Tetragonal-like Phase Research Articles

Low symmetry monoclinic MC phase in epitaxial BiFeO3 thin films on LaSrAlO4 substrates

We reported that the tetragonal-like phase identified in strained epitaxial BiFeO3 films on a (001) LaSrAlO4 single crystal substrates is monoclinic MC, based on high resolution synchrotron x-ray studies and piezoresponse force microscopy measurements. This MC phase has different symmetry with the rhombohedral-like monoclinic MA phase found in BiFeO3 films grown on low mismatch SrTiO3 substrates. Transmission electron microscopy revealed that the films on LaSrAlO4 substrates have a high crystalline quality and coherent interface.

Probing mixed tetragonal/rhombohedral-like monoclinic phases in strained bismuth ferrite films by optical second harmonic generation

Epitaxial strain can induce the formation of morphotropic phase boundary in lead free ferroelectrics like bismuth ferrite, thereby enabling the coexistence of tetragonal and rhombohedral phases in the same film. The relative ratio of these phases is governed by the film thickness and theoretical studies suggest that there exists a monoclinic distortion of both the tetragonal as well as the rhombohedral unit cells due to imposed epitaxial strain. In this work we show that optical second harmonic generation can distinguish the tetragonal-like phase from the rhombohedral-like phase and enable detection of monoclinic distortion in only a pure tetragonal-like phase.

Nanoscale domains in strained epitaxial BiFeO3 thin Films on LaSrAlO4 substrate

BiFeO 3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ∼4.65 Å is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ∼3.99 Å was also detected at film thickness of ∼50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like regions. These findings suggest a complex picture of nanoscale domain patterns in BiFeO3 thin films subjected to large compressive strains.

High-Pressure Effect onPbTiO3: An Investigation by Raman and X-Ray Scattering up to 63 GPa

We report a room-temperature high-pressure x-ray and Raman scattering investigation of lead titanate (PbTiO3) up to 63 GPa. Three continuous phase transitions at 13, 20, and 45 GPa between tetragonal-like phases occur. As a result, no evidence is found for a pressure-induced morphotropic phase boundary. Our study provides experimental evidence that PbTiO3 presents a complex sequence of phases accommodating pressure through mechanisms involving oxygen octahedra tilting and reentrance of ferroelectricity.

Correlation between Cu deficiencies and changes in the structural and electrical properties of Y-Ba-Cu-O films

The experimental results obtained in our previous work included those of the copper composition dependences of structural, dc electrical and microwave properties. From these results, we proposed the inhomogeneous superconductor model that explains the correlation between the dc electrical and the microwave properties. Our recent measurements of the Hall coefficients suggested an additional model to explain the correlation between the structural and the electrical changes. Namely, copper deficiencies are compensated by the oxygen loss due to charge invariance. This oxygen reduction transforms locally the lattice from an orthorhombic phase into tetragonal-like phase. The resultant lattice disorder causes the low mobility and high resistivity.

Effects of annealing on electrical properties of oxygen-deficient YBa 2Cu 3O x

Thermal stability of the physical properties in oxygen-deficient YBa 2Cu 3O x has been investigated. Measurements of dc resistivity showed that metal to semiconductor transition takes place during annealing at temperatures up to 300 °C. Diamagnetic magnetization remarkably decreased with an increase in annealing temperature. This indicates that the volume fraction of superconducting phase diminishes with temperature. The electrical transition accompanies a structural change to tetragonal-like phase which is thought to have the semiconducting nature.

Tetragonal to orthorhombic transformation during mullite formation

The mullite formation process in both single phase and diphasic sol-gel precursors to mullite was studied using dynamic x-ray diffraction (DXRD). A metastable, tetragonal-like mullite phase was observed in all the single gels at temperatures from 980 °C to 1200 °C, but not in any of the other precursors. The tetragonal to orthorhombic mullite transformation was very slow as the lattice parameters, a and b, split and moved gradually away from each other as a result of a gradual decrease of alumina content in the mullite solid solution with increasing temperature from 1100 °C to 1200 °C. The formation of tetragonal mullite coincides with that of the Al–Si spinel. The occurrence of tetragonal mullite or the spinel (or both) is determined mainly by the processing conditions of the sol-gel precursors.