Electron Charge Density Study of (Na_1-xK_x)NbO₃ in Cubic Structure

Abstract

The electron charge density study by synchrotron-radiation powder diffraction has been performed for the cubic structure of (Na1-xKx)NbO3 (NKN: 0≤x≤1) at 1000 K to investigate the atomic substitution effect on chemical bonding. Our precise analysis using the maximum entropy method (MEM)/Rietveld method demonstrates that the covalent bonding is formed on the Nb–O bond whereas the Na/K atoms are ionic in the entire composition range. The thermal motion of the Na ion in NaNbO3 is fairly larger than that of the K ion in KNbO3. The charge density distributions around the O atoms in NaNbO3 are extended in the directions perpendicular to the Nb–O bond, which can be related to the rotational mode of the Nb–O6 octahedron driving the antiferroic phase transition. No such anisotropy is clearly observed in KNbO3 around the O atoms. These behaviors are closely related to the change in the tolerance of the perovskite structure caused by the atomic substitution.

Study of charge density and crystal structure of (La0.75Sr0.25)MnO3.00 and (Ba0.5Sr0.5)(Co0.8Fe0.2)O2.33−δ at 500–900K by in situ synchrotron X-ray diffraction

“…Based on the fact that we obtained FWHM (|F MEM | − |F obs |)/ obs (F MEM : structure factors calculated by MEM analysis, F obs : observed structure factors, and obs : estimated standard deviations of observed structure factors) values of 0.172-0.192 and 0.107-0.157 by MEM of LSM and BSCF, respectively -both of which are considerably smaller than 1 -we confirmed that our MEM were correct. This value is close to that of Zr-O and a Nb-O bonds [17,18]. We assumed that these bonds possess strong covalency in LSM.…”

“…In fact, the oxygen ion conductivity of LSM is lower than that of other perovskite oxides that are used as cathode materials for an SOFC [37]. We speculated that the (La, Sr) site in LSM has a higher valence than the Pb, K, and Na sites in these perovskites [17,18]. Mn-O plane (Fig.…”

“…MEM has been reported in various fields. Kuroiwa et al discussed the charge density of perovskite oxides [17,18]. Yashima reported oxygen ion conduction in fluorite and perovskite oxide via hightemperature neutron diffraction [19].…”

Structure fluctuation in Gd- and Mg-substituted BaTiO3with cubic structure

“…A smaller t can also generate the antiferrodistortive thermal motion as observed in several perovskite-type oxides. [30][31][32][33] Although the Rietveld analysis derives only the average thermal motion, U O22 and t characteristics of seem to support the enhancement of the antiferrodistortive thermal motion in (Ba 1−x Gd x )(Ti 1−x=2 Mg x=2 )O 3 in the low-temperature and high-x regions. Here, the thermal behavior of the off-centered Gd ion is discussed in further detail.…”

Structural Characteristics of (Ba0.94Gd0.06)(Ti0.97Mg0.03)O3in Cubic Structure Determined by High-Energy Synchrotron-Radiation Powder Diffraction

“…Analyzing the synchrotronradiation (SR) powder diffraction data using the maximum entropy method (MEM)/Rietveld method is useful for investigating the crystal structure of cubic perovskite-type oxides ABO 3 at the charge density level. [8][9][10][11][12] Recently, the MEM/Rietveld analysis has provided direct structural evidence for the oxygen stabilization on the Bi/La-O covalent bonds in a rare-earth element La-substituted ferroelectric Bi 3:25 La 0:75 Ti 3 O 12 , which exhibits high endurance to polarization fatigue. 13) The charge density study presented in this paper will provide information essential for a better understanding of the relationship between crystal structure and ferroelectric instability in BaTiO 3 and for designing more promising BaTiO 3 systems for MLCCs.…”