Dhananjai Pandey

Evidence for M B and M C phases in the morphotropic phase boundary region of ( 1 − x ) [ Pb ( Mg 1 / 3 Nb 2 / 3 ) O 3 ] − x PbTiO 3 : A Rietveld study

We present here the results of the room-temperature dielectric measurements and Rietveld analysis of the powder x-ray diffraction data on (1-x)[Pb(Mg 1 / 3 Nb 2 / 3 )O 3 ]-xPbTiO 3 (PMN-xPT) in the composition range 0.20≤x≤0.45 to show that the morphotropic phase boundary region contains two monoclinic phases with space groups Cm (or M B type) and Pm (or M C type) stable in the composition ranges 0.27≤x≤0.30 and 0.31≤x≤0.34, respectively. The structure of PMN-xPT in the composition ranges 0≤x≤0.26 and 0.35≤x ≤1 is found to be rhombohedral (R3m) and tetragonal (P4mm), respectively. These results are compared with the predictions of Vanderbilt & Cohens theory.

Direct evidence for multiferroic magnetoelectric coupling in 0.9BiFeO3-0.1BaTiO3.

Magnetic, dielectric and calorimetric studies on 0.9BiFeO3-0.1BaTiO3 indicate strong magnetoelectric coupling. XRD studies reveal a very remarkable change in the rhombohedral distortion angle and a significant shift in the atomic positions at the magnetic Tc due to an isostructural phase transition. The calculated polarization using Rietveld refined atomic positions scales linearly with magnetization. Our results provide the first unambiguous, atomic level evidence for magnetoelectric coupling of intrinsic multiferroic origin in a BiFeO3-based system.

Dielectric studies of phase transitions in (Pb1−xBax)ZrO3

Phase transitions in semiwet derived (Pb1−xBax)ZrO3 ceramics for the composition range 0⩽x⩽0.30 have been investigated by dielectric measurements at various frequencies during heating and cooling cycles. The paraelectric (PE) to ferroelectric (FE) to antiferroelectric (AFE) sequence of phase transitions is observed for 0⩽x<0.20. On increasing the Ba2+ content from x=0 to x=0.05, the thermal hysteresis associated with the AFE–FE phase transition increases from 11 to 100 °C. This is attributed to the increase in the piezoelectric coupling between the strain and polarization. For x=0.20, the FE phase does not transform into the AFE phase on cooling. Pronounced deviations from the regular FE behavior are observed on increasing the Ba2+ content to x=0.25. For x=0.30, the temperatures corresponding to the peak values of the real and imaginary parts of the dielectric constant become frequency dependent indicating relaxor FE behavior. It is shown that the polar clusters present in the PE phase undergo Vogel–Fulch...

Room temperature structure of Pb(ZrxTi1−xO3) around the morphotropic phase boundary region: A Rietveld study

We have carried out a detailed Rietveld analysis of x-ray powder diffraction data of Pb(ZrxTi1−x)O3 (PZT) compositions across the morphotropic phase boundary (MPB) region (x=0.515, 0.520, 0.525, 0.530). It is shown that the structure of PZT is pure tetragonal for x⩽0.515 with space group P4mm. In the MPB region, 0.515<x<0.530, the tetragonal and monoclinic (space group: Cm) phases are found to coexist as a result of a first order phase transition between the low temperature monoclinic and high temperature tetragonal phases. Further, arguments are advanced to show that the hitherto believed rhombohedral structure (FRHT) of PZT for 0.530⩽x⩽0.62 is more likely to be monoclinic.

Stability of ferroic phases in the highly piezoelectric Pb(ZrxTi1−x)O3 ceramics

The morphotropic phase boundary in the phase diagram of the technologically important Pb(ZrxTi1-x)O3 (PZT) ceramics has been traditionally believed to separate ferroelectric tetragonal and rhombohedral phase regions. This old picture has come under close scrutiny during the last eight years following the discovery of new monoclinic phases in the Cm and Cc space groups. This article presents a brief overview of these discoveries in which the use of multiple diffraction probes (X-ray, electron, neutron diffraction) in conjunction with physical property measurements has played a crucial role. A new phase diagram of PZT showing the stability fields of these structures below room temperature is also presented.

