Gabrielle C. Miles

Coupling between octahedral tilting and ferroelectric order in tetragonal tungsten bronze-structured dielectrics

Strong coupling between local polar displacements and a commensurate octahedral tilting is proposed to explain the onset of classic ferroelectric behavior in tetragonal tungsten bronzelike dielectrics Ba2LaxNd1−xNb3Ti2O15. The ferroelectric phase transition is associated with a discontinuous non-lock-in transformation of an incommensurate tilted structure to a commensurate superstructure. In a manner reminiscent of perovskitelike oxides, the driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion decreases; no classical ferroelectric transition is observed for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

Dielectric and structural studies of Ba2MTi2Nb3O15 (BMTNO15, M=Bi3+,La3+,Nd3+,Sm3+,Gd3+) tetragonal tungsten bronze-structured ceramics

The structure and dielectric properties of a new family of tetragonal tungsten bronze (TTB) ceramics with the general formula, Ba2MTi2Nb3O15 where M=Bi3+,La3+,Nd3+,Sm3+,Gd3+, have been investigated. Hereafter the compositions will be referred to by the abbreviation BMTNO15 where M=B(Bi3+), L(La3+), N(Nd3+), S(Sm3+) or G(Gd3+). BLTNO15 [permittivity maximum (Tm)∼−80°C] and BBTNO15 (Tm∼−100°C) exhibited relaxorlike dielectric behavior. In contrast, BNTNO15 (Tm∼165°C), BSTNO15 (Tm∼250°C), and BGTNO15 (Tm∼320°C) were classic ferroelectrics. Tm increased with decreasing radius of the M ion. Room temperature x-ray powder diffraction (XRD) patterns of all the compounds indexed on a prototype P4∕mbm (or P4bm) space group with lattice parameters a≈12.4A and c≈4A. However, electron diffraction revealed that the relaxor phases, BLTNO15 and BBTNO15, exhibited an incommensurate modulation, whereas the classic ferroelectric BNTNO15, BSTNO15, and BGTNO15 featured an orthorhombic superstructure with lattice parameters a≈...

Temperature-dependent crystal structure of ferroelectric Ba2LaTi2Nb3O15

The crystal structure of Ba2LaTi2Nb3O15 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].

Stoichiometry and doping mechanism of the cubic pyrochlore phase in the system Bi2O3–ZnO–Nb2O5

The subsolidus phase diagram of the system Bi2O3–ZnO–Nb2O5 in the region of the cubic pyrochlore phase has been determined at 950 °C. This phase forms a solid solution area that does not include the so-called ideal composition P, Bi3Zn2Nb3O14. It may be described in terms of two compositional variables: ZnO deficiency compared to P together with variable Bi : Nb ratio. Density measurements, preliminary crystallographic studies and the phase diagram indicate the solid solutions to have general formula, Bi3+yZn2−xNb3−yO14−x−y: −0.11(1) ≤ y ≤ 0.14(1) and −0.03(1) ≤ x ≤ 0.31(1).

Octahedral Tilting and Ferroelectric Order in Tetragonal Tungsten Bronze-Like Dielectrics

The onset of classic ferroelectric behavior in tetragonal tungsten bronze-like dielectrics Ba2LaxNd1-xNb3Ti2O15 was attributed to a strong coupling between local polar displacements and a certain type of octahedral tilting. The ferroelectric phase transition in these systems is associated with a transformation of an incommensurate tilted structure to a distinct commensurate superstructure. The driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion that reside in perovskite type channels decreases. No classical ferroelectric transition is observed (down to 100 K) for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.