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Dielectric Anomalies and Competing Magnetic Interactions in NiFeO-PMN-PT Nanocomposite Materials 2

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K Kamala Bharathi, Tripta Parida, Hanuma Kumar Dara, Ramesh Kumar Kamadurai, André M. Strydom, Sarathbavan Murugan, and K. Ramamurthi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10099 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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The Journal of Physical Chemistry

Dielectric Anomalies and Competing Magnetic Interactions in NiFe2O4-PMNPT Nanocomposite Materials K. Kamala Bharathi1,*, ± Tripta Parida2, ±, Hanuma Kumar Dara2, K. Ramesh Kumar3, André M. Strydom3, M. Sarathbavan,4 K. Ramamurthi4

1

Department of Physics and Nanotechnology, Research Institute, SRM University, Kattankulathur, Chennai – 603203

2

Advanced Magnetic Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India

3

Highly Correlated Matter Research Group, Physics Department, P. O. Box 524, University of Johannesburg, Auckland Park 2006, South Africa. 4

Crystal Growth and Thin Film Laboratory, Department of Physics and Nanotechnology, Faculty of Engineering and Technology, SRM University, Kattankulathur 603 203, Chennai,India

*Corresponding author: [email protected] ± Equal contribution: K. Kamala Bharathi and Tripta Parida have contributed equally

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract We report on dielectric anomalies, competing magnetic interactions and relaxor ferroelectric behaviors of x PMN-PT - (1-x) NiFe2O4 (x=0.2 and 0.4) nanocomposites. The crystal structure and surface morphology of the pure and composite nanomaterials were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Phase formation and purity of the NiFe2O4, PMN-PT and the composites were confirmed from XRD measurements. Uniformly distributed nanoparticles and nanowires and presence of Ni, Fe, O, Pb, Nb, Mg, Ti, in NiFe2O4 and PMN-PT are confirmed from FESEM and energy dispersive X-ray (EDX) measurements. Structural disordering due to phase transition around 453 K in both composites was examined from temperature variation Raman studies. Magnetization measurements at room temperature (RT) and at 40 K, zero field cooled (ZFC) and field cooled (FC) magnetization measurements for both composites were carried out from 2 K to 350 K. Competing magnetic interaction and spin glass behaviors were confirmed from ZFC and FC measurements. Random distribution of spins related with different particle size in both composites was confirmed from the distribution function associated with magnetocrystalline anisotropy energy barrier. Frequency (1 kHz to 40 MHz) and temperature (RT to 553 K) dependent dielectric measurements reveal the existence of dielectric relaxation behaviour, dielectric anomalies and relaxor ferroelectric nature of both composites. Analysis of temperature dependent dielectric constant with modified Curie-Weiss law indicates the diffusion constant value of 1.8 and 1.5 for x = 0.2 and 0.4 composites respectively, clearly indicating the diffusive nature of the transitions and highly disordered nature of the relaxor ferroelectrics.


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Introduction For the past few years, complex oxides exhibiting more than one ferroic properties have been explored and studied by several research groups.1-5 Oxides exhibiting both ferroelectric/ferroelastic properties and ferromagnetic/antiferromagnetic properties in a single phase are very rare (Ex: BiFeO3).6,7 By combing ferro/antiferro magnetic oxide with ferroelectric/ferroelastic oxide, one can have the magnetoelectric (ME) composite which can have more than one order parameter and certain degree of coupling between them.8,9 By applying magnetic field, electric dipoles can be tuned and the magnetization can be altered by the application of electric field in ME composites.8,9 BiFeO3 is one of the well explored single phase complex

oxides

exhibiting

ferroelectric

and

weak

ferromagnetic

properties.6,7,10

(NiFe2O4)/CoFe2O4–BaTiO3 (NFO/CFO-BTO), (NiFe2O4)/CoFe2O4- PbTiO3 and PbZrTiO3CoFe2O4 are few ME composite materials reported to have strong ME coupling.11-15 In ME composites (Ex: CFO-BTO), magnetostriction property of magnetic phase induces strain with the application of magnetic field and lead to change in electric dipole moments (polarization) in ferroelectric phase. ME single phase and composite materials can be utilized for various applications such as low field sensors, magnetoelectric memory devices, magnetic and ferroelectric energy harvesting etc. 16-18 NiFe2O4 (NFO) and CoFe2O4 (CFO) is used as a magnetic phase in ME composites due to their high magnetoelectric coupling coefficient, High Curie temperature and large saturation magnetization at room temperature (RT).19,20 BaTiO3, PbZrTiO3 and PbTiO3 are used as ferroelectric phases due to their high ferroelectric coefficient and high ferroelectric temperature.11-15 (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PMN-PT) in single crystal form has high electromechanical coupling factor of 0.9 (k33) and d33 > 1500 pC/N.21 Structural and dielectric



