Crystal growth and characterizations of ErRh3B2

ELSEVIER

Journal of Alloys and Compounds 248 (1997) 18-23

Crystal growth and characterizations of ErRh,B, Toetsu Shishido”, Jinhua Yeb, Masaoki Oku”, Shigeru Okadac, Kunio Kudou’, Takahiko Sasaki”, Takehiko Matsumotob, Tsuguo Fukuda”

Received 23 July 1996

Single crystals of ternary boride ErRh,B, have been successfully grown by the flux method using molten copper as a solvent: a variety of properties concerning the crystal structure and chemical states of the compound were investigated. ErRh,B, belongs to a monoclinic sysrem isomorphous with Erlr,B,: space group C2/m, a =05355(I), b = 0.9282( 1). c = 0.3102( 1) nm and /3 = 90.89(3)“. A superlattice appears along both the a and c axes with periods of 3a and tic respectively. X-my photoelectron spectroscopic study indicates that boron acts as an electron donor. According to the differential thermal analysis in air. the oxidation of the compound starts at 373 “C. The weight gain of the sample after heating in air up to 12OOT is 15.44% and the mixed phase of ErBO, and Rh is confirmed by X-ray diffraction measurement. The value of the micro-Vicken hardness for (001) and (100) faces of the crystals is 11.3-12.0 GPa and IOA-10.9GPa respectively. Keywords:

ErRh,B::

Cryrtal

smcme:

XPS:

TG-DTA:

Micro-Vicken

hsrdnerr

1. Introduction

2. Experimental 2.1. Sample preparation

Three different compounds ErRh,B. ErRh,B, and ErRh,B, exist at the rhodium-richportion of the Er-Rh-B tentativeternary phasediagram [ 1,2]. These compounds have attractedattentionbecauseof an interestingcompetition betweenmagnetism and superconductivity [3,4]. The physical properties of these &ides have been measured using polycrystallioe samples. In some cases tire results of these measuremenis were not clear because of the existence of a multiphase in the polyctystallitte samples. The growth of singlecrystals of each compound and

systematicmeasurementsusing the single crystals are desirable.However,singlecrystal growth has beenconsidered difficult becausetheir structure is unstableat high temperature,in spite of their high melting point. The prcscnt paper reportstbc single crystal growth of the title compound RrRh,B, by the flux method using molten

Cu as a solvent.

The crystal

structure,

chemical

and

XPS analysis,oxidation rcsistivity and microhardnessfor the singlecrystals obtainedare also reported. 0925~8388/97/517.00 PII S0925-8388(r6)02621-7

8

1997 Elwvier Sctcnce S.A. All rights reserved

The raw materialsused were small piecesof 99.9% Er. Rh powder aad 99.9% B powder. They were weighed at aa atomic ratio Er:Rh:B of l:3:2 (stoichiometric proportion),and mixed with 99999% Cu powderin a weight ratio l:IO. The mixture was placed in a dense aluminacrucible.The crucibtewas insertedinto a vertical electricfurnace. Purified He gas flowed in the furnace as a protectingatmosphereagainstoxidation.Fig. 1 shows the schematic arrangementof the growth apparatus.The mixture was heatedat a rate of 490°C h-’ and held at 1350“C for IO h. The solutionwas cooledto 1080“C at a rate of I “C h-’ and then quenchedto room temperature. The crystals were separatedby dissolving Cu in dilute nitric acid.

99.9%

2.2.

Characterization

The morphologicalpropertiesand impurity of the crys-

7. Shishido

ct al. I Journal

of Alloys

and Compounds

248 11997)

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19

solvent. Idii single crystals of hexqaml prism maximum dimensions lXl~3nn$ with silver met&c lustre as shown in Fig. 2(a) wele extracted f;on lbe

solution. Fig. 2(b) is an e&qemem of the region ia&cated by an arrow in Fig. 2(a));a rare origin&g fn-nn growthstepswasobsmedrtbeqion.CbcmiwIaa&ysisfortbecrystalsshowuithattbccbemiicoqo&km of tbe compoundalmost cWeqs&d tonatomicmtio Er5WB of 19~2; BrRh,B, exists at the &dium-rich portion of the Er-Rh-B tentativeternary phasediagramas shown in Fig. 3. No evidence has been obtainedof the prc;nm of a Cu-coutaining phase in the crystal, as concluded fmm chemical analysis and E P M A of as-grown and fraamed surfaces. The crystal structum investigationof the singk crystal Fig. I. Schematicammgemencof the gmwlh appmrus.

tals were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe micro-

analysis(EPMA). The chemicalcompositionwas analyzed by E P M A and the inductive coupled plasma atomic emissionspectrometry(ICP-AES) method after fusion of the sampleswith NH,HSO,. Crystal structure determination was carried out using an X-my powder diffractometer. a precession camera. and a four-circle X-my diffractometer with graphite monochmmated M O Ka radiation. An X-my photoelectron spectroscopic (XPS) study was performed for a fractured single

crystalline surface to determinethe chemical statesof Er, Rh and B in the compound.The spectrawere taken with a Surface ScienceLaboratoriesmodel SSX-I00 spectrometer, which had a monochromatic Al Ku source with 300x 450 Fm2 spot. Thermogravimctric (TG) analysis and differential thermal analysis (DTA) were performedbetween room temperature and 12C10°C to sudy the oxidation

resistivity of crystals in air. Powders were prepared by grinding in an agate mortar. A pulverized specimenof about 25 mg was heated at a rate of 10°C min-‘. The oxidationproductswere analysedby powder X-ray diffractom&y. The micro-Vickers hardness(MVH) of the asgrown single crystals was measured at room temperature. A load of IOOg was applied for I5 s and at least I5 impressions wem recorded for (001) and (LOO) faces of each crystal. Tbe obtained values were averaged and the

experimentalerror was estimated. 3. 3.1.

