Structural diversity among multinary pnictide oxides: a minireview focused on semiconducting and superconducting heteroanionic materials

Incorporating different anions with varied ionic sizes and charges is a rapidly growing approach to bring out unusual physical properties among various classes of solid-state materials, pnictides and chalcogenides in particular. This minireview is focused on hetero-anionic materials based on the pnictogens, which have been demonstrated to offer an impressive diversity of crystal chemistry and electronic structures. In addition, many pnictide oxides or oxypnictides, over the course of the last decade, have been shown to exhibit a broad spectrum of superconducting, magnetic, and semiconducting properties. However, the structural diversity of the mixed-anion materials is far greater than the several known structure types, or their variants, of the well-known layered superconductive materials. Therefore, with this treatise, we aim to provide a comprehensive overview of the crystal chemistry of pnictide oxides by recounting almost 40 different structures of such ternary and multinary compounds. In addition to the structural aspects, we also highlight some of the challenges associated with the synthesis, and briefly summarize reported, to date, physical properties of this remarkable class of solids.

Bi2+2 nO2+2 nCu2-δSe2+ n-δXδ (X = Cl, Br): A Three-Anion Homologous Series

Both layered multiple-anion compounds and homologous series are of interest for their electronic properties, including the ability to tune the properties by changing the nature or number of the layers. Here we expand, using both computational and experimental techniques, a recently reported three-anion material, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, to the homologous series Bi_{2+2 n}O_{2+2 n}Cu_2-δSe_{2+ n-δ}X_δ (X = Cl, Br), composed of parent blocks that are well-studied thermoelectric materials. All of the materials show exceptionally low thermal conductivity (0.2 W/mK and lower) parallel to the axis of pressing of the pellets, as well as narrow band gaps (as low as 0.28 eV). Changing the number of layers affects the band gap, thermal conductivity, carrier type, and presence of a phase transition. Furthermore, the way in which the different numbers of layers are accessed, by tuning the compensating Cu vacancy concentration and halide substitution, represents a novel route to homologous series. This homologous series shows tunable properties, and the route explored here could be used to build new homologous series out of known structural blocks.

Bi4O4Cu1.7Se2.7Cl0.3: Intergrowth of BiOCuSe and Bi2O2Se 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, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi₂O₂Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi₂O₂Se_0.7Cl_0.3 blocks coupled to Cu-deficient Bi₂O₂Cu_1.7Se₂ blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi₄O₄Cu₂Se₃ is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi₄O₄Cu_1.7Se_2.7Cl_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 Bi₂O₂Se. Bi₄O₄Cu_1.7Se_2.7Cl_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.