A note on the crystal structure of two ScCuSi phases

Journal of the Less-Common

Metals, 81 (1981)

A NOTE ON THE CRYSTAL

B. YA.

KOTUR

University

71

71 - 78

STRUCTURE

OF TWO ScCuSi PHASES

and E. I. GLADYSHEVSKIJ

of Lvov, Lomohosov

Str. 6, 290005

Lvov (U.S.S.R.)

M. SIKIRICA

Laboratory of General and Inorganic Chemisfty, Faculty Zagreb, P.O. Box 153, 41001 Zagreb (Yugoslavia) (Received

March

of Science,

University

of

9, 1981)

summary Two phases in the system Sc-Cu-Si were prepared and investigated. Their compositions were established to be very close to 1 :l :l. The crystal structure was investigated by means of single-crystal and powder methods. The compounds belong to the TiNiSi and FeaP structure types. The peculiarities of the behaviour of scandium in intermetahics are discussed.

1. Introduction During systematic investigations of the phase equilibria in the Sc-T-Si ternary systems with T - Mn, Fe, Co, Ni [ 1 - 41, several new ternary phases of equiatomic composition were found and their crystal structures were investigated [ 1,3 - 51. It was shown that ScCoSi and ScNiSi occur with the TiNiSi-type structure, whereas ScMnSi occurs with the Fe,P-type structure. In the analogous copper system a hexagonal ScCuSi phase of the Fe,P structure type was reported by Dwight et al. [ 61. During a systematic investigation of the Sc-Cu-Si system we obtained such a hexagonal phase [7] as well as another phase which had a composition very close to 1 :l :l and which produced an X-ray powder pattern that was very similar to those of the ScNiSi and ScCoSi phases. The coexistence of similar phases in the Zr-CuSi and Hf-Cu-Si ternary systems has also been shown previously [ 81. The compositions of the FeaP-type phases were determined as ZrsCu& and HfsC&Sia. The results of the crystal structure investigations of the two phases with the composition 1 :l :l in the Sc-Cu-Si system are reported in this paper. 0022-5088/81/0000-0000/$02.50

@ Elsevier Sequoia/Printed

in The Netherlands

72

2. Experimental procedure The alloys were prepared from the elements of high purity (scandium, 99.92% pure; copper, silicon, 99.99%) by arc-melting techniques under an argon atmosphere. The specimens were homogenized at 1073 K for 750 h in evacuated quartz capsules. The X-ray diffraction data from the polycrystalline specimens were taken by means of a DRON-2,O powder diffractometer (filtered Fe Ko radiation) and from a single crystal by means of a Philips PW 1100 four-circle diffractometer (graphite-monochromatized MO Ka radiation).

3. Structure determination and refinement Six ternary alloys with the same scandium content of 33.3 at.% and variable silicon and copper contents in the range 20 - 35 at.% were prepared. A comparison of X-ray diffraction patterns of the specimens before and after heat treatment showed that the hexagonal FesP type appeared only after homogenization and was present in the alloys containing up to about 33 at.% Si. In this range there is an equilibrium with ScCu,. The diffraction patterns of the alloy containing 35 at.% Si were different from those of the FesP type but were very similar to those of ScNiSi and ScCoSi. No phase transformation with temperature was noticed. The compositions of both phases, which coexist in the system at 1073 K, were found to be very close to 1:l:l (a study of the phase equilibria in the Sc-Cu-Si system will be published elsewhere). Our attempts to obtain single crystals were successful only for the phase that was different from the FezP type. Perhaps this was a result of the different modes of formation. A single crystal with a prismatic shape with the approximate dimensions 30 pm X 40 pm X 80 pm isolated from one of the crushed ingots was examined from Laue, rotation and precession photographs. The orthorhombic cell with preliminary dimensions a = 6.54 A, b = 3.98 A, c = 7.23 R , mmm Laue symmetry, with systematic absences Okl with k + I= 212 and hk0 with h = 2n, led to Pnma or PnBla as the possible space groups. These results indicate the TiNiSi-type structure, in agreement with the results obtained earlier from the powder diffraction data. The cell parameters, derived from a least-squares fit of 213values for 21 reflections, obtained with a Philips PW 1100 diffractometer are as follows: a = 6.566(3) W; b = 3.976(2) 8,; c = 7.224(2) A. The integrated intensities of 221 independent reflections were collected to a limit of (sin 0)/h = 0.71 A-’ with MO Kol radiation (h = 0.7107 A). The ScNiSi positional parameters [5] were taken as starting values for a full-matrix least-squares refinement with the CRYLSQ program of the XRAY suite [9] in the space group Pnma. The usual corrections were made for Lorentz-polarization effects but an absorption correction was not applied because of the small size of the crystal. The atomic scattering factors were those of Cromer and Mann [lo] with the cor-

