Novel ferroelectricity and high microwave dielectric permittivity in (Sr,Ce)TiO3 solid solution via reduction-controlled defect ordering

SrTiO 3 is a quantum paraelectric with cubic symmetry that resists long-range ferroelectricity, but targeted substitution and defect ordering can unlock polar states. Here, we report novel ferroelectricity and high microwave permittivity in a novel Sr (1-1.5 x ) Ce x TiO 3 (SCTO@ x for x  = 0.5) (SCTO) solid solution. XRD and Rietveld refinement reveal stabilization of an orthorhombic Pmmm phase, supported by TEM superlattice reflections linked to TiO 6 tilts and antiparallel A-site displacements, while energy landscape analysis confirms this orthorhombic configuration as the strain-relieved ground state. Raman spectra show defect-activated E g and A 1g modes from oxygen-vacancy ordering, and XPS identifies a Ce 3+ /Ce 4+ ↔ Ti 3+ /Ti 4+ redox equilibrium with abundant vacancies. SEM/AFM reveal dense twin-striped grains that grow in size coarsen with sintering, reducing disorder. Dielectric spectra show a low-temperature relaxor response and a high-temperature transition: at 1270 °C both are diffuse and frequency-dependent, whereas at 1300 °C the high-temperature anomaly sharpens into a frequency-independent displacive ferroelectric transition (T C ∼ 449 °C, T CW ∼486 °C), supported by slim P–E loops, higher remanence, and TEM-confirmed domains. The coexistence of relaxor fluctuations with long-range ferroelectric order highlights a crossover from defect-induced short-range polarization to domain-driven coherence, which directly enhances microwave permittivity (ε r ∼82, Q × f∼12800 GHz, and τ f  ∼ 205 ppm/°C) by combining soft-mode contributions with reduced extrinsic losses. These findings establish that reduction-controlled defect ordering stabilizes orthorhombic ferroelectricity absent in pristine SrTiO 3 , providing a clear design pathway for functional dielectrics through defect-engineered lattice symmetry.