Composition and crystal structure of rare-earths–Co–In compounds

Journal of Alloys and Compounds 291 (1999) 80–88

L

Composition and crystal structure of rare-earths–Co–In compounds Ya.M. Kalychak Department of Inorganic Chemistry, I. Franko L’ viv State University, Kirila and Mephodiya Str., 6, 290005 L’ viv, Ukraine Received 10 May 1999; received in revised form 16 June 1999; accepted 16 June 1999

Abstract Data about the composition and crystal structure of the known 66 ternary compounds in rare-earths–Co–In systems have been generalized. Some of their peculiarities and relationships to other structure types have been examined.  1999 Elsevier Science S.A. All rights reserved. Keywords: Intermetallic compound; Crystal structure; Indium; Cobalt; Rare earths

1. Introduction The interaction of the components in rare-earths–Co–In systems has been studied intensively during recent years. As a result, a considerable amount of ternary compounds has been found. The crystal structure has been investigated for 66 of these compounds. Their physical properties, except for the La 6 Co 13 In compound [1], have not been practically investigated although they might be of substantial interest. The aim of this work is to summarize data about composition and crystal structure of R x Co y In z compounds (R5rare earth metals) their similarity and relation to other structure types.

2. Experimental details The alloys were prepared by arc melting of high-purity compact metals in an atmosphere of pure argon. Annealing was carried out at 870 K for 600 h. The crystal structure was determined from single crystal diffraction (autodiffractometers Syntex P1, Syntex P2 1 , DARCH-1, KM-4) and powder diffraction (diffractometers DRON-2.0, DRON3M, DRON-4.07, HZG-4a). The program CSD [2] was used for solving the structure.

of 870 K such compounds were not found in the systems hEu, Ybj–Co–In. The other rare earth metals form approximately 10–15 ternary compounds and there are only two compounds in the system with cerium. It was mentioned in [3] that the rare-earth content for R–hCo, Ni, Cuj–In systems decreases from systems with Co (14.3 at.% of rare earth; compounds Sm 2 Co 9 In 3 , RCoIn 5 ) through Ni (8.3 at.% of rare earth; compound RNi 9 In 2 ) to Cu (7.1 at.% of rare earth; Ce, La, Eu compounds of NaZn 13 structure type). All known compounds of cobalt with rare earths and indium are characterized by practically constant composition while in the nickel and copper systems (to a greater extent) there are compounds with homogeneity regions with transition metal and In. In the case of Ni there are compounds of heavy rare earths with ZrNiAl structure type and some light rare earths with AlB 2 structure type [3] but in the Cu systems there exist compounds of structure types YNi 9 In 2 , CeCu 4.38 In 1.62 , MgCu 4 Sn, ZrNiAl and AlB 2 [4]. It is evident that the affinity of In to Ni and Cu is greater than to Co. There are few common compound compositions for the Co and Ni systems (RT 2 In, RT 4 In, R 10 T 9 In 20 , R 12 T 6 In). Only the RT 4 In compounds having the MgCu 4 Sn structure (not necessarily for the same rare earth) are common to all three transition metals Co, Ni, Cu (5T).

3. Results and discussion Information about R–Co–In systems containing compounds of the established crystal structure (66 compounds) is presented in Table 1. The crystallographic characteristics are shown in Table 2. It should be noted that for the compositions considered in Table 1 and for a temperature

3.1. R2 Co9 In3 and RCo2 In compounds The R 2 Co 9 In 3 composition is realized only for R5Sm forming the Sm 2 Co 9 In 3 compound of its own structure

0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00290-X

Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

81

Table 1 Survey of compounds in the rare-earths–Co–In systems and their structure types a No.

Compound

R (at.%)

