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Keywords:

  • Bitetrahedra;
  • Nitride selenides;
  • Lanthanides;
  • X-ray diffraction;
  • Bond-valence evaluations

Abstract

Long a-axes are a particularity of the crystal structures of the N3–-poor lanthanide(III) nitride selenides with the composition M5NSe6 (monoclinic, C2/c, Z = 16; M = La: a = 5177.2(3), b = 1271.58(9), c = 785.49(5) pm, β = 89.386(7)°; M = Ce: a = 5128.4(3), b = 1260.24(8), c = 778.03(5) pm, β = 90.002(7)°; M = Pr: a = 5096.1(3), b = 1252.47(8), c = 772.61(5) pm, β = 90.705(7)°). Two [NM4]9+ tetrahedra sharing a common edge by forming angled [N2M6]12+ double units represent the main structural feature. These edge-shared bitetrahedra reside in between layers of the defect Th3P4- like structure of the C-type lanthanide sesquiselenides (M2Se3). An alternative description of the structure points out the half-occupied cation positions (M10)3+. The discrete [N2M6]12+ units are linked with each others via four different selenide anions building up cationicequation image{[(N2M6)4Se14]20+} slabs. In the [100] direction they are stacked alternatingly with anionic equation image{[(M14Se34)(M10)2]20–} layers formed by Se2– polyhedra about the M3+ cations without any N3– ligands. Furthermore, these anionic layers offer channels filled up with (M10)3+ cations under half-occupation conditions. The site occupation factor of 1/2 for (M10)3+ is plausible, because a full occupation of this site would reveal to high negative valence values for all Se2– anions (Se4, Se10, Se11 and Se13) that show contacts with (M10)3+.