68 129 313 164 172 112 495 110 58 140 97 67E 103 365 221 288 270 185 83 205E 295 104 135 119 108 173 225 67~ 1lOE 67 49 280 69 72 71E 93E 135

20 4 0 6 0 80 11 21 31 41 51 61 91 02 12 2 2 3 2 4 2 5 2 7 2 82 13 2 3 3 3 4 3 53 6 3 7 3 8 3 14 2 4 34 44 5 4 64 74 15 25 4 5

F,

-18 114 323 157 -171 115 528 -109 -67 -142 104 123 -114 -334 -232 -304 289 -194 -90 -309 338 -113 -141 -133 -119 -199 255 -249 200 -74 -51 330 79 -86 320 267 -179

K=O

F,

h 1

1

if 03 13 2 3 6 3 7 3 83 3 4 54

8 1 12 22 32 4 2 5 2 62

7

20 4 0 60 8 0 11 21 31 41 51 6 1

55 7 5 8 5 06 16 2 6 4 6 3 7

h 1

F,

8': 525

69

17

-410 136 114 -312 210 -462 159 -217 -366 -106 148 -86 -398 -231 187 186 -238 111 -12a -88 700 202 -159 210

X=1 408 132 109 308 223 474 164 225 359 114 142 90 419 237 189 176 242 116 123 80 709 209 169 205 48 79 523

195 238 87 122 119 103 39E -222 55E -260 52E 238 89E 144 77E -307

F,

20 4 0 6 0 a0 11 21 31 41 51 61 0 2 12 22

0 0

64 8 4 0 5 2 5 3 5 4 5 5 5 6 5 7 5 16 2 6 3 6 4 6 5 6 6 6 7 6 17 4 7 57 28 4 8 58

h 1

58 126 111 98 318

07

K=2 830 70 109 271 146 136 104 436

72 61 149E 33 165 155 146 175 132 107E 138 23 114 91 95 206 50E 187 153 96E 61E 79

F,

-859 66 -101 -279 -145 132 -90 -422 87 58 125 -101 89 300

-83 68 270 -39 -168 -157 150 198 -149 -233 168 -35 -117 -98 -101 -222 -198 -212 185 206 -126 a5

F, 3 2 4 2 52 72 8 2 13 23 3 3 4 3 53 6 3 7 3 8 3 0 4 14 24 3 4 4 4 54 6 4 7 4 15 25 4 5 5 5 75 0 6 16 2 6 4 6 2 7 37 4 7 08 28 3 8

h 1

50 294 72 58 288 235 152 220 99 197 227 215 120 66 290 103 144E 100 64

70

201 2-64 246 172 85 264 289 104 123 115 96 176 212 130 212 182

F,,

Observed and calculated structure factors (x10) for ScCuSi (TiNiSi type)

TABLE 1

194 251 -248 173 84 241 -269 94 114 116 104 181 -230 123 199 -171 63 46 -287 -71 74 -269 -224 154 -214 -114 188 225 -208 -127 -54 273 92 239 111 -69

F,

;: 05 35 4 5 55 6 5 16 26 4 6 56 0 7 17 4 7 18 28

:'3 6 3 73

20 4 0 60 11 21 31 41 51 61 7 1 12 2 2 3 2 4 2 52 6 2 7 2 0 3

h 1 K=3 269 87 79 143 326 129 164 276 96 112 278 167 137 138 193 92 108 508 143 126 169 46 411 45 199 131 119 115 151 185 133 98 85 155 163 171 71 178

F,

ii 149 152 177 -72 -166

260 -88 -86 -126 306 -123 152 273 92 -116 257 152 -130 -132 185 -38 106 -476 -141 117 -166 -45 -387 -62 -191 131 119 -115 -160 182 -125