Structure type

Pearson code

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Y

1 2 3

R 2 Co 9 In 3 RCoIn 5 R 6 Co 26 – x In 14

14.3 14.3 15.8

oC28 tP14 cP38

– – –

– 1 –

– 1 –

– 1 –

1 1 –

– – –

– 1 –

– 1 –

– 1 –

– 1 –

– – 1

– 1 1

– – –

– 1 1

– 1 –

4 5 6 7 8 9

RCo 4 In R 2 CoIn 8 RCo 2 In R 10 Co 9 In 20 R 6 Co 13 In R 3 Co 2 In 4

16.7 22.2 25.0 25.6 30.0 33.8

cF24 tP22 oP8 tP78 tI80 hP9

– – – – 1 –

– 1 – – – –

– 1 1 – – –

– 1 1 – – –

– 1 1 – – –

– – – – – –

– 1 1 – – –

– 1 1 – – –

1 1 1 – – 1

1 1 1 – – 1

1 1 – 1 – 1

1 1 – 1 – 1

– – – – – –

1 – – 1 – 1

– 1 1 – – –

10 11 12

R 12 Co 6 In R 6 Co 2 In R 14 Co 2 In 3

63.2 66.7 73.6

Sm 2 Co 9 In 3 HoCoGa 5 Lu 6 Co 26 – x In 14 x58.41 MgCu 4 Sn Ho 2 CoGa 8 PrCo 2 Ga Ho 10 Ni 9 In 20 Nd 6 Fe 13 Si Lu 3 Co 2 – x In 4 x50.13 Sm 12 Ni 6 In Ho 6 Co 2 Ga Lu 14 Co 2 In 3

cI38 oI36 tP76

1 – –

– – –

1 – –

1 – –

– 1 –

– – –

– 1 1

– 1 1

– 1 1

– 1 1

– – 1

– 1 1

– – –

– 1 1

– 1 1

a

1, the compound exists; –, the compound has not been found.

type [5]. Compounds of RCo 2 In composition [6,7] with PrCo 2 Ga structure type [8] are more generally distributed (Tables 1 and 2). They have one peculiarity regarding its prototype PrCo 2 Ga (space group Pmma), namely that the In atoms occupy the position 2(f) but not 2(e) as the Ga atoms in PrCo 2 Ga. As a result (e.g. in the TbCo 2 In structure [6]) the coordination polyhedra (CP) are deformed cubooctahedra that are rather characteristic of these compounds containing rare earth and transition metals. The R 2 Co 9 In 3 and RCo 2 In structures are both orthorhombic with almost equal value of two periods of the unit cell (Table 2). The unit cell of the more complicated Sm 2 Co 9 In 3 structure contains two unit cells of the simpler TbCo 2 In (PrCo 2 Ga–type) structure and two fragments of [Co 5 In] composition with a motive similar to the CaCu 5 structure type (Fig. 1). Both structure types can be considered as being built of fragments of the CsCl type (RIn composition) and fragments with a motive of the CaCu 5 structure type. In the case of the TbCo 2 In compound (Fig. 2) each unit cell has one CsCl and one CaCu 5 fragment (TbCo 4 In composition). Their composition can be deduced by summing up these fragments: TbIn1 TbCo 4 In=Tb 2 Co 4 In 2 =2TbCo 2 In. The Sm 2 Co 9 In 3 structure is shown in Fig. 3 in a similar way, but here the fragments with the CaCu 5 structure type motive have different compositions, Sm 0.5 Co 5 In 0.5 and Sm 0.5 Co 4 In 1.5 . The compound composition is deduced in the following way: 2SmIn12Sm 0.5 Co 5 In 0.5 1 2Sm 0.5 Co 4 In 1.5 =Sm 4 Co 18 In 6 =2Sm 2 Co 9 In 3 . The fact that practically full identity of the CN and the CP atoms of the R-components (CN517), Co (12 vertices, identical to Cu1 and Cu2 atoms in CaCu 5 -type, or 11 vertices, defective polyhedra to Cu1 in CaCu 5 ) and In (CN512, deformed cubooctahedra) is reached in the described structures as a result of their similarity. The only difference is the fact that the In2 atom in the Sm 2 Co 9 In 3 (CN518, hexagonal prism

with additional equatorial atoms) are not analogous to TbCo 2 In.

3.2. RCoIn5 and R2 CoIn8 compounds Compounds of these compositions exist practically in all systems [9] and belong to the structure types HoCoGa 5 and Ho 2 CoGa 8 [10] (Tables 1 and 2), respectively. They are the members of homologous series based on fragments of the AuCu 3 and PtHg 2 type (Fig. 4) and are described by the formula Rm Tn X 3m12n , where m and n are the number of the fragments of the AuCu 3 type of RX 3 composition and PtHg 2 of TX 2 composition in the unit cell. For the HoCoGa 5 -type one has m51, n51, and for the Ho 2 CoGa 8 -type m52, n51. The peculiarity of these structures is a low CN value of the R atoms, which is equal to 12. A part of the In atoms also has the same CN value. Another part of the In atoms have CN511 and the Co atoms have a surrounding in the form of a cube (CN58).

