Symmetry breaking in hexagonal and cubic polymorphs of BaTiO3

Abstract

BaTiO3 appears in cubic and hexagonal variants, both of which are centrosymmetric. Samples of cubic BaTiO3 are known to exhibit breaking of the centric symmetry locally and globally. It has been proposed that the local symmetry breaking originates in polar regions, the precursors of the ferroelectric phase. Origins of the macroscopic symmetry breaking, which are not well understood, have been previously tentatively correlated with inhomogeneities in the samples, such as strain gradients that may align or redistribute objects such as charged point defects or polar regions making material macroscopically polar. No such data are available for BaTiO3 with hexagonal symmetry. We compare dielectric, elastic, and pyroelectric properties of the two materials in polycrystalline form. In contrast to cubic BaTiO3, hexagonal BaTiO3 does not exhibit macroscopic pyroelectric response at room temperature. This is consistent with apparent absence of polar regions in the hexagonal material and the fact that in hexagonal BaTiO3 strain rather then polarization is the order parameter for the phase transition into ferroelectric-ferroelastic phase. The thermally stimulated currents measured in hexagonal and cubic BaTiO3, however, show that both materials exhibit noncentric macroscopic symmetry. This result supports the idea that extrinsic factors such as strain gradients, which are apparently common for both materials, may break the macroscopic symmetry, which may then lead to alignment and redistribution of polar regions or charged defects.Comment: accepted in Journal of Applied Physic

Direct selective laser sintering of hexagonal barium titanate ceramics

“…Later, Yu et al synthesized h‐BaTiO 3 crystals using a pressurized electrostatic levitation furnace, which exhibited a giant dielectric constant ( ε ′ ~ 10 5 at 5 kHz) at room temperature, which may arise from the Maxwell‐Wagner effect contributed by the interfacial boundaries consisting of crystal defects 14,15 . Hashemizadeh et al reported that the permittivity of h‐BaTiO 3 ceramics may be affected by the concentration of defects 16 …”

“…14,15 Hashemizadeh et al reported that the permittivity of h-BaTiO 3 ceramics may be affected by the concentration of defects. 16 SLS, as an additive manufacturing technique, offers freedom to design and produce complex geometric build-parts without the use of molds. 17,18 SLS fabricates dense parts by using a continuous-wave (CW) laser beam to sinter the material powders selectively and additively layer-by-layer to build 3D objects.…”

Flexoelectricity in low densification materials and its implication

“…The possible interpretation of the enhanced flexoelectricity in perovskites was initially accounted for the non-crystalline polar-phases or polar nano-regions existing in the perovskites [27][28][29][30]. Recently, Damjanovic et al contended that the origin of enhanced flexoelectricity was actually attributed to the inner micro strain generated from the material densification process, despite the on-going controversy [31,32]. This paper will study flexoelectricity in BST materials with various densifications, and hence explore the interplay of the material densification and the related flexoelectric performance.…”

Direct Visualization of Polar Nanoregions in BaTiO3-based Ferroelectrics Above Curie Temperature

“…Ferroelectric materials based on BaTiO3 undergo a phase transition from a non-centrosymmetric polar ferroelectric phase to a cubic paraelectric centrosymmetric phase at Curie temperature (Tc) where spontaneous polarization is lost. However, recently it has been shown that cubic BaTiO3 phase exhibits breaking of nominal centric symmetry and exhibits polarization which was explained with the presence of polar nano-regions [1].…”