F,

2 0 11 21 31 4 1 12 2 2 32 0 3 13

4 0 60 11 21 31 41 61 0 2 2 2 3 2 4 2 5 2 13 2 3 3 3 4 3 53 04 14 24 54 15 2 5 4 5 0 6 16 2 6

0 0

h 1

-75 50 266 -56 -93 55

509 86 206

F,

K=5 155 64 208 88 113 168 93 65 300 89

134 183 171 168 185 67 80

-148 64 -197 76 -108 -159 -86 63 292 86

-191 -132 -172 168 -155 165 -73 -71 -91 :4" -84 135 -121 132 120 218 205 200 191 157 149 112 -107 -123 139 -157 155 157 152

67 282 69 97 72 206

K=4 534 91 208 79

F,

74 TABLE 2 Atomic and isotropic thermal parameters for ScCuSi (TiNiSi type) Atom

Site

x

Y

z

u (A2)

SC

4c

0.5089( 4)

+

0.3046( 5)

0.0056(7)

CU

4c

0.1576(3)

+

O&665( 3)

0.0079(6)

Si

4c

0.7702( 6)

+

0.6097( 7)

0.0045(8)

Estimated standard deviations are given in parentheses.

TABLE 3 Interatomic distances in ScCuSi (TiNiSi type) Atoms

Distance

ScSc2 SCSCB SC-cu2 SC-Cu SC--cU SC-cu2 Sc-Si2 S*Si Se-Si2

3.454(4) 3.377( 4) 3.101( 3) 2.983( 4) 2.853( 4) 2.847( 3) 2.835( 4) 2.793(6) 2.773( 3)

(A)

Atoms

Distance

C&SC2 cu-SC cu-SC cu-SC2 Cu-Si Cu-Si Cu-Si2

3.101(3) 2.983(4) 2.853( 4) 2.847( 3) 2.563(4) 2.453( 5) 2.408( 3)

(W)

Atoms

Distance

%-SC2 Si-Sc Si-SC2 Si-Cu Sk-cu Si-Cu2

2.83514) 2.793( 6) 2.773(3) 2.563( 4) 2.453(5) 2.408( 3)

(IL)

rections from Cromer and Liberman [ 111 for anomalous scattering. In the final refinement 17 reflections were omitted because of a large discrepancy between the structure factors F, and F,, probably owing to extinction. The final agreement factor R was 0.059. The list of calculated and observed structure factors is given in Table 1. The final atomic positional and isotropic thermal parameters are listed in Table 2 and interatomic distances are given in Table 3. The values of the isotropic thermal parameters do not indicate the necessity to revise the Cu:Si ratio. The structure of the second phase was refined from the powder data because this phase could not be isolated in the form of single crystals. Unit cell parameters refined with the least-squares method are as follows: a = 6.426(l) A, c = 3.922(l) a. These values are very similar to the values given by Dwight et al. [6], i.e. Q = 6.44 a and c = 3.92 15. The atomic parameters were refined by the use of the POLYCRYSTAL program [ 121. The X-ray diffraction line intensities were calculated taking into account the structure factor, the Lorentz-polarization factor and multiplicity. The assignment of the atomic species to the lattice sites of the Fe,Ptype structure has already been suggested [ 131. The positional parameters

75 TABLE

4

Assignments of atoms for the FenP-type parameters (space group, PG2m)

with refined

atomic

Site

Isign

3&?@, Qf, ZCalcI ZCdC II Zcak III

TABLE

structure

3f (x90, 0)

lb

SC

cu

Si

x = 0.574

x = 0.241

(0,

0.87Si

SC

cu

x = 0.573

x = 0.244

O,$,

2c f$,$O) Si

+ 0.13Cu

Si

cu

SC

cu

x = 0.560

x = 0.250

Si

5

Observed and calculated d values and diffractogram FezP-type structure phase for different assignments (Fe Ko radiation)

hkl

dabs (A)

d,h

100 001 110 101 200 111 201 210 002 300 211 102 301 112 220 202 310 221 311 212 400 302 401 003 320 103 222

5.567 3.927 3.208

5.566 3.922 3.213 3.206 2.783 2.486 2.270 2.104 1.961 1.855 1.854 1.850 t 1.677 1.674 I 1.607 1.603 1 1.544 1.487 1.4361 1.434 j 1.391 1.348 1.311 1.307 1.277 1.273 1.243

