Crystal structure of the R2PbSe4 (R = Er and Yb) compounds

Journal of Alloys and Compounds 429 (2007) 111–115

Crystal structure of the R2PbSe4 (R = Er and Yb) compounds L.D. Gulay a,∗ , M. Daszkiewicz b , J. St˛epie´n-Damm b , A. Pietraszko b a

Department of General and Inorganic Chemistry, Volyn State University, Voli Ave 13, 43009 Lutsk, Ukraine b W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Ok´olna str. 2, P.O. Box 1410, 50-950 Wrocław, Poland Received 17 March 2006; received in revised form 25 March 2006; accepted 27 March 2006 Available online 3 May 2006

Abstract The crystal structure of the R2 PbSe4 (R = Er and Yb) compounds (space group Pnma, Pearson symbol oP28, a = 1.2541(3) nm, b = 0.40810(8) nm, c = 1.4865(3) nm, R1 = 0.0547 for Er2 PbSe4 and a = 1.2501(2) nm, b = 0.40380(8) nm, c = 1.4707(3) nm, R1 = 0.0469 for Yb2 PbSe4 ) was determined by means of X-ray single crystal diffraction. Octahedral surrounding exists for the R atoms. The Pb atoms are surrounded by trigonal prisms with one additional atom. © 2006 Elsevier B.V. All rights reserved. Keywords: Chalcogenides; Rare-earth compounds; Pb compounds; Se compounds; Crystal structure; X-ray single crystal diffraction

1. Introduction Production of the compounds with increasingly complex compositions, such as ternary, quaternary, etc., has become a principle direction in a modern science of materials [1]. Among the multicomponent systems an important place belongs to the complex rare-earth chalcogenides. The rare-earth chalcogenides are being intensively studied during last years due to their specific thermal, electrical and optical properties, which e.g. make them prospective materials in the field of infrared and nonlinear optics. Therefore, the synthesis and the investigation of the crystal structures of complex chalcogenides is important step in a search for new materials. The crystal structure of the Sc2 PbS4 compound (CaFe2 O4 structure type, space group Pnma) has been investigated in Ref. [2] by means of neutron powder diffraction. The crystal structure of the compounds R2 PbS4 (R = Ho, Er, Tm, Yb and Lu) and R2 PbSe4 (R = Er, Tm, Yb and Lu) has been described in Ref. [3]. Orthorhombic lattices have been determined for these compounds and CaFe2 O4 structure type (space group Pnma) has been assumed. The existence of the phases R2 PbS4 (R = La, Ce, Pr, Nd, Sm and Gd) and R2 PbSe4 (R = La, Ce, Pr, Nd and

∗

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

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.087

¯ Sm) with Th3 P4 structure type (space group I43d) has been also reported in Ref. [3]. The crystal structures of the compounds Sc2 PbX4 (X = S, Se) (CaFe2 O4 structure type, space group Pnma) [4], R6 Pb2 Se11 (R = Y, Dy and Ho) (Y6 Pb2 Se11 structure type, space group Cmcm) [5,6], Y4.2 Pb0.7 Se7 (Y5 Se7 structure type, space group Cm) [7] and R2 PbSe4 (R = Tm and Lu) (Tm2 PbSe4 structure type, space group Pnma) [8] have been investigated recently. In the present paper the crystal structure of ternary compounds R2 PbSe4 (R = Er and Yb) is given and discussed. 2. Experimental details The samples were prepared using high purity elements (the purity of the ingredients was better than 99.9 wt.%). The calculated amounts of the components were sealed in evacuated silica ampoules. The synthesis was realized in a tube furnace. The ampoules were gradually heated with 30 K/h to a maximal temperature 1420 K. The samples were kept at maximal temperature for 4 h. After that they were cooled slowly (10 K/h) to 870 K and annealed at this temperature for 240 h. After annealing the ampoules with the samples were quenched in cold water. Diffraction-quality single crystals of R2 PbSe4 (R = Er and Yb) for structure determinations were selected from the samples of the respective compositions. X-ray diffraction data were obtained with the use of graphite-monochromatized Mo K␣ radiation (λ = 0.071073 nm) on a KUMA diffraction KM-4 four-circle single crystal diffractometer equipped with a CCD camera. The intensities of the reflections were corrected for Lorentz and polarization factors. Semiempirical absorption correction was applied. The crystal structure was solved by Patterson methods [9] and refined by full matrix least squares method using SHELX-97 program [10].

