LaTe1.9: a tenfold superstructure of the ZrSSi type

The crystal structure of LaTe1.9 (CeSe1.9 structure type; space group P42/n) was determined from a twinned crystal. LaTe1.9 features puckered [LaTe] slabs and planar [Te] layers consisting of Te2 2− and Te2− anions.


Chemical context
Chalcogenides REX 2-(RE = Y, La-Nd, Sm, Gd-Lu; X = S, Se, Te) of trivalent rare-earth metals comprise a large structural variety in a small compositional range 0 0.2. This variety can mainly be attributed to the amount of vacancies as this strongly affects the final structural motif (Doert & Mü ller, 2016). All crystal structures share a common motif of alternating [REX] and planar [X] layers, related to their common aristotype, the structure of ZrSSi. Here, the same stacking arrangement is observed with a puckered [ZrS] slab and a planar [Si] layer, where an idealized square-planar [Si] layer is realized (Onken et al., 1964;Klein Haneveld & Jellinek, 1964). The chalcogenides, however, do not form a square-planar arrangement for electronic reasons, which can be understood by their charge-balanced formula: considering trivalent rareearth metal cations only, the puckered [REX] slab bears a single positive charge per formula unit, which needs to be compensated by atoms of the planar [X] layer. This is achieved by forming dinuclear X 2 2À anions in the stoichiometric dichacolgenides REX 2 . The formation of such dumbbellshaped anions results in a distortion from the ideal squareplanar layer. Reducing the chalcogenide content results in the formation of vacancies inside the planar [X] layer, which in turn forces a reaction of the remaining atoms to balance the missing charge. Consequently, an isolated X 2À anion per vacancy is formed to maintain a charge-balanced motif, adding two new constituting fragments to the planar layer. As vacancies are not randomly distributed within the layer, commensurate and incommensurately modulated superstructures are found (Doert & Mü ller, 2016). The structural chemistry of the corresponding sulfides and selenides has been thoroughly investigated, revealing several crystal structures that are observed for both chalcogens. The tellurides, however, do not always match the structures of their sulfur and selenium congeners, as shown for LaTe 2 (Stö we, 2000a), CeTe 2 (Stö we, 2000b) and PrTe 2 (Stö we, 2000c). Discrepancies are also observed for the Te-deficient compound NdTe 1.89 (1) (Stö we, 2001). However, the CeSe 1.9 type (Plambeck-Fischer et al., 1989) with a ffiffi ffi 5 p Â ffiffi ffi 5 p Â2 supercell of the basic ZrSSi structure seems common to sulfides, selenides and tellurides. CeTe 1.9 was found to adopt this superstructure in space group P4 2 /n (No. 86) (Ijjaali & Ibers, 2006). The general motif of alternating stacks of [RETe] slabs and planar [Te] layers is preserved in this structure, the planar [Te] layer comprise four Te 2 2À anions surrounding a vacancy, resembling an eightmembered Te ring with alternating long and short distances. Four of these Te rings surround an isolated Te 2À anion in a pinwheel-like arrangement. Rationalizing this motif yields ten negative charges due to four Te 2 2À and a single Te 2À anion, balancing ten positive charges of each [RETe] layer. Here we report on the isotypic compound LaTe 1.9 , for which no structural characterization has been published yet. p Â2 superstructure of the basic ZrSSi unit cell. As indicated above, two stacks of the basic arrangement are present in the structure of LaTe 1.9 as the Te-deficient planar [Te] layers are shifted by an n-glide against each other (Fig. 1). The La atoms are coordinated by eight Te atoms (La2), respectively nine Te atoms (La1, La3) forming a bicapped, respectively a tricapped trigonal prism. The La-Te distances within the slabs range from 3.2637 (2) to 3.3594 (2) Å and from 3.2944 (3) to 3.4480 (3) between the planar [Te] layer and La. Calculating the bond-valence sum bvs (Brese & O'Keeffe, 1991) for each La site results in 2.99 valence units (v.u.) for La1, 3.06 v.u. for La2 and 2.94 v.u. for La3, which are all very close to the expected value of +3 considering the previously discussed charge-balancing situation. The tellurium layer exhibits a pinwheel-like arrangement of four eight-membered Te squares surrounding a single Te 2À anion in its centre (Fig. 2), common to all compounds of the CeSe 1.9 type.

