Ternary Antimony Selenide Semiconductors: Synthesis, Crystal Structure, Electronic Structure, and Optical Properties

Antimony chalcogenides have recently emerged as promising semiconductors for optoelectronic applications. Exploratory synthetic efforts have yielded SrSbSe₃ (I) and Sr₃Sb₄Se₉ (II) semiconductors. Single-crystal X-ray diffraction (XRD) measurements find that I crystallizes in the noncentrosymmetric space group P2₁2₁2₁, while II crystallizes in the centrosymmetric space group Pnma. Both structures are composed of square-pyramidal SbSe₅ units with local distortions on the Sb(III) cations, which are further condensed into [Sb₄Se₉]^6- double-ribbon chains. The structure of I also contains Se₃^2- trimers that form distorted square net ribbons. Its noncentrosymmetric structure has been confirmed by second-harmonic generation, exhibiting a response of ∼0.7 times that of AgGaS₂ at the mid-IR wavelength of 2.09 μm. Both compounds possess optoelectronic properties comparable to those of the intensely studied Sb₂Se₃ semiconductor. These include quasi-direct bandgaps of ∼0.96 and ∼0.98 eV, optical absorptions of >10⁵ cm^-1 which are steeply rising above ∼1.3 eV, and small effective masses in their respective conduction bands of 0.20 and 0.16 m*_e/m_o and valence bands of ∼0.99-1.19 m*_h/m_o. The lowest-energy bandgap transitions and largest optical absorptions are found to occur in directions aligned with the [Sb₄Se₉]^6- double-ribbon chains. These findings highlight the promising properties of ternary antimony chalcogenides as small bandgap semiconductors.