2.782 2.485 2.267 2.103 1.963 1.853 1.676 1.603 1.545 1.485 1.435 1.391 1.347 1.312 1.277 1.244

(A)

Zobs 195 85 102 60 1000 887 670 324 469 14 13 37 29 364 105 116 26 0 14 0 19

intensities for the of the atoms

Zcalcra*b

Zcalc~~’

Zcalc IIId

162 112 68 126 30 979 905 590 301 115 392 17 4 10 21 12 33 29 92 235 64 76 14 1 17 1 22

198 101 74 93 35 983 933 575 304 25 310 17 7 13 19 11 34 29 82 232 67 80 15 1 18 1 21

300 31 84 10 81 693 924 399 263 107 342 17 18 2 24 17 36 46 66 210 83 66 11 2 20 2 20

(continued)

76 TABLE S (continued)

410 321 312 113 203 411 402 500 213 330 501 322 303 420 331 412 421 223

1,213 1.163 1.160 1.135 1.111

1.071

1.033 1.016

1.214 1.214 1.213 1.211 1.183 1.160 1.135 1.113 1.1101 1.071 1.071 1.070 1.069~ 1.052 1.033 1.033) 1.016 1.014I

469

70 74 44 111

172

58 199 33 65 107 43 93 49 83 51 25 33 0

0 156 158

5 6 153 122 37

59

207 33 65 106 48 98 43 89 49 25 35 0 6 6 160 126 35

45 194 38 69 108 61 121 21 63 65 10 43 13 9 2 109 111 36

The R factor is definedas R=-

ElAzl ZIZO&I

*ForZsimseeTable4. bR factor, 0.149. =R faciq0.172. dR factor,0.257.

for ScMnSi [l] were taken as starting values. Refinement was made for three assignments (Table 4). The best agreement, as can be seen from Table 5, is for scandium in 3g sites, copper in 3f sites and silicon in lb and 2c sites of the space group P62m. Attempts to change the occupancies of the silicon and copper atoms led to a rapid increase in the R value. The final atomic positional parameters are listed in Table 4. The interatomic distances are listed in Table 6.

4.Discussion The results of the refinement have confirmed the preiiminary data for the composition of the phases obtained by X-ray powder analysis. The compositions are very close to 1:l:l in contrast with the system Zn(Hf jCu-Si where they exist for the compositions 3:4:2 (Fe,P type) and 1:l:l (TiNiSi type) [Sl.

77 TABLE 6 Interatomic

distances in ScCuSi (FezP type)

Atoms

Distance

Se-SC4 SC-cu4 SC-cu2 Sc-Si4(c) ScSi( b)

3.32 3.08 2.90 2.74 2.74

(A)

Atoms

Distance

cu-SC4 cu-SC2 cu-cu2 Cu-Si2( b) Cu-Si2( c)

3.08 2.90 2.68 2.50 2.49

(A)

Atoms

Distance

Si( b)-Sc3 Si( b)-Cu6 Si(c jSc6 Si( c)-Cu3

2.74 2.50 2.74 2.49

(A)

TABLE 7 Coordination numbers of atoms in some RTSi compounds of the TiNiSi-type structure Atom

R T Si a/c

Compound R=Ti,T=Ni[19]

R=Zr,TzCu[8]

Rz$k,T~Nj[5]