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properties of CoFe2O4-PMN-PT composites have been reported by very few authors.22,23 V.L. Mathe et. al have reported the ME effect in layered CoFe2O4-PMN-PT composites and attributed the high coupling coefficient to the high resistivity and enhanced elastic coupling.22 Anju Ahlawat et. al have reported the spin phonon coupling in NiFe2O4-PMN-PT composites.24 Even though the structural and spin phonon coupling properties of NiFe2O4-PMN-PT composite materials is reported in literature, detailed structural, temperature and frequency dependent dielectric properties and magnetic studies regrading spin glass behaviour assorted with long-range ferromagnetic ordering in NiFe2O4-PMN-PT composites is not reported to our knowledge. In the present case, we have synthesized nanocomposites of x PMN-PT - (1-x) NiFe2O4 (x=0.2 and 0.4) and investigated their temperature dependent structural properties, competing magnetic interactions, relaxor ferroelectric behavior and dielectric anomalies at different frequencies. Obtained dielectric, structural and competing magnetic interaction results are presented and discussed in this manuscript.

Experimental details NiFe2O4 and (0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 nanoparticles were synthesized by solgel and hydrothermal method respectively. Nickel ferrite (NFO) nanocrystalline particles were produced by a sol–gel combustion method. Ni (NO3)2.6H2O, Fe (NO3)3.9H2O, citric acid and urea were employed as starting materials to synthesize NiFe2O4 nanocrystalline particles. All the starting materials were mixed in stoichiometric ratio with continuous rigorous stirring at room temperature (RT) and at 363 K to form a gel, which was then heat treated up to 473 K. The as synthesized material is seen to be amorphous in nature and pure nanocrystalline NiFe2O4 particles were obtained by annealing the powders at 600 ̊C for 3 h. PMN-PT nanowires are synthesized

by

hydrothermal

method.

Stoichiometric


quantity

of

magnesium

2,4-

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pentanedionatedihydrate (Mg(O2C5H7)2.2H2O), lead acetate trihydrate (Pb-(O2C2H3)2.3H2O) and 1,1,1-tris(hydroxymethyl) ethane (THOME) were dissolved in a solution of poly(ethylene glycol)-200 (PEG-200) and methanol (2 : 1 as a volume ratio). In a separate PEG-200/MeOH solution, stoichiometric ratio of titanium di-isopropoxide bis-acetyl acetonate (TIAA) (Ti(O2C5H7)2(OCH(CH3)2)), Niobium ethoxide (Nb(OCH2CH3)5), and THOME were added and well mixed. Both mixtures were rigorously stirred in a beaker employing a magnetic stirrer for 2 h separately at ambient condition. After that, both solutions were added together and stirred for another four hours to obtain homogeneous pale yellow colour solution. Final solution was allowed to dry to eliminate all the organic volatiles employing a rotating evaporator. Obtained viscous solution was mixed with potassium hydroxide (KOH) mineralizer and poured into Teflon autoclave for the hydrothermal reaction. Hydrothermal reaction was carried out at 200 °C for 6 hours. After the reaction, obtained powder was washed with ethanol and DI water (deionized water) several times to eliminate any impurities present and then dried in a vacuum oven at 80 °C for 12 hours. xNiFe2O4-(1-x) PMN-PT (x=0.2 and 0.4) composite powders were prepared by taking the NiFe2O4 and PMN-PT nanomaterials in the appropriate molar ratio and milled in an agate mortar for 2 hours. Well mixed powders were pressed into small pellets and heat treated at 600ºC for 2 h in ambient condition to obtain the final nanocomposite powders. The structural properties of NiFe2O4, PMN-PT powders and x PMN-PT - (1-x) NiFe2O4 (x=0.2 and 0.4) composites were characterized by X-ray diffraction (XRD) using an XRD diffractometer (Rigaku, D/MAX-IIIC X-ray diffractometer, Tokyo, Japan) with Cu Kα radiation (λ = 1.54 Å). The particle size, nanowire shapes and their elemental compositions were examined using a field emission (FE-SEM) scanning electron microscope (Philips XL30 FEG, Eindhoven, Netherland) with energy dispersive X-ray (EDX) analysis. The Raman spectra of



