acetone in avacuum oven at 350 K overnight. Eu2AuSi3 was grown as shinybi-prismatic shaped crystals with an average dimension of 2–3mm, whereas, rod shaped crystals of Yb2AuSi3 were found. Thesingle crystals of Eu2AuSi3 were stable in air and moisture withno decomposition observed even after several months but thecrystals of Yb2AuSi3 sample turned black in aerial conditionsafter 1–2 weeks. Eu2AuSi3 was obtained in relatively high yield(80%) with EuAuIn4 and EuAu2In4 as the main impurityphases whereas the Yb2AuSi3 compound was produced in highyield (of around 97%) with YbAuSi and AuIn2 as the minorimpurity phases. Single crystals were carefully selected fromthese batches for the elemental analysis, structure character-ization and the magnetic measurements.

2.2. Elemental analysis

Quantitative microanalysis on Eu2AuSi3 and Yb2AuSi3 wereperformed with a FEI NOVA NANOSEM 600 instrumentequipped with an EDAX1 instrument. Data were acquiredwith an accelerating voltage of 20 kV and a 100 s accumulationtime. Typical metallic prismatic and rod shaped single crystalsof Eu2AuSi3 and Yb2AuSi3 obtained from the flux method areshown in Fig. 1a and 1b, respectively. The EDAX analysis wasperformed using P/B-ZAF standardless method (where, Z =atomic no. correction factor, A = absorption correction factor,

F = fluorescence factor, P/B = peak to background model) onvisibly clean surfaces of the crystals.

2.3. Powder X-ray diffraction

Powder X-ray diffraction patterns of Eu2AuSi3 and Yb2AuSi3

were collected at room temperature on a Bruker D8 DiscoverX-ray diffractometer with Cu Ka X-ray source (l = 1.5406 Å) todetermine the phase identity and purity. The instrument isequipped with a position sensitive detector. Data werecollected in the angular range 20u ¡ 2h ¡ 90u with the stepsize 0.02u and scan rate of 0.5 s per step calibrated againstcorundum standards. The experimental patterns were com-pared to the pattern simulated from the single crystal structurerefinement and are shown in Fig. S1 and S2, ESI.3

2.4. Single crystal X-ray diffraction

Carefully selected single crystals of Eu2AuSi3 and Yb2AuSi3

were mounted on a Mitegen pin using a minimum amount ofParatone oil. X-ray single crystal data were collected at room-temperature on a Oxford Diffraction Supernova diffractometerequipped with an ATLAS CCD detector. Mo Ka radiation with awavelength of l = 0.71073 Å operating at 50 kV and 0.8 mA. Afull sphere of v scans was recorded using 287 frames with astep of 1u and 22.27 s per frame up to 2h of 58.67u. Data wereprocessed using Crysalis Pro and an analytic absorptioncorrection was applied using a face-indexation of the crystal.Both crystal structures were solved by SHELXS 9738 and refinedby full matrix least-squares method using SHELXL.39 Diamond(version, 3.2) was used for generating packing diagrams.

2.5. Structure refinement

2.5.1. Eu2AuSi3. The single crystal data of Eu2AuSi3 showedan orthorhombic cell and the systematic extinctions werecompatible with the F lattice. The atomic parameters ofCa2AgSi3 were used as the starting parameters and thestructure was refined using SHELXL-97 (full-matrix least-squares on F2) with anisotropic atomic displacement para-meters for all atoms. As a check for the correct composition,the occupancy parameters were refined in a separate series ofleast-squares cycles. Initially, there were five crystallographi-cally different positions in the Eu2AuSi3 structure – two for theEu and Si atoms and one for the Au atom. During the isotropicrefinement, it was observed that the atomic displacementparameters of the gold and silicon atoms were anomalouslylarge. Furthermore, the refinement was largely unsatisfactory

Fig. 1 FE SEM image of (a) Eu2AuSi3 and (b) Yb2AuSi3 single crystals. The edgesof the Eu2AuSi3 crystal are rounded due to the chemical etching with acetic acid.The surface of Yb2AuSi3 crystal is attached with excess indium used in thesynthesis.