Structural commentary
In view of the alternating short and long distances, the Te ring can be understood as being built up from four dinuclear Te 2 2À anions enclosing a vacancy with alternating bonding and non-bonding distances of 2.9224 (3) and 3.1413 (3) Å , respectively.
In accordance with the charge balancing mentioned above and Z = 20, a structured formula of LaTe 1.9 can be written as . This easily explains the anionic motifs and their quantity in the planar [Te] layer: Te5 and Te6 (both on Wyckoff site 8g) form the dumbbell-shaped Te 2 2À anions whereas Te4 (Wyckoff site 2b) represents the isolated Te 2À (Fig. 2).

Database survey
The CeSe 1.9 structure type (Plambeck-Fischer et al., 1989) is realized by several rare-earth metal sulfides and selenides but only by a few tellurides, CeTe 1.9 being one prominent example (Ijjaali & Ibers, 2006). The interatomic distances of LaTe 1.9 match those observed for CeTe 1.9 quite well, including the bonding and non-bonding distances in the planar [Te] layers [Te] layer of LaTe 1.9 with four Te 2 2À anions enclosing a vacancy each and surrounding an isolated Te 2À anion; displacement ellipsoids are drawn at the 99.95% probability level.

Figure 1
Crystal structure of LaTe 1.9 with displacement ellipsoids drawn at the 99.95% probability level.

Synthesis and crystallization
Crystals of LaTe 1.9 were found as a byproduct during the investigation of the system La-Te in chemical transport experiments using iodine as transport agent. All preparation steps were carried out in an argon-filled (5.0, Praxair Deutschland GmbH, Dü sseldorf, Germany) glove box (MBraun, Garching, Germany). Starting from the elements, 300 mg of a stoichiometric mixture of La (99.5%, MaTecK) and Te (Merck, > 99.9%, reduced in H 2 stream at 670 K) were ground and loaded into a silica ampule. A small amount of I 2 (Roth, > 99.8%, purified by sublimating twice prior to use) was added inside the glove box before flame-sealing the ampule under dynamic vacuum (p 1Â10 À3 mbar). The ampule was heated with a ramp of 2 K min À1 to 1173 K before applying a gradient from 1173 ! 1073 K, where the actual transport took place. After seven days, the ampule was cooled down to room temperature. A synthesis resulting in a phase pure product of LaTe 1. 9 has not yet been successful. The reason is most probably that two other Te-deficient compounds also exist in the composition range LaTe 2-(0 0.2) along the stoichiometric ditelluride, namely LaTe 1.94 (1) and LaTe 1.82 (1) . To address the stability ranges of the individual phases, the chalcogen vapor pressures and temperatures have to be evaluated and controlled precisely during synthesis (Mü ller et al., 2010).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All investigated crystals of LaTe 1.9 were found as reticular merohedric twins with a twin index n = 5. On a first glance, the diffraction patterns seem to suggest a large tetragonal unit cell with apparent lattice parameters of a = 22.6211 (6) Å and c = 18.3135 (5) Å , corresponding to a 50fold superstructure of the basic ZrSSi structure (Fig. 3). Similar apparent supercells have been reported for the sulfides SmS 1.9 (Tamazyan et al., 2000) or TmS 1.9 (Mü ller et al., 2012), and can be explained by twinning along the mirror planes in (100) and (110) of the twin lattice. A schematic scheme drawn along [001] is depicted in Fig. 3, illustrating the lattices of each domain. The corresponding twin law calculated by the diffractometer software (Bruker, 2016) corresponds to the twin law derived for SmS 1.9 (0.6 À0.8 0 À0.8 À0.6 0 0 0 À1). Both domains were handled during the process of integrating and correcting the data, and the refinements were performed on a HKLF5 format file. The twin ratio of the two domains calculated by SHELXL is 0.57 (1):43 (1).  Computer programs: APEX2 and SAINT (Bruker, 2016), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg, 2018) and publCIF (Westrip, 2010).

Lanthanum telluride (1/1.9)
Crystal data 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. Refined as a 2-component twin.