Growth and Characterization of Rb _0.146 K _0.854 Ta _0.61 Nb _0.39 O ₃ Crystal

High-quality Rb1–xKxTa1–yNbyO3 (RKTN) single crystals were synthesized via a solid-state reaction route and successfully grown by the Czochralski method. The obtained crystal (Rb0.146K0.854Ta0.61Nb0.39O3) exhibits high optical transparency and a cubic perovskite structure with a lattice constant of 3.9934 Å. The calculated tolerance factor (t = 1.067) suggests slightly oversized A-site cations, which is consistent with the behavior of typical ferroelectric perovskites. Dielectric measurements show Curie–Weiss behavior with TC = 10.1 °C and three successive phase transitions, confirming the material as a normal ferroelectric. The experimental density (6.26 g·cm–3) agrees well with the theoretical value (6.271 g·cm–3), confirming the high crystalline quality. Optical and microscopic analyses reveal a refractive index of 2.49 and well-developed 90° domain structures. DFT calculations yield a formation energy of −279.10 eV/supercell, indicating excellent thermodynamic stability. Electro-optic measurements at 26 °C show a half-wave voltage of 91.76 V and a quadratic electro-optic coefficient of 1.623 × 10–15 m2·V–2, comparable to KTN. These results demonstrate that RKTN is a stable, lead-free perovskite with promising electro-optic modulation, offering promising potential for adaptive photonic and electro-optic applications.