Crystal structures of the ScCuSe2 and Sc3CuSn3Se11 compounds

Journal of Alloys and Compounds 393 (2005) 174–179

Crystal structures of the ScCuSe2 and Sc3CuSn3Se11 compounds L.D. Gulay∗ , V.Ya. Shemet, I.D. Olekseyuk Department of General and Inorganic Chemistry, Volyn State University, Voli Ave 13, 43009 Lutsk, Ukraine Received 29 September 2004; accepted 7 October 2004 Available online 10 December 2004

Abstract The crystal structure of the ScCuSe2 compound was investigated using X-ray powder diffraction (space group P2, a = 0.67108(5) nm, ◦ b = √ 0.38949(3) nm, c = 1.27879(6) nm, β = 90.281(6) , Pearson symbol mP16.08, RI = 0.0839). The structure of ScCuSe2 represents a distinctive 3a × a × 2c superstructure of Er2/3 Cu2 S2 . The Se atoms are stacked in a close-packed arrangement with layers in the sequence AB. The Sc atoms occupy half of the octahedral sites, the Cu atoms are located in 3/8 of the tetrahedral sites. The crystal structure of the Sc3 CuSn3 Se11 compound (space group Fd3m, a = 1.08827(4) nm, Pearson symbol cF52.48, RI = 0.0386) was also determined by means of X-ray powder diffraction. The Se atoms are stacked in a close-packed arrangement with layers in the sequence ABC. The Sc atoms and a statistical mixture M (Sc + Sn) are located in all octahedral sites, the Cu atoms in 1/8 of the tetrahedral sites. © 2004 Elsevier B.V. All rights reserved. Keywords: Chalcogenides; Sc compounds; Cu compounds; Sn compounds; Se compounds; Crystal structure; X-ray powder diffraction

The crystal structure of the ScCuSe2 and Sc2/3 Cu2 Se2 compounds has been described as Er2/3 Cu2 S2 structure ¯ in [1] and [2]. The lattice patype (space group P 3) rameters have been refined. No quaternary phases in the Sc2 Se3 –Cu2 Se–SnSe2 system have been reported yet in the literature. The crystal structures of the ScCuSe2 and Sc3 CuSn3 Se11 compounds are given in the present paper.

rate of 10 K/h and annealed at respective temperature during 240 h. After annealing the ampoules with the samples were quenched in cold water. X-ray powder diffraction patterns of the samples for the crystal structure determination were recorded using a DRON-4-13 powder diffractometer (Cu K␣ radiation, 10◦ ≤ 2Θ ≤ 100◦ , step scan mode with a step size of 0.05◦ and counting time of 20 s per data point). The crystal structure determination was performed using the CSD [3] and DBWS-9411 [4] programs.

2. Experimental details

3. Results and discussion

The alloys were prepared by melting the high purity elements (the purity of the ingredients was better than 99.9 wt.%) in evacuated quartz ampoules. The synthesis was realized in a shaft furnace with a heating rate of 20 K/h. The ampoules with the samples were heated to a maximal temperature of 1420 K. The samples were kept at the maximal temperature during 4 h. After that they were cooled slowly to 870 K with a

3.1. Crystal structure of the ScCuSe2 compound

1. Introduction

∗

Corresponding author. E-mail address: [email protected] (L.D. Gulay).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.10.037

The existence of the ternary ScCuSe2 compound was observed during an investigation of the phase relations in the Sc2 Se3 –Cu2 Se system. According to Refs. [1,2] the ScCuSe2 compound crystallizes in the Er2/3 Cu2 S2 struc¯ The basic peaks of the X-ray ture type (space group P 3). powder diffraction pattern of the sample ScCuSe2 were really indexed in a hexagonal unit cell. The obtained lattice parameters were close to those reported in [1,2] for

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Table 1 Results of the crystal structure determination of the ScCuSe2 and Sc3 CuSn3 Se11 compounds

Number of formula units per unit cell Space group a (nm) b (nm) c (nm) β (◦ ) Cell volume (nm3 ) Number of atoms in cell Calculated density (g/cm3 ) Radiation and wavelength Diffractometer Mode of refinement Number of atom sites Structure solution and refinement RI RP Texture axis and parameter a

ScCuSe2

Sc3 CuSn3 Se11

4 P2 0.67108(5) 0.38949(3) 1.27879(6) 90.281(6) 0.33425(7) 16.08 5.319 Cu (0.154178 nm) Powder DRON-4-13 Full profile 11 CSD 0.0839 0.1355 [0 0 1] 0.51(1)