NiFe2O4, PMN-PT powders and x PMN-PT - (1-x) NiFe2O4 (x=0.2 and 0.4) composite powders were measured using a HR 800 Raman spectrophotometer (Jobin Yvon- Horiba, France) employing He-Ne monochromatic LASER (632.8 nm). The magnetization measurements at various temperatures and zero field cooled – field cooled (ZFC-FC) measurements between 2 K to 350 K were measured employing the commercial Quantum Design SQUID magnetometer.

Results and Discussion XRD patterns of NiFe2O4 and PMN-PT nanomaterials is shown in figure 1 (a) and (b) respectively. NiFe2O4 and PMN-PT are seen to crystalize in inverse spinel structure with the space group Fd-3m (JCPDS-ICDD Card # 22-1086) and perovskite tetragonal structure with space group P4mm (JCPDS no. 010-075-8020) respectively. The lattice constants of NiFe2O4 nanoparticle is calculated to be a = b = c = 8.329 Å. Lattice parameter of PMN-PT nanowires is calculated to be a = b = 3.989 Å and c = 4.041 Å, agreeing well with the reported value.19-21 Diffraction peaks are seen to split into two (c/a = 1.012), confirming the characteristic of tetragonal nature of the PMN-PT structure. XRD pattern of x PMN-PT - (1-x) NiFe2O4 (x=0.2 and 0.4) composites are shown in figure 1 (c), clearly indicating the presence of both NiFe2O4 and PMN-PT phases. With increasing PMN-PT composition, the XRD peak intensity of tetragonal perovskite phase is seen to increase. Figure 1 (d) to (g) shows the high resolution TEM/SEM images of pure NiFe2O4, PMNPT and x=0.2 and 0.4 composites captured at RT. TEM image of pure NiFe2O4 is seen to be spherical particles (average particle size of 10 nm) which are homogeneously distributed. SEM image of PMN-PT nanowires shows large amount of arbitrarily oriented nanowires with length


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of 10 nm and diameter of around 100 nm respectively. Morphology of x=0.2 and 0.4 composites (Fig 1 (f) and (g)) is seen to be a collection of nanoparticles with average size of around 100 nm. The EDX measurements for pure NiFe2O4 and PMN-PT nanomaterials were recorded at RT and shown in figure 2 (a) and (b) respectively. A Few peaks without indexing correspond to the K and L lines of the respective elements. The representative peaks of Ni, Fe and O are obvious in the EDX spectra. Fig 2 (b) shows the EDX spectra of PMN-PT nanowires, clearly indicating the presence of Pb, Nb, Mg, Ti, and O. Raman spectra of x = 0.2 and 0.4 composites were recorded from 290 K to 573 K and are shown in figure 3 (a) and (b) respectively along with the magnified spectra (fig 3 (c), (d) and (e)). Both composite nanomaterials are seen to exhibit Raman peaks corresponding to NiFe2O4 and PMN-PT phases (♦ and # represents NiFe2O4 and PMN-PT phase respectively). Peak position of few modes are seen to be little shifted and overlapped in both composites due to the presence of two phases (NiFe2O4 and PMN-PT) in the composites. NiFe2O4 nanoparticles exhibit five Raman active modes such as A1g, Eg and 3F2g.25,26 In the inverse spinel phase (space group Fd-3m) of NiFe2O4, oxygen atoms occupy the general positions 32e and the octahedral A (8a) and tetrahedral B (16d) are occupied by metal ions (Ni and Fe) coordinated by oxygen ions.25,26 A1g peak around 700 cm-1 in the Raman spectra arises because of stretching of oxygen atoms alongside Fe-O bond in the tetrahedral site. 25,26 Symmetric bending and asymmetric breathing of oxygen atoms with respect to Fe atoms at A site gives rise to the Eg mode at 290 cm-1 and the peak at 571 cm-1 respectively. 25,26 Fe-O bond symmetric stretching vibration at B site causes the F2g Raman mode at 478 cm-1. 25,26 Recently it has been reported from the lattice dynamic studies that there exist a small disorder in the NiFe2O4, with the local formation of small amount of P4122/P4322 symmetry.27 Macroscopically NiFe2O4 exhibits inverse spinel symmetry with the



