Structural, electronic and optical characteristics of SrGe2 and BaGe2: A combined experimental and computational study

• Charge transfer between cation and anion atoms observed first time in digermandies.
• Study yields a band gap of ∼1 eV and ∼0.85 eV for SrGe2 and BaGe2, respectively.
• Band gap decrease with the application of hydrostatic pressure.
• Localized cation d states lead to a large absorption coefficient (>7.5 × 104 cm−1).

SrGe2 and BaGe2 were characterized for structural, electronic and optical properties by means of diffuse reflectance and first-principles density functional theory. These two germanides crystallize in the BaSi2-type structure, in which Ge atoms are arranged in tetrahedral configuration. The calculation indicates a charge transfer from Sr (or Ba) atoms to Ge atoms along with the formation of covalent bonds among Ge atoms in Ge tetrahedral. The computational results confirm that these two germanies are Zintl phase described as Sr2Ge4 (or Ba2Ge4), which are characterized by positively charged [Sr2 (or Ba2)]2.59+ and negatively charged [Ge4]2.59− units acting as cation and anion, respectively. These compounds are indirect gap semiconductors with band gap estimated to be Eg = 1.02 eV for BaGe2 and Eg = 0.89 eV for SrGe2 which are in good agreement with our experimental measured values (Eg = 0.97 eV for BaGe2 and Eg = 0.82 eV for SrGe2). Our calculations demonstrate that the band gaps are narrowed by application of hydrostatic pressure; the pressure coefficients are estimated to be −10.54 for SrGe2 and −10.06 meV/GPa for BaGe2. Optical properties reveal that these compounds have large absorption coefficient (∼7.5 × 104 cm−1 at 1.5 eV) and the estimated high frequency (static) dielectric constant are, ε∞(ε0) ≈ 12.8(20.97) for BaGe2 and ε∞(ε0) ≈ 14.27(22.87) for SrGe2.