Diffuse ferroelectric transition and relaxational dipolar freezing in (Ba,Sr)TiO3

The effect of Sr2+ substitution on the structure and dielectric behaviour of BaTiO3 is reported. It is shown that the smearing of the variation of dielectric constant with temperature in such samples is not due to compositional inhomogeneities. Observation of relaxational dipolar freezing below the hump temperature indicates the similarity of diffuse ferroelectric transition and orientational glass behaviour.

Effect of phase coexistence at morphotropic phase boundary on the properties of Pb(ZrxTi1−x)O3 ceramics

The temperature variation of planar electromechanical coupling coefficient (kp) in Pb(ZrxTi1−x)O3(PZT) ceramics near the morphotropic phase boundary (MPB) compositions has been correlated with structural changes to show that the maximum electromechanical response at a given composition (x) is a function of temperature (T) and occurs in the tetragonal phase field just outside the two‐phase MPB region in the (x,T) plane. This clearly disproves the hypothesis that the coexistence of rhombohedral and tetragonal phases at MPB is responsible for the enhancement of the electromechanical response. In fact, coexistence of the rhombohedral phase with the tetragonal phase is shown to lead to the lowering of the electromechanical response.

Morphotropic phase boundary in (1−x)BiFeO3–xPbTiO3: phase coexistence region and unusually large tetragonality

It is shown that the tetragonal (T) and rhombohedral (R) structures of (1−x)BiFeO3–xPbTiO3 are stable for xT⩾0.31 and xR⩽0.27, respectively, giving the narrowest width of 0.03, reported so far, for the morphotropic phase boundary region in this system. The Rietveld refined structure of the T phase for x=0.31 reveals partial covalent character of the Ti∕Fe–O and Pb∕Bi–O bonds which may be responsible for the unusually large tetragonality.

Irreversibility of the antiferroelectric to ferroelectric phase transition in (Pb0.90Ba0.10)ZrO3 ceramics

Dielectric and x-ray diffraction evidences are presented to show that the antiferroelectric to ferroelectric phase transition in (Pb0.90Ba0.10)ZrO3 ceramics is not reversible during the cooling cycle. It is shown that the stable antiferroelectric phase recovers from the metastable ferroelectric matrix on aging at room temperature. The kinetics of recovery of the antiferroelectric phase is very sluggish. It is pointed out that this type of irreversibility occurs in field induced antiferroelectric to ferroelectric transitions also. This may have serious implications for the actuator applications of these materials. It is proposed that the large transformation strains associated with the antiferroelectric orthorhombic to ferroelectric rhombohedral phase transition is responsible for this irreversibility.

Stability of the various crystallographic phases of the multiferroic (1-x)BiFeO3-xPbTiO3 system as a function of composition and temperature

In this report, we have studied the stability of various crystallographic phases of (1−x)BiFeO3–xPbTiO3 as a function of composition and temperature. The structure of BF-xPT is reconfirmed to be tetragonal and monoclinic in the P4mm and Cc space groups for x>0.31 and 0.10≤x≤0.27, whereas the two phases coexist in the morphotropic phase boundary (MPB) region 0.27<x<0.31. A recent proposition of an orthorhombic phase in the MPB region has been ruled out comprehensively. From the high temperature powder x-ray diffraction (XRD) data it is shown that the MPB in this system is tilted towards the tetragonal side, a unique feature among the family of PbTiO3 based MPB systems. We have established accurately the room temperature phase diagram for this solid solution. By comparing the observed bond lengths between oxygen and other cations, obtained from Rietveld analysis of the room temperature powder XRD data with expected ionic bond lengths, we have shown that the very high c/a ratio in the tetragonal phase of thi...