Phase transition characteristics, electrical and optical properties of AgNbO₃crystals grown by flux method

Abstract

<sec>AgNbO₃, with the antiferroelectric ordering and huge polarization (>50 μC/cm²), has potential applications in smart electronic devices, such as energy storage dielectrics, large displacement actuators, and electrocaloric cooling device. Large electro-strain and excellent energy storage properties have been reported in AgNbO₃-based ceramics. Nevertheless, the lack of systematic research on the AbNbO₃ single crystals increases the difficulty in further understanding their structure-property relationship.</sec><sec>In this work, <inline-formula><tex-math id="M3">\begin{document}${\left\langle {001} \right\rangle _c}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20230984_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20230984_M3.png"/></alternatives></inline-formula> oriented AgNbO₃ single crystals with a large size (maximum size 5 mm×4 mm×4 mm) and high quality are successfully grown by the flux method. The phase transition characteristics are studied by the X-ray diffraction, temperature dependence of dielectric data and AC impedance, polarized light microscope photos, and differential scanning calorimetry curves. The electrical and optical properties are studied by the ferroelectric response and electro-strain response, optical absorbance spectrum and photo-dielectric effect.</sec><sec>The AgNbO₃ single crystals with the <i>M</i> phase exhibit the same domain structure. When the structure changes from <i>M</i>₂ to <i>M</i>₃, the contrast of the PLM image is darkened. Correspondingly, the conductivity and dielectric loss significantly increase, which relates to the dynamic behaviors of the carriers. Interestingly, neither exothermic peak nor endothermic peak relating to the <i>M</i>₂-<i>M</i>₃ transition is observed. The active energy for the <i>M</i>₃ phase AgNbO₃ single crystal is ~1.24 eV. When the structure changes from orthogonal <i>M</i>₃ to paraelectric orthogonal <i>O</i>, the domain structure disappears and the contrast becomes darker. The finding indicates that the anisotropy of the crystals disappears. At the same time, an obvious thermal hysteresis observed in the DSC curve reveals that the <i>M</i>₃-<i>O</i> transition is first-order. At room temperature, the direct band gap of AgNbO₃ single crystal is ~2.73 eV, which is slightly narrower than that of AgNbO₃ ceramic. Below the critical electric field, AgNbO₃ single crystal shows an electro-strain of 0.076% under <i>E</i>_m = 130 kV/cm. The obtained electro-strain value is much higher than that of AgNbO₃ ceramic under the same electric field. The relative permittivity increases from 70 to 73 under the green laser irradiation, showing an apparent photo-dielectric effect. We believe that our study can assist in the further understanding of the phase transition characteristics and physical properties in AgNbO₃ single crystals.</sec>