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owing to high residuals (R1 . 20%), and large electron densityresiduals (45–23 e Å23) around the gold and silicon atoms.Anisotropic refinement did not improve the refinement andalso resulted in abnormal cigar-shaped gold and siliconatomic displacement ellipsoids (U11 = 0.050 Å2). The anom-alous atomic displacement parameter could not be resolved bysubsequent refinement of the occupancy parameters. All thesefeatures indicated a crystallographic disorder associated withthe Si and Au atoms. Consequently, gold and silicon atomswere refined by mixing silicon and gold sites, respectively. Theresulting atomic displacement parameters of mixed positionsbecame well-behaved and the final difference maps showedresiduals that were reasonably acceptable. The final refine-ment gives an atomic ratio of Eu2AuSi3.

2.5.2. Yb2AuSi3. The data for Yb2AuSi3 suggested anF-centered orthorhombic lattice with lattice parameters a =8.1886(3), b = 14.1771(5), c = 16.8218(6). The atomiccoordinates of the Ba2LiSi3 structure type were used as astarting model for the refinement. The preliminary isotropicrefinement in least square method with 10 cycles yielded veryhigh residuals (R1 . 11%) and electron density residuals alsovery high (.80 e Å3). Anisotropic refinement could not solvethe problems of high residuals. In the next step Au and Sipositions were mixed as similar to Eu2AgGe3 and the structurewas refined with reasonable residual parameters.

Relevant crystallographic data for the data collections andrefinement parameters for both compounds are summarizedin Table 1. The atomic coordinates and equivalent atomicdisplacement parameters, anisotropic atomic displacementparameters and important bond lengths are listed in Tables 2–4, respectively.

2.6. Magnetic measurements

Magnetic susceptibility measurements for the Eu2AuSi3 andYb2AuSi3 samples were carried out with a Quantum DesignMPMS SQUID magnetometer. Around 16 crystals of Eu2AuSi3

and 13 crystals of Yb2AuSi3 were respectively mounted withoutgrounding to a brass holder and affixed to the end of a carbonfiber rod in random fashion in a small butter paper forcarrying out the magnetic measurements. Temperaturedependent data were collected between 2 and 300 K, with anapplied magnetic field of 1000 Oe. In a typical measurement,data were collected while cooling the sample from 2 to 300 K.Field dependent magnetic measurements were acquired at 2and 300 K with field sweeping from 260 kOe up to +60 kOe forboth the samples. The raw data were corrected for the sampleholder (brass) contribution.

3. Results and discussion

3.1. Crystal structure

3.1.1. Eu2AuSi3. Eu2AuSi3 crystallizes in the Ca2AgSi3

structure type which is an ordered orthorhombic super-structure derivative of the AlB2 structure type. The overallcrystal structure of Eu2AuSi3 shown in Fig. 2 contains fivedifferent kinds of atomic sites – two Eu, two [Au + Si] mixedsites (represented as M1 and M2) and one Si site. The structureis comprised of infinite hexagonal network of [M12M24] and[Si2M24] and [Si2M12M22] rings as shown in Fig. 2b. Theinterlayer Eu–Eu distance in this compound ranges from4.4850(9) to 4.5519(9) Å. This Eu–Eu bond distance matcheswith the same distance in other Eu based compounds such asEu2AuGe3,4 EuCu2Si2,40,41 EuGe2

42 etc. with divalent europium.

