APdCu(Se₂)(Se₃) (A = K and Rb): New quaternary copper palladium polyselenides with unusual metal-selenium coordination

Two novel metal polyselenides, KPdCu(Se₂)(Se₃) (I) and RbPdCu(Se₂)(Se₃) (II), have been synthesized from solvothermal reactions in superheated ethylenediamine at 160 °C. The isostructural compounds crystallize in the monoclinic space group P2₁/m, Z = 2, with a = 6.145(1) Å, b = 7.268(1) Å, c = 8.865(2) Å, β = 102.41(3)° for I, and a = 6.253(1) Å, b = 7.267(1) Å, c = 8.993(2) Å, β = 102.28(3)° for II. Their crystal structures are two-dimensional networks with _∞²[PdCu(Se₂) (Se₃)]^- anionic layers built from one-dimensional _∞¹[Pd(Se₂) (Se₃)]^2- "chains" that are "stitched" together by tetrahedrally coordinated Cu atoms. The DSC data show that I and II are stable up to 400 °C and decompose at ca. 436 and 424 °C, respectively. Both compounds are narrow band-gap semiconductors with estimated band gaps of about 0.7 eV (I) and 0.8 eV (II), respectively. They are the first structurally characterized quaternary copper palladium polychalcogenides with a (Se₂)^2- and a (Se₃)^2- fragment, both exhibiting interesting and unusual metal-selenium coordination.