R=Sc,T=Cu

15 12 9

15 12 9

16 10 10

15 10 9

0.88

0.89

0.91

0.93

Among the family of rare earth elements (REEs) scandium occupies a particular position. It has no 4f electron level and it differs greatly in atomic size from the other REEs. Since the size factor is of great importance in the structure of intermetallics, the behaviour of scandium is more striking here than in other inorganic compounds. This has already been observed during the investigation of Sc-T-Si ternary systems with T = Mn, Fe, Co, Ni, Cu [ 1 - 4,7] . In these systems scandium forms nearly the same phases, with regard to stoichiometry and crystal structure, as zirconium and hafnium, the metals with atomic sizes which are similar to that of scandium. The same is also true for scandium-containing binary intermetallic systems [ 14 - 161. Perhaps this is the reason that among the REEs only scandium forms silicides and germanides (ScCuSi and ScCuGe) of the FesP type [6] . The same result applies to the structure of ScMnSi [l].Among other manganesecontaining ternary systems, this structure was found also in NbMnSi [ 171 but not in any REE-containing system. The same conclusion is valid for the TiNiSi-type structure. The family of compounds with this structure is large for the ternary systems of titanium, zirconium, hafnium, niobium and tantalum but there are only few such compounds in the ternary systems containing REEs [ 181. The formation of ScCoSi and ScNiSi was observed by Dwight et al. [6]. They noticed the similarity of their structure to that of the CeCu, type which occurs in the systems with large REE atoms but they did not

78

determine the structural details of these compounds. ScNiSi crystallizes with the TiNiSi-type structure as was shown [5] from a single-crystal investigation. The increase in the a/c value, compared with other TiNiSi-type structures, is due to the variation in the coordination of atoms in ScNiSi as well as in ScCuSi (Table 7). The arrangement of atoms in ScNiSi is very similar to that in the CeCuz-type structure [20] and accounts for the similarity in the X-ray patterns of these compounds. Interatomic distances in the phases investigated (Tables 2 and 5) do not exhibit any contraction compared with the sum of the atomic radii and are nearly the same as in ZrCuSi and Zr3CuqSi2 on taking into account an increase in the radius of scandium to 1.64 _&(1.60 A for zirconium).

Acknowledgments The authors thank Professor D. Grdenic for his interest in this work and M. Bruvo for collecting the diffractometer data.

References 1 B. Ya. Kotur, 0. I. Bodak and 0. Ya. Kotur, Dokl. Akad. Nauk Ukr. S.S.R., Ser. A, (8) (1980) 82. 2 E. I. Giadyshevskij, B. Ya. Kotur, 0. I. Bodak and V. P. Skvorchuk, Dokl. Akad. Nauk Ukr. S.S.R., Ser. A, (1977) 751. 3 B. Ya. Kotur, 0. I. Bodak and E. I. Gladyshevskij, Dokl. Akad. Nauk Ukr. S.S.R., Ser. A, (1977) 666. 4 0. I. Bodak, B. Ya. Kotur and E. I. Gladyshevskij, Dokl. Akad. Nauk Ukr. S.S.R., Ser. A, (1976) 656. 5 B. Ya. Kotur and 0. I. Bodak, Kristallografiya, 22 (1977) 1209. 6 A. E. Dwight, W. C. Harper and C. W. Kimball, J. Less-Common Met., 30 (1973) 1. 7 B. Ya. Kotur and 0. I. Bodak, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 16 (1980) 459. 8 H. Sprenger, J. Less-Common Met., 34 (1974) 39. 9 J. M. Stewart, G. J. Kruger, H. L. Ammon, C. Dickinson and S. R. Hall, The XRAY system: June 1972 version, Tech. Rep. TR-192,1972 (Computer Science Center, University of Maryland, College Park, MD). 10 D. T. Cromer and J. Mann, Acta Crystallogr., Sect. A, 24 (1968) 321. 11 D. T. Cromer and D. Liberman, J. Chem. Phys., 53 (1970) 1891. 12 Yu. G. Titov, L. F. Verkhorobin and N. N. Mat’yushenko, Kristallografiya, 17 (1972) 1053. 13 W. Jeitschko, Acta Crystallogr., Sect. B, 26 (1970) 815. 14 M. Hansen and K. Anderko, Constitution ofBinary Alloys, McGraw-Hill, New York, 1958. 15 R. P. Elliot, Constitution ofBinary Alloys, McGraw-Hill, New York, 1st Suppl., 1965. 16 F. A. Shunk, Constitution ofBinary Alloys, McGraw-Hill, New York, 2nd Suppl., 1969. 17 B. Deyris, J. Roy-Montreuil, R. Fruchart and A. Michel, Bull. Sot. Chim. Fr., (1968) 1303. 18 E. I. Gladyshevskij, Crystal Chemistry ofsilicides and Germanides, Metallurgiya, Moscow, 1971 (in Russian). 19 C. B. Shoemaker and D. P. Shoemaker, Acta Crystallogr., 18 (1965) 900. 20 W. B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, Wiley, New York, 1972.