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X-ray scattering and lattice dynamics studies for impurity effects on the para-to-ferroelectric phase transformation behavior in lead titanate and barium titanate

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Title: X-ray scattering and lattice dynamics studies for impurity effects on the para-to-ferroelectric phase transformation behavior in lead titanate and barium titanate
Author(s): Takesue, Naohisa
Doctoral Committee Chair(s): Chen, Haydn
Department / Program: Chemistry, PhysicalPhysics, Condensed MatterEngineering, Materials Science
Discipline: Chemistry, PhysicalPhysics, Condensed MatterEngineering, Materials Science
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Chemistry, Physical Physics, Condensed Matter Engineering, Materials Science
Abstract: Doping of impurities may affect the Curie temperature (T$\sb{\rm c})$ associated with the ferroelectric phase transition in terms of lattice harmonicity and anharmonicity including the creation of the defect energy. Its role in the para-to-ferroelectric transition has been investigated on two prototypical perovskite ferroelectrics, PbTiO$\sb3$ and BaTiO$\sb3.$In-situ x-ray diffuse-scattering technique has been applied to a PbTiO$\sb3$ single crystal containing $\sim$1 at.% of impurities. Tetragonal fluctuations heralding the ferroelectric phase were observed at temperatures considerably above T$\sb{\rm c}$ in the cubic paraelectric phase. The magnitude of this fluctuation continued to increase upon cooling and eventually caused the heterogeneous nucleation of the ferroelectric phase. Local atomic distortions caused by impurities apparently assist the transition.Two BaTiO$\sb3$ single crystals with one containing 0.3 at.% Fe$\sp{3+},$ heterovalently substituted for Ti$\sp{4+}$, have been examined by in-situ x-ray thermal diffuse scattering (TDS) measurement near but above T$\sb{\rm c}$. By monitoring the Bragg peak, T$\sb{\rm c}$s were found to be 129.0$\sp\circ$ and 132.5$\sp\circ$ C on cooling and heating for the pure sample, and correspondingly, 127.2$\sp\circ$ and 128.8$\sp\circ$ C for the doped sample. Results from the pure sample show two-dimensional TDS distribution around the Bragg peaks on the $\{100\}$ relplane; they are attributed to the overdamped (010) $\sb{\rm TO}$ mode and to the anisotropic on-(100) TA modes. The TDS distribution and negative temperature dependence of the intensity near the zone center agree well with neutron inelastic scattering results in the literature.The TDS distribution from the Fe-doped sample shows a similar pattern to that of the pure sample. However, some differences are found at small $\rm \vec q$; it displays positive temperature dependence of the intensity and shows more isotropic distribution. The heterovalent doping is believed to have changed the lattice anharmonicity.The TDS intensity calculations at 200$\sp\circ$, 150$\sp\circ$, and 135$\sp\circ$ C have been carried out using the normal mode analysis based upon the rigid shell model under the harmonic approximation. The second-order TDS intensity is $\sim$10-17% of that of the first order TDS. The calculated results show good agreement with the observed data at $\vert{\vec\rm q}\vert > 0.25$ for the two samples. The anharmonic correction was made to the intensities near the zone center for the (010) $\sb{\rm TO}$ mode using the imaginary part of the response function; the damping coefficient was adjusted to fit the calculated data to the observed data. At the final refinement, the residue factor is $\sim$11.5% for the pure sample, and $\sim$9.3% for the doped sample. The damping constants are almost independent of temperature for the two samples. The damping constants of the pure sample are slightly larger than those of the doped sample. This implies that the heterovalence due to doping attenuates the anharmonicity. This effect can be understood using the hole-relaxation coupling concept.
Issue Date: 1996
Type: Text
Language: English
URI: http://hdl.handle.net/2142/22965
ISBN: 9780591200096
Rights Information: Copyright 1996 Takesue, Naohisa
Date Available in IDEALS: 2011-05-07
Identifier in Online Catalog: AAI9712452
OCLC Identifier: (UMI)AAI9712452
 

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