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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.
The synthesis of compounds with increasingly complex compositions, such as ternary, quaternary, etc., has become a principal direction in modern materials science (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, a series of quarternary rare-earth chalcogenides with Pb have been obtained from the R 2S3–PbS–SnS2 system (Marchuk et al., 2007 ; Gulay et al., 2008 ). These R 2Pb3Sn3S12 (R = La–Nd, Sm, Gd–Tm) compounds crystallize in the noncentrosymmetric space group Pmc21 (Y2Pb3Sn3S12 structure type). However, a thorough investigation of the similar La2S3–PbS–SiS2 system shows that a different quaternary 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 . 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 La3Ag0.90SiS7 (Daszkiewicz et al., 2008 ). In the title compound, the La and Pb atoms occupy the same site, with occupancy factors of 0.696 (9) and 0.304 (9), respectively. Therefore, these atoms have the same coordination environment of eight S atoms, creating an [(La1/Pb1)S12S26] bicapped trigonal prism (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 Ho5Cu1+xPb3−x/2S11 (x = ) 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, viz. [(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-dimensional 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.
The sample was prepared by sintering the elemental constituents (La:Pb:Si:S atomic ratio = 2:1:2:8), 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.
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 R c, 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 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 0.696 (9) and 0.304 (9), respectively. These values are in good agreement with the requirements of charge balance. The positions of the other atoms are fully occupied.
Data collection: CrysAlis CCD (Oxford Diffraction, 2007 ); cell refinement: CrysAlis RED (Oxford Diffraction, 2007 ); data reduction: CrysAlis RED; 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 ).
Crystal structure: contains datablocks I, global. DOI: 10.1107/S0108270110000247/fn3045sup1.cif
Supplementary data for this paper are available from the IUCr electronic archives (Reference: FN3045). Services for accessing these data are described at the back of the journal.