space group Fd-3m and there is a tetragonal P4122/P4322 symmetry locally and as a whole both phases coexist. Raman mode at 634 cm-1 (marked by ⊗ symbol in fig. 3 (e)) is caused by the existence of local P4122 symmetry in addition to the primary cubic inverse spinel phase. Raman spectra of PMN-PT tetragonal structure is mainly due to the longitudinal optical (LO) modes such as A1 and E mode. In the present case, XRD studies clearly confirmed the tetragonal structure of the PMN-PT phase. PMN-PT with tetragonal structure gives the A1, B1 and E modes in Raman spectra and the peaks appear at 99, 273, 570, 746 and 806 cm-1.25,28 The Raman modes at high frequencies (753 and 806 cm-1) are the characteristic peaks of perovskite structure due to the stretching of different size cations at B site ( B’-O-B’’, where B’, B’’ = Ti, Mg and Nb). In the present case, these peaks can be attributed to the Ti-O and Mg/Nb-O bond stretching. Raman mode at 570 cm-1 can be assigned to the Ti-O-Ti modes and the peak at 450 cm-1 can be consigned to the Mg-O-Mg vibrational modes.28 Perovskite relaxor ferroelectric PMN-PT oxide is reported to exhibit structural transition (ferroelectric to paraelectric) at 453 K while heating the sample.29 In the present case, the Raman modes correspond to PMN-PT phase (around 453 K) is seen to broaden and shift confirming the structural disordering at the phase transition. Magnified region of Raman spectra recorded between 453 to 493 K (around the transition temperature) for x = 0.2 and 0.4 compositions (Figure 3 (c) and (d)) clearly shows the rise in disorder in the PMN-PT phase around the structural transition. Raman peak positions for both the compositions (x = 0.2 and 0.4) around the transition is seen to have little shift towards the lower wavenumber due to the structural disordering at the transition temperature. Structural disordering at the transition could be due to the thermal fluctuations in atomic positions and the variation in amount of oxygen vacancies etc.


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Magnetization measurements for pure Ni ferrite, x = 0.2 and 0.4 composites were carried out at RT and at 40 K and are shown in Figure 4 (a) and (b) respectively. In the case of pure NiFe2O4 (Inset of fig 4 (a)), saturation magnetization value is seen to be 48 emu/g at RT, agreeing with the reported value.30 For x = 0.2 and 0.4 composites, saturation magnetization value at RT and 40 K is seen to be 23, 27 emu/g and 19, 22 emu/g respectively. Due to the presence of a nonmagnetic phase (ferroelectric PMN-PT phase) in the composites, saturation magnetization value is seen to be lower than that of pure NiFe2O4. With increasing PMN-PT phase ( x from 0.2 to 0.4), saturation magnetization value is seen to decrease at RT as well as at 40 K. Magnetization for both composites are not observed to saturate even at 2 Tesla, which is due to the uncompensated surface spins and the paramagnetic contributions from the PNM-PT phase. Both composites are exhibiting a higher saturation magnetization value at 40 K than that of at 300 K due to reduction of randomizing thermal vibrations and magnetocrystalline anisotropy. Coercivity for the x = 0.2 and 0.4 composites at RT and 40 K are seen to be 55, 270 Oe and 62, 300 Oe respectively. Coercivity is seen to increase with increasing the amount of nonmagnetic phase (PMN-PT) due to increase in magnetocrystalline anisotropy. Field cooled (FC) and zero field cooled (ZFC) measurements were carried out at 100 Oe from 2 K to 350 K for both the composites and is shown in figure 5 (a) and (b). For both x = 0.2 and 0.4 composites, ZFC magnetization is observed to decrease with decreasing the temperature from 350 to 2 K. For x = 0.2 and 0.4 compositions, ZFC curve is seen to exhibit a hump (maximum) at Tm1~ 170 K and 150 K respectively and magnetization is seen to continuously decrease with decreasing the temperature below the hump. Observation of Tm1 at ZFC curve indicated that there exists an arbitrary freezing of magnetic spin clusters into diverse metastable energy states and short range ferromagnetic clusters in both composites.30-32 In addition to that,