3.3. R6 Co262 x In14 (x58.41) compounds R 6 Co 262x In 14 (x58.41) compounds with the cubic Lu 6 Co 262x In 14 structure type are formed only in the systems Er, Tm [13] and Lu [11,12] (Tables 1 and 2). The peculiarity of this structure type is the defect of Co atoms. Moreover, the Co4 and Co5 atoms in Lu 6 Co 262x In 14 that occupy the position 1 / 2 y 1 / 2 for 45 and 26% and have close positional parameter values equal to 0.287 and 0.237, respectively. In spite of the relatively small rare-earth content (15.8 at.%), the Lu–Lu distances are rather short and equal to 0.3187 nm while the sum of the Lu radii equals 0.348 nm. Thus, double nuclear clusters of [Lu 2 Co 8 In 14 ] composition in the form of hexagonal antiprisms coupled by base can be pointed out in the structure. Every antiprism contains also two additional atoms opposite the other base. The clusters are joined by the edges and

Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

82

Table 2 Crystallographic data of the R–Co–In system compounds Compound

Structure type

Space group

Lattice parameters (nm)

References

1

2

3

a 4

b 5

c 6

7

Sm 2 Co 9 In 3

Sm 2 Co 9 In 3

Cmmm

2.2834

0.5020

0.40842

[5]

CeCoIn 5 PrCoIn 5 NdCoIn 5 SmCoIn 5 GdCoIn 5 TbCoIn 5 DyCoIn 5 HoCoIn 5 TmCoIn 5 LuCoIn 5 YCoIn 5

HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5 HoCoGa 5

P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm

0.4601 0.4596 0.4590 0.4577 0.4567 0.4549 0.4545 0.4547 0.4532 0.4527 0.4551

– – – – – – – – – – –

0.7540 0.7503 0.7502 0.7463 0.7461 0.7425 0.7418 0.7411 0.7387 0.7359 0.7433

[9] [9] [9] [9] [9] [9] [9] [9] This work [9] [9]

Er 6 Co 262x In 14 (x(8) Tm 6 Co 262x In 14 (x57.42) Lu 6 Co 262x In 14 (x58.41)

Lu 6 Co 262x In 14

Pm3

0.8663

–

–

[13]

Lu 6 Co 262x In 14

Pm3

0.8655

–

–

[13]

Lu 6 Co 262x In 14

Pm3

0.8652

–

–

[11,12]

DyCo 4 In HoCo 4 In ErCo 4 In TmCo 4 In LuCo 4 In

MgCu 4 Sn MgCu 4 Sn MgCu 4 Sn MgCu 4 Sn MgCu 4 Sn

F4¯ 3 m F4¯ 3 m F4¯ 3 m F4¯ 3 m F4¯ 3 m

0.7087 0.7068 0.7049 0.7042 0.7029

– – – – –

– – – – –

[14] [14] [14] [14] [14]

Ce 2 CoIn 8 Pr 2 CoIn 8 Nd 2 CoIn 8 Sm 2 CoIn 8 Gd 2 CoIn 8 Tb 2 CoIn 8 Dy 2 CoIn 8 Ho 2 CoIn 8 Er 2 CoIn 8 Tm 2 CoIn 8 Y 2 CoIn 8

Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8 Ho 2 CoGa 8

P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm P4 /mmm

0.4640 0.4605 0.4608 0.4583 0.4569 0.4568 0.4561 0.4540 0.4560 0.4544 0.4574

– – – – – – – – – – –

1.2251 1.2193 1.2172 1.2101 1.2021 1.2008 1.1994 1.1984 1.1958 1.1934 1.2013

[9] [9] [9] [9] [9] This work [9] [9] [9] [9] [9]

PrCo 2 In NdCo 2 In SmCo 2 In GdCo 2 In TbCo 2 In DyCo 2 In HoCo 2 In YCo 2 In

PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga PrCo 2 Ga

Pmma Pmma Pmma Pmma Pmma Pmma Pmma Pmma

0.5119 0.5096 0.5080 0.5052 0.5033 0.4998 0.4993 0.5027

0.4089 0.4082 0.4060 0.4055 0.4050 0.4034 0.4029 0.4038

0.7197 0.7158 0.7127 0.7124 0.7122 0.7060 0.7054 0.7108

[7] [7] [7] [7] [6,7] [7] [7] [7]

