Density Functional Theory‐Driven Design of FrBaX 3 (X = Br, I) Perovskites for Stable Optoelectronic and Thermoelectric Applications

We present a first‐principles analysis of the hypothetical FrBaX 3 (X = Br, I) perovskites using density functional theory (DFT) with the generalized gradient approximation‐Perdew–Burke–Ernzerhof (GGA‐PBE) functional and semiclassical Boltzmann transport theory. The GGA‐PBE functional underestimates bandgaps, and the constant relaxation time approximation ( τ = 10 −14 s) neglects energy‐dependent scattering mechanisms; both limit quantitative predictions. Our calculations indicate that these compounds exhibit indirect bandgaps of 3.88 (FrBaBr 3 ) and 3.38 eV (FrBaI 3 ) at the GGA‐PBE level, with corrected TB‐mBJ + SOC values of 6.00 and 4.69 eV, respectively. Optical property calculations reveal high absorption coefficients (~10 6 cm −1 ) in the ultraviolet region, with optical onsets corresponding to the calculated bandgaps (3.82 and 3.29 eV at the PBE + SOC level, or 6.00 and 4.69 eV at the TB‐mBJ + SOC level). No significant visible‐light absorption is observed, consistent with the wide bandgap nature of these compounds. The thermoelectric figure of merit (ZT) reaches 0.70 for FrBaBr 3 and 0.66 for FrBaI 3 at 1200 K, assuming ideal crystal structures and within the limitations of the constant relaxation time approximation. All conclusions regarding application potential are theoretical and require experimental validation. Francium's radioactivity and extreme scarcity constrain the practical feasibility of these materials.