Two structurally related oxide nitride chalcogenides of the lanthanoids. Tb₃ONSe₂ and La₅O₂NS₄

The structurally related oxide nitride chalcogenides Tb₃ONSe₂ and La₅O₂NS₄ can be prepared by the oxidation of the respective lanthanoid metal (M = Tb and La) with selenium or sulfur and sodium azide (NaN₃) in the presence of oxygen-containing impurities, when iodine (I₂) or the corresponding lanthanoid trichloride (MCl₃) is used as sodium trap for the formation of NaI or NaCl, respectively. The thermal treatment of these mixtures along with additional NaI or NaCl as flux at 900 C in evacuated silica tubes yielded transparent, yellow polyhedra in the case of Tb₃ONSe₂ or orange, lath-shaped crystals of La₅O₂NS₄. Both monoclinic structures (C2/m, Z = 4) were determined from X-ray single-crystal diffraction data (Tb₃ONSe₂: a = 1548.85(9), b = 392.02(2), c = 968.61(6) pm, β = 97.194(3) ; La₅O₂NS₄: a = 1571.13(8), b = 409.67(2), c = 1681.03(9) pm, β = 101.204(3) ). The crystal structure of Tb₃ONSe₂ comprises three crystallographically different Tb³⁺ cations residing in six- and sevenfold coordination of the respective anions, whereas the crystal structure of La₅O₂NS₄ shows five different La³⁺ cations with coordination numbers from six to eight. For the first time in the chemistry of oxide nitride chalcogenides of the lanthanides, an ordering of the light anions (O^2- and N^3-) is observed, and proper structure descriptions based on vertex- and edge-connected [OM₄]¹⁰⁺ and [NM₄]⁹⁺ tetrahedra (M = Tb and La) becomes possible. As main structural feature, a bulky chain ¹_∞{[ONTb₃]⁴⁺} running along [010] can be detected in the crystal structure of Tb₃ONSe₂. The same morphology, but a different chemistry is realized in the related crystal structure of La₅O₂NS₄, where these topologically equivalent strands ¹_∞{[O₂La₃]⁵⁺} and an additional second chain ¹_∞{[NLa₂]³⁺}, built up by vertex- and edge connected [NLa₄]⁹⁺ tetrahedra, compose the compound. As supporting tool for the distinction of the O^2- and N^3- anions in both crystal structures, the valence-bond concept is also applied. (Abstract Copyright [2010], Wiley Periodicals, Inc.)