In this study, the combinatory effects of co-doping TiO₂ ceramics with trivalent scandium (Sc³⁺) and pentavalent vanadium (V⁵⁺) on dielectric behavior are systematically investigated. A series of (Sc_0.5V_0.5)_xTi_1-xO₂ (SVTO-x) ceramics, with nominal dopant concentrations with nominal dopant concentrations x=0.05, 0.10, 0.15, and 0.20 were synthesized via conventional solid-state reaction. Structural, microstructural, and dielectric characterizations reveal that moderate co-doping (x ≤ 0.15) preserves phase-pure rutile structure, enhances grain uniformity, and promotes the formation of thermally stable defect complexes, comprising electron-pinned defect-dipoles and internal barrier layer capacitance mechanisms. Notably, the SVTO-0.05 composition achieves an exceptionally high relative permittivity (~3.9 × 10⁴) and low dielectric loss (tanδ≈0.0128) at 1 kHz, along with excellent frequency and thermal stability. X-ray photoelectron spectroscopy further confirms the simultaneous presence of Sc³⁺, V⁵⁺, Ti³⁺, and oxygen vacancies, supporting the formation of localized triangular and rhombic defect clusters that govern the observed colossal dielectric response. These findings establish that strategic (Sc,V) co-doping constitutes an effective defect-engineering approach for designing high-performance dielectric ceramics.
Methods:
A series of (Sc_0.5V_0.5)_xTi_1-xO₂ (SVTO-x) ceramics with nominal dopant concentrations of x=0.05, 0.10, 0.15, and 0.20 were synthesized via a conventional solid-state reaction method. Phase purity and crystal structure were analyzed by X-ray diffraction (XRD) with Rietveld refinement. Microstructure was examined using scanning electron microscopy (SEM). Chemical states and defect chemistry were probed by X-ray photoelectron spectroscopy (XPS). Dielectric properties (permittivity ε' and loss tangent tanδ) were measured over a frequency range of 100 Hz–1 MHz and a temperature range of 180–380 K using an impedance analyzer. Complex impedance and electric modulus spectroscopy were employed to deconvolute grain and grain-boundary contributions.
Results:
Structural evolution: Single-phase rutile solid solutions were maintained up to x=0.15. At x=0.20, a secondary ScVO₄ phase appeared, indicating the solubility limit. Lattice parameters increased with x due to the larger Sc³⁺ substitution.
Microstructure: Average grain size increased from ~19.4 μm (x=0.05) to ~25.4 μm (x=0.15), then slightly decreased at x=0.20 due to Zener pinning by ScVO4 at grain boundaries.
Dielectric performance: The SVTO-0.05 ceramic exhibited an optimal combination: ε'≈3.9×10⁴ and tanδ≈0.0128 at 1 kHz (room temperature), with good frequency and thermal stability. Both ε' and tanδ increased monotonically with temperature, showing thermally activated behavior.
Relaxation mechanisms: Electric modulus spectra revealed two relaxation peaks, attributed to grain-boundary (IBLC) and bulk (defect-dipole) processes. Activation energies were ~0.11 eV (low-frequency, IBLC) and ~0.16 eV (high-frequency, electron hopping).
Defect chemistry: XPS confirmed the presence of Sc³⁺, V⁵⁺, Ti³⁺, and oxygen vacancies, supporting the formation of triangular and rhombic defect clusters. These complexes pin free electrons, suppress long-range conduction, and enable short-range hopping polarization.