Er 10 Co 9 In 20 Tm 10 Co 9 In 20 Lu 10 Co 9 In 20

Ho 10 Ni 9 In 20 Ho 10 Ni 9 In 20 Ho 10 Ni 9 In 20

P4 /nmm P4 /nmm P4 /nmm

1.3253 1.3202 1.3160

– – –

0.9078 0.9105 0.9106

[15,16] [15,16] [15,16]

La 6 Co 13 In

Nd 6 Fe 13 Si

I4 /mcm

0.8102

–

2.3576

[1]

Dy 3 Co 2 In 4 Ho 3 Co 2 In 4 Er 3 Co 2 In 4 Tm 3 Co 2 In 4 Lu 3 Co 22x In 4 (x50.13)

Lu 3 Co 22x In 4 Lu 3 Co 22x In 4 Lu 3 Co 22x In 4 Lu 3 Co 22x In 4 Lu 3 Co 22x In 4

P6¯ P6¯ P6¯ P6¯ P6¯

0.7867 0.7866 0.7850 0.7843 0.7814

– – – – –

0.3645 0.3605 0.3583 0.3556 0.3521

[21] [21] [21] [21] [21]

Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

83

Table 2. Continued Compound

Structure type

Space group

Lattice parameters (nm)

References

1

2

3

a 4

b 5

c 6

7

La 12 Co 6 In Pr 12 Co 6 In Nd 12 Co 6 In

Sm 12 Ni 6 In Sm 12 Ni 6 In Sm 12 Ni 6 In

Im3 Im3 Im3

1.0165 0.9920 0.9866

– – –

– – –

[24] [24] [24]

Sm 6 Co 2 In Gd 6 Co 2.135 In 0.865 Tb 6 Co 2.135 In 0.865 Dy 6 Co 2.135 In 0.865 Ho 6 Co 2.135 In 0.865 Tm 6 Co 2.135 In 0.865 Lu 6 Co 2.135 In 0.865 Y 6 Co 2.135 In 0.865

Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga Ho 6 Co 2 Ga

Immm Immm Immm Immm Immm Immm Immm Immm

0.9549 0.9544 0.9428 0.9401 0.9348 0.9288 0.9238 0.9506

0.9583 0.9597 0.9450 0.9438 0.9430 0.9301 0.9241 0.9511

1.0068 1.0041 0.9969 0.9938 0.9906 0.9793 0.9727 0.9992

This work [25] [25] [25] [25] [25] [25] [25]

Gd 14 Co 2 In 3 Tb 14 Co 2 In 3 Dy 14 Co 2 In 3 Ho 14 Co 2 In 3 Er 14 Co 2 In 3 Tm 14 Co 2 In 3 Lu 14 Co 2 In 3 Y 14 Co 2 In 3

Lu 14 Co 2 In 3 Lu 4 Co 2 In 3 Lu 14 Co 2 In 3 Lu 14 Co 2 In 3 Lu 14 Co 2 In 3 Lu 14 Co 2 In 3 Lu 14 Co 2 In 3 Lu 14 Co 2 In 3

P42 /nmc P42 /nmc P42 /nmc P42 /nmc P42 /nmc P42 /nmc P42 /nmc P42 /nmc

0.9615 0.9544 0.9615 0.9459 0.9413 0.9368 0.9333 0.9530

– – – – – – – –

2.3336 2.3225 2.3336 2.2913 2.2793 2.2691 2.2633 2.3269

[26] [26] [26] [26] [26] [26] [26] [26]

Fig. 1. Structure of TbCo 2 In (shadowed cell) (b) as part of the structure Sm 2 Co 9 In 3 (a). The unit cell of Sm 2 Co 9 In 3 is bordered by a dotted line.

Fig. 2. Packing of the CsCl-type fragments (composition TbIn) (b) and CaCu 5 -type fragments (composition TbCo 4 In) (c) in the TbCo 2 In structure (a).

