Tuning electrical properties and phase transitions through strain engineering in lead-free ferroelectric K0.5Na0.5NbO3-LiTaO3-CaZrO3 thin films

La0.07Sr0.93SnO3 and SrRuO3 are used as bottom electrodes to provide in-plane tensile and compressive stress, respectively. Our results show that the La0.07Sr0.93SnO3-buffered KNNLT-CZ films are mostly strain-relaxed with an orthorhombic (O) and tetragonal (T) mixed phase and a tetragonality of 1.002, which have a twice remnant polarization (2P(r)) of 14.29 mu C/cm(2), an effective piezoelectric strain coefficient (d(33)*) of similar to 60 pm/V, and an O to T phase transition temperature (TO-T) of 140 degrees C, while the SrRuO3-buffered KNNLT-CZ films are only partially strain-relaxed with a pure O phase and a larger tetragonality of 1.011, resulting in an increased 2P(r) value of 33.63 mu C/cm(2), an improved d(33)* value of similar to 80 pm/V, and an enhanced TO-T value of 200 degrees C. Both films show a high Curie temperature above 380 degrees C and stable hysteresis loops from room temperature to 225 degrees C. These results highlight the feasibility to improve the performance of KNN-based materials via epitaxial strain.