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Structural Transformations during Formation of Quasi‐Amorphous BaTiO 3

2006; Wiley; Volume: 17; Issue: 7 Linguagem: Inglês

10.1002/adfm.200600190

1616-3028

David Ehre, Hagai Cohen, V. Lyahovitskaya, A. K. Tagantsev, Igor Lubomirsky,

Multiferroics and related materials

Abstract A model of structural transformations of amorphous into quasi‐amorphous BaTiO 3 is suggested. The model is based on previously published data and on X‐ray photoelectron spectroscopy data presented in the current report. Both amorphous and quasi‐amorphous phases of BaTiO 3 are made up of a network of slightly distorted TiO 6 octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical‐charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO 6 network. Upon heating, the edge‐to‐edge and face‐to‐face connections between TiO 6 octahedra are severed and then reconnected via apices. Severing the connections between TiO 6 octahedra requires a volume increase, suppression of which keeps some of the edge‐to‐edge and face‐to‐face connections intact. Transformation of the amorphous thin films into the quasi‐amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO 6 octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non‐centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi‐amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.