Origin of ferroelectric polarization in tetragonal tungsten-bronze-type oxides

Abstract

The origin of ferroelectric polarization in the tetragonal tungsten-bronze-type oxide strontium barium niobate (SBN) is investigated using first-principles density functional calculations. We study in particular the relationship between the polarization and the cation and vacancy ordering on the alkali earth lattice sites. Lattice dynamical calculations for paraelectric structures demonstrate that all cation configurations that can be accommodated in a 1 × 1 × 2 supercell result in a single unstable polar phonon, composed primarily of relative Nb-O displacements along the polar axis, as their dominant instability. The majority of the configurations also have a second octahedral tilt-mode instability which couples weakly to the polar mode. The existence of the tilt mode is strongly dependent on the local cation ordering, consistent with the fact that it is not found experimentally. Our results suggest that ferroelectricity in the SBN system is driven by a conventional second-order Jahn-Teller mechanism caused by the d 0 Nb 5+ cations, and demonstrate the strong influence of the size of Sr and Ba on the lattice distortions associated with polarization and octahedral tilting. Finally, we suggest a mechanism for the relaxor behavior in Sr-rich SBN based on Sr displacement inside pentagonal channels in the TTB structure. PACS numbers: 71.15.Mb, 77.80.Jk, 77.80.bg Ferroelectric oxides with tetragonal tungsten-bronze (TTB) structure have been known since 1953 [3], and their polar properties are both widely studied and used in applications. One of the simplest TTB compounds, lead metaniobate (Pb 5 Nb 10 O 30 or PN), is commercially available and attractive for hightemperature piezoelectric sensor applications. In spite of their popularity, however, a fundamental explanation of the mechanism leading to spontaneous polarization in TTBs has to our knowledge not been reported. One possible origin is Nb 5+ , which is a ferroelectrically active cation in perovskites owing to a second-order Jahn-Teller mechanism enabled by its d 0 electron configuration [4,5]. Fundamental studies on TTBs are challenging, partly due to multiple competing phases: PN, for example, is at ambient temperature metastable in the TTB structure with respect to a rhombohedral polymorph [6][7][8]. Most other tungsten-bronze oxides are solid solutions with structural and chemical disorder, consistent with many of them being relaxor-type ferroelectrics [9][10][11].The aristotype unit cell with the general formula (A1) 2 (A2) 4 C 4 (B1) 2 (B2) 8 O 30 has six sites available for A-type cations and ten for B-type. The structure is highly anisotropic, with the A1 and A2 sites forming channels along the c direction. The A1 sites are formally 12-coordinated and similar to the A sites in perovskites, while the A2 sites are pentagonal and formally 15-coordinated. The B1 and B2 sites are octahedrally coordinated, with corner-sharing BO 6 octahedra forming the structural framework. Many niobates and tantalates crystallize as TTBs where Nb or Ta resides on the octa...

The Effect of Cation Disorder on Ferroelectric Properties of SrxBa1−xNb2O6 Tungsten Bronzes

“…This is because the temperature where complete site order was expected was too low for the cations to be mobile [12]. The Curie temperature decreased with smaller average A cation size because the octahedra tilt to optimize the coordination of the smaller cation, and this tilt coupled to and suppressed the second-order Jahn–Teller distortion, causing ferroelectric polarization [16]. Finally, the incommensurate modulations found in the ab plane in the SBN system may also contribute to the non-linear composition dependence of the a parameter.…”

Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze K3Li2Ta5OSmith1, Gardner2, Morrison3 et al. 2018Phys. Rev. Materials2020View full textAdd to dashboardCite

“…Those reported [63] in Ba 4 (La 1−x Nd x ) 0.67 Nb 10 O 30 are not likely to be seen in PXRD analysis, especially in KLT, due to the presence of heavier Ta. In addition, these subtle tilts affect the ferroelectric properties, as shown [64] in (Sr x Ba 1−x ) 5 Nb 10 O 30 . Neutron diffraction studies are required to probe O and Li effects on structure further.…”

Local environments and transport properties of heavily doped strontium barium niobates Sr0.5Ba0.5Nb2O6

“…In comparison with thermo-electric (TE) materials containing elements such as Cd, Tl, Se, Te, Sb, Bi, the TE-oxides of alkali, alkali earth, and transition elements are generally cheaper and more health and environmental friendly, and the abundance of their constituent elements on Earth's crust is orders of magnitude higher. Among them, strontium barium niobates, SrxBa1-xNb2O6 (SBN) are interesting because of their low thermal conductivity, which is related to structural features [1,2] Actually, SBNs have been investigated since long time not only as TE materials [4][5][6][7][8][9][10], but also for ferro-electricity [11][12][13][14][15][16][17][18][19] , dielectric properties [20][21][22][23], photocatalysis [24,25], and photorefractive, electro-optic, nonlinear optic, and fluorescence properties [26][27][28][29][30][31][32]. The structural aspects and their relation with the functional properties have been comprehensively discussed by Zhu et al [18] using a crystal-chemical model for the whole family of tetragonal bronzes.…”