Tm1 indicates the presence of a small range of ferromagnetic spin groups with different particle size and magnetically coupled with long range ferromagnetic phase. 30-32 The FC magnetization curve is seen to continuously increase with decreasing temperature for the x = 0.4 composition. For x = 0.2 composition, FC magnetization is seen to decrease first and have a minimum around 268 K and then start to increase with increasing the temperature, confirming the competing magnetic interactions in the x = 0.2 composition. For x = 0.4 composition, FC is seen to increase (with decreasing temperature) in very slow rate, which generally happens in weakly coupled magnetic systems.29-32 For both the compositions, ZFC and FC curves are not seen to merge (irreversibility) even at 350 K, indicating that the blocking temperature (Tb) is above 350 K. Magnetocrystalline energy barriers associated with anisotropy of the nanoparticles are responsible for such irreversibility between ZFC and FC magnetizations. Magnetic moments in inverse spinel NiFe2O4 structure is arranged in two different sub-lattices namely tetrahedral (A site) and octahedral (B) sites. Large irreversibility between ZFC and FC can also be explained with the help of cluster mean field theory of spin glasses as follows. As the nanoparticles of x = 0.2 and 0.4 samples are cooled from 350 to 2 K at the magnetic field of 100 Oe, the energetic antiferromagnetic nature of inter spin group exchanges are diminished by alignment of the few spin groups along the magnetic field direction which causes the large opening between ZFC and FC magnetization. Dispersion function related with magnetocrystalline anisotropy energy (MAE) barrier for x = 0.2 and 0.4 composites are shown in figure 6. ZFE and FC magnetization (MZFC and MFC) can be related to the MAE by the following relationship: 33,34 =

---{1}


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FC magnetization is related with global contribution to the total observed magnetization. ZFC Magnetization has the numerous contributions such as short range ordered spins from small nanocrystals, remote spin collections (isolated clusters) and minor matrices comprising frustrated magnetic moments in which the anisotropy energy barriers exceed the energy associated with temperature (kBT) at the measuring temperature.33,34 Particle size distribution is also reflected in the MAE distribution function f (T) for both the composites. x = 0.2 and 0.4 composites are seen to exhibit broad peaks in the f (T) function, supporting the maximum Tm1 observed in ZFC curves. Broad nature of the peaks and more than one peak in f (T) indicates the random distribution of spins related with magnetic nanoparticles as well as the existence of different size particles in both the composites. In addition to that, the broad nature of the peaks indicates the existence of a range of thermal energies associated with spin cluster which can overcome the MAE barriers. The dielectric constant as a function of frequency and temperature was measured for both the composites in the frequency range 1 kHz - 40 MHz and RT to 573 K respectively. Real part of the dielectric constant (ɛ’) in the frequency range 1 kHz - 40 MHz for x=0.2 and 0.4 composition is shown in figure 7 (a) and (b) respectively. ɛ’ value at 553 K for x=0.2 and 0.4 composition is 6.3x103 and 1.3x104 respectively. The value of the dielectric constant is seen to increase with increasing temperature. Predominant contribution from various polarizations such as atomic, ionic, electronic, bulk grains, interfacial and grain boundary contributions to the dielectric polarization causes the large ɛ’ at low frequencies.35 With increasing frequency from 1 kHz to 40 MHz, contributions from ionic and orientation polarization decrease and resulted in decreasing ɛ’, indicating the existence of distribution of dielectric relaxation in both composites. In the NFO-PMN-PT composites, orientation polarization arises from the displacement of



