Satendra Pal Singh

Crystallographic phases, phase transitions, and barrier layer formation in (1 - x) [Pb(Fe1/2Nb1/2)O3]-xPbTiO3

Powders of (1 - x) Pb(Fe 1 / 2 Nb 1 / 2 )O 3 -xPbTiO 3 (PFN-PT) with x = 0.00, 0.10, 0.13, 0.15, 0.20, and 0.25 were prepared by the conventional solid-state route. Structure of PFN-PT was tetragonal for x ≥ 0.10, which indicates that the morphotropic phase boundary (MPB) may be between 0 < x < 0.10. The nature of phase transition in PFN-PT changed from diffuse ferroelectric to relaxor ferroelectric to normal ferroelectric on increasing the PT content. The effect of the PT content and sintering temperature on barrier layer formation in the PFN-PT system was studied using complex impedance spectroscopy. With increasing PT content, the possibility of the barrier layer formation decreased while with increasing sintering temperature, the barrier layer formation was promoted.

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.

A succession of relaxor ferroelectric transitions in Ba0.55Sr0.45TiO3

We present here the results of frequency dependent dielectric, polarization, and powder x-ray powder diffraction studies in the 300–100K temperature range for Ba0.55Sr0.45TiO3. The dielectric results indicate a succession of three relaxor ferroelectric transitions accompanying the cubic to tetragonal to orthorhombic to rhombohedral phase transitions confirmed by x-ray diffraction studies.

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.

Evidence for a monoclinic MA to tetragonal morphotropic phase transition in (1−x)[Pb(Fe1/2Nb1/2)O3]–xPbTiO3 ceramics

The results of a powder x-ray diffraction study of the evolution of the crystal structure in pyrochlore-free single-phase powders of (1−x)[Pb(Fe1/2Nb1/2)]O3–xPbTiO3 (PFN–xPT) are presented. Rietveld analysis of the powder diffraction data reveals that the morphotropic phase transition in PFN–xPT corresponds to a change of structure from MA-type monoclinic with Cm space group to tetragonal with P4mm space group at 0.06<x<0.08. It is argued that this monoclinic phase persists in the entire composition range 0.0≤x≤0.06, as evidenced by the presence of anomalous broadening of the 200 profile. The similarity of the morphotropic phase boundary (MPB) in PFN–xPT with that in the Pb(ZrxTi1−x)O3 (PZT) is pointed out.

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.

Barrier Layer Formation and PTCR Effect in (1 − x) [Pb(Fe1/2Nb1/2)O3]-xPbTiO3(x= 0.13) Ceramics

(1 − x)Pb(Fe 1/2 Nb 1/2 )O 3 -xPbTiO 3 (PFN-xPT) ceramics with x = 0.13 sintered at 1200°C show diffuse phase transition and very high dielectric constant at lower frequencies. The high value of dielectric constant at lower frequencies is shown to be due to the barrier layer formation. The resistivity of the PFN-xPT ceramics, obtained by complex impedance analysis, initially decreases with temperature and then shows an upward trend beyond the ferroelectric Curie point reminiscent of BaTiO 3 based thermistors with PTCR effect.