S. M. Gupta

Multiferroic properties of polycrystalline Bi1−xCaxFeO3

It is shown that Ca2+ doping at Bi-site results in the release of weak ferromagnetism in BiFeO3. Structural transformation from rhombohedral to triclinic is observed with 10% Ca doping. Raman measurements show the presence of oxygen vacancies with Ca doping and no evidence of either intermediate valence or the tetravalence of iron is observed from Mossbauer measurements. No significant change in Neel temperature is observed with Ca doping. The observed weak ferromagnetism and ferroelectric nature at room temperature indicates the multiferroic nature of Bi1−xCaxFeO3 (x=5% and 10%) samples.

Large dielectric permittivity and possible correlation between magnetic and dielectric properties in bulk BaFeO3−δ

We report structural, magnetic, and dielectric properties of oxygen deficient hexagonal BaFeO3−δ. A large dielectric permittivity comparable to that of other semiconducting oxides is observed in BaFeO3−δ. Magnetization measurements indicate magnetic inhomogeneity and the system shows a paramagnetic to antiferromagnetic transition at ∼160 K. Remarkably, the temperature, at which paramagnetic to antiferromagnetic transition occurs, around this temperature, a huge drop in the dissipation factor takes place and resistivity shoots up; this indicates the possible correlation among magnetic and dielectric properties. First principle simulations reveal that some of these behaviors may be explained in terms of many body electron correlation effect in the presence of oxygen vacancy present in BaFeO3−δ indicating its importance in both fundamental science as well as in applications.

Investigation of structural, dielectric and ferroelectric properties of Gd-doped lead magnesium niobate ceramics

The structural, microstructural, local ordering of B-site cations, dielectric and ferroelectric properties have been investigated for Pb1-xGdx(Mg1/3Nb2/3)1−x/4O3; 0 ≤ x ≤ 0.1. Charge imbalance due to Gd3+ ion substitution at Pb2+ is compensated by creating B-site vacancies. The Gd-ion doping at Pb sites in PMN results in the formation of a secondary GdNbO4 phase, which is explained by considering the ionic radii of the Gd ion and its solubility limit at Pb-ion sites. A reaction mechanism for the formation of GdNbO4 has been proposed. Selected area electron diffraction along the 〈110〉 axis reveals enhancement in size of the non-stoichiometric chemical ordered regions leading to a decrease in the size of polar nano regions and their correlations, which manifests in a decrease of Tm, a more diffused max peak and enhancement of the dielectric relaxation strength calculated from fitting of the frequency and temperature dependence of the dielectric behavior. Linear dependence between polarization and electric field also signifies reduction in the correlation between the PNRs resulting in the decrease of the value of Pmax (at ~ 30 kV cm−1).

Structural analysis of lead magnesium niobate using synchrotron powder X‐ray diffraction and the Rietveld method

The room-temperature synchrotron powder X-ray diffraction pattern of the single phase perovskite lead magnesium niobate (PMN) has shown significant broadening in the q range ∼ 5-7 Å(-1) compared with standard LaB6 synchrotron powder X-ray diffraction data, taken under similar conditions. This broadening/asymmetry lies mainly towards the lower 2θ side of the Bragg peaks. Attempts to fit this data with the paraelectric cubic phase (Pm\bar 3m) and the local rhombohedral phase (R3m) corresponding to polar nanoregions (PNRs) are made using the Rietveld method. Rietveld refinements show that neither cubic (Pm\bar 3m) nor rhombohedral (R3m) symmetry can fit this XRD pattern satisfactorily. The two-phase refinement fits the experimental data satisfactorily and suggests that the weight percentage of the PNRs is approximately 12-16% at room temperature. The unit-cell volume of these rhombohedral PNRs is approximately 0.15% larger than that of the unit cell volume of the paraelectric cubic phase.

Study of structural disorder in Pb(Mg1/3Nb2/3)O3

Lead Magnesium Niobate (PMN) is prepared by two step solid state reaction i.e. Columbite route. X-ray diffraction pattern of powder of calcined PMN at 800 °C and powder of PMN pellet sintered at 1200 °C have shown some systematic differences in their relative intensity ratios measured by a 2D detector on Indus-2 synchrotron source. Rietveld method is used to study these differences taking split-atom approach. We have observed that statistical disorder observed in both cases is different. Such multisite disorder is consistent with other Pb based systems like PZN-PT.