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Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

Fig. 3. Packing of the CsCl-type fragments (composition SmIn) (b) and CaCu 5 -type fragments (composition Sm 0.5 Co 5 In 0.5 (c) and Sm 0.5 Co 4 In 1.5 (d) in the Sm 2 Co 9 In 3 structure (a).

form infinite chains along the three coordinate axes. Clusters of different directions are joined by two triangular faces. The gaps are occupied by icosahedra [CoCo 12 ], cubes [CoIn 8 ], trigonal prisms [CoIn 6 ] and empty (E) tetrahedra [ECo 3 In] (Fig. 5). The CP of the Co atoms are icosahedra and tetrahexahedra and that of In atoms are 13-vertices derivatives of icosahedra and deformed cubooctahedra. For the compound Tm 6 Co 262x In 14 the x value is somewhat different from that of Lu 6 Co 262x In 14 and is equal to 7.42 [13].

3.4. RCo4 In compounds RCo 4 In compounds with MgCu 4 Sn structure type, superstructure to AuBe 5 and MgCu 2 types exist only with Dy, Ho, Er, Tm and Lu [14] (Tables 1 and 2), while in the Ni systems they are characteristic of practically all rare

Fig. 4. The structures HoCoIn 5 (HoCoGa 5 -type) (a) and Ho 2 CoIn 8 (Ho 2 CoGa 8 -type) (b) as linear homologues which are built from AuCu 3 (composition HoIn 3 ) (c) and PtHg 2 (composition CoIn 2 ) (d) types.

earths except La and Eu [3], and in the Cu systems they have homogeneity regions for Cu and In (compounds are not found for La, Ce, Pr and Eu) [4]. Frank-Kasper’s 16 vertices are characteristic of the R-component and In. The CP in the form of icosahedra are characteristic of Co.

3.5. R10 Co9 In20 compounds R 10 Co 9 In 20 compounds with tetragonal Ho 10 Ni 9 In 20 type structure exist only in the systems with Er, Tm and Lu [15,16] (Tables 1 and 2). In the nickel systems, besides the mentioned rare earth metals, also Ho forms a compound of such a composition [3]. In the Ho 10 Ni 9 In 20 structure one can discern four columns of polyhedra formed only by In

Fig. 5. Packing of the clusters [Lu 2 Co 8 In 14 ] (a) and icosahedra [CoCo 12 ] (b) in the Lu 6 Co 262x In 14 .

Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

atoms along the Z-axis. Two columns contain anticubes [HoIn 8 ] around the Ho1 and Ho2 atoms conjugated by square faces. Another two columns are formed by empty trigonal prisms [EIn 6 ] conjugated along the X- and Y-axes by mutual edges and along the Z-axes by four mutual vertices. The spaces between these prisms are packed doubly by square face trigonal prisms which are interchangeable empty [EIn 8 ] or filled by Co1 atoms [CoIn 8 ] (Fig. 6). The space between the columns is packed by blocks of deformed cubes [Tm3Co 2 In 6 ], trigonal prisms [Co2In 6 ], empty trigonal prisms [ECoIn 5 ] along the X- and Y-axes and by columns of deformed tetragonal prisms [Tm4Co 2 In 6 ] along the Z-axis.

3.6. La6 Co13 In compound La 6 Co 13 In compound [1] belongs to Nd 6 Fe 13 Si structure type which is an atomic redistribution structure of the La 6 Co 11 Ga 3 type [17]. It is the only representative in the systems of In with rare earth and Co (Tables 1 and 2). This structure is peculiar because it is formed for a small In content. The In atoms have a coordination number of 10 and a coordination polyhedron in the form of an anticube with additional atoms opposite the square faces which is not characteristic of compounds of rare earth and transitions metals. In La 6 Co 13 In there are layers of icosahedra [CoCo 12 ] and anticubes with two additional atoms opposite the square faced [InLa 10 ] lying in the X–Y plane. The centers of the first ones are at the height z50; 1 / 2, and the

85

Fig. 7. Packing of the anticubes with two additional atoms [InLa 10 ] (a) and icosahedra [CoCo 12 ] (b) in the La 6 Co 13 In structure.

