Up until now many nitride chalcogenides of the lanthanides with the formulae Ln₃NCh₃ (Ch = S, Se) [1-5] and Ln₄N₂Ch₃ (Ch = S, Se, Te) [4,6-10] are known in the literature, mainly as compounds with the lighter representatives (Ln = La−Ho). Thus, the quest for a possible formation of nitride chalcogenides of the heaviest lanthanoid leads to formulation of the question, which nitrogen source would be most suitable for their synthesis? In order to test new educts for such experiments, the recently described barium azide chloride BaN₃Cl [11] offers a promising option. Unfortunately, these experiments failed in the case of lutetium and ternary chalcogenides with the general composition BaLu₂Ch₄ were obtained instead of the primary target products Lu₃NCh₃ or Lu₄N₂Ch₃ (Ch = S, Se, Te). This type of compound (M²⁺)(M³⁺)₂(Ch²⁻)₄ has been known for quite a long time with a huge range of combinations for chalcogenide anions (Ch²⁻), divalent (M²⁺) and trivalent cations (M³⁺), even for the lanthanides as rather large trivalent species (Ln³⁺). First studies within the systems alkaline-earth metals, rare-earth metals and chalcogens were carried out by Flahaut et al. [12] in the early 1960s. Since this time further groups have published many reports dealing with this class of compounds. Although several examples for lutetium-bearing chalcogenides were characterized, the combination of barium as divalent (Ba²⁺) and lutetium as trivalent cation (Lu³⁺) was barely investigated. The so far identified compounds M^IILu₂Ch₄ crystallize in three different structure types: MgAl₂O₄ (for M^II = Mg, Mn, Fe and Ch = S) [13-17], CaFe₂O₄ (for M^II = Ca, Eu, Pb and Ch = O, S, Se) [18-23] and Th₃P₄ (for M^II = Eu and Ch = S) [24]. Furthermore, besides BaY₂S₄ [25] and the defective crystal structure of Ba_0.9Sm₂S_3.9 [26] no other examples for sulfide-containing compounds with barium and rare-earth metal cations are known with the CaFe₂O₄-type arrangement hitherto, while many representatives for the compositions BaLn₂Se₄ [27] and BaLn₂Te₄ [28] could be synthesized. In this short paper the crystal structures of the three isotypic barium lutetium chalcogenides BaLu₂Ch₄ (Ch = S, Se and Te) will be presented and discussed.

Initially, nitride chalcogenides of the heavy lanthanoids should be synthesized by using barium azide chloride BaN₃Cl as a nitrogen source and fluxing agent. Instead of the target compounds several ternary chalcogenides with the formula type BaLn₂Ch₄ (Ln = Gd, Tb, Er, Tm, Lu; Ch = S, Se and Te) could be isolated and identified by X-ray single crystal and powder diffraction after washing the products with water. Although the first attempts to synthesize almost all of the compounds BaLn₂Ch₄ were successful and described by Bugaris and Ibers [27] and Narducci et al. [28], the lutetium representatives were not described in their papers.

The ternary chalcogenides BaLu₂Ch₄ crystallize in the orthorhombic system with the space group Pnma (Ch = S: a = 1211.43(6), b = 395.56(2), c = 1427.81(4) pm; Ch = Se: a = 1261.32(4), b = 410.89(1), c = 1487.74(4) pm; Ch = Te: a = 1353.58(8), b = 438.47(3), c = 1593.62(8) pm) and four formula units per unit cell representing the CaFe₂O₄-structure type (see Table 1 for atomic coordinates). All seven crystallographically independent atoms reside at Wyckoff positions 4c with the site symmetry m. The two trivalent lutetium cations are surrounded by six chalcogenide anions forming slightly distorted [(Lu1)(Ch2)(Ch3)₃(Ch4)₂]⁹⁻ and [(Lu2)(Ch1)₃(Ch2)₂(Ch4)]⁹⁻ octahedra (Figure 1, mid and right), while the divalent barium cations are coordinated by eight chalcogenide anions in the shape of bicapped trigonal prisms (Figure 1, left). Each of the four crystallographically different chalcogenide anions shows three contacts to lutetium and just two bonds to barium. The polyhedra can be described as more or less distorted square pyramids for (Ch1)²⁻, (Ch2)²⁻ and (Ch3)²⁻ as well as distorted trigonal bipyramids for (Ch4)²⁻.