32/11 Fd3m 1.08827(4)

1.2889(1) 52.48 5.3461 Cu (0.154178 nm) Powder DRON-4-13 Full profile 4 DBWS-9411 0.0386 0.0536a

Presence of the phases Sc2 Se3 and Cu2 SnSe3 was taken into account during the refinement procedure (see Fig. 4).

a sample of the corresponding composition. Many additional peaks of low intensity were observed in the X-ray powder diffraction pattern. Taking into account all peaks of the X-ray powder diffraction pattern a monoclinic lattice with the parameters a = 0.67108(5) nm, b = 0.38949(3) nm, c = 1.27879(6) nm, β = 90.281(6)◦ was found for the ScCuSe2 compound. By assuming space group symmetry P2 we were able to extract a plausible structural model from the powder X-ray intensities by means of direct methods and difference Fourier syntheses. Table 1 contains the essential technical and crystallographic data of the crystal structure determination. The atomic coordinates and isotropic temperature factors are given in Table 2, whereas the interatomic distances and coordination numbers of the atoms are listed in Table 3 . The positions of the Sc and Se atoms are fully occupied. All positions of the Cu atoms are partially occupied. The experimental and calculated diffractograms and the corresponding difference diagram for ScCuSe2 are shown in Fig. 1. The interatomic distances agree well with the sum of the corresponding ionic radii [5]. The unit cell, the coordination polyhedra of the Sc1 (a), Sc2 (b), Sc3 (c), Sc4 (d), Cu1 (e), Cu2 (f), Cu3 (g), Se1 (h),

Se2 (i), Se3 (j), Se4 (k) atoms and the layers of the Se atoms of hexagonal topology in the structure of the ScCuSe2 compound are shown in Fig. 2. Octahedral surrounding exists for the Sc atoms, tetrahedral for the Cu atoms. The Se1 and Se2 atoms are surrounded by seven and five cations, respectively. The neighbors of the Se3 and Se4 atoms form distorted octahedra. The Se atoms in the structure of the ScCuSe2 compound are stacked in a close-packed arrangement with layers in the sequence AB. The Sc atoms occupy half of the octahedral sites. The Cu atoms are located in 3/8 of the tetrahedral sites. Taking into account the occupation factors of the positions of the Cu atoms we can assume that only one quarter of the tetrahedral sites is really occupied by Cu atoms. The layers of the Sc(Er)-centered octahedra and Cu-centered tetrahedra in the structures of the compounds ScCuSe2 and Er2/3 Cu2 S2 [6] are shown in Fig. 3. According to Refs. [1,2] the compounds with composition RCuSe2 (R = Y, Tb, Dy, Ho, Er, Tm, Yb and Lu) crystallize in the Er2/3 Cu2 S2 structure type. The Se atoms are stacked in a close-packed arrangement with layers in the sequence AB. The R atoms occupy half of the octahedral sites. The Cu atoms of the RCuSe2 compounds are located in half of the tetrahedral sites. Since the occupation

Table 2 Atomic coordinates and isotropic temperature factors for the ScCuSe2 compound Atom

Position

x/a

y/b

z/c

Occupation

Biso ×102 (nm2 )

Sc1 Sc2 Sc3 Sc4 Cu1 Cu2 Cu3 Se1 Se2 Se3 Se4

1(a) 1(b) 1(c) 1(d) 2(e) 2(e) 2(e) 2(e) 2(e) 2(e) 2(e)

0 0 1/2 1/2 0.649(1) 0.719(2) 0.153(2) 0.6603(9) 0.6653(9) 0.1644(9) 0.1646(9)

0.000a 0.080(4) 0.556(4) 0.485(5) 0.021(4) 0.047(7) 0.503(4) 0.025(2) 0.983(2) 0.500(2) 0.532(2)

0 1/2 0 1/2 0.1906(4) 0.7146(8) 0.6875(4) 0.3801(3) 0.8838(3) 0.3815(3) 0.8796(3)

1.000 1.000 1.000 1.000 0.823(6) 0.496(6) 0.722(6) 1.000 1.000 1.000 1.000

0.3(3) 0.4(3) 0.4(3) 0.4(3) 0.7(2) 1.6(4) 0.4(2) 0.2(1) 0.28(9) 0.25(9) 0.25(9)

a

Fixed.

