Structure–optical–dielectric correlations in ordered double perovskites Sr2(Co Zn )TeO6 (0 ≤ x ≤ 1)

The double perovskite series Sr₂(Co₁₋ₓZnₓ)TeO₆ (x = 0, 0.25, 0.50, 0.75, 1) has been synthesized and systematically investigated to establish the correlations between structure, optical response, and dielectric behavior. Powder X-ray diffraction and Rietveld refinement confirm that Sr₂CoTeO₆ crystallizes in the monoclinic P2₁/n structure, Sr₂ZnTeO₆ in I2/m, and the intermediate compositions in I2/m as well. Structural refinements reveal opposite compositional trends in Co/Zn–O and Te–O bond lengths, directly influencing the vibrational dynamics: Raman spectra display progressive blue-shifts of Te–O stretching modes, particularly near 750 cm⁻¹. Optical diffuse reflectance measurements, analyzed via Tauc and derivative absorption spectroscopy fitting, show a systematic widening of the band gap from 3.21 eV (x = 0) to 4.13 eV (x = 1), consistent with the reduction of Co⁺² contributions near the band edge. Dielectric spectroscopy further highlights a strong dependence on composition. While Zn-rich members exhibit insulating behavior with stable permittivity and low loss over 100 Hz–1 MHz, the Co-rich end member Sr₂CoTeO₆ displays an anomalous dispersion with a crossover from Maxwell–Wagner interfacial polarization to dipolar polarization at ∼300 kHz, as confirmed by Cole–Cole analysis (εs ≈ 320, τ ≈ 3.3 μs). This unusually high transition frequency stabilizes the dielectric response over a broad frequency window. Taken together, these results demonstrate clear structure–optical–dielectric correlations across the Sr₂(Co₁₋ₓZnₓ)TeO₆ series. The Co-rich composition combines a high permittivity and frequency stability, underscoring its potential for dielectric capacitor applications, while Zn substitution tunes the material toward wider band gaps and enhanced optical insulation.