All bond lengths between Ba²⁺ and its eight Ch²⁻ ligands (315−333 pm for Ch = S, 327−349 pm for Ch = Se, and 350−367 pm for Ch = Te, for details see Table 2) are similar to those obtained for the other representatives of the ternary barium sulfides BaLn₂S₄ (318−343 pm for Ln = Sm, Y) [25,26], selenides BaLn₂Se₄ (327–346 pm for Ln = Er−Yb) [28] and tellurides BaLn₂Te₄ (350−375 pm for Ln = Sm−Tm, Y) [27]. Comparable short distances between Lu³⁺ and Ch²⁻ as in the BaLu₂Ch₄ series (264−272 pm for Ch = S, 277−285 pm for Ch = Se, 298−306 pm for Ch = Te) can also be observed in Lu₂S₃ (E-type: 264−274 pm) [30], CaLu₂S4 (265−277 pm) [19], Lu₂Se₃ (Z-type: 279−286 pm) [31], EuLu₂Se₄ (275−282 pm) [21], CsCdLuTe₃ (307−311 pm) [32] and CsCuLu₂Te₄ (297–314 pm) [33]. A periodic effect regarding ionic radii and volume increments can be stated by a comparison of their influence on the molar volumes V_m of the title compounds in the order of increasing radii of the chalcogenide anions: sulfide (r = 184 pm), selenide (r = 198 pm) and telluride (r = 221 pm) [34]. The difference of the volume increments of these chalcogenide anions (16 cm³/mol between 4 × S²⁻ and 4 × Se²⁻ as well as 30 cm³/mol between 4 × Se²⁻ and 4 × Te²⁻) [35] are very similar to the obtained V_m differences of the ternary compounds (13 cm³/mol between BaLu₂S₄ and BaLu₂Se₄, 26 cm³/mol between BaLu₂Se₄ and BaLu₂Te₄).

The different surrounding of the two crystallographically distinct lutetium cations is manifested in the crystal structure as the [(Lu1)(Ch2)(Ch3)₃(Ch4)₂]⁹⁻ and [(Lu2)(Ch1)₃(Ch2)₂(Ch4)]⁹⁻ octahedra form two different kinds of { [ ( Lu ) Ch 4 / 4 e Ch 2 / 2 t ] 8 − } ∞ 1 chains by edge linkage. Interestingly, the chalcogenide anions (Ch3)²⁻ connect two { [ ( Lu 1 ) Ch 4 / 4 e Ch 2 / 2 t ] 8 − } ∞ 1 strands to double chains running parallel to the [010] direction (Figure 2, left). The same holds for the (Ch1)²⁻ anions in the case of the (Lu2)²⁺-bearing congeners (Figure 2, right) and so, besides two extra Ba²⁺ contacts for all chalcogenide anions, (Ch1)²⁻ and (Ch3)²⁻ exhibit three contacts to only one sort of trivalent lutetium cation, while the (Ch2)²⁻ and (Ch4)²⁻ anions are bonded to both types of Lu³⁺. Both kinds of double chains are finally corner-connected according to the pattern in Figure 3, forming a three-dimensional framework { [ Lu 2 Ch 4 ] 2 − } ∞ 3 with channels that are occupied by the octacoordinated barium cations.

Single crystals of BaLu₂S₄, BaLu₂Se₄ and BaLu₂Te₄ were accidentally (but reproducible) obtained after heating mixtures of lutetium metal, its trichloride, chalcogen and barium azide chloride in molar ratios of 26:2:27:6 designed to produce Lu₃NCh₃ [1-5] or Lu₄N₂Ch₃ [4,6-10] along with an excess of barium chloride as flux at 920 °C for ten days.

All three water- and air-stable products exhibit the shape of needles with different colors (BaLu₂S₄: colorless, BaLu₂Se₄: dark red, BaLu₂Te₄: black) and were characterized by single crystal X-ray diffraction (κ-CCD, Bruker-Nonius, Mo-Kα radiation with graphite monochromator: λ = 71.01 pm) at room temperature. Essential information about the structure solutions and refinements for the BaLu₂Ch₄ series (Ch = S, Se, Te) by using the program package SHELXS-97 and SHELX-97 [36] as well as X-SHAPE for correction for absorption [37] and scattering factors from the International Tables, Vol. C [38], is available in Table 3. Further details may be obtained from the Fachinformationszentrum (FIZ) Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (Fax: (+49)7247-808-666, E-mail: crysdata@fizkarlsruhe.de), on quoting the depository numbers CSD-422891 (BaLu₂S₄), CSD-422892 (BaLu₂Se₄) and CSD-422893 (BaLu₂Te₄).

The crystal structures of all three barium lutetium chalcogenides BaLu₂Ch₄ (Ch = S, Se and Te) exhibit a CaFe₂O₄-type arrangement. As a particularity, the two crystallographically distinct trivalent lutetium cations can be found in two different types of { [ Lu Ch 4 / 4 e Ch 2 / 2 t ] 8 − } ∞ 1 chains. These first build double chains by edge-linkage, which are corner-connected further to form a three-dimensional { [ Lu 2 Ch 4 ] 2 − } ∞ 3 framework apt to embed divalent barium cations. It should be noted that these compounds were obtained in order to produce lutetium nitride chalcogenides (Lu₃NCh₃ or Lu₄N₂Ch₃) by using barium azide chloride as flux and a nitrogen source, which leads to the assumption that this starting material is not suitable as a source of nitrogen for these kinds of synthetic experiments.