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
system (Marchuk et al.
; Gulay et al.
). These R
= La–Nd, Sm, Gd–Tm) compounds crystallize in the noncentrosymmetric space group Pmc
structure type). However, a thorough investigation of the similar La2
system shows that a different quaternary compound of formula La2
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 La2
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
point-group symmetry. A similar, but compressed, environment for an Si atom was found in the recently published hexagonal compound La3
(Daszkiewicz et al.
). 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)S12
] 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 La2
(Basançon et al.
) is 2.91 (1) Å and the shortest Pb—S distance in Ho5
) is 2.822 (8) Å (Gulay et al.
). 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+
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.
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. Displacement ellipsoids are shown at the 50% probability level. See Comment for descriptions of (1) and (2).
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 36 net of the (La+Pb) atoms (top) and the honeycomb-like 63 net of the Si atoms (bottom), both viewed down the c axis.