IMF2017

High quality thin films of the lead-free (K_0.5Na_0.5)_0.985La_0.005NbO₃ (KNNLa) compound were successfully deposited on Nb-doped SrTiO₃ substrates with orientation [100] (NSTO100) and [110] (NSTO110) by rf-sputtering. Highly textured thin films, 200 nm thick, grew conditioned by the substrate surface. The KNNLa films on NSTO100 are characterized by the coexistence, at room temperature, of a commensurate [001]-oriented tetragonal phase and two incommensurate [010]-oriented monoclinic phases; while on NSTO110 the KNNLa films grew only in an incommensurate [101]-oriented monoclinic phase. Local ferroelectric and piezoelectric properties were evaluated by piezoresponse force microscopy. Both samples show excellent and homogenous out-of-plane polarization switching patterns of up and down ferroelectric domains corresponding with 180° domains walls. For the KNNLa/NSTO100 sample, asymmetric switching voltages of -30 V and +15 V were required because the KNNLa film grew with net polarization pointing down, with high coercivity depending on the spontaneous polarization direction in each of the coexisting phases. For the KNNLa/NSTO110 sample, lower symmetric switching voltages of +5 V and -5 V were required since the KNNLa film grew with polarization pointing up along the [110] direction (~45° away from the out-of-plane direction). The KNNLa/NSTO100 film exhibits the higher piezoelectric constant, d₃₃ = 29 pm/V and global electromechanical resonance response at ~396 kHz.

This work was supported by PAPIIT-DGAPA-UNAM (Grants IN110315 and IN105317) and CONACYT (Grants Fronteras-2026, 282778 and 280309). Two of the authors, H.-H. and E. M-A. thank CoNaCyT for their scholarships. The authors acknowledge E. Aparicio, E.A. Murillo, and P. Casillas for their technical assistance.