Influence of erbium substitution on structural, electrical, and up-conversion photoluminescence properties of unfilled tungsten bronze oxides Ba3.75La0.833-xErxNb10O30

Abstract

Tetragonal tungsten bronze (TTB) oxides represent a huge family of materials which exhibit rich electric and magnetic functionalities. Recently, multifunctional materials based on some members of this family have attracted considerable attention. In this work, new plumbum-free multifunctional oxides of Ba3.75La0.833-xErxNb10O30 (BLN-x) were prepared. Their crystal structure, dielectric behavior, ferroelectricity, up-conversion (UC) photoluminescence, temperature sensing behavior, and water-resistance properties were discussed in detail. Based on Rietveld structural refinement, the crystal structure is determined as single phase unfilled TTB structure which can be refined in P4bm space group. Dielectric and ferroelectric results indicates that phase transition and ferroelectric state are sensitive to x. The phase transition temperature, Tm, increases with the increasing of size difference between A1-and A2-site cations. Meanwhile, tight correlation between Tm and tetragonality is also revealed. Moreover, A1 tolerance factor (tA1) plays an important role to determine the polarization state and phase transition behavior of BLN-x. Normal ferroelectric phase in BLN-x (x = 0.7 and 0.8) is driven by the low tA1 (0.9025 and 0.89476) value. However, the competition interaction between average A-site size ((A1 + A2)/2) and tA1 results in the onset of relaxor ferroelectric state for BLN-x (0.2 ≤ x ≤ 0.6). Furthermore, under 980 nm excitation, bright UC green and weak red emissions are observed which correspond to the transitions from 2H11/2/4S3/2 and 4F9/2 to 4I15/2 level, respectively. Color-tunable behavior is achieved with x increase, which can be illustrated by the cross relaxation (CR) processes. The dependence of UC emission intensity on pumping power shows that two photon absorption process is involved in the UC green and red emissions. Temperature sensing properties are investigated according to the fluorescence intensity ratio of UC green emissions in the temperature range of 160–480 K. The maximum sensing sensitivity is found to be 0.0025 K−1. In addition, superior water-resistance feature of UC photoluminescence is also obtained for BLN-x. These results revealed that BLN-x oxides may have promising applications in future multifunctional devices designing.