others are at z51 / 4; 3 / 4 (Fig. 7). If icosahedra within the layer are isolated then every anticube has one common face with four neighbouring ones. The layers of icosahedra and anticubes do not have mutual contact. One unit cell of La 6 Co 13 In has four icosahedra [CoCo 12 ] and four anticubes [InLa 10 ], but the composition of the latter ones thanks to the presence of common faces has the formula [InLa 6 ]. The compound composition is described by the sum of the components: 4CoCo 12 1 4InLa 6 =La 24 Co 52 In 4 =4La 6 Co 13 In. On the other hand, like the La 6 Co 11 Ga 3 structure [17], the La 6 Co 13 In compound can be considered as a hybrid structure consisting of the Ce(Mn 0.55 Ni 0.45 ) 11 structure type (composition RT 11 ; space group P4 /mbm, a|0.83, c|0.49 nm) [18] and the Cr 5 B 3 structure type (with the composition R 5 T 2 X; space group P4 /mcm, a|0.76, c|1.40 nm for Ho 5 Ga 3 ) [19] in a ratio 2:1 (Fig. 8). Here, the compound composition is deduced as follows: 2?2RT 11 11?4R 5 T 2 X=R 4 T 44 1 R 20 T 8 X 4 =R 24 T 52 X 4 =4R 6 T 13 X. As mentioned in [18], the RT 11 structure is, in turn, a hybrid of the simpler types Zr 4 Al 3 and CeMg 2 Si 2 in a ratio 2:2. The Cr 5 B 3 structure [20] is a hybrid of Al 2 Cu and U 3 Si 2 in a ratio 1:2.

3.7. R3 Co2 In4 compounds

Fig. 6. Packing of the anticubes [TmIn 8 ] (a), empty trigonal prisms [EIn 6 ] (b), and empty [EIn 8 ] and filled [CoIn 8 ] (c) doubled by square face trigonal prisms in the Ho 10 Ni 9 In 20 structure type.

R 3 Co 2 In 4 compounds with Lu 3 Co 22x In 4 (x50.13) structure type are formed by the heavy rare earth Dy–Tm and Lu [21] (Tables 1 and 2). This structure type is closely connected with the ZrNiAl structure type which is characteristic for RNiIn [22] and RCuIn [23] compounds. The Lu 3 Co 22x In 4 (R 3 T 2 X 4 ) structure can be obtained from ZrNiAl (R 3 T 3 X 3 ) by means of substitutions of X (In) T =X atoms for T atoms: R 3 T 3 X 3 → R 3 T 2 X 4 (R 3 Co 2 In 4 ).The decrease of the space group symmetry from P6¯2 m to P6¯ occurs under preservation of the hexagonal symmetry. Further substitution of one more X atom for a T atom leads to the Lu 3 CoGa 5 structure type which increases the space group symmetry again to P6¯2 m [19] (Fig. 9) occurs. However, there are no representatives in the In systems. In the Lu 3 Co 22x In 4 structure a part of the In atoms has

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Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

Fig. 8. The La 6 Co 13 In structure (a) as a combination of the Cr 5 B 3 -type fragments (composition R 5 T 2 X) (b) and Ce(Mn 0.55 Ni 0.45 ) 11 -type fragments (composition RT 11 ) (c). Cr 5 B 3 structure (b) as a hybrid of the U 3 Si 2 -type (composition R 3 T 2 ) (d) and Al 2 Cu-type (composition R 2 X) (e), and Ce(Mn 0.55 Ni 0.45 ) 11 structure as a hybrid of the CeMg 2 Si 2 -type (composition RT 4 ) (g) and Zr 4 Al 3 -type (composition T 7 ) (h).

CN511 and a coordination polyhedron in the form of a trigonal prism with five additional atoms. The indium atoms have similar coordination polyhedra in the compounds of about the same composition in the systems with Ni and Cu (|RNi 0.5 In 1.5 [3], |RCu 0.5 In 1.5 [4]), but which belong to the AlB 2 structure type.

3.8. R12 Co6 In and R6 Co2 In compounds R 12 Co 6 In and R 6 Co 2 In compounds belong to the Sm 12 Ni 6 In- [24] and Ho 6 Co 2 Ga-structure types [25] (Tables 1 and 2), respectively. Compounds of the R 12 Co 6 In composition form with the light rare earths La,

Fig. 9. Relation between ZrNiAl- (a), Lu 3 Co 22x In 4 - (b) and Lu 3 CoGa 5 -types (c). The origin of the unit cell of the Lu 3 Co 22x In 4 structure (shadowed cell) is shifted to the position of the Co2 atom (1 / 3 2 / 3 1 / 2).

