Features on the vibrational properties in La-doped AgNbO₃ ferroelectric ceramics

Ferroelectric and related materials have been extensively studied due to their promising technological applications, allowing their insertion in strategic areas involving electronics, automotive, medicine, and other industries. Special attention has been paid to antiferroelectrics, because of their potential applications in energy storage devices, due to their high-power density, ultrafast charge/discharge speed, and low dielectric losses. In this way, lead-free materials based on silver niobate (AgNbO₃) have shown high piezoelectric performance and large saturation polarization, thus emerging as interesting candidates to be used in energy storage devices. AgNbO₃ (AN) has a perovskite structure with an interesting sequence of structural phase transitions upon heating, which include both ferroelectric and antiferroelectric phases. Therefore, their physical properties need to be carefully investigated, in order to better understand the real mechanisms behind the observed anomalies. In this work, a systematic study on the vibrational properties in the Ag_1-3xLa_xNbO₃ system (where x = 0, 0.005, 0.010, and 0.015) has been carried out in a wide temperature range. Results on the temperature-dependent Raman spectra revealed anomalous behaviors in the frequency of the active vibrational modes in the temperature ranges of 50–100 °C, 150–200 °C and 350–400 °C, for all the studied compositions, being strongly dependent on the doping content. In particular, the Raman active mode observed for wavenumbers around 100 cm^–1 for all compositions exhibited a temperature dependence, which shows to be consistent with the behavior of the ferroelectric soft-mode, being this behavior, therefore, associated with a transition to the paraelectric phase in the highest temperature region (350–400 °C). The selected temperature ranges are consistent with previously reported anomalies in the dielectric spectra for the AgNbO₃ system. ACKNOWLEDGMENTS: The authors thank to CNPq (303447/2019-2), FAPEMIG (PPM-00661-16 and APQ-02875-18) and CAPES (Finance Code 001) Brazilian agencies for the financial support.