Bipolar electrocaloric effect in Pb1−Gd (Mg1+/3Nb2–/3)O3 ceramic: Relaxor with super-dipolar glass state

Abstract The electrocaloric (EC) effect is calculated for Pb1−xGdx(Mg1+x/3Nb2–x/3)O3; x = 0 to 0.1 relaxors from temperature dependent heat capacity and polarization measurements, using a thermodynamic Maxwell equation. Polarization change with temperature reveals an anomalous behaviour around the glass transition temperature for x ≥ 0.05, which is in the vicinity of a crossover from positive to negative EC effect, which is explained evoking the critical slowing-down of the polar nano-domains dynamics to super-dipolar glass state upon cooling. The maximum negative EC coefficient ξmax is observed to decrease from ~0.4 K-mm/kV at 164 K to ~0.1 K-mm/kV with x.

Critical slowing down of polar nano regions ensemble in Gd3+-substituted PbMg1/3Nb2/3O3 ceramics

Investigations on Gd-substituted lead magnesium niobate (Pb1-xGdxMg(1+x)/3Nb(2-x)/3O3; varying x = 0.01–0.1) ceramics have revealed critical slowing down of the polar nano regions (PNRs) ensemble into a “super-dipolar glass state” for higher Gd-substitution x ≥ 0.05. Low temperature electric field induced polarization switching study (P-E) has revealed a sharp decrease in the remanent polarization up to x = 0.03, which strengthen the critical slowing down of polar nano-domains dynamics, suggesting a reduction in the correlation between or within polar nano regions (PNRs) leading to a reduction in its size. Bright field imaging by using transmission electron microscope has also confirmed the reduction of the size of polar nano regions with increasing “x.” Selected area electron diffraction pattern along ⟨110⟩ unit axis has revealed enhancement in intensity of the superlattice reflections spot at ½ ½ ½ along ⟨111⟩ unit axis with increasing “x,” which is associated with the enhancement of chemical ordered re...

Investigation of phase and dielectric properties of lead nickel tantalate ceramics

ABSTRACT Lead nickel tantalate (PNT) ceramic was prepared by solid-state reaction method using 2-step columbite precursor route. Phase analysis of samples sintered at 1473 K shows PNT pyrochlore phase with minute peaks of NiO rich phase. 2-phase Rietveld refinement of x-ray diffraction patterns reveal ∼92.4 (0.33) % pyrochlore phase (cubic structure; space group: Fd-3m; a = 10.5867(4) Å). From temperature dependent dielectric constant and dissipation factor study, strong frequency dependent dielectric anomaly within 140–240 K, related to relaxation rather than structural changes in PNT ceramic is obtained. Observed Arrhenius behavior of dielectric relaxation is believed to be associated with single-ionized oxygen defect hopping.

Cluster glass like behavior of magnetoelectric PbNi1/3Nb2/3O3 ceramics

Dielectric properties of single phase PbNi1/3Nb2/3O3 (PNN) ceramic prepared by two step coulmbite method has been investigates thoroughly. The value of degree of relaxational constant ΔTm ∼24, and diffuseness parameter calculated from the modified Curie-Weiss fitting δA ∼111, suggests higher dielectric relaxation strength for PNN in comparison to well-known PbMg1/3Nb2/3O3 PMN. Frequency dependence of temperature of dielectric maximum (e’m) is fitted with Arrhenius, Dipolar glass, stretched exponential and cluster glass model. The attempt frequency of physically acceptable process has been obtained from cluster glass model, which indicates critical slowing down of PNR dynamics.

Empirical model studies on relaxor behaviour in Bi2.5La1.5Ti3O12 ceramic

Relaxor like dielectric characteristics have been observed for high La-concentration substituted bismuth titanate Bi2.5La1.5Ti3O12 ceramic. Frequency dependent temperature of dielectric maxima has been fitted with models of non-interacting and interacting polar region. This study clearly reveals slowing down of nano-polar dynamics into cluster like glass as proposed by cluster glass model. La-substitution at Bi-site of Bi2O2 layer is responsible for the random field generation.