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L.D. Gulay et al. / Journal of Alloys and Compounds 429 (2007) 111–115

Table 1 Crystal data and structure refinement details for the R2 PbSe4 (R = Er and Yb) compounds Empirical formula

Er2 PbSe4

Yb2 PbSe4

Formula weight Space group

857.55 Pnma (No. 62)

869.11 Pnma (No. 62)

Unit cell dimensions

a = 1.2541(3) nm b = 0.40810(8) nm c = 1.4865(3) nm

a = 1.2501(2) nm b = 0.40380(8) nm c = 1.4707(3) nm

Volume Number of formula units per unit cell Calculated density Absorption coefficient F(0 0 0) Crystal size Θ range for data collection

0.7608(3) nm3 4 7.487 g/cm3 62.956 mm−1 1416 0.05 mm × 0.12 mm × 0.06 mm 2.74–25.67

0.7424(2) nm3 4 7.776 g/cm3 67.102 mm−1 1432 0.07 mm × 0.11 mm × 0.08 mm 3.21–25.67

Index ranges

−15 ≤ h ≤ 15 −4 ≤ k ≤ 4 −18 ≤ l ≤ 17

−15 ≤ h ≤ 15 −4 ≤ k ≤ 4 −17 ≤ l ≤ 17

Reflections collected Independent reflections Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole × 10−3

7531 825 [R(int.) = 0.2611a /0.1604b ] Full-matrix least-square on F2 825/0/50 1.021 R1 = 0.0547, wR2 = 0.1145 R1 = 0.0885, wR2 = 0.1309 0.00005(9) 2.978 and −3.601 e/nm3

6868 805 [R(int.) = 0.2559a /0.1734b ] Full-matrix least-square on F2 805/0/50 1.047 R1 = 0.0469, wR2 = 0.0867 R1 = 0.0748, wR2 = 0.0950 0.00003(6) 3.338 and −3.075 e/nm3

a b

Before absorption correction. After absorption correction.

3. Results and discussion The formation of the R2 PbSe4 (R = Er and Yb) compounds was confirmed during the investigation of the phase relations in the respective R2 Se3 –PbSe systems. The crystal structure of these compounds was determined using X-ray single crystal

diffraction. The extinctions observed were found to be consistent with the space group Pnma which was used for the crystal structure refinement. At the first stage of the refinement two positions of R, one position of Pb and four positions of Se were determined for both compounds. Unusual values of anisotropic thermal parameters for Pb and residual electron densities of about

Table 2 Atomic coordinates and temperature factors for the R2 PbSe4 (R = Er and Yb) compounds x/a

y/b

z/c

Occupation

Ueq. × 102 (nm2 )

U11 × 102 (nm2 )

U22 × 102 (nm2 )

U33 × 102 (nm2 )

U23 × 102 (nm2 )

U13 × 102 (nm2 )

U12 × 102 (nm2 )

Er2 PbSe4 Er1 4c Er2 4c Pb1 4c Pb2 4c Se1 4c Se2 4c Se3 4c Se4 4c

0.4381(1) 0.4160(1) 0.7904(3) 0.7312(3) 0.2061(2) 0.1286(2) 0.5259(3) 0.4099(2)

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4

0.3905(1) 0.9041(1) 0.3369(2) 0.3213(2) 0.8316(2) 0.5305(2) 0.2150(2) 0.5766(2)

1.00 1.00 0.50 0.50 1.00 1.00 1.00 1.00

0.0169(4) 0.0162(4) 0.0278(8) 0.0257(8) 0.0163(8) 0.0154(8) 0.0194(8) 0.0160(8)

0.0130(7) 0.0100(8) 0.020(1) 0.014(1) 0.006(1) 0.010(1) 0.018(1) 0.006(1)

0.0157(8) 0.0143(9) 0.030(1) 0.030(1) 0.014(1) 0.014(1) 0.016(1) 0.018(1)

0.0219(9) 0.0242(9) 0.032(1) 0.031(1) 0.027(1) 0.021(1) 0.023(1) 0.023(1)