Table 1 Crystal data and structure refinement for Eu2AuSi3 and Yb2AuSi3 at 293(2) Ka

Empirical formula Eu2AuSi3 Yb2Au0.98Si3.02

Formula weight 584.52 624.15Temperature (K) 293(2) 293(2)Wavelength (Å) 0.71073 0.71073Crystal system Orthorhombic OrthorhombicSpace group Fmmm FdddUnit cell dimensions (Å) a = 8.306(2), b = 9.0369(18), c = 14.377(3) a = 8.2003(16), b = 14.187(3), c = 16.869(3)Volume (Å3) 1079.2(4) 1962.5(7)Z 8 16Density (calculated) (g cm23) 7.195 8.450Absorption coefficient (mm21) 50.479 67.628F(000) 1974 4157Crystal size (mm3) 0.086 6 0.060 6 0.048 0.082 6 0.014 6 0.012h range for data collection (u) 3.62 to 45.58 3.75 to 45.52Index ranges 216 ¡ h ¡ 14, 218 ¡ k ¡ 16, 228 ¡ l ¡ 28 216 ¡ h ¡ 16, 228 ¡ k ¡ 18, 233 ¡ l ¡ 33Reflections collected 9130 14 628Independent reflections 1279 [Rint = 0.1074] 2092 [Rint = 0.0637]Completeness to h = 45.52u (%) 99.8 99.8Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters 1279/0/25 2092/0/34Goodness-of-fit 1.008 1.034Final R indices [.2s(I)] Robs = 0.0320, wRobs = 0.0709 Robs = 0.0433, wRobs = 0.0880R indices [all data] Rall = 0.0346, wRall = 0.0740 Rall = 0.0793, wRall = 0.1017Extinction coefficient 0.00045(3) 0.000021(5)Largest diff. peak and hole (e Å23) 8.195 and 27.414 9.737 and 26.506

a R = S||Fo| 2 |Fc||/S|Fo|, wR = 371/2 and calc w = 1/[s2(Fo2) + (0.0333P)2 + 0.0000P] where P = (Fo

2 + 2Fc2)/3.

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The distance between two adjacent hexagonal layers isprecisely 4.5186(9) Å, means that the hexagonal rings arecompletely flat and planar. In comparison to the Eu2AuGe3

4

and Yb2AuGe3 compounds,5 Au and Si atoms in the Eu2AuSi3

structure have well behaved anisotropic thermal parameters

and we anticipate that no probable phase transitions can beexpected in higher and lower temperature. The shortestdistance between the sites comprising the hexagonal networkis 2.3714(4) Å for M2–Si suggests a strong interaction betweenthem.

Due to the difference in ordering among Au and Si sites inthe hexagonal array, the coordination environment aroundEu1 and Eu2 slightly varies. There are two crystallographicallydifferent Eu sites in Eu2AuSi3. Both of them reside in ahexagonal bipyramidal environment built up of Au and Si sites(Fig. 4). Eu1 is sandwiched between [M12M24] and [Si2M24]hexagon rings. The Eu1–Si and Eu1–M distances are 3.2565(5)Å and 3.2692(4)–3.3119(4) Å respectively. Eu2, on the otherhand, is sandwiched between two [Si2M12M22] hexagon rings.The Eu2–M distance is in the narrow range of 3.2952(4)–3.3007(5) Å and the distance between Eu2 and Si is precisely3.3070(4) Å.

3.1.2. Yb2AuSi3. The crystal structure of Yb2AuSi3 is shown inFig. 3. Yb2AuSi3 crystallizes in the Ba2LiSi3 structure type,another ordered superstructure derivative of the hexagonalAlB2 type with Fddd space group and lattice parameters are a =8.2003(16) Å, b = 14.187(3) Å, c = 16.869(3) Å. Among fivedifferent crystallographic positions, there are three crystal-

Table 2 Atomic coordinates and equivalent isotropic displacement parameters (Å2 6 103) for Eu2AuSi3 and Yb2AuSi3 at 293(2) K with estimated standard deviationsin parentheses

