Ta-induced electronic reconstruction in CaSiO₃: wide-band-gap modulation, emergent metallic behavior, and...

A comprehensive first-principles investigation was conducted to clarify the impact of Ta substitution at the Si site on the structural, electronic, optical, thermoelectric, mechanical, and magnetic properties of CaSiO₃. The pristine cubic model has an optimized lattice parameter of 3.607 Å, whereas Ta incorporation at 12.5% and 25% preserves the host framework while inducing local B-O coordination rearrangement. The equilibrium volume increases from 2533.4554 Bohr³ for pristine CaSiO₃ to 2755.1025 and 3155.7362 Bohr³ for the 12.5% and 25% Ta-substituted systems, respectively. The EOS-derived bulk modulus decreases from 215.3005 GPa to 167.1973 and 169.5482 GPa, indicating greater hydrostatic compressibility of both substituted compositions relative to pristine CaSiO₃, with a slight recovery at 25% Ta. The negative formation energies of -4.11, -3.28, and -2.96 eV per atom support the energetic feasibility of the modeled compositions relative to the chosen elemental reference states. The absence of imaginary phonon branches further confirms the dynamical stability of both pristine and Ta-substituted structures. Electronically, pristine CaSiO₃ exhibits an indirect band gap of 3.432 eV dominated by O-2p states at the valence band and Si-3p states at the conduction band, whereas Ta substitution introduces Ta-5d-dominated donor-derived states with O-2p participation near the Fermi level, producing a degenerate n-type/incipient metallic regime at 12.5% Ta and a metallic n-type regime at 25% Ta. This concentration-dependent electronic evolution is independently supported by the Fermi-surface topology. Within the stoichiometric defect-free model, the corresponding nominal donor-density estimates increase from the negligible-carrier parent limit to 2.45 × 10²¹ cm^-3 and 4.28 × 10²¹ cm^-3 for 12.5% and 25% Ta, respectively; these values represent formal donor-density estimates rather than independently calculated equilibrium free-carrier concentrations. The electronic restructuring is also reflected in the calculated interband optical response. Pristine CaSiO₃ exhibits strong deep-ultraviolet absorption, whereas Ta incorporation reduces the intensity of interband transitions, redistributes the spectral weight, and smooths the profiles of ε ₂(ω), α(ω), k(ω), σ(ω), and R(ω). In addition, Ta substitution enhances the calculated plasmon-related loss features in the ∼8-13 eV energy range. The thermoelectric response is strongly composition dependent. Pristine CaSiO₃ retains a positive Seebeck coefficient and weak band-derived charge transport, whereas 12.5% Ta produces negative electron-like thermopower and the most favorable calculated PF/τ balance over much of the investigated temperature range. At 25% Ta, σ/τ and the electronic heat-transport contribution increase markedly, but the small Seebeck magnitude suppresses the power factor and produces a near-zero ZT over most of the calculated range. Thus, increased Ta-induced conductivity does not translate monotonically into improved thermoelectric performance. Mechanical analysis shows a systematic increase in the elastic-constant-derived response with Ta content, quantified by C ₁₁ (248 → 296 GPa), C ₁₂ (115 → 144 GPa), C ₄₄ (90 → 117 GPa), B (160 → 194 GPa), G (84 → 106 GPa), E (212 → 261 GPa), θD (590 → 672 K), and H (7.5 → 11.2 GPa).These small-strain trends are treated separately from the EOS-derived bulk modulus and are interpreted together with Ta-induced local B-O coordination changes and complementary COHP/ELF evidence for electronic reorganization, without using the mechanical data as direct proof of monotonic strengthening or increasing covalency of individual Ta-O bonds. Magnetically, pristine CaSiO₃ remains non-magnetic, whereas Ta incorporation induces finite spin polarization at 12.5% (0.43417 µ _B), associated with Ta-5d-dominated near-Fermi-level states, O-2p participation, and a spatially distributed Ta/O-centered spin imbalance. At 25% Ta, the magnetic response is nearly quenched (-0.00132 µ _B), with very small mixed-sign Ta/O local moments and a negligible interstitial contribution (0.00001 µ _B), indicating strong spatial compensation of the induced spin polarization. Spin-resolved PDOS, site-resolved magnetic moments, and real-space spin-density distributions consistently show that the suppression at higher Ta content originates from the reduction and spatial compensation of the local spin polarization rather than from an assumed long-range magnetic-ordering mechanism. These results demonstrate that Ta substitution enables composition-dependent modulation of the electronic, optical, thermoelectric, mechanical, and magnetic responses of CaSiO₃ while preserving the underlying perovskite framework. The calculated trends therefore support further experimental and application-specific evaluation of Ta-substituted CaSiO₃ as a compositionally tunable functional oxide.