dipoles (in PMN-PT phase) and exchange of electrons between metal ions (Fe3+/Fe2+ in NFO phase) with the application of electric field. Relaxation polarization arises from the inertia to the polarization and charge movement. Charge accumulation at the grain boundaries of nanocomposites give raise to the interfacial polarization in both compositions. At high frequencies, inability of dipoles to respond to the applied ac electric field causes the dielectric constant to decrease and attain a constant value. Dielectric loss (tanδ) as a function of frequency for both the composites is shown in figures 7 (c) and (d) respectively. Dielectric loss for both the composites is seen to continuously decrease with increasing frequency and the resonance peak is not observed up to 40 MHz. Dielectric loss appears in the nanocomposites due to the lagging of polarization with the applied electric field. Tanδ is seen to be low for both the samples, indicating the perfect homogenous nature of the samples. Temperature dependent dielectric constant for x = 0.2 and 0.4 composition is measured from 300 K to 573 K at various frequencies and is shown in figure 8 (a) and (b). Dielectric constant is seen to increase with increasing temperature. For x = 0.2 composition, small but broad peak is observed at the temperature between 450 to 550 K due to the relaxor ferroelectric nature of the PMN-PT. For x = 0.4 composition, peak position is seen to shift to high temperature. Any local crystal defects such as presence of oxygen vacancies and amount of ferroelectric phase can modify (shift) or influence the peak position.36 Peak position is seen to shift to high temperature with increasing frequency. Around 560 K, the dielectric constant is seen to increase rapidly. Due to our limitation in increasing temperature beyond 573 K, a peak could not be established unambiguously at this temperature. A dielectric anomaly at low temperature in PMN-PT is reported due to the ferroelectric to paraelectric transition. Anomaly at high temperature is generally attributed to the rhombohedral to tetragonal phase transition. Anomaly


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at low temperatures is seen to disappear for x = 0.4 composition and the sharp transition around 570 K is clearly observed.

Shifting of peak position with increasing frequency is clearly

indicating the relaxor ferroelectric nature of the samples. For relaxor ferroelectrics, the dielectric dispersion obeys the modified Curie-Weiss law:29 1/ɛ - 1/ɛm = (T-Tm)γ/C----{2} Where ɛm,Tm, γ and C are maximum dielectric constant value, transition temperature, diffusion coefficient having value from 1 to 2 and Curie constant respectively. γ has the value of 1 for a perfect ferroelectric and 2 for a perfect relaxor ferroelectric.29 In order to confirm the nature of these composites, the plot of ln[(ɛm/ ɛr-1)] vs ln[T-Tm] at 1 kHz is carried out and shown in figure 9. From figure 9, it is clear that the relationship between ln [(ɛm/ ɛr-1)] vs ln [T-Tm] is linear and the slope of the linear curve gives the value of γ. For x = 0.2 and 0.4 composition, the γ value is calculated to be 1.8 and 1.5 respectively, clearly indicating the diffusive nature of the transitions and highly disordered nature of the relaxor ferroelectrics. Disorder in these composites could be raised from oxygen deficiency and displacement of ions at various sites. In addition to that, the local structural disorder was observed from the Raman peak corresponding to tetragonal P4122 symmetry.

Conclusions Nanoparticles and nanowires of NiFe2O4 and PMN-PT were prepared by sol-gel and hydrothermal method respectively. NiFe2O4 nanoparticles and PMN-PT nanowires are seen to crystalize in inverse spinel and perovskite tetragonal structures respectively. XRD analysis of composite nanomaterials indicates the presence of both phases and increasing intensity of PMNPT phase with increasing its concentration. Temperature variation Raman studies clearly



indicates the structural disordering around 453 K due to PMN-PT phase transition. With increasing non-magnetic PMN-PT phase in the composite, saturation magnetization at RT and at 40 K is seen to decrease. ZFC and FC magnetization measurements show the existence of spin glass behavior and competing magnetic interactions in both composites. Dielectric anomalies and relaxor ferroelectric behavior in both the composites are analyzed using temperature dependent dielectric studies. Analysis of temperature dependent dielectric constant with modified CurieWeiss law indicates the diffusive nature of the phase transitions and highly disordered nature of the relaxor ferroelectrics.

Acknowledgement AMS acknowledges funding support from the SA-NRF (93549) and the Science Faculty and University Research Committees of the University of Johannesburg. KRK acknowledges the FRC/URC Postdoctoral fellowship.


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