Journal of the Less-Common Metals. 31 (1973) 21 l-220 N$, Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

211

CRYSTAL STRUCTURE OF SmCo,

Y. KHAN and D. FELDMANN

Iustitur far WerksroJfe der Elektrorechnik der R&r-Unitlersitiit Bochum ( West Germany)

(Received October 16, 1972)

SUMMARY

An alloy having the composition SmCo, has so far been considered to be a single phase compound with the CaZns-type structure. It is shown by X-ray analysis and other methods that this material is actually always obtained as a composite of two phases, one having the CaZn,-structure and the other being a variant of that structure with doubled lattice constants.

INTRODUCTION

In recent years, much interest has been taken in the RCo,(R=Y, La, Ce, Sm) alloys because of their magnetic properties which make them very favourable from the permanent magnet viewpoint. All of the work on these alloys has been based on the assumption that RCo, is invariably a single phase compound with the CaZn,-structure. In this paper preliminary results are reported of a structural reinvestigation of SmCo, undertaken to determine whether it is in fact single phase.

Wernick and Geller have reported crystal structure data of most of the RCo, alloys, except LaCo,, SmCo, and a few of the heavy RCo, compounds. Starting from a very detailed structural investigation of the compound ErNi, which they found to have the CaZn, structure, they assumed all the RCo, compounds to be isotypes. Haszko investigated SmCo, and confirmed this assumption. Later, LeMaire3, Buschow4, and Lihl et ah5 independently confirmed the results of Haszko and reported the lattice parameters of materials at the Co-rich and Sm-rich borders of the homogeneity region of SmCo,. None of these authors presented a detailed comparison of the calculated and observed structure data of SmCo,. Shibata6, in an investigation of the effect of heat-treatment, was the only author to present X-ray diffractometer patterns of SmCo, powder. Apparently no thorough structural investigation of SmCo, has been carried out, either on powder or on single crystals.

EXPERIMENTAL

Alloys in the composition range SmCo,.,-SmCo,,, were prepared. The Sm and Co metals used were supplied by HEK, GmbH, Liibeck (Germany) and had reported purities of 99.9 and 99.999 wt.%, respectively. A second series of alloys

212 Y. KHAN, D. FELDMANN

was prepared using Co and Sm of industrial purity, supplied by Th, Goldschmidt AG, Essen (Germany). In order to avoid crucible contamination, alloys of 1-3 g were melted with a tungsten electrode on a water-cooled copper hearth in an atmosphere of highly-purified argon. Alloy buttons were crushed into several pieces and remelted to obtain homogeneity. With this procedure weight losses were found to be virtually negligible. Besides these alloys, three commercial samples with the approximate composition SmCo,. 2 were investigated.

The as-cast buttons were homogenized for 30-120 h between 1000 and 1100C. Heat-treatments of bulk and powder samples were performed in the temperature range between 500 and 1300C in argon-filled quartz tubes with tantalum foil linings.

Most of the X-ray measurements were made with an Enraf-Nonius Guinier camera, Model II. In addition, a Philips horizontal goniometer with scintillation counter for powder analysis and a Weissenberg camera (Enraf-Nonius) for single crystals were used.

Chemical analyses were made with a Philips X-ray fluorescence diffractometer and a Unicam atomic absorption spectrometer. The results were checked using classical gravimetric methods.

RESULTS AND DISCUSSION

Of the 10 alloys, repeatedly prepared in the investigated composition range, those close to the 1:5 stoichiometry were free from the known neighboring phases. Figure 1 shows Guinier patterns of Sm-Co alloys, containing: (a) SmCo, and Sm,Co,, (b) SmCo, and Sm$or,, and (c) apparently single phase SmCo,. The Guinier pattern of Fig. l(c) can super~cially be indexed as the hexagonal CaZn,-type structure. After long exposure, however, three additional weak lines were observed at 46=35.2, 41.3 and 48.8 (Fig. 2(b)). It would seem reasonable to explain the existence of these three additional lines in terms of the presence of the neighboring phases, Sm,Co, or Sm,Cor 7. However, a close examination of the pattern in Fig. 1 and the overexposed photograph in Fig. Z(b) shows that these Iines do not belong to any of these phases, as the strongest lines of either phase are missing. The possibility that these lines are reflections due to the half wavelength harmonic of the CoKa radiation has been excluded on the basis of corresponding experiments on Si and CaCu,.

