Synthesis and Crystal Structure of the Layered Lanthanide Oxychlorides Ba₃Ln₂O₅Cl₂

Single crystals of a new family of layered lanthanide oxychlorides, Ba₃Ln₂O₅Cl₂ (Ln = Gd-Lu), have been synthesized from a molten barium flux. This family crystallizes in the space group I4/mmm (No. 139; Z = 2) with lattice parameters a = 4.3384(1)-4.4541(1) Å and c = 24.5108(7)-24.8448(9) Å. Ba₃Ln₂O₅Cl₂ phases are built up of two different blocks: a perovskite double layer of stoichiometry Ba₂Ln₂O₅ formed by corner-connected LnO₅ tetragonal bipyramids and a puckered rock-salt-like interlayer of composition BaCl₂. A complete structural study along with bond-valence-sum calculations shows that, for lanthanides larger than gadolinium, the structure becomes unstable. Density functional theory calculations show that the valence-band edge is dominated by oxygen orbitals, whereas the conduction band forms from Ba 5d orbitals. The synthesis of this family suggests a route to other potential multianion phases.