Ya.M. Kalychak / Journal of Alloys and Compounds 291 (1999) 80 – 88

87

Pr, Nd and compounds having R 6 Co 2 In composition form with the heavy rare earths starting with Sm and Y. The R 6 Co 2 In compounds are characterized by the presence of a statistical mixture of Co and In atoms. In Ho 6 Co 2 In the composition of the statistical mixture is 0.27Co10.73In. As a result of this the composition of the compound is Ho 6 Co 2.135 In 0.865 [25]. The given structures are closely connected between themselves: the cubic Sm 12 Ni 6 In (R 12 T 6 X) structure can be obtained from the Ho 6 Co 2 Ga (R 6 T 2 X)-type by means of the replacement of two X atoms by four T atoms in the unit cell according to the 2X=4T scheme: (4Ho 6 Co 2 Ga=) R 24 T 8 X 4 → R 24 T 12 X 2 (=2Sm 12 Ni 16 In). Both structures can be considered as forming a lattice of empty octahedra [ER 6 ] with T atoms and X components between them or with [InR 12 ] icosahedra and T atoms between them (Fig. 10). In both structures CP of In atoms are icosahedra, and the R 6 Co 21x In 12x structures for statistical mixture atoms there are cubes.

3.9. R14 Co2 In3 compounds

Fig. 11. Packing of the clusters [In 2 Lu 15 ] (a), pentagonal prisms [LuLu 10 ] (b) and icosahedra [InLu 10 Co 2 ] (c) in the Lu 14 Co 2 In 3 structure.

R 14 Co 2 In 3 compounds with the Lu 14 Co 2 In 3 structure type [26] have the highest rare-earths content among the nowadays known ternary compounds in rare-earth–transition metal–indium systems. They are formed by the metals of the yttrium subgroup. In this structure the In and Co atoms have CP in the form of icosahedra and three-capped trigonal prisms, respectively. In the compound Lu 14 Co 2 In 3 , in spite of its low In content, one observes a considerable reduction of the In–In distance. It is equal to 0.2974 nm, smaller than the sum of In radii, 0.322 nm. Thus, there are [In 2 In 15 ] clusters represented by pentagonal antiprisms coupled by their bases (icosahedra without two vertices). The clusters are situated along the X- and

Y-axes and are separated by pentagonal prisms around the Lu1 atom in this direction (Fig. 11). The change of the coordination number values and the forms of the coordination polyhedra of the compounds in the R–Co–In systems correlate with the general tendency of these characteristics changes in compounds of R–T–X systems, i.e. the largest CN values are characteristic of the compounds rich in the T element (CP atoms of T elements are icosahedra). A shift of the composition to higher In content leads to a decrease of the CN (the CP atoms of the T elements are cubes, anticubes, trigonal prisms with additional atoms). A composition shift to higher rare-earth content also leads to a decrease of the CN values,

Fig. 10. Empty octahedra [ER 6 ], icosahedra [InR 12 ] and atoms of T or (T1In) components in the Sm 12 Ni 6 In (a) and Ho 6 Co 21x In 12x (b) structure types, respectively.

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especially for the rare earths and T elements (the CP of the latter ones are anticubes, trigonal prisms). In the compounds with low rare-earths content the CP of the In atoms are more often cubooctahedra (often deformed), but in the compounds with high rare-earth content the CP of the In atoms are icosahedra (in multilayer structures). There is one more tendency for the compounds of the R–Co–In systems, which is also characteristic of the compounds in the nickel and copper systems. With increasing rare-earth content one observes a decrease of the shortest lattice constant in compounds having the same number of layers in this direction. In the R–Co–In systems the double layer compounds R 2 Co 9 In 3 and RCo 2 In have a lattice constant of about 0.41 nm, the RCoIn 5 and R 2 CoIn 8 compounds have a somewhat larger lattice constant (|0.45 nm) because of the larger In content. In R 3 Co 2 In 4 , in spite of the larger rare-earth and In contents, the value of the smallest lattice constant has decreased to about 0.36 nm. Previously [3] we attributed this fact to a considerable polarization of the In atoms in the rare-earth rich compounds.

