Intrinsic origin of enhanced piezoelectricity in alkali niobate‐based lead‐free ceramics

Abstract

An orientational average method is used to calculate the piezoelectric coefficients of KNbO3 and K0.5Na0.5NbO3 (KN and KNN) piezoceramics, which is the first time that an ab initio method has been used to directly compare the piezoelectric coefficients with the experimental results of the orthorhombic alkali niobate‐based lead‐free piezoceramics. It is found that the longitudinal piezoelectric coefficient d¯33∗ of orthorhombic KN and KNN piezoceramics is mainly contributed by d33 and d15 of their single crystal. The approximately 70% increase in the calculated d¯33∗ is confirmed in pure orthorhombic KNN over KN piezoceramics. The increased response of the internal coordinates of Na and O atoms to macroscopic strain is considered the main origin of the enhanced piezoelectricity. The underlying mechanism is that the smaller atomic size of Na provides more room for itself and the adjacent O atoms in the perovskite structure, which leads to the increased local structural instability. In this case, the Na and its nearby O atoms are easier to move when an external stress is applied on the KNN and then induce the substantial change in polarization. This work provides a microscopic insight to understand the enhanced piezoelectricity in KNN piezoceramics.

Realizing High Comprehensive Energy Storage and Ultrahigh Hardness in Lead-Free Ceramics

“…After the geometry optimizations, the strongly constrained and appropriately normed semilocal density functional (SCAN) is included to better describe the band structure of KNN. , The calculated lattice parameters of the Bmm 2 cell are 5.613, 3.947, and 5.639 Å for La-doped KNN, 5.627, 3.961, and 5.644 Å for Bi-doped KNN, 5.638, 3.938, and 5.665 Å for pure KNN. The pure KNN exhibits the high-accurate lattice parameters compared with experimental results, suggesting the PBEsol is better than the conventional PBE method to describe the KNN lattice . The lattice parameters of La-doped and Bi-doped KNNs are smaller than that of the pure KNN, which can be due to KNNs containing two A-site vacancies in their supercell.…”

“…The pure KNN exhibits the highaccurate lattice parameters compared with experimental results, suggesting the PBEsol is better than the conventional PBE method to describe the KNN lattice. 53 The lattice parameters of La-doped and Bi-doped KNNs are smaller than that of the pure KNN, which can be due to KNNs containing external electric field is the intrinsic DBS. The larger the band gap, the higher the intrinsic DBS.…”

Energy Storage Behavior in ErBiO3-Doped (K,Na)NbO3 Lead-Free Piezoelectric Ceramics

“…[010], and [101] directions of the Bmm2 space group, 41 respectively. The coordinates of the second phase ErNbO 4 use monoclinic C2/c as the initial model.…”

Critical Role of Order–Disorder Behavior in Perovskite Ferroelectric KNbO3

“…It is well known that the spontaneous polarization of BaTiO 3 and PbTiO 3 derive from the hybridization between the Ti 3d states and the O 2p states, and the ferroelectricity in BaTiO 3 is less related to the Ba, which exhibits completely ionic and weak Ba–O interaction. The Born effective charges of orthorhombic KNbO 3 are calculated in our previous work and the effective charge of K is very close to its nominal charge, 1, indicating that the K–O interaction is fundamentally ionic and there is no orbital hybridization between K and O atoms. However, the Born effective charges of Nb and O are 8.868 and −6.827, respectively, along the nonpolarized direction of the Nb–O bond, much larger than their nominal charge.…”