P. L. Zhang

Phase transition in PbTiO3 ultrafine particles of different sizes

The size effect on the ferroelectric phase transition in PbTiO3 ultra-fine particles is reported. Samples with particle sizes from 20 to 200 nm were prepared by a sol-gel process followed by calcining at different temperatures. The particle size was determined by X-ray diffraction from the integrated width of diffractions. The soft-mode frequency at room temperature was measured by Raman scattering. It decreases with decreasing particle size. The ferroelectric phase transition was traced by specific-heat measurement. The transition temperature decreases and the transition becomes diffused as the particle size decreases. The size dependence of TC can be described by TC(D)=766-256/(D-8.8) (K), where 766 K is the TC for bulk PbTiO3 and D (nm) is the particle size. This equation gives a critical size of 9.1 nm below which ferroelectricity disappears.

The Curie temperature of ultra-thin ferroelectric films

The Curie temperature of ferroelectric films described by the transverse Ising model was studied under the mean-field theory. The film layer number, the surface interaction and the surface layer number dependence of the Curie temperature were obtained. There is a critical surface interaction strength, which is JSC=1.25 J for single-surface-layer films, and JSC=1.078 J for multiple-surface-layer films. If the surface interaction strength exceeds the critical value, there exists an optimum film thickness which possesses the maximum Curie temperature; then the surface interaction strength is weaker than the critical value, the Curie temperature decreases monotonically with increasing film thickness, and there exist critical sizes or critical thicknesses at which the ferroelectricity will disappear if the surface interaction is weak enough.

Size driven phase transition in ferroelectric particles

Abstract The phenomenological Landau-Devonshire theory is used for finite size ferroelectric particles. The spatial distribution and size dependence of polarization are calculated. The latter reveals a size driven phase transition from ferroelectric to paraelectric. The ferroelectric critical size predicted is compared with that of PbTiO 3 and BaTiO 3 particles.

Size effects on the Curie temperature of ferroelectric particles

Abstract Landau-Devonshire theory for ferroelectric phase transition is applied to ferroelectric spherical particles. The extrapolation length is shown to be size dependent. The Curie temperature as a function of size is calculated. Theoretical results are compared with experimental data in the literature and good agreement is obtained.

Curie temperature of a ferroelectric superlattice described by the transverse Ising model

Abstract The Curie temperature of a ferroelectric superlattice formed from two alterating materials was studied using the transverse Ising model in the mean-field theory. The dependence of the Curie temperature on the interface coupling strength J AB and the layer-number of the two slabs in one period were obtained. Assuming the bulk Curie temperature of slab B is higher than that of slab A, the Curie temperature of the superlattice increases with the increases in J AB , but decreases with increasing the layer- number of slab A. While fixing the layer-number of slab A, there exists a critical interface coupling strength J c ; as the layer- number of slab B increases, the Curie temperature increases for J AB J c but decreases for J AB > J c . The temperature dependence of the spin average in the two slabs we also calculated.

The cell volume effect in barium strontium titanate

Abstract The Curie temperature Tc BaμSr1−μTiO3 as a function of the composition and the lattice parameter γ (i.e. a or a 2 C 3 ) is examined. Near the end of μ ≈ 1 and μ ≈ 0, it is compared with the lattice parameter dependence of Tc of BaTiO3 and SrTiO3 respectively. It is argued that the variation of Tc in BaμSr1−μTiO3 is determined by changes in the cell volume whether the change is produced by a change in pressure or a change in composition.

Ferroelectric films described by transverse Ising model with long-range interactions

By considering pseudo-spin interactions of different range, the polarization and Curie temperature of ferroelectric films are studied using transverse Ising model. They are shown to decrease with the interaction range.

Phase transition properties of ferroelectric superlattices with three alternative layers

Abstract The Curie temperature, polarization and susceptiblity of ferroelectric superlattices with three alternative layers have been obtained using the transverse Ising model in the mean field approximation. The interface layers show crucial influence on the properties of superlattices when the period is small enough.

Size effects on phase transitions in ferroelectric films

Abstract Using the Landau phenomenological theory, we studied the size effects on phase transitions in ferroelectric films in the presence of depolarization. The thickness dependence of the polarization at 0 K and the Curie temperature were calculated. In the case of positive extrapolation length, a size-driven phase transition was revealed and the corresponding ferroelectric critical thickness was obtained.

First-principles study on the optical properties of SrHfO3 and SrTiO3

Abstract The optical properties of cubic SrTiO3 and SrHfO3 has been calculated. The full potential linearized augmented plane wave method is used and exchange-correlation effects are treated by the generalized gradient approximation. In order to understand the optical properties of SrTiO3 and SrHfO3, the imaginary part of dielectric function ϵ2(ω), the optical absorption coefficient I(ω), the reflectivity R(ω) and the optical conductivity σ(ω) were calculated. The calculated ϵ2(ω) is in agreement with experimental results.