Evaluation of covalency of ions in lead-free perovskite-type dielectric oxides

Abstract

Electronic states of ions in lead-free perovskite-type dielectric oxides have been investigated with a first-principle cluster calculation. For this calculation a double-perovskite cluster model based upon the simple cubic ABO3 was used; A and B are both the cations, and O is the oxygen anion. Systematic variations of ionic species for A and B, and lengths of the model cube edge were given to the model. Results of charge transfers of the ions show that their magnitudes depend on the edge length; the lager length leads to the higher transfer magnitude. This tendency implies spatial tolerance of the ions to the clusters, and are expected to correlate with electric polarizability and dipole reversibility of this kind of oxides. The density of states and the overlap population indicate that the higher cation valence causes the higher covalency of the anions. Considering all results together provides us an idea to obtain lead-free high-performance ferroelectrics, as high as the lead-based solid solutions.

Molecular Orbital Calculation of Lead-Free Perovskite Compounds for Efficient Use of Alkaline and Alkaline Earth Metals

“…In this way, both charges for each model give a coordinate in the plane. We marked, with a dot, each coordinate determined from the charges in Tables 5-7 including those of the BNT of the Na-centered model employed in a previous study [9]. The dots are given in Figure 7, which shows the following features.…”

“…Therefore, other ionic species that cause covalency at the site of Pb can be substitutes for harmless, lead-free piezoelectrics and their promising application. One of the authors, N. Takesue [9], confirmed with molecular orbital calculation that (Bi 1/2 Na 1/2 )TiO 3 (BNT) [10,11] could be such a substitute. This substance is now widely recognized as a potential substitute and has been studied through many types of fabrication [11][12][13], long before N. Takesue's confirmation [9].…”

“…One of the authors, N. Takesue [9], confirmed with molecular orbital calculation that (Bi 1/2 Na 1/2 )TiO 3 (BNT) [10,11] could be such a substitute. This substance is now widely recognized as a potential substitute and has been studied through many types of fabrication [11][12][13], long before N. Takesue's confirmation [9]. In addition to BNT, many excellent Pb-free piezoelectrics have been discovered [14][15][16].…”

“…The sites of ions in each model are indicated by colored spheres, and the correspondence of the colors to the ions is summarized in Table 1. Each model is a rectangular cluster and was given a nonpolar symmetry D 2 (Schoenflies notation), common to all perovskite models employed; the reason is that the preliminary calculation with ferroelectric strains with a smaller symmetry than D 2 indicated no essential effect on the bonds [9]. Furthermore, this simplified modeling reduced the time needed to complete the calculations while providing the essential chemical features.…”