Centrosymmetric Tetragonal Tungsten Bronzes A₄Bi₂Nb₁₀O₃₀ (A = Na, K, Rb) with a Bi 6s Lone Pair

Abstract

A first-principles study of the tetragonal tungsten bronze (TTB) K4Bi2Nb10O30 has suggested that the Bi 6s lone pair causes in-plane polarization (within the a–b plane), corresponding to the one found in Pb5Nb10O30 (PN), in contrast to the out-of-plane polarization (along c) found in most TTBs. Replacing PN with KBN potentially opens for a lead-free analogue to morphotropic phase boundaries known in TTBs based on PN. Here, we report on the synthesis and properties of A4Bi2Nb10O30 (ABN, A = Na, K, Rb) with the objective to determine the structure and electrical properties, paying particular attention to the role of the Bi 6s lone pair. The ABN materials were synthesized via conventional solid-state synthesis in a two-step process. Convergent-beam electron diffraction demonstrated a centrosymmetric tetragonal space group for the two compounds KBN and RBN, and ferroelectric polarization–electric field measurements confirmed the lack of hysteretic behavior in line with the observed centrosymmetric symmetry. Non-ambient powder X-ray diffraction demonstrated the signature of a phase transition for KBN and RBN, as several weak satellite reflections vanished during heating and reappeared upon cooling. Dielectric spectroscopy supported the observation of an anomaly due to the presence of a weak maximum in the electrical permittivity at temperatures corresponding to the disappearance of the satellite reflections. Possible explanations for the absence of polarization in ABN TTBs are discussed with particular attention to the suppression of the 6s2 lone pair effect of Bi and the size of A-site cations in the TTB crystal structure.

The effects of Sr-substitution in Ba2SmTi2Nb3O15 ceramics: structural study, optical properties, and complex impedance spectroscopy

“…These compounds exhibit ferroelectric behavior, and the Curie temperature (T C ) gradually decreases from 332 °C for Sr 2 SmTi 2 Nb 3 O 15 to 246 °C for Sr 2 NdTi 2 Nb 3 O 15 . This trend correlates with the center displacement of the cation Ti/Nb (1) in the (B1) site. 16 In another study, ceramics with the composition (Sr x Ba 1−x ) 4 Sm 2 Fe 2 Nb 8 O 30 (x = 0, 0.1, 0.2, 0.3, 0.4) were investigated.…”

“…The standard formula for a TTB is (A1) 2 (A2) 4 (C) 4 (B1) 2 (B2) 8 O 30 . [1][2][3][4][5][6] The pentagonal sites (A2) with a coordination number of 15 are capable of accepting sizable cations, including alkaline earth elements. On the other hand, the square sites (A1) with a coordination number of 12 are designed to accommodate moderately sized cations, such as rare earth elements.…”

Single crystal growth and phase transition mechanism of KCsMoP2O9 with efficient second harmonic generation via the emerging additional periodic phase technology

“…The observed weak reflections vanished at about ∼300 °C, indicating that there may be structural anomalies. 46…”

“…The observed weak reflections vanished at about B300 1C, indicating that there may be structural anomalies. 46 The in situ XRD data were processed using Rietveld refinement (Fullprof), and the temperature dependence of the lattice parameters for the KCMP crystal is illustrated in Fig. 8a and b.…”

“…It has been proved that the modulated structure phase transition is generally associated with the octahedral tilting or rigid unit mode. 46,[56][57][58] In particular, the concept of a rigid unit mode has been successfully applied to investigate the soft vibrational modes involved in incommensurately modulated structures. 47,52,59,60 Therefore, the rigid unit mode approach was used to analyse the mechanisms of phase transitions of the KCMP crystal.…”