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 Ba 2 La x Nd 1−x Nb 3 Ti 2 O 15. 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 Ͼ 0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

Articles you may be interested inEvolution of structure, dielectric properties, and re-entrant relaxor behavior in Ba5LaxSm1−xTi3Nb7O30 (x = 0.1, 0.25, 0.5) tungsten bronze ceramics

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].

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The composition Zn2.33Sb0.67O4 (or Zn7Sb2O12) exists in two polymorphic forms. The thermodynamically stable, low‐temperature orthorhombic β form transforms to the high‐temperature cubic α‐polymorph with a spinel structure at 1225°±25°C. The transformation is fully reversible but slower in the α→β direction and therefore, it is easy to preserve the high‐temperature α‐polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the α‐polymorph directly at low temperatures, e.g., 900°C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the α‐polymorph has been confirmed by Rietveld refinement of X‐ray powder diffraction data to be an inverse spinel, (Zn)[Sb2/3Zn4/3]O4, in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn.