La₂Pb(SiS₄)₂

Crystals of La₂Pb(SiS₄)₂, dilanthanum(III) lead(II) bis­[tetra­sulfido­sili­cate(IV)], were obtained from the La-Pb-Si-S system and structurally characterized using X-ray single-crystal diffraction. The La and Pb atoms are coordinated in bicapped trigonal prisms of S atoms, with the Si atoms in tetra­hedra. An occupational disorder of the La and Pb centres was refined for one position in the structure. The bicapped trigonal prisms and tetra­hedra share edges. A gap located 2.629 (1) Å from the sulfide anions was found around the coordination polyhedra, which makes La₂Pb(SiS₄)₂ a prospective material in crystal engineering. The Si and one S atom lie on a threefold axis.

S1. Comment

The synthesis of compounds with increasingly complex compositions, such as ternary, quaternary etc., has become a principal direction in modern material sciences (Eliseev & Kuzmichyeva, 1990; Mitchell & Ibers, 2002). Among multicomponent systems an important place belongs to the complex rare-earth chalcogenides. They have been intensively studied over recent years owing to their specific thermal, electrical and optical properties, which make them prospective materials in the field of IR and nonlinear optics. Therefore, the synthesis and investigation of the crystal structures of complex chalcogenides are important in the search for new materials. So far, the series of quarternary rare-earth chalcogenides with Pb have been obtained from the R₂S₃–PbS–SnS₂ system (Marchuk et al., 2007; Gulay et al., 2008). These R₂Pb₃Sn₃S₁₂ (R = La–Nd, Sm, Gd–Tm) compounds crystallize in the non-centrosymmetric space group Pmc2₁ (Y₂Pb₃Sn₃S₁₂ structure type). However, a thorough investigation of the similar La₂S₃–PbS–SiS₂ system shows that a different quarternary compound of formula La₂Pb(SiS₄)₂ can be obtained. The crystal structure of this new chalcogenide is presented here.

Relevant interatomic distances and coordination numbers of the La, Pb and Si atoms in the structure of La₂Pb(SiS₄)₂ are listed in Table 1 [Coordination numbers not given - do you wish to add them?]. Overall, the distances are close to the sums of the respective ionic radii (Wiberg, 1995). The Si atom lies on a threefold rotation axis and is surrounded by one S1 and three S2 atoms, resulting in a slightly elongated [Si1S1S2₃] tetrahedron of C_3v point-group symmetry. A similar, but compressed, environment for an Si atom was found in the recently published hexagonal compound La₃Ag_0.90SiS₇ (Daszkiewicz et al., 2008). In the title compound the La and Pb atoms occupy the same site, with occupancy factors of 0.69 (1) and 0.31 (1), respectively. Therefore, these atoms have the same coordination environment of eight S atoms, creating a bicapped trigonal prism, [(La1/Pb1)S1₂S2₆] (Fig. 1). Similar values for La—S and Pb—S distances have also been observed in the previously reported lanthanum and lead sulfides. For example, the shortest La—S distance in La₂S₃ (Basançon et al., 1969) is 2.91 (1) Å and the shortest Pb—S distance in Ho₅Cu_1+xPb_3-x/2S₁₁ (x = 1/4) is 2.822 (8) Å (Gulay et al., 2007). In the title compound the two longest (La/Pb)—S distances of 3.2784 (10) Å contribute 0.178 of a valence unit (Brown, 1996). However, the bond-valence sums of the La³⁺, Pb²⁺ and Si⁴⁺ ions are 2.722, 2.040 and 4.077, respectively. These values suggest that the La³⁺ ion is underbonded in its eight-coordinate site. On the other hand, the bond-valence sums for both symmetry-independent S atoms are 1.901 for S1 and 2.087 for S2. Thus, atom S1 is underbonded and S2 is overbonded, despite both anions having similar pyramidal trigonal surroundings, [(La1/Pb1)₃Si1].

The [(La1/Pb1)S1₂S2₆] bicapped trigonal prisms and [Si1S1S2₃] tetrahedra are connected to each other in two ways. Firstly, three prisms connect the tetrahedra by the edges and the prisms are connected to each other only by one corner [denoted (1) in Fig. 1], and secondly three prisms are connected by edges around the threefold axis and an empty trigonal prism exists inside this block [denoted (2) in Fig. 1]. In addition, two [Si1S1S2₃] tetrahedra share edges, resulting in a closed empty trigonal prism in the structure. The centre of gravity of this gap is located 2.629 (1) Å from the S atoms, which makes La₂Pb(SiS₄)₂ a prospective material in crystal engineering.

Overall, the (La+Pb) and Si atoms in the structure of La₂Pb(SiS₄)₂ form separated two-dimmensional nets which are parallel to the ab plane (Fig. 2). A 3⁶ net is created by the (La+Pb) atoms, whereas the Si atoms form a honeycomb-like 6³ net. However, the S atoms do not create a layer. Thus, the cationic (La³⁺+Pb²⁺) and Si⁴⁺ layers are arranged in an alternating manner and they are immersed in the anionic sublattice.

S2. Experimental

The sample was prepared by sintering the elemental constituents [Molar ratio?], of purity better than 99.9 wt.%, in an evacuated quartz ampoule in a tube furnace. The ampoule was heated at a rate of 30 K h^-1 to a maximum temperature of 1370 K and kept at this temperature for 4 h. It was then cooled slowly (10 K h^-1) to 770 K and annealed at this temperature for 500 h. After annealing the ampoule, the sample was quenched in cold water. A diffraction-quality single crystal of the title compound was selected from the sample.

S3. Refinement

The formation of La₂Pb(SiS₄)₂ was established during the investigation of the phase relations in the respective La₂S₃–PbS–SiS₂ system. The systematic absences were found to be consistent with the space group R3c which was applied for the crystal structure determination. One position for La and Pb, one position for Si and two positions for S were determined at the first stage of refinement. However, a statistical mixture of the La and Pb was assumed in the refinement, with the same anisotropic displacement parameters for the La and Pb atoms. The site-occupancy factors for the positions of the La and Pb atoms refined to 0.69 (1) and 0.31 (1), respectively [Please check rounding - 0.696 (9) and 0.304 (9) in CIF tables]. These values are in good agreement with the requirements of charge balance. The positions of the other atoms are fully occupied.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2007); cell refinement: CrysAlis RED (Oxford Diffraction, 2007); data reduction: CrysAlis RED (Oxford Diffraction, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. The unit cell and the coordination polyhedra of the La, Pb and Si atoms in the structure of La2Pb(SiS4)2, viewed down the c axis.
	Fig. 2. The 3^6 net of the (La+Pb) atoms and the honeycomb-like 6^3 net of the Si atoms, viewed down the c axis.

dilanthanum(III) lead(II) bis[tetrasulfidosilicate(IV)]

Crystal data

La2Pb(SiS4)2	Dx = 4.153 Mg m^−3
Mr = 797.67	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3c	Cell parameters from 475 reflections
Hall symbol: -R 3 2"c	θ = 3.0–27.5°
a = 9.0522 (13) Å	µ = 21.19 mm^−1
c = 26.964 (5) Å	T = 295 K
V = 1913.5 (5) Å^3	Prism, yellow
Z = 6	0.25 × 0.15 × 0.08 mm
F(000) = 2112

Data collection

Kuma KM-4 with CCD area-detector diffractometer	487 independent reflections
Radiation source: fine-focus sealed tube	475 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.034
Detector resolution: 1024x1024 with blocks 2x2, 33.133pixel/mm pixels mm^-1	θmax = 27.5°, θmin = 3.0°
ω scans	h = −11→11
Absorption correction: numerical CrysAlis (Oxford Diffraction, 2007)	k = −11→11
Tmin = 0.059, Tmax = 0.414	l = −30→35
6359 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.015	w = 1/[σ^2(Fo^2) + (0.0241P)^2 + 4.5858P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.043	(Δ/σ)max < 0.001
S = 1.22	Δρmax = 0.53 e Å^−3
487 reflections	Δρmin = −0.67 e Å^−3
23 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00030 (3)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
La1	0.34803 (3)	0.01470 (3)	0.0833	0.01932 (15)	0.696 (9)
Pb1	0.34803 (3)	0.01470 (3)	0.0833	0.01932 (15)	0.304 (9)
Si1	0.6667	0.3333	−0.00684 (5)	0.0182 (4)
S1	0.3333	−0.3333	0.08575 (4)	0.0215 (4)
S2	0.43233 (11)	0.30201 (11)	0.01982 (3)	0.0238 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
La1	0.02208 (17)	0.02208 (17)	0.0174 (2)	0.01376 (11)	−0.00096 (3)	0.00096 (3)
Pb1	0.02208 (17)	0.02208 (17)	0.0174 (2)	0.01376 (11)	−0.00096 (3)	0.00096 (3)
Si1	0.0199 (5)	0.0199 (5)	0.0148 (6)	0.0099 (2)	0.000	0.000
S1	0.0256 (5)	0.0256 (5)	0.0133 (6)	0.0128 (3)	0.000	0.000
S2	0.0276 (5)	0.0253 (5)	0.0229 (4)	0.0166 (4)	0.0099 (3)	0.0074 (3)

Geometric parameters (Å, º)

La1—S1	3.0868 (5)	Si1—S2^vii	2.1203 (9)
La1—S1^i	3.0867 (5)	Si1—S2^v	2.1203 (9)
La1—S2	2.8801 (8)	S1—La1^viii	3.0868 (5)
La1—S2^i	2.8800 (8)	S1—Pb1^viii	3.0868 (5)
La1—S2^ii	3.0143 (9)	S1—La1^ix	3.0868 (5)
La1—S2^iii	3.0144 (9)	S1—Pb1^ix	3.0868 (5)
La1—S2^iv	3.2784 (10)	S2—Pb1^x	3.0144 (9)
La1—S2^v	3.2784 (10)	S2—La1^x	3.0144 (9)
Si1—S1^vi	2.1277 (17)	S2—La1^vii	3.2784 (10)
Si1—S2	2.1203 (9)
S2^i—La1—S2	108.54 (4)	Si1^vi—S1—La1	88.79 (2)
S2^i—La1—S2^ii	76.180 (19)	Si1^vi—S1—La1^viii	88.79 (2)
S2—La1—S2^ii	125.02 (2)	La1—S1—La1^viii	119.956 (1)
S2^i—La1—S2^iii	125.02 (2)	Si1^vi—S1—Pb1^viii	88.79 (2)
S2—La1—S2^iii	76.179 (19)	La1—S1—Pb1^viii	119.956 (1)
S2^ii—La1—S2^iii	146.66 (3)	Si1^vi—S1—La1^ix	88.79 (2)
S2^i—La1—S1^i	142.28 (2)	La1—S1—La1^ix	119.956 (2)
S2—La1—S1^i	80.23 (2)	La1^viii—S1—La1^ix	119.956 (1)
S2^ii—La1—S1^i	69.45 (2)	Pb1^viii—S1—La1^ix	119.956 (1)
S2^iii—La1—S1^i	92.63 (3)	Si1^vi—S1—Pb1^ix	88.79 (2)
S2^i—La1—S1	80.23 (2)	La1—S1—Pb1^ix	119.956 (2)
S2—La1—S1	142.28 (2)	La1^viii—S1—Pb1^ix	119.956 (1)
S2^ii—La1—S1	92.63 (3)	Pb1^viii—S1—Pb1^ix	119.956 (1)
S2^iii—La1—S1	69.45 (2)	Si1—S2—La1	96.78 (3)
S1^i—La1—S1	115.736 (8)	Si1—S2—Pb1^x	90.88 (4)
S2^i—La1—S2^iv	67.90 (3)	La1—S2—Pb1^x	135.29 (3)
S2—La1—S2^iv	68.01 (3)	Si1—S2—La1^x	90.88 (4)
S2^ii—La1—S2^iv	63.78 (3)	La1—S2—La1^x	135.29 (3)
S2^iii—La1—S2^iv	144.16 (2)	Si1—S2—La1^vii	85.82 (3)
S1^i—La1—S2^iv	83.023 (17)	La1—S2—La1^vii	108.26 (3)
S1—La1—S2^iv	143.63 (2)	Pb1^x—S2—La1^vii	116.23 (3)
S2^i—La1—S2^v	68.01 (3)	La1^x—S2—La1^vii	116.23 (3)
S2—La1—S2^v	67.89 (3)	S2^v—Si1—S2^vii	109.12 (4)
S2^ii—La1—S2^v	144.16 (2)	S2^v—Si1—S2	109.12 (4)
S2^iii—La1—S2^v	63.77 (3)	S2^vii—Si1—S2	109.12 (4)
S1^i—La1—S2^v	143.63 (2)	S2^v—Si1—S1^vi	109.82 (4)
S1—La1—S2^v	83.022 (17)	S2^vii—Si1—S1^vi	109.82 (4)
S2^iv—La1—S2^v	100.00 (3)	S2—Si1—S1^vi	109.82 (4)
S2^i—La1—S1—Si1^vi	−126.859 (18)	S2^iv—La1—S2—Si1	107.53 (4)
S2—La1—S1—Si1^vi	−19.13 (3)	S2^v—La1—S2—Si1	−3.95 (4)
S2^ii—La1—S1—Si1^vi	157.651 (16)	S2^i—La1—S2—Pb1^x	149.99 (4)
S2^iii—La1—S1—Si1^vi	6.495 (18)	S2^ii—La1—S2—Pb1^x	−124.18 (5)
S2^iv—La1—S1—Si1^vi	−155.47 (3)	S2^iii—La1—S2—Pb1^x	27.24 (4)
S2^v—La1—S1—Si1^vi	−58.065 (14)	S1^i—La1—S2—Pb1^x	−67.94 (4)
S2^i—La1—S1—La1^viii	145.23 (4)	S1—La1—S2—Pb1^x	51.89 (5)
S2—La1—S1—La1^viii	−107.04 (4)	S2^iv—La1—S2—Pb1^x	−154.31 (3)
S2^ii—La1—S1—La1^viii	69.74 (4)	S2^v—La1—S2—Pb1^x	94.21 (4)
S2^iii—La1—S1—La1^viii	−81.41 (4)	S2^i—La1—S2—La1^x	149.99 (4)
S2^iv—La1—S1—La1^viii	116.62 (3)	S2^ii—La1—S2—La1^x	−124.18 (5)
S2^v—La1—S1—La1^viii	−145.97 (4)	S2^iii—La1—S2—La1^x	27.24 (4)
S2^i—La1—S1—Pb1^viii	145.23 (4)	S1^i—La1—S2—La1^x	−67.94 (4)
S2—La1—S1—Pb1^viii	−107.04 (4)	S1—La1—S2—La1^x	51.89 (5)
S2^ii—La1—S1—Pb1^viii	69.74 (4)	S2^iv—La1—S2—La1^x	−154.31 (3)
S2^iii—La1—S1—Pb1^viii	−81.41 (4)	S2^v—La1—S2—La1^x	94.21 (4)
S2^iv—La1—S1—Pb1^viii	116.62 (3)	S2^i—La1—S2—La1^vii	−35.995 (14)
S2^v—La1—S1—Pb1^viii	−145.97 (4)	S2^ii—La1—S2—La1^vii	49.84 (3)
S2—La1—S1—La1^ix	68.78 (5)	S2^iii—La1—S2—La1^vii	−158.74 (2)
S2^ii—La1—S1—La1^ix	−114.44 (4)	S1^i—La1—S2—La1^vii	106.08 (3)
S2^iii—La1—S1—La1^ix	94.40 (4)	S1—La1—S2—La1^vii	−134.09 (2)
S1^i—La1—S1—La1^ix	177.15 (5)	S2^iv—La1—S2—La1^vii	19.71 (3)
S2^iv—La1—S1—La1^ix	−67.57 (5)	S2^v—La1—S2—La1^vii	−91.77 (2)
S2^v—La1—S1—La1^ix	29.84 (3)	La1—S2—Si1—S2^v	5.99 (6)
S2—La1—S1—Pb1^ix	68.78 (5)	Pb1^x—S2—Si1—S2^v	−129.86 (5)
S2^ii—La1—S1—Pb1^ix	−114.44 (4)	La1^x—S2—Si1—S2^v	−129.86 (5)
S2^iii—La1—S1—Pb1^ix	94.40 (4)	La1^vii—S2—Si1—S2^v	113.91 (4)
S1^i—La1—S1—Pb1^ix	177.15 (5)	La1—S2—Si1—S2^vii	−113.16 (4)
S2^iv—La1—S1—Pb1^ix	−67.57 (5)	Pb1^x—S2—Si1—S2^vii	110.99 (6)
S2^v—La1—S1—Pb1^ix	29.84 (3)	La1^x—S2—Si1—S2^vii	110.99 (6)
S2^i—La1—S2—Si1	51.83 (3)	La1^vii—S2—Si1—S2^vii	−5.24 (5)
S2^ii—La1—S2—Si1	137.66 (3)	La1—S2—Si1—S1^vi	126.418 (17)
S2^iii—La1—S2—Si1	−70.92 (5)	Pb1^x—S2—Si1—S1^vi	−9.43 (2)
S1^i—La1—S2—Si1	−166.10 (4)	La1^x—S2—Si1—S1^vi	−9.43 (2)
S1—La1—S2—Si1	−46.27 (5)

Symmetry codes: (i) y+1/3, x−1/3, −z+1/6; (ii) −x+y+1/3, y−1/3, z+1/6; (iii) y, −x+y, −z; (iv) x−y+1/3, −y+2/3, −z+1/6; (v) −y+1, x−y, z; (vi) −x+1, −y, −z; (vii) −x+y+1, −x+1, z; (viii) −y, x−y−1, z; (ix) −x+y+1, −x, z; (x) x−y, x, −z.