0 0 0 0 0 0 0 0

−0.0025(7) −0.0001(6) 0.000(1) 0.001(1) −0.001(1) −0.002(1) 0.004(1) 0.000(1)

0 0 0 0 0 0 0 0

Yb2 PbSe4 Yb1 4c Yb2 4c Pb1 4c Pb2 4c Se1 4c Se2 4c Se3 4c Se4 4c

0.4404(1) 0.4150(1) 0.7784(4) 0.7388(4) 0.2049(3) 0.1282(3) 0.5299(3) 0.4085(2)

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4

0.38890(8) 0.90455(7) 0.3344(6) 0.3248(6) 0.8321(1) 0.5324(1) 0.2153(1) 0.5755(1)

1.00 1.00 0.50 0.50 1.00 1.00 1.00 1.00

0.0126(4) 0.0119(4) 0.027(1) 0.023(1) 0.0122(7) 0.0122(7) 0.0159(7) 0.0112(7)

0.0145(8) 0.0115(8) 0.036(4) 0.020(3) 0.006(1) 0.011(1) 0.022(2) 0.009(1)

0.0112(7) 0.0096(7) 0.020(2) 0.024(2) 0.013(1) 0.012(1) 0.012(1) 0.011(1)

0.0122(6) 0.0147(7) 0.028(2) 0.027(2) 0.017(1) 0.012(1) 0.013(1) 0.012(1)

0 0 0 0 0 0 0 0

−0.0015(6) −0.0006(6) 0.002(3) 0.002(3) −0.001(1) 0.000(1) 0.004(1) 0.000(1)

0 0 0 0 0 0 0 0

Atom

Position

Ueq. is defined as one-third of the trace of the orthogonalized Uij tensor. The anisotropic temperature factor exponent takes the form: −2π2 [h2 a*2 U11 + . . . + 2hka* b* U12 ].

L.D. Gulay et al. / Journal of Alloys and Compounds 429 (2007) 111–115

35(32) e/10−3 nm3 at the distances ∼0.078(0.052) nm near these atoms were observed at this stage for Er(Yb)2 PbSe4 , respectively. At the second stage additional Pb atoms were located at these extra positions. The occupation factors for the positions of both Pb atoms were fixed at the values 0.50 closed to calculated ones to satisfy charge balance requirements. At this stage the anisotropic thermal parameters for Pb were significantly reduced. The final value of R1 factors was also improved significantly for both compounds. The crystal data and refinement information on the R2 PbSe4 (R = Er and Yb) compounds are summarized in Table 1, whereas atomic coordinates and temperature factors are given in Table 2. The interatomic distances and coordination numbers of the Er, Yb and Pb atoms are listed in Table 3. The interatomic distances (except the Pb1–Pb2 distances) agree well with the sum of the respective ionic radii [11]. Since the Pb1 and Pb2 sites are only occupied by 50%, no more than one of two adjacent positions will be occupied. Thus, such close contacts between the Pb1 and Pb2 atoms (0.0778(3) and 0.0515(8) for Er(Yb)2 PbSe4 , respectively) do not really exist. Similar structure models were determined for the compounds Tm2 PbSe4 (X-ray single crystal data) and Lu2 PbSe4 (X-ray powder diffraction data) in Ref. [8]. The disorder of the Pb atoms was observed. At the same time only one position of the Pb atoms was determined in the crystal structures of the Sc2 PbX4 (X = S, Se) compounds [4]. The disorder of the Pb atoms was

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Table 3 Interatomic distances (δ, nm) and coordination numbers (c.n.) of the Er, Yb, Pb atoms in the structures of the R2 PbSe4 (R = Er and Yb) compounds Atoms

δ

c.n.

