Bi 4 O 4 Cu 1.7 Se 2.7 Cl 0.3 : Intergrowth of BiOCuSe and Bi 2 O 2 Se Stabilized by the Addition of a Third Anion.

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi ₄ O ₄ Cu _1.7 Se _2.7 Cl _0.3 , which adopts a new structure type composed of alternately stacked BiOCuSe and Bi ₂ O ₂ Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi ₂ O ₂ Se _0.7 Cl _0.3 blocks coupled to Cu-deficient Bi ₂ O ₂ Cu _1.7 Se ₂ blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi ₄ O ₄ Cu ₂ Se ₃ is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi ₄ O ₄ Cu _1.7 Se _2.7 Cl _0.3 : a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi ₂ O ₂ Se. Bi ₄ O ₄ Cu _1.7 Se _2.7 Cl _0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units.