References ¨ G. [1] F. Weitzer, A. Leithe-Jasper, P. Rogl, K. Hiebl, H. Noel, Wiesinger, W. Steiner, J. Solid State Chemistry 104 (1993) 368– 376. [2] L.G. Akselrud, Y.N. Gryn’, P.Y. Zavalij, V.K. Pecharsky, V.S. Fundamensky, XII. Eur. Crystallographic. Meet. Coll. Abstr. 3 (1989) 155. [3] Ya.M. Kalychak, J. Alloys Compounds 262–263 (1997) 341–345. [4] Ya.M. Kalychak, Izv. Russ. Acad. Nauk., Metally. 4 (1998) 110– 118. [5] V.M. Baranyak, Ya.M. Kalychak, P.Yu. Zavalij, Kristallografiya 38 (1993) 268–270. [6] Ya.M. Kalychak, V.I. Zaremba, V.K. Pecharsky, Z. Kristallographie 205 (1993) 333–334.

[7] Ya.M. Kalychak, V.I. Zaremba, Kristallografiya 39 (1994) 923–924. [8] Ya.P. Yarmolyuk, P.I. Krypyakevich, Dopov. Acad. Nauk. Ukr. RSR. Ser.A 1 (1976) 86–88. [9] Ya.M. Kalychak, V.I. Zaremba, V.M. Baranyak, V.A. Bruskov, P.Yu. Zavalij, Izv. Acad. Nauk SSSR, Metally 1 (1989) 209–210. [10] Yu.N. Grin’, Ya.P. Yarmolyuk, E.I. Gladyshevsky, Kristallografiya 24 (1979) 242–246. [11] Ya.M. Kalychak, V.I. Zaremba, E.I. Gladyshevsky, XXII Eur. Crystallographic Meet. Coll. Abstr. 2 (1989) 86. [12] V.I. Zaremba, Ya.M. Kalychak, P.Yu. Zavalij, V.E. Zavodnik, Kristallografiya 35 (1990) 493–499. [13] V.P. Dubenskyy, Ya.M. Kalychak, V.I. Zaremba, M.O. Lukachuk, E.A. Goreshnik, J. Alloys Compounds 284 (1999) 228–231. [14] L.V. Sysa, V.I. Zaremba, Ya.M. Kalychak, V.M. Baranyak, Visn. L’viv. Univ., Ser. Khim. 29 (1988) 32–34. [15] Ya.V. Galadzhun, Ya.M. Kalychak, V.P. Dubenskyy, V.I. Zaremba, 6th Intl. Conf. on Crystal Chemistry of Intermetallic Compounds, L’viv, 1995, September 26–29, Abstr., p. 72. [16] V.P. Dubenskyy, Ya.M. Kalychak, V.I. Zaremba, E.A. Goreshnik, J. Alloys Compounds 280 (1998) 199–203. [17] O.M. Sichevich, R.P. Lapunova, A.N. Sobolev, Yu.N. Grin’, Ya.P. Yarmolyuk, Kristallografiya 30 (1985) 1077–1080. [18] Ya.M. Kalychak, L.G. Akselrud, Ya.P. Yarmolyuk, O.I. Bodak, E.I. Gladyshevsky, Kristallografiya 20 (1975) 1045–1047. [19] Yu.N. Grin’, R.E. Gladyshevsky, Metallurgiya, Gallidy, Moscow, 1989, 303 pp. [20] Yu.B. Kuz’ma, Crystal Chemistry of Borides, Vyshcha Shkola, L’viv, 1983, 161 pp. [21] V.I. Zaremba, Ya.M. Kalychak, P.Yu. Zavalij, O.I. Sobolev, Dopov. Akad.Nauk Ukr. RSR, Ser.B 2 (1989) 37–39. [22] R. Ferro, R. Marazza, G. Rambaldi, Z. Metallkunde 65 (1974) 37–39. [23] P. Villars, L.D. Calvert, in: Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, Vol. 1–3, American Society for Metals, OH, 1985, p. 6087. [24] Ya.M. Kalychak, V.I. Zaremba, J. Stepien’-Damm, Ya.V. Galadzhun, L.G. Akselrud, Kristallografiya 43 (1998) 17–20. [25] Ya.M. Kalychak, V.I. Zaremba, P.Yu. Zavalij, Z. Kristallographie 208 (1993) 380–381. [26] V.I. Zaremba, Ya.M. Kalychak, P.Yu. Zavalij, Kristallografiya 37 (1992) 352–355.