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.