- Fig. 1. Guinier X-ray photographs of SmCo,. (a) Two-phase with Sm,Co,, (b) two-phase with Sm,Co,,, (c) apparently single phase.

This suggests that either (a) SmCo, is single phase with a structure other than CaZn,, or (b) samples with SmCo, composition are two-phase, one phase having the CaZn,-type structure and the other a yet unknown variant of this

213

Fig. 2. Guinier X-ray photographs of SmCo, for various exposures. (a) Calibration substance (Si +a little SiO,), 14.0 h CoKa; (b) SmCo, +calibration substance, 14.0 h Co&; (c) SmCo, +calibration substance, 3.5 II Co&; (d) SmCo, +calibration substance, 1.0 h CoKa.

structure where the main diffraction lines coincide with those of the CaZn, structure. In order to decide between these two possibilities, a series of etching

experiments were made and photomicrographs, with and without polarized light, were taken. X-ray powder diffraction intensities of SmCo, (CaZn,-type) were calculated and compared with those found experimentally (Table I).

Most of the polished samples of the homogen~ed SmCo, compound

TABLE I

SmCo, (CaZns-TYPE) STRUCTURAL DATA

Experiment: SmCo, (alloy annealed at 1000C for 72 h; powder 14 h 55OC/wa) Guinier-photograph 4 h CoKa, calibrated with Si.

Structure: CaZn, Typea = 4.99, c = 3.97, c/a = 0.795,,.

Atom-parameters were taken from CaZn, (Haucke4). Remarks: I&,,~, CI lo- HPLGIFJ*

Where H is the multiplicity of the reflex, PLG is calculated for Guinier Camera Mode&II, Enraf-Nonius, Hotland, after K. Sagel. All reflexes permitted by the space group are included.

_~_..

(h k 1) sin 6corc. sin & leolc. I ohs. ^

100 0.0428 0.0428 38 w 001 0.0506 0.0506 143 mw 101 0.0934 0.0934 1256 st 110 0.1283 0.1284 849 m+ 200 0.1711 0.1712 990 mst Ill 0.1789 0.1789 2799 VSt 002 0.2025 0.2026 742 m 201 0.2237 0.2218 153 mw 102 0.2453 0.2455 18 VVW 210 0.2994 0.2995 15 ww 112 0.3308 0.3309 554 m- 211 0.3500 0.3500 543 m- 202 0.3736 0.3737 799 m 300 0.3849 0.3849 238 mw+ 301 0.4355 0.4354 1035 mst

Y. KHAN, D. F~LDM~NN

CRYSTAL STRUCTURE OF SmCo, 215

Fig. 3. Photomicrographs of SmCo,, expected to be single-phase from Fig. I(c). (a) Grain boundary etching, (b) grain surface etching, (c) Kerr domain pattern before etching. (d) Kerr domain pattern after grain boundary etching.

216 Y. KHAN, D. F~LDMANN

TABLE II

ETCHING PROCEDURES

Ser. no. Nature of etching Etchants Procedure

1. Grain-boundary of Fig 3(a)

(A) 5 ~01% H,O,+ 10 ~01% corm. HCI +85 ~01% H20.

(Bf 5 ~01% cont. HNU, + balance C,HSOH.

2. Grain-surface of Fig. 3(b)

(A) 5 ~01% cont. 30 s in etchant (A). HNO, + balance Washed out in H,O and C,H,OH. left in air for 10 min.

(B) 5 vol~ HF+ 2 min again in (A). 10 ~01% C,H,OH+ Washed in H,O and finally 30 s in balance HrO. etchant and washed out in C,H,OH.

30 s in etchant (A). Washed out in H,O and left in air for 5 mm. IO min again in (A). Washed in H,O and left in air again for 5 min. Finally, 30 s in etchant (B) and then washed out in &H,OH.

convening neither Srn,CO~ nor Sm,Co,, showed a homogeneous magnetic domain structure as given in Fig. 3(c). This means that the samples consist of either extremely large grains or fine grains with parallel orientation.