Results and dIscussion Morphology

and structure

Thesingle crystals of ternary boride were successfully grown by the flux method using molten copper as a

Fig. 2. (al Ed~llM T-n

20

T. Shishido

et al. I Journal

of A//ow

and Compounds

248 (1997)

18-23

-B Fig.

3. Er-Rh-B

tentative

phase diagram

(rhodium-rich

portion).

BrRh,B, shows that it belongs to a monoclinic system isomorphous with the ErIr,Bz structure, which is slightly distorted from the hexagonal CeCo,B, structure within the basal plane. This result is in agreement with the literature [S]. Tk perspective drawing of the structure of ErRh,Bz along the c direction is shown in Fig. 4. The large, medium and small circles represent Er, Rh and B atoms respectively. The height of the layers is also indicated. The crystallographic data on ErRh,B, are summarized in Table 1. The polyhedron coordination of the boron atom in ErRhsB, is shown in Fig. 5. A boron atom is surrounded by six Rb atoms and three Er atoms. The six Rh atoms form a prism,

Fig. 5. The polyhedron open circles small

Table

coordination

of the boron

Er ~~mms. intemxediate

amm

in Erftb,B,.

ones are Rh atoms

Large and the

and the three Er atoms locate at the corner of a triangle. The interatomic distances M-B (M = Er or Rh) are listed in Table 2. Comparing with the sum of Pauling’s single bond metallic radii, the Er-B and Rh-B distances enlarge with the compound formation. Precession photography reveals first that a superlattice exists along both then and c axes with periods of 3a and 6c respectively (Fig. 6). Details of the superstructure are under investigation [6]. 3.2. X-ray

Fig. 4. Perspective drawing of the structure of ErRh,B, along the c dir&on. Large circks represent Er aloms, medium circles are rhodium atoms and small circks are boron atoms. Haghts of layers are indicated.

represent

one is dte B awm~.

photoelectron

spectroscopy

XP8 was performed to study the chemical states of Er, Rh and B atoms. The Er 4d. Rh 3d and B Is XP spectra are shown in Fig. 7. The Er 4d spectrum for metal Er was taken during light argon ion sputtering in order to prevent oxidation of the metal. The spectra of the fractured sample did not change during the XP8 measurement. The remarkable difference in the Er 4d region between the metal and the compound was found in the intensity ratio of the peaks at 167.5 and 170 eV. The intensity ratio reversed from the metal to the compound. The broad peaks at I85 and 195 eV were observed for the metal. The existence of tb peaks in the spectrum of the compound could not be confirmed, because of its low

I

Crystal Chemical

data of ErRh,B, formula

Cryshll system snlJc1ure type

ErRb,BI monoclinic Erlr,B2 C2lm

Tabk 2 Interatomic M-B

a mm b @III)

0.5355( I ) 0.9282( I )

c (nm) B (9 Formula unhs per unit cell X-ray density (g cm-‘)

0.3102(I)

Er-B Er4B Er-2B Rb-B Rb( I )-4B

90.89(3) 2

Rb(2)-2B Rh(2)-2B

Swe gmuP Unit cell parameter

10.72

distances

in the coordination

@I = Er M Rh) bond

’ Sum of Pading’s

Disunce

._

Dolvbedmn (nm)

0.238 0.308(3) 0.312(7)

0.206 0.217(S) 0.218(2) 0.221(2) single

bawd metallic

radii.

._

of boron in ErRb.B.

Close-packed

kngth’

(nm)

T. Shishido

et al. I Jmmal

of Alloys

a+

and Compounds

248 (1997)

Id-23

21

ofthecompoundishugerthanthatoftheBammsinthe metal.Tbismeansthatboronactsasanckctrondunorin the compound. 3.3. TG-DTA and hardness

Fig. 6. Illusuation of m X-my O-level precession photograph around tk b wis in ErRh,B>. Numenlr indicate the index of fundmenml reflections. The splitting of some r&&m is due to the existence of twinning in the a-h plane.

