Songhak Yoon

High-resolution synchrotron x-ray diffraction study of Zr-rich compositions of Pb(ZrxTi1- x)O3 (0.525<x<0.60) : Evidence for the absence of the rhombohedral phase

Results of Rietveld analysis of the synchrotron x-ray diffraction data on Pb(ZrxTi1−x)O3 (PZT) for 0.525⩽x⩽0.60 are presented to show the absence of rhombohedral phase on the Zr-rich side of the morphotropic phase boundary. Our results reveal that the structure of PZT is monoclinic in the Cm space group for 0.525⩽x⩽0.60. The nature of the monoclinic distortion changes from pseudotetragonal for 0.525⩽x⩽0.54 to pseudo-rhombohedral for x>0.54.

Origin of high piezoelectric response of Pb(ZrxTi1−x)O3 at the morphotropic phase boundary: Role of elastic instability

Temperature dependent structural changes in a nearly pure monoclinic phase composition (x=0.525) of Pb(ZrxTi1−x)O3 (PZT) have been investigated using Rietveld analysis of high-resolution synchrotron powder x-ray diffraction data and correlated with changes in the dielectric constant and planar electromechanical coupling coefficient. Our results show that the intrinsic piezoelectric response of the tetragonal phase of PZT is higher than that of the monoclinic phase. It is also shown that the high piezoelectric response of PZT may be linked with an anomalous softening of the elastic modulus (1∕S11E) of the tetragonal compositions closest to the morphotropic phase boundary.

Evidence for monoclinic crystal structure and negative thermal expansion below magnetic transition temperature in Pb(Fe1∕2Nb1∕2)O3

The existing controversy about the room temperature structure of multiferroic Pb(Fe1∕2Nb1∕2)O3 is settled using synchrotron powder x-ray diffraction data. Results of Rietveld refinements in the temperature range of 300–12K reveal that the structure remains monoclinic in the Cm space group down to 12K, but the lattice parameters show anomalies at the magnetic transition temperature (TN) due to spin-lattice coupling. The lattice volume exhibits negative thermal expansion behavior, with α=−4.64×10−6K−1, below TN.

Resolving the characteristics of morphotropic phase boundary in the (1 -x)Pb(Fe1/2Nb1/2)O3-xPbTiO3 system : A combined dielectric and synchrotron x-ray diffraction study

Frequency dependent dielectric studies on the mixed system (1−x)Pb(Fe1/2Nb1/2)O3–xPbTiO3 reveal contributions from the electrodes, grain boundaries, and grains. It is shown that the intrinsic value of the dielectric permittivity coming only from the grains peaks at x=0.08. Analysis of synchrotron x-ray diffraction profiles using the Rietveld technique shows that this peaking is linked with a monoclinic to tetragonal morphotropic phase transition in the multiferroic system.

Confirmation of the monoclinic Cc space group for the ground state phase of Pb(Zr0.525Ti0.475)O3: A combined synchrotron X-ray and neutron powder diffraction study

The low temperature antiferrodistortive phase transition in a pseudo-tetragonal composition of Pb(ZrxTi1-x)O3 (PZT) with x = 0.525 is investigated through a combined synchrotron x-ray and neutron powder diffraction study. It is shown that the superlattice peaks cannot be correctly accounted for in the Rietveld refinement using R3c or R3c + Cm structural models, whereas the Cc space group gives excellent fits to the superlattice peaks as well as to the perovskite peaks. This settles at rest the existing controversies about the structure of the ground state phase of PZT in the morphotropic phase boundary region.

A study of phase coexistence and temperature dependent monoclinic to tetragonal phase transition in the multiferroic (1−x)Pb(Fe1/2Nb1/2)O3−xPbTiO3 (x=0.08)

The origin of coexistence of tetragonal and monoclinic phases in the morphotropic phase boundary (MPB) region of (1−x)Pb(Fe1/2Nb1/2)O3−xPbTiO3 has been investigated for x=0.08 by synchrotron x-ray powder diffraction in the 10–550 K range. It is shown that the phase coexistence observed at room temperature in the MPB region is essentially due to a first order phase transition between the low temperature monoclinic and high temperature tetragonal phases. This transition is shown to be accompanied with a phase coexistence over a very wide temperature range (ΔT∼200 K) across the room temperature.

Synthesis of Monodispersed Barium Titanate Nanopowders by Alkoxide-Hydroxide Sol-Precipitation Method

Barium titanate nanoparticles were synthesized under N2 atmosphere by the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide. The synthesized particles were aggregates of nanosized primary particles. The primary particles of about 20 ?50 ㎚ in diameter became building blocks of larger secondary particles, which are in most cases spherical in shape. The size and morphological evolution of secondary particles are strongly related to the precursor concentration. The observations suggest that formation and control of secondary particles is an essential step in the alkoxidehydroxide sol-precipitation process to obtain monodispersed barium titanate nanopowders.