Etching was found to be very difficult and frequently gave the impression that the whole of the polished sample was a single crystal. A sequential application of various mixtures of etching agents (see Table II) finally led to a completely different result, namely the occurrence of one phase precipitated in another (Fig. 3(a) and (b)), with a fixed correlation between the o~e~tations of the magnetic axes of matrix and pr~ipi~tes (Fig. 3(d)). This metallographic examination clearly supports the second possibility mentioned above, i.e., a two-phase structure of a SmCo, alloy. An analogous phenomenon has been found by Jain et aLi in the Cu-In system. The X-ray patterns of these two-phase SmCo, alloys may therefore be interpreted as a superposition of the patterns of SmCo, with the CaZns-type structure and of a new phase variant of the CaZn,-type structure having doubled lattice constants (u = 9.995, c = 7.955, c/a = 0.795) and unchanged group-symmetry. A comparison between calculated and observed sin e-values is given in Table III.

In order to check whether the atomic positions in the new CaZn, structure sariant are greatly different from those in the CaZns-type structure, the relative X-ray intensities of SmCo, were calculated on the basis of the CaZn,-type structure and are given in Table I and Fig. 4. It is seen that the calculated relative intensities of the main lines do not differ from the observed ones in the composite diagram. This suggests that the atomic positions in the new phase variant may not be much different from those in the CaZn,-type structure, and doubling of the cell may take place by some process of faulting.

With this support for the existence of two phases in the composition range between SmCo,, and SmCo,.,, which hitherto has been believed to be a homogeneity region of SmCo5, experiments were done with the aim of obtaining either of the two phases in the pure state, but neither variation in ~om~sition nor in heat-

CRYSTAL STRUCTURE OF SmCo,

TABLE III

SmCo, (NEW VARIANT) TRANSLATION GROUP DATA

Experiment

Structure:

SmCo, (alloy annealed at 1000C for 72 h; powder 14 h 55OC/wa) Guinier-photograph with CoKcc. 4 h. Calibrated with Si for the following lattice-constants. Hexagonal with a = 9.995,s

c = 1.955,, c/a = 0.795,,

Remarks: Variant of the CaZns-type. All reflexes permitted by the above mentioned translation group are included. no. = not observed.

(h k 1) Sin Oobs.

100 0.01069 n.0.

001 0.01266 no.

10 1. 0.02335 0.02335

110 0.03208 0.03207

200 0.04277 0.04277

1 1 1 0.04474 0.04463

002 0.05063 0.05063

201 0.05543 no.

102 0.06133 n.0.

120 0.07485 no.

112 0.08271 no. 21 I 0.0875 1 n.0. 202 0.09340 0.09344

300 0.09624 no.

301 0.10890 n.0.

003 0.11392 n.0.

103 0.12462 n.0. 212 0.12548 n.0.

220 0.12832 0.12840

310 0.13901 n.0.

221 0.14097 n.0.

113 0.14600 n.0.

302 0.14687 n.0.

311 0.15167 n.0.

203 0.15669 no.

400 0.17109 0.17118

222 0.17895 0.17894 401 0.18375 n.0.

213 0.18877 no.

312 0.18964 n.0.

004 0.20253 0.20262

320 0.203 17 no.

303 0.21016 n.0.

104 0.21322 n.0.

321 0.21583 n.0. 402 0.22172 0.22180

410 0.22455 n.0.

114 0.2346 1 n.0.

411 0.23721 n.0.

223 0.24224 n.0.

1. (Guinier )

n.0. n.0. vvw vvw W

VW

mw n.0. n.0. no. n.0. n.0. st no. n.0. n.0. no. n.0. m+ no. X0.

n.0. n.0. no. n.0. mst vst n.0. n.0. n.0. m n.0. n.0. n.0. n.0. mw no. ll.0.

no. n.0.

(continued)

217

218 Y. KHAN. D. FELDMANN

TABLE III (contd.)

(h k 1) Sin Oobs. I, (Guinier)

204 0.24530 0.24550 vvw

313 0.25293 n.0. n.0.

322 0.25380 n.0. n.0.

500 0.26733 n.0. n.0.

412 0.27519 n.0. n.0.

214 0.27738 n.0. n.0.

501 0.27998 n.0. n.0.

