Effect of cation vacancies on the optical and dielectric properties of KSr₂Nb₅O₁₅: A first‐principles study

Abstract

Using first‐principles calculations, the effect of cation vacancies on the electronic structures and optical characters of KSr2Nb5O15 (KSN) lead‐free ferroelectrics are investigated. The calculated dielectric properties are demonstrated by the experimental results. The cation vacancies involve K+ vacancies (KSN‐K), Sr2+ vacancies (KSN‐Sr), and coexisting K+ and Sr2+ vacancies (KSN‐K&Sr). When these cation vacancies exist in KSN, the unit cell volumes decrease, leading to phase transition from tetragonal to orthorhombic, and the cation vacancies show strong effects on the band gap of KSN, declining by 1.46%‐9.46%. The optical properties including the static dielectric constants, refraction, and extinction coefficient of KSN‐K, KSN‐Sr, and KSN‐K&Sr increase more than those of KSN without vacancies, but the reflectivity and loss function decrease. All structures with cation vacancies are mainly refractive in the 0‐4 eV photon energy range and are reflective at 5‐8 eV. The refractivity increases and reflectivity decreases after vacancies occur. KSN‐Sr has the largest static dielectric constant while KSN‐K&Sr has the smallest values. The dielectric constant can be adjusted in the range of 25% by controlling the cation vacancies. The calculated dielectric properties are in good agreement with the experimental results. The results pave the way to regulate the optical and dielectric properties of lead‐free ferroelectrics by controlling different cation vacancies.

Effect of Intrinsic Nb5+ Vacancy on Dielectric and Polarization Behaviors of KSr2Nb5O15: First‐Principles Investigation

“…The band structures of KSN‐Nb1 and KSN‐Nb2 are shown in Figure 4 . When comparing the PDOS and band structure of defect‐free KSN [ 9 ] with those of KSN‐V B , the PDOS and band structures of KSN with Nb 5+ vacancy show significant differences. When the Nb1 and Nb2 sites of KSN are vacant, the bandgap of KSN‐Nb1 remains indirect, the bandgap generates between the Q point (valence band) and the F point (conduction band), whereas KSN‐Nb2 becomes a direct bandgap compound, the bandgap generates between the G point (valence band) and the G point (conduction band).…”

“…This means that when the Nb 5+ vacancy is generated, the point defect formed is (), (), (, or (. According to the results of the vacancy formation energy of the presence of cationic vacancies in the KSN, [ 9 ] K + and O 2− form a weak ionic bond, Sr 2+ and O 2− form a weak covalent bond, and Nb 5+ and O 2− form a strong covalent bond; so, the E f of K + vacancy in KSN is the lowest and K + vacancy is the easiest to produce, followed by Sr 2+ vacancy, K + and Sr 2+ vacancies coexistence, and Nb 5+ vacancy is the most difficult to produce. However, according to a recent research on the microstructure and electrical performance of KSN, energy dispersive spectroscopy (EDS) analysis of KSN microcrystalline powder and ceramic revealed that K + and Sr 2+ vacancies exist in KSN, with a tiny quantity of Nb 5+ vacancies remaining.…”

“…[ 6 ] The Curie temperature of KSN is reduced by cation vacancies at the A site. [ 7,8 ] Furthermore, in the previous study, [ 9 ] it was shown that when Sr 2+ vacancies exist in KSN, the dielectric properties of KSN can be greatly improved, 4.647 times higher than defect‐free KSN, implying that the dielectric constant of KSN may be adjusted by the cation vacancies.…”