176

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Fig. 1. The experimental and calculated diffractograms and the corresponding difference diagram for ScCuSe2 .

factor of the Cu atoms is 0.5 we can assume that only quarter of the tetrahedral sites is really occupied by Cu in the RCuSe2 (R = Y, Tb, Dy, Ho, Er, Tm, Yb and Lu) compounds. Since the Cu atoms are located in 3/8 of the tetrahedral interstices formed by Se of the ScCuSe2 compound and in half of the tetrahedral interstices formed by Se in the RCuSe2 (R = Y, Tb,

Dy, Ho, Er, Tm, Yb and Lu) compounds additional ordering of the Cu atoms and vacancies takes place in case of ScCuSe2 due to the lower symmetry of the ScCuSe2 compound compared with the RCuSe2 (R = Y, Tb, Dy, Ho, Er, Tm, Yb and Lu) compounds. The structure of ScCuSe2 represents a dis√ tinctive 3a × a × 2c superstructure of Er2/3 Cu2 S2 .

Fig. 2. The unit cell, coordination polyhedra of the Sc1 (a), Sc2 (b), Sc3 (c), Sc4 (d), Cu1 (e), Cu2 (f), Cu3 (g), Se1 (h), Se2 (i), Se3 (j), Se4 (k) atoms and the layers of the Se atoms of hexagonal topology in the structure of the ScCuSe2 compound.

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177

Fig. 3. The layers of the Sc(Er)-centered octahedra and Cu-centered tetrahedra in the structures of the compounds ScCuSe2 and Er2/3 Cu2 S2 .

3.2. Crystal structure of the Sc3 CuSn3 Se11 compound The formation of a new quaternary compound with the composition Sc3 CuSn3 Se11 was observed during an investigation of the phase relations in the Sc2 Se3 –Cu2 Se–SnSe2 system. According to the results of the phase analysis the presence of peaks of a new unknown compound and weak additional peaks of the phases Sc2 Se3 [7], Cu2 SnSe3 [8] in the X-ray powder diffraction patter of the sample with composition Sc3 CuSn3 Se11 was observed. The peaks of unknown phase were indexed on the basis of a face-centered

cubic lattice with a = 1.08827(4) nm. According to the obtained results the crystal structure of the Yb1.84 Fe1.23 S4 compound (space group Fd3m, a = 1.0830 nm) [9] was used as a starting model for the crystal structure determination of the Sc3 CuSn3 Se11 compound. The presence of Sc2 Se3 and Cu2 SnSe3 as minority phases was taken into account during the crystal structure investigation. Results of the crystal structure determination are given in Table 1. Atomic coordinates and isotropic temperature factors are listed in Table 4 . The experimental and calculated diffractograms and the corresponding difference diagram for

Fig. 4. The experimental and calculated diffractograms and the corresponding difference diagram for Sc3 CuSn3 Se11 (1: Sc3 CuSn3 Se11 , 2: Sc2 Se3 , 3: Cu2 SnSe3 ).

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Table 3 Interatomic distances δ (nm) and coordination numbers (CN) of the atoms in the ScCuSe2 structure

Table 4 Atomic coordinates and isotropic temperature factors for the Sc3 CuSn3 Se11 compound

δ (nm)

CN

Atom

Position

x/a

y/b

z/c

Occupation

Biso (×102 nm2 )

2Se4 2Se2 2Se4

0.2632 0.2688 0.2811

6

Sc M

16(c) 16(d)

0 1/2

0 1/2

0 1/2

2.7(8) 0.98(6)

2Se3 2Se1 2Se3

0.249(1) 0.2749(6) 0.293(1)

6

Cu Se

8(a) 32(e)

1/8 0.2524(2)

1/8 x

1/8 x

0.10 0.45Sc, 0.55Sn 0.36 1.00

2Se2 2Se4 2Se2

0.249(1) 0.2722(6) 0.290(1)

6

Table 5 Interatomic distances δ (nm) and coordination numbers (CN) of the atoms in the Sc3 CuSn3 Se11 structure

2Se1 2Se3 2Se1

0.260(2) 0.2709(6) 0.282(2)

6

1Se2 1Se1 1Se4 1Se4

0.231(1) 0.2424(7) 0.245(1) 0.251(1)

4

1Se2 1Se3 1Se3 1Se1

0.221(1) 0.229(2) 0.258(2) 0.281(1)

4

1Se3 1Se1 1Se4 1Se1

0.230(1) 0.241(2) 0.2460(7) 0.254(2)