Label Wyckoff site x y z Occupancy Ueqa

Eu2AuSi3

Eu(1) 8h 0.0000 0.2518(1) 0.0000 1 8(1)Eu(2) 8f 0.2500 0.2500 0.2500 1 9(1)Si 8i 0.0000 0.0000 0.6642(2) 1 10(1)M(1) 8i 0.0000 0.0000 0.1665(1) 0.79(2)Au + 0.21Si 8(1)M(2) 16n 0.2519(2) 0.0000 0.0826(1) 0.89(13)Au + 0.11Si 9(1)Yb2AuSi3Yb(1) 16g 0.1250 0.1250 0.0005(1) 1 8(1)Yb(2) 16g 0.1250 0.1250 0.5003(1) 1 8(1)M(1) 16f 0.1250 0.4582(1) 0.1250 0.77(1)Au + 0.23Si 8(1)M(2) 16f 0.1250 0.2904(1) 0.1250 0.08(1)Au + 0.92Si 5(1)M(3) 32h 20.1277(1) 0.5422(1) 0.1249(1) 0.06 (1)Au + 0.94Si 6(1)

a Ueq is defined as one third of the trace of the orthogonalized Uij tensor. M = [Au + Si].

Table 3 Anisotropic displacement parameters (Å2 6 103) for Eu2AuSi3 andYb2AuSi3 at 293(2) K with estimated standard deviations in parenthesesa

Label U11 U22 U33 U12 U13 U23

Eu2AuSi3Eu(1) 8(1) 8(1) 9(1) 0 0 0Eu(2) 8(1) 9(1) 9(1) 0 0 0Si 5(1) 16(1) 8(1) 0 0 0M(1) 8(1) 11(1) 7(1) 0 0 0M(2) 5(1) 14(1) 7(1) 0 0(1) 0Yb2AuSi3Yb(1) 8(1) 8(1) 7(1) 1(1) 0 0Yb(2) 8(1) 8(1) 7(1) 1(1) 0 0M(1) 6(1) 6(1) 11(1) 0 1(1) 0M(2) 3(1) 2(1) 10(1) 0 22(1) 0M(3) 4(1) 3(1) 11(1) 21(1) 2(1) 0(1)

a The anisotropic displacement factor exponent takes the form:22p2[h2a*2U11 + … + 2hka*b*U12]. M = [Au + Si].

Table 4 Bond lengths [Å] for Eu2AuSi3 and Yb2AuSi3 at 293(2) K with estimatedstandard deviations in parenthesesa

Label Distances Label Distances

Eu2AuSi3Eu(1)–Si 3.2565(16) Si–M(2) 2.3714(15)Eu(1)–M(2) 3.2692(8) Si–M(1) 2.434(2)Eu(1)–M(1) 3.3026(6) M(3)–M(2) 2.3761(14)Eu(2)–M(1) 3.2952(4) M(1)–M(2) 2.4144(10)Eu(2)–M(2) 3.3007(7)Yb2AuSi3Yb(1)–M(3) 3.1455(10) M(1)–M(3) 2.3901(10)Yb(1)–M(2) 3.1491(10) M(1)–Yb(1) 3.1747(4)Yb(2)–M(1) 3.1659(5) M(2)–M(3) 2.3427(12)Yb(2)–M(3) 3.1760(11) M(2)–Yb(2) 3.1797(6)M(1)–M(2) 2.3816(14) M(3)–M(3) 2.348(2)

a M = [Au + Si].

Fig. 2 Crystal structure of Eu2AuSi3 viewed along a- and b-axis. The unit cells aremarked with red solid lines. [Si2M24] and [M12M24] hexagons are shaded withblue and pink colors, respectively and [Si2M12M22] kept open.

