Materials Research Bulletin - Ferroelectrics Turns 100! Special Issue

This year we commemorate the first report of ferroelectricity a century ago with a special issue, the following papers and editorial have been made free to view for the next 12 months:

Editorial: 

Ferroelectrics turn 100!

This year we commemorate the first report of ferroelectricity a century ago, but the full story goes much further back.

Featured Papers:

Structural and impedance properties of KBa₂V₅O₁₅ ceramics

The polycrystalline sample of KBa₂V₅O₁₅ ceramics was prepared by a mixed oxide method at low temperature (i.e., at 560 °C). The formation of the compound was confirmed using an X-ray diffraction technique at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 323 °C, and exhibits diffuse phase transition. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of both grain (>10³ Hz) and the grain boundary (<10³ Hz) effects in the material. Studies of electrical conductivity over a wide temperature range suggest that the compound exhibits the negative temperature coefficient of resistance behavior. The ac conductivity spectrum was found to obey Jonscher's universal power law.

Abnormal electrocaloric effect of Na_0.5Bi_0.5TiO ₃-BaTiO₃ lead-free ferroelectric ceramics above room temperature

We demonstrate the electrocaloric effect (ECE) of Na_0.5Bi _0.5TiO₃-BaTiO₃ (NBT-BT) lead-free ferroelectric ceramics, which were fabricated by the solid-state reaction method. Based on a Maxwell relation, the ECE was characterized via P-T curves under different electric fields. The polarization of NBT increases monotonically within the temperature range of 25-145 °C. It indicates that the NBT has an abnormal ECE with a negative temperature change (ΔT₁₄₀ = -0.33 K at E = 50 kV/cm) opposite to that of the normal ferroelectrics. The 0.92NBT-0.08BT composition near the morphotropic phase boundary has a normal ECE under low electric fields and an abnormal ECE under high electric fields. The abnormal ECE character originates from the relaxor characteristic between ferroelectric and antiferroelectric phases, while the common ECE is always related to the normal ferroelectric-paraelectric phase transition.

Unusual dynamic polarization response and scaling behaviors in Bi_1/2Na_1/2TiO₃ ceramics

The excellent electrical properties and potential applications of ferroelectric ceramics are intrinsically correlated with the dynamic polarization response under applied electric field. However, the lack of clarity with regard to global polarization reversal process has hindered the understanding of the functional properties. Here, the polarization reversal and dynamic hysteresis were investigated in (Bi_1/2Na_1/2)TiO₃ (BNT) ceramics. The scaling behaviors were presented by studying the field amplitude E₀, frequency f₀ and temperature T₀ dependence of hysteresis loops. Three new features of polarization responses and scaling behaviors were obtained. First, the exponent α in the hysteresis loop area ∝E₀ ^α equation displayed large values in high-E₀ stage. Then, the displayed an abnormal increase with increasing temperature. Finally, a near frequency-invariant coercive field E_C, accompanied by a weak dependent P_r and was observed. The correlation between these new features and polarization extension/rotation as well as complex domain structure and field-induced phase transition was demonstrated.

Fabrication of perovskite lead magnesium niobate

The perovskite relaxor ferroelectric lead magnesium niobate (PbMg_{1 3}Nb_{2 3}O₃) is an important material because of its high dielectric constant and correspondingly large electrostrictive strains. However, it is difficult to prepare in polycrystalline ceramic form because of the formation of a stable pyrochlore phase. The reaction kinetics during calcining were investigated and an improved fabrication scheme was developed.

Structural basis of ferroelectricity in the bismuth titanate family

More than fifty ferroelectrics belong to the Bi₄Ti₃O₁₂ family, and all consist of Bi₂O₂ layers interleaved with perovskite-like M_n-1R_nO_3n+1 layers. Crystal structures of three members of the family have been refined from x-ray and neutron diffraction data, elucidating the distortions responsible for ferroelectricity. Bi₂WO₆ (n = 1) is orthorhombic, space group B2cb; Bi₃TiNbO₉ (n = 2) orthorhombic, A2₁am; Bi₄Ti₃O₁₂ (n = 3) monoclinic, Pc, but very nearly orthorhombic, B2cb. Similar distortions occur in all three structures, with large rotational motions accompanying the polarization along a. Below the transition, a strong Bi-O bond is formed to the apex oxygen of the perovskite layer, tilting the octahedra and producing antiparallel shifts along b. Symmetry differences in the even- and odd-layered compounds can be explained by the type of strains produced in the perovskite layer. The octahedral cations (W, Ti, Nb) are the major contributors to the spontaneous polarization, moving about 0.4Å toward an octahedral edge.