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 Å to 1056 Å 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 responsible for the upward frequency shift in the soft modes, especially, the E(1TO) mode, which is markedly altered with respect to its analogs in BT-bulk crystal and BT thin film. Ferroelectric hysteresis loops measurements, confirmed the x-ray diffraction and Raman results, and revealed a clear ferroelectric behavior for the constrained SLs (32 Å≤Λ≤256 Å) while a weak ferroelectricity is observed for the large periods (Λ=500 Å and 1056 Å). This is attributed to the strain-induced ferroelectricity in BZ layers for the constrained SLs.
X-Ant-Em instrument operated at 300 kV. The temperature dependent resistivities were measured with a van der Pauw geometry on a Quantum Design physical property measurement system. The XAS experiments were performed on the XTreme beamline at Swiss Light Source. [45]
In this work structural refinement, complex impedance spectroscopy (CIS) and Raman spectroscopy have been investigated on Na0.5(Bi1-xDyx)0.5TiO3 (xDyNBT) ceramic systems with x = 0, 2, 5 and 15%. The pure NBT, 2DyNBT and 5DyNBT compounds crystallize in a rhombohedral R3c structure while the 15DyNBT composition crystallizes in an orthorhombic Pnma structure. We reported that dysprosium addition affects the phase transition temperatures as well as the dielectric losses. The electrical transport at high temperatures was investigated using the CIS over a wide frequency range (10-10 6 Hz). The studied samples showed a non-Debye type process, with a short-range relaxation for the pure NBT and a coexistence of both localized and long-range relaxations of charge carriers for the 2DyNBT and 5DyNBT compounds. For the high concentration, 15DyNBT, a short-range relaxation is observed. Moreover, using a bricklayer model we discuss the resistance and capacitance of the different contributors (grain and grain boundaries) in our samples. High temperature Raman spectroscopy investigation was performed in order to follow the temperature evolution of the structural transformations on ferroelectric compounds. Anomalies in the temperature evolution of the vibrational modes are seen to correlate well with the temperature transitions observed from dielectric measurements.
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