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lographically non-equivalent sites (M1, M2 and M3) withmixed positions of Au and Si atoms building up the infinitehexagonal network of Yb2AuSi3. These layers are stackedparallelly along the c-axis with two crystallographicallydifferent Yb atoms embedded between two adjacent layers.The interlayer distance between adjacent Yb–Yb ranges from4.2010–4.2302 Å. Considering the fact that divalent Yb is largerin size (1.86 Å)43 compared to trivalent Yb (1.66 Å),44 both Ybatoms in Yb2AuSi3 are in the nonmagnetic divalent state,which is later confirmed by the magnetic susceptibilitymeasurements. The distance between two adjacent hexagonallayers is 4.2150(8)–4.2198(9) Å suggests that the rings are

highly planar which is in sharp contrast to the prototypecompound Eu2AgGe3, where the interlayer distance betweenthe hexagonal rings and adjacent Eu–Eu vary in the wide rangeof 4.3699 to 4.5396 Å and 4.3086 to 4.3129 Å, respectively,indicating puckered hexagonal rings owing to high anisotropicthermal displacement parameters of Ag and Ge mixed sitesalong the c-axis (U33). Yb1 and Yb2 atoms reside in a hexagonalbiprismatic coordination environment built up completely byAu and Si mixed positions (Fig. 4). The Yb1–M and Yb2–Mdistances are in the range of 3.1455(4)–3.1820(4) Å and3.1659(4)–3.1797(4) Å suggesting a strong interaction betweenthem.

3.2. Magnetic properties

3.2.1. Eu2AuSi3. Temperature dependent magnetic suscept-ibility and inverse susceptibility of Eu2AuSi3 at an applied fieldof 1 kOe in both ZFC and FC modes are plotted in Fig. 5a. Themagnetic susceptibility sharply increases with decreasingtemperature, reaching a maximum at 26 K followed by sharpdecrease indicates antiferromagnetic ordering. The two curvesin ZFC and FC modes curves do not bifurcate at any pointhinting towards absence of any kind of spin-disorder (spinglass behavior).45 The temperature dependence of inversemagnetic susceptibility curve (Fig. 5a) follows Curie–Weiss law

Fig. 4 Coordination environments of Eu1 (a), Eu2 (b), Yb1 (c) and Yb2 (d) in thecompounds Eu2AuSi3 and Yb2AuSi3.

Fig. 3 Crystal structure of Yb2AuSi3 viewed along a- and c-axis. The unit cells aremarked with red solid lines. [M12M34] and [M22M34] hexagons are shaded withblue and pink color and [M12M22M32] kept open.

Fig. 5 (a) Temperature dependent molar magnetic and inverse susceptibilities,the inset shows the variation of the product of susceptibility and temperaturewith temperature and (b) field dependence magnetic moment of Eu2AuSi3.

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above 30 K. A fit to the curve within the range 30–300 K gave aneffective magnetic moment (meff) of 8.13 mB per Eu which isclose to the spin only magnetic moment of divalent europium(7.94 mB per Eu) and paramagnetic Curie temperature (hp) 14.6K. The positive and low value of hp indicates an overall weakferromagnetic interaction present at low temperature. At thispoint, it is worthwhile to mention that the prototypecompound Eu2AuGe3 orders antiferromagnetically at 18 Kand there is a ferromagnetic clustering at 25 K.4 Similarcoexistence of antiferromagnetic and ferromagnetic interac-tions has been reported for EuSnP which has layered structurecontaining Sn sheets.46,47 A possible existence of spin-cantingis hinted by the initial increase of xT value with temperature atlow temperature range (inset of Fig. 5a)48 as well as by thesigmoidal curve in field dependent magnetization plot at 2 K(Fig. 5b).49

The field dependence of magnetization has been shown inFig. 5b. The low temperature (2 K) curve is sigmoid shapedcarrying an overall signature of an antiferromagneticallyordered state. It is to be noted that at the low field (210 to10 kOe), the curve is wavy in nature hinting a probable weakmetamagnetic transition which was also observed in case ofpreviously reported Eu2AuGe3.4 The curve shows no sign ofsaturation upto maximum applied magnetic field 60 kOe.

