Unlocking High Dielectric Tunability and Exceptional Electrocaloric Performance via Growth-Driven Domain Dynamics

Achieving simultaneously high dielectric tunability, thermal and frequency stability, and efficient electrocaloric performance remains a major unresolved challenge in lead-free ferroelectric films. These constraints limit the practical deployment of environmentally benign tunable components and solid-state refrigeration technologies. To address this gap, we investigate Ba₀.₇Ca₀.₃TiO₃ thin films grown at 630, 670, and 700°C, establishing quantitative growth-structure-property correlations that enable co-optimization of these functionalities. By integrating dielectric spectroscopy, Rayleigh analysis, phase-field simulations, and electrocaloric measurements, we disentangle intrinsic and extrinsic contributions governing dielectric and electrocaloric behaviour. Film grown at 630°C exhibit the highest tunability (∼90%), dominated by extrinsic mechanisms with mixture of a/c nano-domains, whereas film grown at 670°C yields a stable tunability (∼85%), low dielectric loss (<0.05), high cumulative quality factor (CQF∼1.6 × 10⁴), and excellent thermal (300-420 K) and frequency (10 kHz-1 MHz) stability. In contrast, films grown at 700°C display the best performance of electrocaloric coefficient (ξ ∼0.025 K cm kV^{- 1}), refrigerant capacity (RC of ∼1900 J kg^{- 1}) and an outstanding relative cooling power (RCP ≈ 1755 K²), among the highest reported for lead-free films. These results establish growth temperature as an effective control parameter for overcoming tunability-stability trade-offs in adaptive microelectronics and solid-state refrigeration.