A near top down view of a 3 × 3 × 3 supercell of the rectangular unit cell of Cs2Te5. Cell data are from Ref. 10 . Bronze spheres denote Te, blue ones are Cs. Notice the quasi 1D [ ] n polytelluride ions embedded in Cs matrix in the form of 4 Å wide wavy Te-ribbons, showed in detail in Fig. 2 .
An isolated Te-ribbon of the 3 × 3 × 3 supercell of Cs2Te5. In the wavy Te-ribbons, six-membered rings of Te in chair-conformation are connected via common vortices into quasi 1D chains.
Panels (A) and (B) show the (bc) and (ba) plane views of a 3 × 3 × 3 supercell of the Cs2Te5 crystal, respectively, where a, b, and c denote the crystallographic axes. Only those crystal surfaces have been considered that do not cleave polytelluride ions and have small Miller indices. These are (110), (010)-C1, and (010)-C2. The (010)-C1 surface slab leaves some Te atoms directly exposed on both of its surfaces, while the (010)-C2 one has one fully Cs covered surface and one partially Cs covered one.
Band structure of Cs2Te5. Energy levels are relative to the top of the valence band. The band gap is 0.19 eV. The selection of the special k-points is based on the orthorhombic symmetry of the cell and is identical with that used for Cs2Te in Ref. 5 , as both Cs2Te (space group: Pnma) and Cs2Te5 (space group: Cmcm) crystallize in the orthorhombic system.
Optical absorption spectra in terms of the macroscopic dielectric constant for Cs2Te5 as compared to that of Cs2Te and Cs2TeC2. The energy of the incident photons is denoted by ω, while the polarization of the photons is indicated by the coordinate directions in the curve-keys with z being parallel with the main crystallographic axis. The calculations predict that Cs2Te5 would have a significantly higher absorption probability at lower photon energies, also at 1.9 eV which is the workfunction value of the Cs-covered Cs2Te5(010) surface.
Calculated properties of Cs2Te5 surfaces: workfunctions ( ), bandgaps at the Γ-point , and surface energies (σ). For the Cs2Te5(010)-C2 cleavage, data refer to the fully Cs-covered surface. The workfunction of this surface has been calculated both from the asymmetrically cesiated Cs2Te5(010)-C2 slab ( ) and from the symmetrized (with additional Cs) and relaxed version of it ( ). The average surface energy of the asymmetrically cesiated Cs2Te5(010)-C2 slab was 22.6 meV/Å2, and the contribution of the Cs-rich side is estimated to be close to the Cs2Te5(010)-C1 value ( ).
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