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 (<b>I</b>) and Sr₃Sb₄Se₉ (<b>II</b>) semiconductors. Single-crystal X-ray diffraction (XRD) measurements find that <b>I</b> crystallizes in the noncentrosymmetric space group <i>P</i>2₁2₁2₁, while <b>II</b> crystallizes in the centrosymmetric space group <i>Pnma</i>. 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 <b>I</b> 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 <i>m</i>*_e/<i>m</i>_o and valence bands of ∼0.99-1.19 <i>m</i>*_h/<i>m</i>_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.