Er2 PbSe

Yb2 PbSe

R1 1Se4 1Se3 2Se4 2Se1

0.2788(4) 0.2833(4) 0.2834(2) 0.2864(2)

0.2774(3) 0.2787(3) 0.2813(2) 0.2841(2)

6

R2 2Se3 2Se2 1Se2 1Se1

0.2798(3) 0.2830(2) 0.2837(3) 0.2845(3)

0.2767(2) 0.2811(2) 0.2821(3) 0.2835(3)

6

Pb1 2Se2 1Se3 2Se1 2Se4

0.3013(4) 0.3053(5) 0.3232(4) 0.3482(4)

0.3046(6) 0.3228(7) 0.3181(8) 0.3360(6)

7

Pb2 1Se3 2Se4 2Se1 2Se2

0.3020(5) 0.3098(3) 0.3155(3) 0.3480(4)

0.3067(3) 0.3100(7) 0.3147(8) 0.3354(7)

7

Fig. 1. The unit cell and coordination polyhedra of the Er1 (a), Er2 (b), Pb1 (c), Pb2 (d), Se1 (e), Se2 (f), Se3 (g), Se4 (h) atoms in the crystal structure of the Er2 PbSe4 compound.

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L.D. Gulay et al. / Journal of Alloys and Compounds 429 (2007) 111–115

Fig. 2. The (Pb1 + Pb2)-centered trigonal prism as combination of the Pb1- and Pb2-centered trigonal prisms in the structure of the Er2 PbSe4 compound.

Fig. 3. Packing of the Y(Er)-centered octahedra, M(Y + Pb)- and Pb-centered trigonal prisms in the structures of the compounds Er2 PbSe4 , Y6 Pb2 Se11 and Y4.2 Pb0.7 Se7 .

also observed in the crystal structures of the recently investigated compounds RCuPbS3 (R = Tb, Dy, Ho, Er, Tm, Yb and Lu) [12] and R3.33 CuPb1.5 Se7 (R = Tb, Dy, Ho, Er, Tm, Yb and Lu) [13]. The unit cell and coordination polyhedra of the Er1 (a), Er2 (b), Pb1 (c), Pb2 (d), Se1 (e), Se2 (f), Se3 (g), Se4 (h) atoms in the crystal structure of the Er2 PbSe4 compound are shown in Fig. 1. Octahedral surrounding exists for the Er atoms. The Pb atoms are surrounded by trigonal prisms with one additional atom. Taking into account partial occupation of the positions of the Pb atoms the Se1-, Se2- and Se4-centered polyhedra can be described as tetragonal pyramids. The Se3 atoms are located in trigonal bipyramids. Similar surrounding of the respective atoms exists in the crystal structures of the compounds investigated in Refs. [4–8]. The Pb1 and Pb2 atoms are located in the same trigonal prism with two additional atoms (c.n. = 8). The Pb1 atom is shifted from the center of this prism in one direction, the Pb2 atom in opposite direction. Separately the Pb1 and Pb2 atoms are

Fig. 4. The chains of the Er-centered octahedra and columns of the Pb-centered trigonal prisms in the crystal structure of the Er2 PbSe4 compound.

L.D. Gulay et al. / Journal of Alloys and Compounds 429 (2007) 111–115

115

Fig. 5. Crystal structure of the Y6 Pb2 Se11 compound (dotted line) as combination of the fragments of the Er2 PbSe4 and Y4.2 Pb0.7 Se7 compounds.

located in trigonal prism with one additional atom (c.n. = 7). The (Pb1 + Pb2)-centered trigonal prism as combination of the Pb1- and Pb2-centered trigonal prisms in the structure of the Er2 PbSe4 compound is shown in Fig. 2. The packing of the chains of the Er-centered octahedra and the columns of the Pb-centered trigonal prisms along y-axis in the crystal structure of the Er2 PbSe4 compound is shown in Fig. 3. Similar fragments of the chains of the Y-centered octahedra and the columns of the M (Y + Pb)- and Pb-centered trigonal prism exist in the crystal structure of the Y6 Pb2 Se11 compound. Separately the chains of the Er-centered octahedra and columns of the Pb-centered trigonal prisms in the crystal structure of the Er2 PbSe4 compound are shown in Fig. 4. The double chains of the Er-centered octahedra connected by edges located along y-axis. These double chains are connected to each other by common vertices. The Pb-centered trigonal prisms are connected by common faces. The Y6 Pb2 Se11 compound (Y6 Pb2 Se11 structure type, space group Cmcm) [5] can be completely constructed from the fragments of the compounds Y4.2 Pb0.7 Se7 (Y5 Se7 structure type, space group Cm) [7] and Er2 PbSe4 (Tm2 PbSe4 structure type, space group Pnma) (Figs. 3 and 5).

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