Ferroelectric Tungsten Bronze-Type Crystal Structures. I. Barium Strontium Niobate Ba0.27Sr0.75Nb2O5.78

Abstract

Ferroelectric Ba0.27Sr0.75Nb2O5.78, with Tc = 348° ± 15°K, is a tungsten bronze-type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024 ± 0.00002 and c = 3.91341 ± 0.00001 Å at 298°K, space group P4bm, and five formulas in the unit cell. The integrated intensities of 6781 structure factors were measured with PEXRAD, 875 symmetry-independent structure factors being significantly above background. The metal-atom positions were determined from the three-dimensional Patterson function and the oxygen atoms from subsequent Fourier series. The final agreement factor between measured and calculated structure factors is 0.0508. The structure consists of close-packed slightly puckered layers of oxygen atoms separated by nearly c / 2. The Nb atoms are slightly displaced from one layer, the Ba and Sr atoms from the other and in the same sense. The oxygen atoms in the Ba and Sr layer are disordered. Neither of the two independent sites occupied by the Ba and Sr atoms is fully filled. The 2a site is preferred by Sr, which is in distorted cubo-octahedral coordination; the Sr–O distances range from 2.547 ± 0.008 Å to 2.919 ± 0.008 Å. The 4c site, occupied by Ba and Sr, has distorted tricapped trigonal prismatic symmetry; the Ba/Sr–O distances range from 2.604 ± 0.009 Å to 3.035 ± 0.010 Å. The two independent Nb atoms fully occupy the 2b and 8d sites and are each in relatively regular octahedral coordination. The 2b site has Nb–O distances ranging from 1.92 ± 0.03 Å to 2.03 ± 0.03 Å; the 8d-site Nb–O distances range from 1.88 ± 0.03 Å to 2.11 ± 0.03 Å. A ferroelectric mechanism is proposed in which the metal atoms in the 2a, 2b, and 4c sites move, at the Curie temperature, into the oxygen layers. These metal atoms then acquire 4̄,222, or 2 point symmetry, respectively, as the space group transforms to P4̄b2. The remaining Nb atoms, in the 8d position, also rearrange so as to have half above and half below the oxygen layer. The absolute displacements of the metal atoms from their nearest mean planes of oxygen atoms, in space group P4̄bm, are directly related to the ferroelectric polarization sense.

On the energetics of cation ordering in tungsten-bronze-type oxides

“…[3][4][5] A lead-free analogue to PN is strontium barium niobate, Sr x Ba 1-x Nb 2 O 6 or SBN, which is apparently stable in the TTB structure down to ambient temperature, although with a lower T C of 70-200 • C, depending on composition. 6,7 Both PN and SBN have the general formula (A1) 2 (A2) 4 C 4 (B1) 2 (B2) 8 O 30 , with five formula units in the unit cell, and space group symmetry P4/mbm in the aristotype structure. The A1 and A2 sites form, respectively, pentagonal and square channels that run parallel to the tetragonal axis, and which accomodate the Pb 2+ , Sr 2+ and Ba 2+ cations.…”

“…PN is a classical ferroelectric, while SBN is ferroelectric for barium-rich compositions, but becomes relaxor-like for strontium contents higher than approximately x = 0.6. 10 The ferroelectric transitions are also different in terms of symmetry: While SBN remains tetragonal at all temperatures, with a spontaneous polarization in the [001] direction below T C , 6 PN has an in-plane polarization along the [110] direction referred to the aristotype cell, leading to an orthorhombic distortion in the ferroelectric state. 11 The explanation of this difference, or indeed the mechanism for ferroelectricity in general, is not fully understood for these compounds.…”

“…Intuitively, the larger ionic radius of Ba 2+ relative to Sr 2+ and Pb 2+ implies that the former will preferentially occupy the larger pentagonal sites, which has also been found experimentally. 6,18 However, even this simple effect of atomic size is not obvious: Large cations preferentially occupy the larger octahedral sites in 4-2 spinels, while the opposite is actually true for 2-3 spinels. 16 There is therefore no guarantee that a simple argument based on atomic size also holds for a different crystal structure such as the TTB-type.…”

Dielectric Relaxation Phenomena in SBN Single Crystals Doped with Ce

“…Equations (1) and (2) should be considered as sequences of relation (3). In this case the relaxation time follows a temperature dependence of the Arrhenius type [18]:…”

“…A first structural characterization of this type of materials has been made in 1968 [3] for the composition x = 0.75. The ferroelectric relaxor SBN belongs to the structure type of tetragonal tungsten bronze (TTB) with the space group P 4bm at room temperature (see Fig.…”