Electronic Polarization in KNbO₃Visualized by Synchrotron Radiation Powder Diffraction

Abstract

Electron charge density studies of a potassium niobate KNbO3 perovskite-type crystal have been carried out by analyzing synchrotron radiation X-ray powder diffraction data using a combination of the Rietveld method and the maximum entropy method. The crystal structures of the cubic, tetragonal, orthorhombic, and rhombohedral phases are accurately analyzed associated with the successive ferroelectric phase transitions by taking the multicomponent structures in the ferroelectric phases into account. The electrostatic potential analyses reveal the emergence of electronic polarization arising from the anisotropic covalent bonding on the Nb and O atoms, while no significant electronic polarization is detected on the isolated K ion. Thus, the spontaneous polarization is caused by the ionic displacements of the constituent atoms and the imbalance of the hybrid atomic orbitals on the Nb–O bonding.

Introducing a New d0 Sc3+ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO6 in Ba3Sc2(BO3)4

“…[33] Ba atoms occupy the interlayer space of [Sc(BO 3 ) 2 ] ∞ layers in two coordination environments: Ba [34][35] Such C3 distortion of octahedra is commonly observed in the crystal structures of LiNbO 3 and low temperature form of KNbO 3 , NaNbO 3 and BaTiO 3 . [36][37][38][39] All values for bond distances and bond angles are consistent with previously reported values (Table S2) and the calculated bond valence sums for Sc, B and O atoms also match well with their oxidation states (Table S4). The BVS values of the two different Ba 2 + ions show strong deviations from their ideal values, which has been reported previously in the similar compound of Ba 3 Y(BO 3 ) 3 .…”

One Site, Two Cations, Three Environments: s2 and s0 Electronic Configurations Generate Pb-Free Relaxor Behavior in a Perovskite Oxide

“…For the two BiO 12 polyhedra in KBMN the Bi–O bond lengths are very close to those observed for the 100% Bi 3+ A-site perovskites Bi­(Zn 1/2 Ti 1/2 )­O 3 (BZT, [001] ordered A-site displacement P 4 mm , shortest Bi–O 4 × 2.384 Å) and BiFeO 3 (BFO, [111] ordered A-site displacement R 3 c , shortest Bi–O 3 × 2.365 Å), which feature the same displacement directions, as seen in Table S8. , Lower valence A-sites (K + , Pb 2+ ) have longer bond lengths in comparison to Bi 3+ . The bond lengths for both PMN (2.861 Å, [000], Pm m ) and KNbO 3 (2.763 Å, [011], Amm 2) are longer than the measured Bi–O bond lengths in KBMN and are at the maximum of the rms displacement (2.772 Å) for the [001] displaced Bi 3+ in KBMN. , For disordered cubic models of PMN with [111] displacements, the Pb–O bond length (2.599 Å) is closer to Bi–O bonding environments but is still at least 0.126 Å longer. The A-site bond lengths in PMN ([000]) and KNbO 3 are comparable to the isotropic KO 12 bond length (2.861301(2) Å) in KBMN.…”

“…The bond lengths for both PMN (2.861 Å, [000], Pm m ) and KNbO 3 (2.763 Å, [011], Amm 2) are longer than the measured Bi–O bond lengths in KBMN and are at the maximum of the rms displacement (2.772 Å) for the [001] displaced Bi 3+ in KBMN. 47 , 48 For disordered cubic models of PMN with [111] displacements, the Pb–O bond length (2.599 Å) is closer to Bi–O bonding environments but is still at least 0.126 Å longer. The A-site bond lengths in PMN ([000]) and KNbO 3 are comparable to the isotropic KO 12 bond length (2.861301(2) Å) in KBMN.…”

“…The bond lengths for both PMN (2.861 Å, [000], Pm3m) and KNbO 3 (2.763 Å, [011], Amm2) are longer than the measured Bi−O bond lengths in KBMN and are at the maximum of the rms displacement (2.772 Å) for the [001] displaced Bi 3+ in KBMN. 47,48 For disordered cubic models of PMN with [111] 1. The Rietveld model can only define one oxygen position; thus, additional information about local displacements is contained in the atomic displacement parameters.…”

Perovskite-Type CuNbO3 Exhibiting Unusual Noncollinear Ferrielectric to Collinear Ferroelectric Dipole Order Transition

“…For example, KNbO 3 is ferroelectric with t > 1 (t = 1.06), displaying so-called B-site driven ferroelectricity. [9][10][11] The B-site cation (i.e., Nb 5+ ) is nominally too small for its octahedral site and can off center. In the room-temperature phase of KNbO 3 , second-order Jahn-Teller distortions [12][13][14] for Nb 5+ (4d 0 ) ions yield their off-center displacements along the pseudo-cubic <110> pc axis, leading to an orthorhombic ferroelectric structure (space group Amm2).…”