J. Belhadi

Ferroelectric BaTiO3/BaZrO3 superlattices: X-ray diffraction, Raman spectroscopy, and polarization hysteresis loops

Artificial superlattices (SLs) consisting of ferroelectric BaTiO3 (BT) and paraelectric BaZrO3 (BZ) have been growth by a pulsed laser deposition technique. The epitaxial BT and BZ layers with a periodicity from 16 A to 1056 A were sequentially deposited on (001) MgO substrate buffered with an oxide conducting layer of La1/2Sr1/2CoO3. The out-of-plane lattice parameters of the SLs constituents were determined by modeling of the x-ray diffractograms. The results indicate that the polar c-axis of the BT layers lies in the plane of the substrate and BZ layers exhibit enhanced tetragonal distortion which is induced by the mismatch between the alternating BZ and BT layers. The Raman data reinforces this interpretation and suggest a monoclinic phase in BT layers and polar phase in BZ layers. The Raman spectra give evidence of coupling between the constituent layers and a narrowing of the Raman peaks is attributed to a reduction in the disorder of the Ti4+ ions due to the epitaxial strain. This strain is respons...

Highly constrained ferroelectric [BaTiO3](1−x)Λ/[BaZrO3]xΛ superlattices: X-ray diffraction and Raman spectroscopy

We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO3] (1−x)Λ/[BaZrO3]xΛ for which the composition varied, 0.15 ≤ x ≤ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100 A. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La0.5Sr0.5CoO3. Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO3 (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO3 (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 c...

Giant increase of ferroelectric phase transition temperature in highly strained ferroelectric [BaTiO3]0.7Λ/[BaZrO3]0.3Λ superlattice

We report epitaxial strain effects in the lead-free ferroelectric/paraelectric superlattice (SL) for a modulation period Λ of about 100 A. We have demonstrated that the large strain in SL induces ferroelectricity in BaZrO3 layers, a material that is paraelectric at any temperature in the bulk form. The induced polar axis in BaZrO3 layers is perpendicular to the plane of the substrate while BaTiO3 layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SL due to the misfit strain generated by the large lattice mismatch between the alternating BaZrO3 and BaTiO3 layers. Such strain induces a huge upward frequency shift of the lowest E(1TO) soft mode from in the BaTiO3 single film to in the SL. The temperature of the ferroelectric phase transition in the SL was found to be upshifted by about with respect to the BaTiO3 single crystal.

Phase transitions in BaTiO3 thin films and BaTiO3/BaZrO3 superlattices

Using pulsed laser deposition, we grew a ferroelectric [BaTiO3]Λ/2/[BaZrO3]Λ/2 (BT/BZ) superlattice with a stacking periodicity of Λ = 256 A and a BT single film on a La1/2Sr1/2CoO3-buffered (001)MgO substrate, and then studied the phase transitions of these materials. At room temperature, the polarized Raman spectra of the BT film corresponded to a ferroelectric orthorhombic C2V14 phase with the polar axis oriented in the plane of the substrate. A ferroelectric-paraelectric phase transition in the BT film occurred at ∼450 K. Upon cooling to ∼300 K, a phase transition to the monoclinic Cs3 phase occurred. These experimental results agree well with a theoretical “temperature-misfit strain” phase diagram of the BT film. We found no evidence of phase transitions in the BT/BZ superlattice below room temperature. The phase transition to the paraelectric phase in the BT/BZ superlattice increased in temperature because of lattice mismatch between the BT and BZ layers. A desirable Curie temperature can be tailore...

Energy storage property of Lead-free Na 0.5 Bi 0.5 TiO 3 ceramic and thin film

LeaA-iree ferroelectric Nao.sBio.sTiOa (NBT) in both bulk and thin film forms we processed via solid state reaction and solution deposition respectively. SEM investigations showed dense samples with fine grained and smooth microstructure for thin films. A pure perovskite structure was observed by the X-ray diffraction for both specimens. The frequency dependencies of the dielectric constant and loss tangent are investigated at room temperature and revealed a decrease in the dielectric losses in the case of thin film. The energy storage density (W) was calculated using the obtained ΡΕ loops. The stored and delivered energies are found better in the thin film sample.

Structural, vibrational, and dielectric investigations of Ba0.925Bi0.05(Ti0.95−xZrx)Sn0.05O3 ceramics

Ceramics of Ba0.925Bi0.05(Ti0.95−xZrx)Sn0.05O3 (0.05 ≤ x ≤ 0.30) were prepared by conventional solid-state reaction technique. The effects of the Zr4+ addition to the structural and dielectric properties were investigated. The room temperature X-ray study has allowed to evidence a tetragonal symmetry up to x = 0.10, and beyond it to a cubic single phase. Thermal dielectric study at different frequencies has revealed two different behaviors, a normal ferroelectric for x ≤ 0.10, and a relaxor character for the rest of the solid solution. High temperature Raman spectra corroborates the arguments of the phase transition revealed by dielectric results. In addition, a remarkable line broadening was observed in Raman spectra, which can be attributed to a certain disorder in the structure lattice. The low Zr-composition was found to exhibit a flat εr′(T) curve with stable dielectric permittivity in a large range of temperature, promising for the X7R specifications.

Structural investigation of (111) oriented (BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices by X-ray diffraction and Raman spectroscopy

(BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices (SLs) with varying x have been grown by pulsed laser deposition on (111) oriented SrTiO3 substrates. In order to obtain good epitaxy and flat samples, a conducting SrRuO3 buffer has been deposited prior to the superlattices to screen the polar mismatch for such (111) SrTiO3 orientation. X-ray diffraction reciprocal space mapping on a different family of planes was collected and evidenced a room temperature structural change at x = 0.5 from a rhombohedral/monoclinic structure for rich BiFeO3 to an orthorhombic symmetry for rich LaFeO3. This symmetry change has been confirmed by Raman spectroscopy and demonstrates the different phase stability compared to similar SLs grown on (100) SrTiO3. The strongly anisotropic strain and oxygen octahedral rotation/tilt system compatibility at the interfaces probably explain the orientation dependence of the phase stability in such superlattices.