Crystal and Electronic Structures of Palladium Sesquichalcogenides

Despite having a relatively simple binary stoichiometry, palladium sesquichalcogenides [i.e., Pd2Q3 (Q = O, S, Se, Te)] are absent in the reported Pd–Q phase diagrams. To search for the possibility of missing phases, we performed an extensive ab initio structural search and report on the surprising discovery of thermodynamically stable and metastable Pd2Q3 compounds. Our results show that all the "missing" phases adopt new structure types (as yet unobserved structures) with remarkable structural and electronic properties. Their characteristics depend on the approach they take to satisfy the octet rule. Pd2O3, for which the first report—though without structural determination—dates back to 1908, exhibits a mixed valency of Pd2+ and Pd4+ cations with alternating square-planar and octahedral units, respectively. Unlike most other valence-trapped mixed-valence systems, Pd2O3 has a metallic nature, consistent with the black-brown color observed in the early experiment. For Pd2Te3, we find dynamical mixed-valence states of Pd2+/Pd4+, resulting in an intermediate oxidation state of +3 for palladium and a subsequent metallic electronic ground state. Adopting a different approach, Pd2S3/Se3 structures satisfy the octet rule by the formation of polyanionic chalcogen dimers, known as Zintl ions, and thus are best formulated as [Pd2+]2[Q2–][Q2]2– (Q = S, Se). Our results suggest that Pd2S3/Se3 with hierarchical chemical bonds, that is, the coexistence of different types of chemical bonds, are highly efficient light absorbers for solar cell applications. The discovery of thermodynamically stable and synthesizable palladium sesquichalcogenide phases could provide an opportunity to fabricate new materials for energy applications.