Theoretical Prediction of Mixed-Valence Layered Halide Perovskites Cs 4 M(IV)M(II) 2 X 12 (M = Ge, Sn; X = Cl, Br).

Charge-ordered compounds (i.e., Cu + /Cu 2+ , Au + /Au 3+ , In + /In 3+ , Tl + /Tl 3+ , Sb 3+ /Sb 5+ , and Bi 3+ /Bi 5+ ) have been widely explored because of their unique physical properties. Here, a new class of ⟨111⟩-oriented mixed-valence layered halide perovskites Cs 4 M(IV)M(II) 2 X 12 (M = Ge, Sn; X = Cl, Br) with C 2/ m , R -3 m , and I 41/ amd space groups was predicted by first-principles calculations. Based on the decomposition enthalpy, the phonon spectrum, and the mechanical stability criteria, we found that Cs 4 GeGe 2 Cl 12 ( C 2 /m and R- 3 m ), Cs 4 GeGe 2 Br 12 ( R -3 m ), and Cs 4 GeGe 2 Br 6 Cl 6 ( R -3 m ) exhibit thermodynamic, dynamical, and mechanical stability. The electronic structure calculations show that the predicted band gap of stable Cs 4 Ge(IV)Ge(II) 2 X 12 varies from 1.16 to 2.25 eV. And an isolated intermediate conduction band contributed by the Ge(IV) 4 s states below the Ge(II)/Ge(IV) 4 p states is observed in these compounds, which is similar to previously reported Cs 4 CuSb 2 Cl 12 but different from Cs 4 CdM(III) 2 Cl 12 (M = Sb, Bi). In addition, the calculated static dielectric constant and optical absorption coefficient of Cs 4 GeGe 2 Br 12 are close to those of typical double perovskites (e.g., Cs 2 AgBiBr 6 ). We believe that our work enriches the family of mixed-valence halide perovskites and provides a new platform for potential optoelectronic semiconductor design.