4

1Cu3 1Cu1 1Cu3 1Sc4 1Sc2 1Cu2 1Sc4

0.241(2) 0.2424(7) 0.254(2) 0.260(2) 0.2749(6) 0.281(1) 0.282(2)

7

1Cu2 1Cu1 1Sc3 1Sc1 1Sc3

0.221(1) 0.231(1) 0.249(1) 0.2688 0.290(1)

5

1Cu2 1Cu3 1Sc2 1Cu2 1Sc4 1Sc2

0.229(2) 0.230(1) 0.249(1) 0.258(2) 0.2709(6) 0.293(1)

6

1Cu1 1Cu3 1Cu1 1Sc1 1Sc3 1Sc1

0.245(1) 0.2460(7) 0.251(1) 0.2632 0.2722(6) 0.2811

6

Atoms Sc1

Sc2

Sc3

Sc4

0.9(3) 1.49(5)

Atoms

δ (nm)

CN

Sc 6Se M 6Se Cu 4Se

0.2747(2) 0.2695(2) 0.2401(2)

6 6 4

0.2401(2) 0.2695(2) 0.2747(2)

7

Se

Cu1

Cu2

Cu3

Se1

1Cu 3M 3Sc

Sc3 CuSn3 Se11 (1: Sc3 CuSn3 Se11 , 2: Sc2 Se3 , 3: Cu2 SnSe3 ) are shown in Fig. 4. The cation–anion distances (Table 5) agree well with the sum of the corresponding ionic radii [5]. The projection of the unit cell of the Sc3 CuSn3 Se11 compound on the XY plane and the coordination polyhedra of the Sc (a) M (Sc + Sn) (b), Cu (c) and Se (d) atoms are shown in Fig. 5. The Se atoms are stacked in a close-packed arrangement with layers in the sequence ABC. The atoms of Sc and statistical mixture M (Sc + Sn) are located in all octahedral sites, the Cu atoms in 1/8 of the tetrahedral sites. The comparison of the crystal structures of the compounds Sc3 CuSn3 Se11 , Yb1.84 Fe1.23 S4 and Cr1.3 Cu1.1 Sn0.7 S3.9 [10]

Se2

Se3

Se4

Fig. 5. The projection of the crystal structure of the Sc3 CuSn3 Se11 compound on XY plane and coordination polyhedra of the Sc (a) M (Sc + Sn) (b), Cu (c) and Se (d) atoms.

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Table 6 The comparison of the crystal structures of the compounds Sc3 CuSn3 Se11 , Yb1.84 Fe1.23 S4 and Cr1.3 Cu1.1 Sn0.7 S3.9 Position

Sc3 CuSn3 Se11 space group Fd3m (a = 1.08827 nm)

Yb1.84 Fe1.23 S4 space group Fd3m (a = 1.0830 nm)

Cr1.3 Cu1.1 Sn0.7 S3.9 space group Fd3m (a = 1.0101 nm)

8(a) (1/8, 1/8, 1/8) 16(c) (0, 0, 0) 16(d) (1/2, 1/2, 1/2) 32(e) (x, x, x)

Cu Sc M (Sc + Sn) Se (x = 0.2524)

Fe M1 (Yb + Fe) M2 (Yb + Fe) S (x = 0.2531)

Cu1 Cu2 M (Cr + Sn) S (x = 0.259)

are given in Table 6 . The S atoms in the structures of the compounds Yb1.84 Fe1.23 S4 and Cr1.3 Cu1.1 Sn0.7 S3.9 are also stacked in a close-packed arrangement with layers in the sequence ABC. One-eighth of the tetrahedral sites (8a position) are occupied by Cu (Fe) atoms in the Sc3 CuSn3 Se11 , Yb1.84 Fe1.23 S4 and Cr1.3 Cu1.1 Sn0.7 S3.9 structures. Half of the octahedral sites (16d position) are occupied by a statistical mixtures of atoms (Sc + Sn, Yb + Fe or Cr + Sn) in all structures. Half of the octahedral sites (16c position) are occupied by Sc atoms in Sc3 CuSn3 Se11 , by a statistical mixture of (Yb + Fe) atoms in Yb1.84 Fe1.23 S4 and by Cu atoms in Cr1.3 Cu1.1 Sn0.7 S3.9 . An octahedral surrounding of the Sc atoms and a tetrahedral surrounding of the Cu atoms is also found in the crystal structure of the ScCuSe2 compound investigated in the present paper.

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