3.2.2. Yb2AuSi3. Temperature dependent magnetic suscept-ibility in both ZFC and FC modes at an applied field of 1000 Oefor the compound Yb2AuSi3 are plotted in Fig. 6a. The molarmagnetic susceptibility data at room temperature suggeststhat Yb2AuSi3 is a diamagnet wherein the magnetic suscept-ibility does not vary much with temperature down to 50 Kbelow which it starts increasing exponentially upto 2 K owingto the dominance of field induced magnetic ordering overthermal agitation, although no sign of magnetic orderingdown to the lowest attainable temperature is observed. Thefield dependence of magnetic moment is plotted in Fig. 6b.The overall feature of both plots is consistent with that of adiamagnet. At low temperature, the curve decreases logarith-mically with increasing field; whereas at higher temperature(300 K), the variation is almost linear similar to a diamagnet.Interestingly, a strange splitting in the fifth quadrant of M vs.H curve was noticed for this compound. This could beattributed to the experimental error due to resolution limitof SQUID for diamagnetic samples where the magneticsusceptibility is very small.50

To find out the effect of aerial oxidation on the magneticproperty of Yb2AuSi3, we have performed the magneticmeasurement on the sample kept in aerial conditions fortwo weeks and found out that the effective magnetic moment(meff) of the oxidized sample to be 2.80 mB which indicates thataround 62% of Yb in the sample is converted from divalent totrivalent state based on the calculations from xT vs. T plot at300 K (Fig. S3 in the ESI3). The field dependence of magneticmoment (Fig. S4 in the ESI3) also suggests that the compoundbehaves like a paramagnet at 300 K, whereas in the case of lowtemperature (2 K) curve, slope changes continuously with fieldwithout any saturation upto the maximum applied field. Thisindicates that at low temperature spin-ordering occurs withincreasing field but long range ordering does not exist in thepartially oxidized sample.5,45

4. Conclusion

Two new intermetallic compounds Eu2AuSi3 and Yb2AuSi3

were synthesized using molten indium over a broad range ofsynthetic conditions. Eu2AuSi3 and Yb2AuSi3 crystallize in theorthorhombic Ca2AgGe3 and Ba2LiSi3 structure types, respec-tively. Unlike the previously reported RE2TGe3 (RE = Eu, Yb; T= Ag, Au), these two compounds are more stable having flathexagonal rings and fairly low anisotropic displacementparameters (all below 10 6 1023 Å2). Our work establishesthe fact that the Ge atom which is comparatively larger in sizethan the Si atom could possibly be responsible for moredisordered systems and cause phase transitions upon varyingthe temperature. The magnetic susceptibility studies show thatEu2AuSi3 is an antiferromagnetic below 26 K whereas Yb2AuSi3

is a diamagnet at room temperature. The discovery of thesenew compounds motivated us to search new RE2TX3 com-pounds in the silicide family using the metal flux technique.Further research in this series of compounds may pave the wayto interesting structural and physical properties.

Fig. 6 (a) Temperature dependent molar magnetic susceptibility the inset showsthe variation of the product of susceptibility and temperature with temperatureand (b) field dependence of magnetic moment of Yb2AuSi3.

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Acknowledgements

We thank Jawaharlal Nehru Centre for Advanced ScientificResearch, Sheikh Saqr Laboratory and Department of Scienceand Technology, India (DST) for financial support. S. S. thanksthe Council of Scientific and Industrial Research (CSIR) forresearch fellowship and S. C. P. thanks the DST for theRamanujan fellowship. We are grateful to Prof. C. N. R. Rao forhis constant support and encouragement. We thank Mrs. N. R.Selvi and Prof. G. U. Kulkarni for SEM measurements. Wegratefully acknowledge the access to the X-ray diffractionfacilities at the Research Complex at the Rutherford AppletonLaboratory (Oxfordshire, United Kingdom).

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