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, Ba 9 Hf 3 Sn 2 Se 19 ( 1 ) and Ba 8 Zr 2 SnSe 13 (Se 2 ) ( 2 ), have been synthesized. The structure of 1 is formed from isolated SnSe 4 4- tetrahedra and distorted HfSe 6 octahedra. The latter condense via face-sharing trimeric motifs that are further vertex-bridged into chains of 1 ∞ [Hf(1) 2 Hf(2)Se 11 ] 10- . The structure of 2 is comprised of SnSe 4 4- tetrahedra, Se 2 2- dimers, and face-sharing dimers of distorted ZrSe 6 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 Δ U 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 ∼10 5 cm -1 at greater than 1.8 eV. Thus, promising optical properties are demonstrated for solar energy conversion within these synthetically challenging chemical systems.