Syntheses, Crystal Structures, and Electronic Structures of Quaternary Group IV-Selenide Semiconductors

Early transition-metal chalcogenides have garnered recent attention for their optoelectronic properties for solar energy conversion. Herein, the first Zr-/Hf-chalcogenides with a main group cation, Ba₉Hf₃Sn₂Se₁₉ (1) and Ba₈Zr₂SnSe₁₃(Se₂) (2), have been synthesized. The structure of 1 is formed from isolated SnSe₄^4- tetrahedra and distorted HfSe₆ octahedra. The latter condense via face-sharing trimeric motifs that are further vertex-bridged into chains of ₁^∞[Hf(1)₂Hf(2)Se₁₁]^10-. The structure of 2 is comprised of SnSe₄^4- tetrahedra, Se₂^2- dimers, and face-sharing dimers of distorted ZrSe₆ octahedra. These represent the first reported examples of Hf-/Zr-chalcogenides exhibiting face-sharing octahedra with relatively short Hf-Hf and Zr-Zr distances. Their preparation in high purity is inhibited by their low thermodynamic stability, with calculations showing small calculated ΔU_dec values of +7 and +9 meV atom^-1 for 1 and 2, respectively. Diffuse reflectance measurements confirm the semiconducting nature of 1 with an indirect band gap of ∼1.4(1) eV. Electronic structure calculations show that the band gap absorptions arise from transitions between predominantly Se-4p valence bands and mixed Hf-5d/Sn-5p or Zr-4d/Sn-5p conduction bands. Optical absorption coefficients were calculated to be more than ∼10⁵ cm^-1 at greater than 1.8 eV. Thus, promising optical properties are demonstrated for solar energy conversion within these synthetically challenging chemical systems.