CaZrO₃-Mediated Structural Instability and Electrical Properties in Doped Ferroelectric (K,Na)NbO₃-LiTaO₃ Films

Chemical doping has been widely applied to modify the lattice structure and functionalities in perovskite oxide layers. Here, the CaZrO3-doped (K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3 films [i.e., KNNLT(1 – x)-CZx, with nominal doping levels x of CaZrO3 selected at 0, 3, and 5%] are chosen to investigate how the structural instability, as well as dielectric and ferroelectric properties, changes with x. Compared to the undoped film, the introduction of CaZrO3 suppresses the coexistence of tetragonal and orthorhombic phases, leaving the orthorhombic phase more favored at room temperature. With x increasing from 0 to 5%, the room-temperature dielectric constant is improved by 60% (i.e., from 736 to 1183 at 1 kHz). On the other hand, the temperature-dependent dielectric constant reveals that the transition temperature TMIX-T, where the almost equivalently mixed phase turns into the high-temperature tetragonal-dominant phase on heating, is reduced with an increase in x. This suggests the doped CaZrO3 can effectively reduce the energy barrier between the coexisting orthorhombic and tetragonal phases. More importantly, the ferroelectric nature is maintained up to 200 °C in all those samples, showing great application potentials in lead-free electronic devices with tunable structural stability.