The Short Series of the Oxygen-poor Lanthanide Oxide Selenides M 10 Ose 14 with M = La–nd

Single crystals and phase pure samples of oxygen-poor ternary lanthanide oxide selenides with the composition M 10 OSe 14 (M = La–Nd; tetragonal, I4 1 /acd; a = 1592.0–1559.8 pm, c = 2106.5–2062.9 pm) could be obtained by reacting the corresponding metals, selenium and selenium dioxide as oxygen source. Their crystal structures are isotypic with Pr 10 OS 14 and thus contain isolated [OM 4 ] 10+ tetrahedra (d(O 2– –M 3+) = 243–248 pm) embedded in a complex anionic 3 ∞ {[M 6 Se 14 ] 10– } lanthanide selenide matrix (d(M 3+ –Se 2–) = 288–358 pm). All three crystallographically independent M 3+ cations exhibit eight contacts to chalcogenide anions (O 2– and/or Se 2–) resulting in the formation of bicapped trigonal prismatic coordination polyhedra. The optical band gaps of the oxide selenides M 10 OSe 14 amount to values between 1.89 and 2.04 eV indicating wide band-gap semiconductors.


OPEN ACCESS
(M = La-Nd, Sm) [1,2,6,7].Oxide centered metal tetrahedra [OM 4 ] 10+ represent the dominating structural feature of all, but occurs as isolated entities just in the M 10 OSe 14 -type compounds.While the M 2 OSe 2 -I-type representatives exhibit their condensation to a chain via common cis-oriented edges, the remaining three examples show different kinds of layers built up of these [OM 4 ] 10+ tetrahedra also by vertex-and edge-condensation.Furthermore, selenide and diselenide anions coexist in the crystal structure of the M 4 O 4 Se 3 -type compounds according to M 4 O 4 Se[Se 2 ].The title compounds M 10 OSe 14 (M = La-Nd) follow up and suffer from the same kind of problem as the homologous oxide sulfides M 10 OS 14 [8,9]: Both were formerly addressed as B-type modifications of the corresponding lanthanide sesquichalcogenides M 2 Ch 3 (Ch = S and Se) [10,11].Besançon and coworkers [12,13] have refined Pr 10 OS 14 as first example of this structure type.More recently, Meerschaut et al. [14] refined a structure model for La 10 O 0.945 Se 14.055 with a mixed occupation of O 2-and Se 2-anions at a common Wyckoff position (8a), which raises some questions.We are now presenting the crystal structures of the complete short M 10 OSe 14 series with M = La-Nd here.A comment on La 10 O 0.945 Se 14.055 is given as well as a detailed comparison between the title compounds and the isostructural lanthanide oxide sulfides M 10 OS 14 (M = La-Nd, Sm, Gd) [8,9].Furthermore, we compare the optical band gaps of the title compounds with those of the C-type sesquiselenides of the lanthanides M 2 Se 3 and the oxide selenide diselenides M 4 O 4 Se[Se 2 ].

Optical Band Gaps
The optical band gaps of the oxide selenides M 10 OSe 14 amount to values between 1.89 eV and 2.04 eV (see Table 4 and Figure    ).Furthermore, it opens not only for the oxide selenides with the general formula M 10 OSe 14 (M = La-Nd) but a new scientific field for their applications as red pigments without toxic metals like cadmium [22].

Experimental Section
Single crystals and phase pure samples of the M 10 OSe 14 representatives with M = La-Nd were obtained after heating mixtures of the respective lanthanide metal (ChemPur: 99.9%), selenium (Merck: 99.9%) and selenium dioxide (SeO 2 , ChemPur: 99.999%) in molar ratios of 20:27 : 1 along with an excess of caesium chloride (CsCl, ChemPur: 99.9%) as flux at 800 °C for four days in evacuated silica ampoules according to: 20 M + 27 Se + SeO 2 2 M 10 OSe 14 (1) For crystals of high quality, these mixtures had to be cooled within four days from 800 to 500 °C followed by a subsequent slow cooling process down to room temperature within ten hours.
All four water-and air-stable products were characterized by single-crystal X-ray diffraction (IPDS-I, Stoe, Mo-Kα radiation with graphite monochromator: λ = 71.01pm) at room temperature.Essential information for the structure solutions and refinements for the representatives of the M 10 OSe 14 series (M = La-Nd) using the program packages SHELXS-97 and SHELX-97 [23] as well as X-RED (within X-SHAPE) for correction of absorption [24] and scattering factors from the International Tables (Volume C) [25] is available from Table 5.Further details can be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository numbers CSD-424095 (La 10 OSe  Diffuse reflectance spectra (DRS) were recorded on a TIDAS UV-VIS-spectrometer (J&M) equipped with optic fibers.As reference, a Ba[SO 4 ] standard found application.For converting the reflectance into absorbance and obtaining the band gap information, the Kubelka-Munk function was applied.This approximation relates the absorbance coefficient (α) and the diffusion coefficient (S) of the compounds.The absorption-edge energies (E g ) were derived by the intersection points of the particular baseline along the energy axis and the extrapolated line of the linear part of the threshold.

Conclusions
The crystal structures of all four representatives of the oxide selenides M 10 OSe 14 with M = La-Nd exhibit the Pr 10 OS 14 -type arrangement.Hence, the lanthanide selenide matrix 3  ∞ {[(M1) 3 (M3) 3 Se 14 ] 10-} embed isolated [O(M2) 4 ] 10+ tetrahedra.It should be noted that no hint of the existence of representatives with heavier lanthanides (M = Sm-Lu) could be obtained so far, but we are still busy trying to synthesize them.The optical band gaps amount to values between 1.89 and 2.04 eV encouraging investigations in their ability to be used for application as red pigments.Based on the interplay of the light anions O 2-and N 3-in the perovskite-type compounds Ca (1-x) La x TaO (2-x) N (1+x) [26], we are also actively investigating the band-gap changes in correlation with nitride incorporation in this structure type represented by the recently published compound La 10.25 O 0.25 N 0.75 Se 14 [27,28].

Figure 2 .
Figure 2. View at (a) the unit-cell representation; and (b) the isolated [O(M2) 4 ] 10+ tetrahedra with their full Se 2-surrounding in the crystal structure of the M 10 OSe 4 representatives (M = La-Nd).
3), so they should represent typical semiconducting materials.In comparison with the oxygen-richer oxide selenide diselenides M 4 O 4 Se[Se 2 ] the band gaps are exhibiting lower values than those of the title compounds.Similar to the M 4 O 4 Se[Se 2 ] series the transitions of electrons from the 4f to the 5d orbitals or within the 4f shell of the lanthanides in the related M 10 OSe 14 representatives can be detected below the actual band gap for the corresponding cerium (mainly 4f-5d), praseodymium and neodymium compounds (mainly intra 4f).

Table 1 .
Fractional atomic coordinates for the four M 10 OSe 14 representatives with M = La-Nd.

Table 3 .
Selected internuclear distances (d/pm) and angles (∢/deg) for the M 10 OSe 14 representatives (M = La-Nd) in comparison to those for known related compounds (in italics).Distances/

Table 4 .
Comparison of the optical band gaps of the M 10 OSe 14 , M 4 O 4 Se[Se 2 ] and C-M 2 Se 3 representatives (M = La-Nd).

Table 5 .
Crystallographic data for the four M 10 OSe 14 representatives (M = La-Nd).