The Effects of Low Oxygen Activity Conditions on the Phase Equilibria and Cation Occupancy of Strontium Barium Niobate

Abstract

Strontium barium niobate (SBN) is a tungsten bronze family ferroelectric which shows promising thermoelectric properties under reducing conditions. It is found here that the enhanced electrical conductivity of oxygen‐deficient SBN correlates with the formation of a NbO2 secondary phase. The effects of the reducing environment and the NbO2 phase formation are studied via a detailed defect chemistry analysis. Increasing amounts of the NbO2 phase are accompanied by an interesting mechanism where the A‐site occupancy of the SBN matrix increases. The resulting donor defects source the large carrier concentrations which cause the enhanced electrical conductivity necessary for thermoelectric performance. In investigation of the phase equilibria, it is found that a solid solution between (Sr0.6,Ba0.4)Nb2O6 and (Sr0.6,Ba0.4)1.2Nb2O6 exists and that the A‐site filling is found to occur at more modest reduction conditions in Sr‐ and Ba‐rich compositions. Finally, thermogravimetric analysis of the reoxidation process is performed, and the results suggest that the A‐site filling is compensated ionically. Not only do the presented results explain the enhanced electrical conductivity of oxygen‐deficient strontium barium niobate but also modification of the site occupancies by reduction and reoxidation may widen the design space for property modification in tungsten bronze‐structured materials in general.

High-temperature thermoelectric characterization of filled strontium barium niobates: power factors and carrier concentrations

“…10 The large increase in conductivity was associated with the filling of the A-site vacancy in the structure, and the resulting generation of carriers. 9,11,12 Here, the filling of the A-site acts as the other source of dopants. With respect to unfilled SBN, the effect of the A-site occupancy can be demonstrated with the expression:…”

“…12 The filling of the A-sites was supported by the observation that NbO 2 precipitation was reversible, showing that neither Ba nor Sr were volatilizing out of the system. 11 For unfilled SBNs, the amount of NbO 2 precipitated (which is a function of pO 2 and temperature) controlled the degree of A-site occupancy. 11 It was found that 100% A-site occupancy could not be reached in unfilled SBN.…”

“…11 For unfilled SBNs, the amount of NbO 2 precipitated (which is a function of pO 2 and temperature) controlled the degree of A-site occupancy. 11 It was found that 100% A-site occupancy could not be reached in unfilled SBN. However, Chan et al and Kolodiazhnyi et al have shown that the "filled" chemistry can be produced where all A-sites are occupied and where NbO 2 is absent.…”

Enhanced electrical properties and strong red light‐emitting in Eu3+‐doped Sr1.90Ca0.15Na0.9Nb5O15 ceramics

“…1-3 TB niobate oxides with general formula of (A1) 4 (A2) 2 (C) 4 Nb 10 O 30 are based on ab-planar array of corner-shared NbO 6 octahedrons that are stacked along the c-axis to form different tunnels with pentagonal (A1), quadrangular (A2), and triangular (C) cross-sections in the crystal lattice. [5][6][7][8][9][10][11] In addition, niobate materials with the 'filled' TB structure usually exhibit much better dielectric and ferroelectric properties compared to those with the 'unfilled' TB structure. 4 It is reported that the distortion of NbO 6 octahedral polar unit can be greatly affected by the ion-occupying-situation, ionic size, and electronegativity of substituted ions, resulting in some special dielectric and ferroelectric behaviors in TB niobate materials.…”

“…4 It is reported that the distortion of NbO 6 octahedral polar unit can be greatly affected by the ion-occupying-situation, ionic size, and electronegativity of substituted ions, resulting in some special dielectric and ferroelectric behaviors in TB niobate materials. [5][6][7][8][9][10][11] In addition, niobate materials with the 'filled' TB structure usually exhibit much better dielectric and ferroelectric properties compared to those with the 'unfilled' TB structure. 5,8,12 In the past decades, luminescent-ferroelectic materials have aroused considerable attention as that they can be developed for future multifunctional optoelectronic devices within a single material.…”