Rhenium substitution drives band-gap collapse and concentration-selective multifunctionality in CaSiO₃: A first-principles study

Rhenium substitution at the Si site is explored as a concentration-dependent substitution approach to the design of multifunctional cubic perovskite-type CaSiO₃ with coupled structural, electronic, optical, thermoelectric, magnetic, mechanical, phonon and electromechanical response. The adopted parent structure corresponds to the perovskite-type CaSiO₃ framework associated with its high-pressure polymorphism and is distinct from ambient-pressure wollastonite. Full-potential density-functional calculations and Boltzmann transport analysis were carried out for perovskite-type CaSiO₃ and Re -substituted CaSi₁₋ₓReₓO₃ with x = 0.125 (12.5%) and 0.25 (25%), where x represents the Re occupancy of the Si sublattice . The calculated formation energies are −4.11, −3.32, and − 3.05 eV atom −1 for pristine, 12.5% Re-substituted, and 25% Re-substituted CaSiO₃, respectively, indicating energetic favorability relative to the adopted elemental reference states, while the absence of imaginary phonon frequencies along the sampled reciprocal-space paths supports the dynamical stability of the corresponding optimized structures. Re maintains the cubic structure at 12.5% but causes the tetragonal P4/mmm structure at 25%, with the bulk modulus changing from 216.18 GPa to 154.86 and 187.72 GPa, respectively, at the 12.5% Re concentration and the higher substitution level. The electronic structure undergoes pronounced concentration-dependent reconstruction: the 3.432 eV indirect gap of pristine CaSiO₃ decreases to direct spin-dependent gaps of 0.1672 and 0.191 eV at 12.5% Re and closes at 25% Re. Despite the narrow fundamental gaps at 12.5% Re, appreciable interband optical spectral weight develops mainly around 3.5–4.5 eV, reflecting the transition matrix elements and available spectral weight of the Re-5d/O-2p-derived states. At 25% Re, the calculated optical spectra describe the finite-energy interband response and exclude the separate low-frequency intraband (Drude) contribution expected for the metallic-like state. Re also induces a non-monotonic magnetic state: the total moment is 3.00020 μ B cell-1 at 12.5% Re and 1.16148 μ B cell-1 at 25% Re, and the spin-density maps show strong polarization of Re -centered spins that is followed by partial compensation driven by the spin density. The mechanical response is non-monotonic, with Young's modulus decreasing from 314.4 GPa in pristine CaSiO₃ to 226.2 GPa at 12.5% Re and recovering to 274.0 GPa at 25% Re, while the corresponding Vickers hardness values are 14.76, 11.74, and 13.48 GPa, respectively. Thermoelectric transport is strongest at 25% Re, reaching ~9.2 × 10 19 Ω −1 m −1 s −1 at 800 K and a peak power factor of ~4.3 × 10 12 W m −1 K −2 s −1 at 550–650 K. The calculated effective electromechanical response is also concentration selective, with 12.5% Re exhibiting the highest effective strain-response quantities (d₃₃ = 4.42 and d₃₁ = 3.28 pC N −1 ), whereas 25% Re exhibits the highest effective stress-response quantities (e₃₃ = 2.08 and e₃₁ = 1.74C m −2 ); these quantities are interpreted as effective response descriptors rather than intrinsic bulk piezoelectric coefficients of the centrosymmetric structures. These findings signal the possibility of using concentration-dependent Re substitution to engineer perovskite-type CaSiO₃ for UV active coatings, spin responsive oxide devices, electromechanical-response applications, conductive layers, and medium temperature energy conversion.