Temperature-dependent crystal structure of ferroelectric Ba2LaTi2Nb3O15

Abstract

The crystal structure of Ba 2 LaTi 2 Nb 3 O 15 was determined using high resolution powder neutron diffraction data collected at 100 K and 400 K. Both structures, refined in space groups P4bm (100 K) and P4/mbm (400 K), are closely related tetragonal tungsten bronzes with Ba in large 15-coordinate sites, La in 12-coordinate sites and Nb,Ti disordered over octahedral sites. The structural origin of the low temperature ferroelectricity is attributed primarily, from variation in bond lengths with temperature, to off-centre displacement of Ti/Nb atoms from their octahedral sites. At room temperature, selected area electron diffraction showed a weak superstructure leading to a doubling of a and c but which is incommensurate parallel to [110].

A new family of ferroelectric tetragonal tungsten bronze phases, Ba2MTi2X3O15

“…5. 5,6 Additional rows of weak spots are seen, consistent with a doubling of the c axis. However, the superlattice is also present in the [1 1 0] direction, but its periodicity is incommensurate with that of the sublattice.…”

“…However, the superlattice is also present in the [1 1 0] direction, but its periodicity is incommensurate with that of the sublattice. 5 More recent studies on other M analogues 6 show that all possess a superlattice with a doubling of c; however, the periodicity parallel to [1 1 0] is incommensurate for M = La, Bi but commensurate for smaller rare earths, e.g. Nd, Sm, Gd.…”

“…Nb and Ti are disordered over the octahedral sites, but the distribution is non-statistical, with a slight preference of Nb for site (2) and conversely, of Ti for site (1). 5 At 100 K, the structure has the non-centrosymmetric space group P4bm, which allows Ti and/or Nb to undergo small, offcentre displacements within the (Nb/Ti)O 6 octahedra, leading to a polar c axis. At 400 K, the structure is centrosymmetric, space group P4/mbm and there is little evidence of polarity within the largely undistorted (Nb/Ti)O 6 octahedra.…”

Preparation and dielectric properties of unfilled tungsten bronze ferroelectrics Ba4RETiNb9O30

“…But for the Ba 4 Ln 2 Ti 4 M 6 O 30 compounds, not all the ordered tungsten bronze structure compounds tend to be normal ferroelectric nature, such as Ba 4 La 2 Ti 4 Nb 6 O 30 [15,16]. Stennett et al [17] suggested that the lack of commensurate periodicity may contribute to the disruption of long range dipolar coupling in these compositions, resulting in relaxor rather than classic ferroelectric behavior, which leading by the effects of the ionic radius of rare-earth ion in interstice A1.…”

Growth and Characterization of Centimeter‐Sized Ba2LaFeNb4O15 Crystals from High‐Temperature Solution under a Controlled Atmosphere

“…Recently, the incommensurate structure of SBN was solved and refined as a five dimensional (5D) modulated structure in the superspace formalism. [49] In 2012, Graetsch et al [50][51] [52][53][54][55] demonstrated that both Bi-and La-based compounds exhibit relaxor behavior, whereas Nd-, Sm-, and Gd-based analogs are classical ferroelectrics. In Ba 2 RENb 3 Ti 2 O 15 compounds, deviations from the ideal TTB symmetry caused by an incommensurate modulation was associated with relaxor behavior, whereas a commensurate modulation was observed for phases exhibiting ferroelectric behavior.…”