We present a systematic computational study based on density functional theory (DFT) of the Janus-Kagome Nb3−xTaxTeI7 system, generated through progressive Nb-to-Ta substitution from Nb3TeI7 to Ta3TeI7. Two stacking configurations were considered to investigate the role of polytypism on the structural and electronic properties. The optimized structures reveal that Nb-to-Ta substitution does not occur as a purely random process within the Kagome lattice. Instead, heteronuclear Nb2Ta and NbTa2 triangular clusters are energetically favored at intermediate compositions, providing an atomistic description of the local substitution mechanism. Relative-energy calculations show small differences between the two stacking arrangements, while formation-energy analysis indicates that Ta incorporation is favored under Nb-poor conditions. The substituted structures remain thermally stable at room temperature, as confirmed by ab initio molecular dynamics (AIMD). Electronic structure analysis shows a gradual evolution of the transition-metal contributions near the band edges, from Nb-dominated to Ta-dominated states. All compositions preserve a semiconducting character, with a systematic increase of the bandgap as Ta concentration increases. These results demonstrate that chemical substitution provides an effective strategy to tune the structural and electronic properties of Janus-Kagome layered materials.
Scheelite-type molybdates and tungstates have attracted considerable interest in condensed-matter physics because of their versatile electronic and optical properties. In this work, the electronic structure, chemical bonding, and carrier effective masses of scheelite-type ABO4 compounds (A = Ca, Sr, Ba; B = Mo, W) were systematically investigated using density functional theory calculations. Crystal structures were optimized using the GGA-PBEsol functional, while the modified Becke–Johnson (mBJ) exchange potential was employed for the electronic-structure calculations. All investigated AMoO4 compounds, as well as SrWO4 and CaWO4, exhibit direct band gaps at Γ, whereas BaWO4 has an indirect gap, with the valence-band maximum at Γ and the conduction-band minimum at Z. The mBJ band gaps range from 4.0 to 4.6 eV for the molybdates and 5.1–5.7 eV for the tungstates, increasing systematically from Ca to Sr to Ba. Bonding analysis reveals predominantly ionic A–O interactions and stronger covalent B–O bonding. The effective-mass tensors reveal pronounced transport anisotropy, particularly for holes, with conductivity effective masses below 3m0 for electrons and 6m0 for holes for both tungstates and molybdates. Moreover, the density-of-states effective mass is systematically larger for holes than for electrons. The hole-to-electron effective-mass ratio further quantifies the asymmetry between electron and hole transport characteristics.
