Two new multinary chalcogenides with (Se2)2- dimers: Ba8Hf2Se11(Se2) and Ba9Hf3Se14(Se2)

Two multinary selenides, Ba8Hf2Se11(Se-2) and Ba9Hf3Se14(Se-2), with unprecedented structure types have been prepared using high-temperature synthesis techniques and represent the first known compounds in the Ba-Hf-Se system. Their structures were determined from single crystal X-ray diffraction (XRD) data. The Ba8Hf2Se11(Se-2) compound crystallizes in the monoclinic C2/c space group with a = 12.3962(15) angstrom, b = 12.8928(15) angstrom, c = 18.1768(17) angstrom, and beta = 90.685(4)degrees, while Ba9Hf3Se14(Se-2) forms in the rhombohedral R (3) over bar space group with a = b = 19.4907(6) angstrom and c = 23.6407(11) angstrom. Both have pseudo-zero-dimensional structures with homoatomic Se-Se bonding in the form of (Se-2)(2-) at distances of 2.400-2.402 angstrom. The structure of Ba8Hf2Se11(Se-2) is comprised of [Hf2Se11](14-), Ba2+, and (Se-2)(2-) dimers. Conversely, the Ba9Hf3Se14(Se-2) structure contains a novel perovskitetype cluster constructed from eight octahedrally-coordinated Hf cations, i.e., [Hf8Se36](40-), and isolated [HfSe6](8-) units which are separated by (Se-2)(2-) dimers and Ba2+ cations. Polycrystalline Ba8Hf2Se11(Se-2) is synthesized at 1073 K using a two-step solid-state synthesis method, with the co-formation of a small amount of a BaSe secondary phase. A direct bandgap of 2.2(2) eV is obtained for the polycrystalline sample of Ba8Hf2Se11(Se-2), which is consistent with its yellow color. Density functional theory calculations reveal their bandgap transitions stem from predominantly filled Se-4p to empty Hf-5d at the edges of the valence bands (VB) and conduction bands (CB), respectively. The optical absorption coefficients are calculated to be relatively large, exceeding similar to 105 cm(-1) at about >2.0 eV with effective masses in the CB varying from similar to 0.5 m(e) (Gamma -> A) in Ba8Hf2Se11(Se-2) to similar to 1.0 m(e) (Gamma -> L) in Ba9Hf3Se14(Se-2). Thus, their optoelectronic properties are shown to be competitive with existing perovskite-type chalcogenides that have been a focus of recent research efforts.

Automated summary

This study reports the synthesis and characterization of two previously unknown multinary selenides, Ba8Hf2Se11(Se-2) and Ba9Hf3Se14(Se-2), which display unique structures and optoelectronic properties competitive with existing perovskite-type chalcogenides.