The TG-DTA curves in air for ErRh,B, am shown in Fig. 8. According to the TG curve, o&ation of the compound begins at 373 “c aad hecomes even strouger at 975°C. The weight gain of the sample a&r TG determination, heated to 1200°C in air, was 15.44%. The DTA curve reveals two distinct exothermk peaks at 798 and 1014OC. and oue emb&mdc peak at lu7S”c. The mixed phase of ErBG, and Rh was identified by X-ray diffraction determination. The exothermk peaks are artributed to oxidation and the e&o&m& peak may be due to thermal decomposition and forma&m of metallic Rh. In our recent paper on the oxidation resistivity of ErRh,B and ErRh,B, [12], both m showed high thenno& emical stability. In contrast. ll%IHdkOxidrtioniSC!bserved for ErRh,Bx. The oxkBtion resistivity of the compounds seems to be rehued to these crystal smmtmes. In the case of ErRh,B,, its crysml sb\wn*cbelongs to the cazn,-lype [13]. It is well known that CaZn, structure type compounds take Byer slructme and generally show strong n5activeaess. for example agahkst I-I, [ 141, because the inlerlayer bond sbength of this family is weak and reveals such a high reactivity. In the case of ErRh,B?, as shown in Fig. 4, it has two kinds of Lyer one consistsofErandB,Pndthe~hPS~yRh.Thtsctwo kinds of layer may stpclr loosely aBemately. hence the oxidation resistivity of the ErRh,B, is so low. Fur&r discussions on Ute relation between the crystal stnmtum aod oxidation resislivity among these lhme compom& will be given elsewhere. As presented in Table 3, the valoe of the M V H for (001) and (100) planes of the ErRhsB, crystals is 11.3-12.OGPa and 10.4- 10.9 GPa mpeetively. Ibe data for ErRhsB and ErRh,B, crystals are co-Iii in the table [12]. It can he seent~tttvalueofthcMVHfathe~al~iskrger thanthevahtefortheiMerp&ne.Thisindkateslhatlhe bondingstrcngthofthei&aplaaeislargerlhanlhatofIhe interplane. lo a previous study, the v&e of the M V H increases with increasing boron content of the borides ]lS]. This tendency is essentially in agreement with our dam.

4. cnnelusions The single crystals of ternary boride BrRhsB, have been grown and structurally chamcmrixed. The authors can draw the following conchusions fnnn this study. (I) The single erystak oftemmy boride wem successfully grown by the Bux meIhod using mohen copper as a solvent. Rliomorphic single crysIak3 with sliver mUaRk. lustre, whose shape is a hexagonal prism wilh maximum

22

T. Shishido

et al. I Journal

of Alloys

and Compounds

Binding energy

248 (1997)

18-23

(ev)

@I ErRh,B, _, ..,.

. .._.

u, I

320

310

i3itdingm~~

(ev)

Fig. 7. XP spectra of (a) ErRh,B, and Er, (b) ErRb,B, and Rb. (c) ErRb,B, and 6. specua respeclively. whew the background is Shirley-type.

dimensions I X I X3 mm3, were extracted from the. solutioa. (2) Chemical analysis for the crystals shows that the chemical composition of the compound corresponds almost to aa atomic ratio Er:RhzB of 1:3:2.

Tbc tine and bold lines arz the observedand backgroundsubtracted

(3) ErRh,B, belongs to a monoclinic system isomorphous with Brlr,B,: space group C2/m, o =0.5355(l), b= 0.9282’.1). c =0.3102(l) nm aad p = 90.89(3)9 A superlattice appears along both the 0 and c axes with periods of 3a and 6c respectively.

T. Shishido et al. I Journal of Alloys and Cmn+ Table 3 Micro-Vickers hardness of the sin&

-.

.

the svstem Er-Rh-B

248 (1997) M-23

23

- -

tloadinr IO0a.X 153

Testing compound. stmcwe

B ~ontenf in the compound (at.%)

LoadingorknUIioa

nwH (GR)

ErRh,B, cubic ErRh,B,. monoclinic

20 33.3

4.77497

ErRh,B,, Ievdgonnl Rh,B,. hexagonal RhB. hexagonal

44.4 30 50

( 100) to01 ) (loo) ~I00)or(ll0) -

IL3-12.0 10.4-10.9 10.9-11.3 7.7r 12.13’

1121 ilBiil*ar -ardy

I121 IlSl WI

‘Data in the literature on the compounds of the binary system Rh-B ye co-listed.

Acknowkdgocds The present study was performed under the coupemtive research program of the IMR, Tohok~ University. The authorsarepleasedtoacktwwkdgethecwsidenMe assistance of Mr. R. Note, Mr. K. Gbara, Mr. T. T&ah&i, Mr. T. Satou, Mr. S. Tozawa, Mr. K. Murakami and Dr. K. Takada of Tohoku University.

References

Fig. 6. TG-DTA curves for ErRh,B: IO°Cmin-‘).

(determined ~a sir, heaang rate

(4) According to X-ray photoelectron spectroscopic study. significant evidence of weak metal-boron bonding is found. This indicates that boron acts as an electron donor. (5) By means of TG-DTA between room temperature and 12WC, the oxidation of the compound in air starts at 373°C and the final weight gain is 15.44%. The mixed phase of ErBG, and Rh is identified by XRD as a resultant product. (6)

The

value

of the micro-Vickers

hardness

for

(001) and (100) faces of the crystals is l1.3-12.OGPa 10.4-10.9 GPa respectively.

the

and

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