Krishna Nittala

The role of spontaneous polarization in the negative thermal expansion of tetragonal PbTiO3-based compounds

PbTiO(3)-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO(3)-0.3BiFeO(3) and 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3), whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz(Pb/Bi)) is weakened in 0.7PbTiO(3)-0.3BiFeO(3) but well-maintained in 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3). There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.

Phase and texture evolution in solution deposited lead zirconate titanate thin films: Formation and role of the Pt3Pb intermetallic phase

Solution deposition is widely used for the fabrication of lead zirconate titanate (PZT) thin films on platinized silicon substrates. However, phase and texture evolution during the crystallization process is not well understood, particularly due to the difficulty in tracking changes in the thin films in situ during heating. In this work, we characterized phase and texture evolution in situ during heating and crystallization of PZT thin films using high-energy X-ray diffraction. Films were pyrolyzed at either 300 °C or 400 °C and heated at various rates between 0.5 °C/s and ∼150 °C/s. For films that were pyrolyzed at 300 °C, the most rapid heating rates first induced strong intensities from a transient Pt3Pb phase. The Pt3Pb phase inherited the texture of the pre-existing platinum layer. Combined with other observations, the results suggest the conversion of the platinum to the intermetallic phase near the interface due to the interdiffusion of lead. In all experimental variations, the pyrochlore phase was...

In situ x-ray diffraction of solution-derived ferroelectric thin films for quantitative phase and texture evolution measurement

An in situ measurement technique is developed and presented, which utilizes x-rays from a synchrotron source with a two-dimensional detector to measure thin film microstructural and crystallographic evolution during heating. A demonstration experiment is also shown wherein the measured diffraction patterns are used to describe phase and texture evolution during heating and crystallization of solution-derived thin films. The diffraction images are measured sequentially while heating the thin film with an infrared lamp. Data reduction methodologies and representations are also outlined to extract phase and texture information from the diffraction images as a function of time and temperature. These techniques and data reduction methods are demonstrated during crystallization of solution-derived lead zirconate titanate ferroelectric thin films heated at a rate of 30 °C/min and using an acquisition time of 8 s. During heating and crystallization, a PtxPb type phase was not observed. A pyrochlore phase was obse...

Structural evidence for the nonmonotonic trend of TC in tetragonal PbTiO3BiScO3 solid solutions

High-TC piezoelectric (1−x)PbTiO3xBiScO3 shows a nonmonotonic trend of TC in the tetragonal phase with respect to content of BiScO3. To understand this behavior, the structure of (1−x)PbTiO3xBiScO3 solid solutions is studied by means of neutron powder diffraction. The cation displacements of Pb/Bi and Ti/Sc exhibit a coupling property and a different impact by the substitution content of BiScO3. Its nonmonotonic trend of TC is quantitatively related to the calculated spontaneous polarization in the whole tetragonal range. The unique role of Bi-substitution not only contributes to enhance the component of polarization of Pb/Bi but also to increase the TC.