Interplay between local distortion at lattice sites with optical and electrical properties of Eu3+-doped MNbO3 (M = Na and K) compounds

Abstract

Local distortion at lattice sites plays a pivotal role in determining many important physical properties. In the present study, our focus has been to examine such crucial roles for the optical and ferroelectric properties in MNbO₃ (M = Na and K) compounds. This study on dual property has significant importance in view of the potential application for piezophotonic devices; herein, an effort has been made to develop compounds with the simultaneous control of such properties in the desired range. For the optical properties, we doped Eu³⁺ ions, which not only give efficient red emission but also act as a spectroscopic probe for the local structure investigation. It was observed from a photoluminescence (PL) study that the Eu³⁺:NaNbO₃ compound is a pink-colour-emitting phosphor material while the Eu³⁺:KNbO₃ compound is a single phase white-light-emitting material. It was also observed that doping Eu³⁺ ions had a significant impact on ferroelectric properties of these compounds. While such doping increased the remnant polarization of NaNbO₃, the impact was completely reversed in KNbO₃. A PL study confirmed that the local structure surrounding the Eu³⁺ ion in NaNbO₃ and KNbO₃ compounds was different, and there was a more distorted environment surrounding the Eu³⁺ ion in the NaNbO₃ compound. It was confirmed that Eu³⁺ ions, which exist in an asymmetric environment, have a major contribution among the PL lifetime components in the NaNbO₃ compound. On the other hand in the KNbO₃ compound, Eu³⁺ ions, which exist in a comparatively more symmetric environment, make the major contribution. From the PL study, we also conclude that Eu³⁺ ions preferably occupy the Na-site in NaNbO₃ compound, which may distort the nearby NbO₆ more and are responsible for a higher polarization. On the contrary in KNbO₃ compound, majority of Eu³⁺ ions prefer symmetrical octahedral sites, which may not have an additional impact on the distortion of the NbO₆ octahedron. Further, it was also observed from the extended X-ray absorption fine structure (EXAFS) study that upon Eu³⁺ doping the distortion parameter σ² increased for Nb–O bonds in NaNbO₃ compounds, while it decreased in KNbO₃ compounds.