403 0.28501 n.0. n.0.

330 0.28871 n.0. n.0.

304 0.29877 n.0. n.0.

420 0.29940 0.2994 vvw

331 0.30137 n.0. n.0.

421 0.31206 n.0. n.0.

005 0.3 1645 n.0. n.0.

323 0.31709 n.0. n.0.

502 0.31796 n.0. n.0.

105 0.32714 n.0. n.0.

224 0.33084 0.33094 m-

510 0.33148 n.0. no.

413 0.33848 n.0. n.0.

332 0.33934 x0. n.0.

314 0.34154 n.0. n.0.

511 0.344 14 no. n.0.

115 0.34853 n.0. n.0.

422 0.35004 0.35004 m-

205 0.35922 n.0. n.0.

404 0.37362 0.37373 m

503 0.38125 n.0. n.0.

512 0.38212 n.0. n.0.

600 0.38495 0.38495 mw+

215 0.39130 n.0. n.0.

430 0.39564 n.0. n.0.

601 0.39761 n.0. n.0.

333 0.40263 n.0 n.0.

324 0.40570 n.0. n.0.

431 0.40830 n.0. n.0.

305 0.41269 no. n.0. 423 0.41333 n.0. no.

520 0.41703 n.0. n.0.

414 0.42708 n.0. n.0.

521 0.42969 n.0. n.0.

602 0.43558 0.43550 mst.

treatment at various temperatures gave one of the two phases alone. The only conclusion we can draw from these experiments is that the observed three extra X-ray diffraction lines become stronger with increasing Co-concentration. The in- vestigation of single crystals of SmCo, is complicated by twin formation. This is always expected in derivative structures *. The equilibrium phase diagram around the composition of SmCo, and effects of the new phase on the magnetic properties of SmCo, are being investigated 9,10. Moreover, the results of the heat-

CRYSTAL STRUCTURE OF SmCo, 219

Fig. 4. Calculated powder X-ray intensities, LS. 40 for SmCo, including correction for Guinier-type camera.

treatment of the bulk as well as powder samples of the SmCo, intermetallic compound do not agree with those found by Den Broeder and Buschow and Westendorpt4, according to whom the SmCo, compound, as a single phase, is only stable above 1080C (I&,) below which it decomposes into Sm,Co, +Sm,Co,,. We annealed our samples (powder) for many hours at 550C (see Tables I and III), but have not observed any such decomposition. A detailed study of the stability of SmCo, will be presented in a separate paper.

ACKNOWLEDGEMENTS

The authors are indebted to Prof. E. Kneller for many stimulating discussions, and to Dr. C. Herget of the Th. Goldschmidt A.G. for providing research material.

REFERENCES

1 J. Wernick and S. Geller, Acta Crysr., 12 (1959) 662665. 2 S. E. Haszko, Trans. AIM& 218 (1960) 763. 3 R. LeMaire, Cobair, 32 (1966) 117P125.

220 Y. KHAN, D. FELDMANN

4 K. Buschow, A. Van der Goot, J. Less-Common Metals, 14 (1968) 323-328. 5 F. Lihl, J. R. Ehold, H. R. Kirchmayr and Ii. D. Wolf, Acta Phys. Austr., 30 (1969) 164-175. 6 T. Shibata and T. Katayama, Japan J. Appt. Phys., 10 (1971) 510. 7 K. Sage& Iabelien zur S~rukturanalyse, Springer, Berlin, 1958, pp. 77-80. 8 M. H. Buerger, Crystal Structure Analyses, Wiley, London, 1960, pp. 53-69. 9 Y. Khan and D. Feldmann, to be published.

10 D. Feldmann, Y. Khan and M. Krone, Magnetic and X-ray investigations of SmCo, magnetic single crystals, to be published.

11 W. Haucke, 2. Anorg. ANgem. Ckem., 244(1940) 17. 12 K. C. Jain, M. Ellner and K. Schubert, %r die Phasen in der NIhe der Zu~mmensetzung

Cu.&,,, to be published in 2. Meralfk. 13 F. J. A. Den Broeder and K. I-I. J. Buschow, J. Less-Common Metals, 29 (1972) 65. 14 F. F. Westendorp, Solid State Commun., 8 (1970) 139.