Structural, Electrical, Optical and Magnetic properties of Y1-xBixCrO3and Nd1-xBixCrO3 (x=0 to 0.15) Ceramics

Mannepalli, Venkateswara Rao and Ramadurai, Ranjit (2017) Structural, Electrical, Optical and Magnetic properties of Y1-xBixCrO3and Nd1-xBixCrO3 (x=0 to 0.15) Ceramics. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

Multiferroics exhibit the unusual combination of ferro/Antiferro electricity and ferro/Antiferro magnetism in the single phase material. Multiferroics are known to be potential candidate for various applications such as memory, data storage , sensors etc. Rare earth orthochromites (RCrO 3 ) are interesting group of compounds that were studied in search of multiferroic features in recent years. Some of them possess a structural heterogeneity at local scale which is crucial for the observed macroscopic electrical properties such as dielectric and ferroelectric hysteresis behavior. The Neutron diffraction and phonon studies on RCrO 3 confirm the structural distortions whi ch lead to non - centrosymmetry at local scale irrespective of possessing a global /average orthorhombic ( i.e. centrosymmetric) structure. The pyroelectric measurements on RCrO 3 ceramics confirm structure - electrical property correlation at magnetic ordering temperature. The essential point to be noted is that such local non - centrosymmetry or the structural heterogeneity in chromites is due to presence of magnetic rare earth and its interaction with Cr 3+ ion. Thus the nonmagnetic rare earth in RCrO 3 leads to non - ferroelectric conclusion i.e. local structural distortions are absent in such chromites as observed from pyroelectric measurements. However the neutron and phonon studies on YCrO 3 (Y - nonmagnetic) exhibits the structural distortions a s well as hysterisis loops indicative of existence of ferroelectricity. It is worth to remember that RCrO 3 samples are associated with high dc - leakage currents which inhibit the polarization hysteresis loops irrespective of magnetic or nonmagnetic nature o f R - ion. Hence, the structural studies give more insight on inherent structural distortions rather than electrical studies on RCrO 3 samples , where electrical measurements mislead the existence of possible ferroelectricity. Several structural studies are ca rried out such as Neutron diffraction; Transmission electron microscopy (TEM) and Phonon scattering to understand local structural distortions in RCrO 3 samples. Neutron studies confirm the existence of global Pnma orthorhombic (centrosymmetric) and local P2 1 monoclinc (non - centrosymmetric) structure at different temperatures . Phonon studies revealed the presence of spin - phonon coupling (R - ion is magnetic) and magnetostriction (R - ion is non - magnetic) in RCrO 3 samples. The Pyroelectric and phonon studies at magnetic ordering temperature indicate that nonmagnetic R - ion is unlikely to exhibit polar behavior. However, the local non centrosymmetry is observed from neutron studies on nonmagnetic Y - ion in YCrO 3 (YCO) raises a question that whether such local structu ral changes are really associated with magnetic nature of R - ion? In literature, there are no extensive studies related to phonon studies on nonmagnetic rare earth RCrO 3 which can rule out possible polar nature in these samples. Hence, the detailed investigations on phonon studies are carried out in nonmagnetic rare earth RCrO 3 sample i.e.YCrO 3 (YCO). vii YCO possess a global orthorhombic structure with Pnma space group at RT observed from neutron and x - ray diffraction measurements. YCO ex hibits the antiferromagnetic phase transition (T N ~140K), as well as ferroelectric curie transition temperature (T c ) around 473K, reveals the presence of multiferroic nature. The weak ferroelectric nature of YCO understood from the dielectric and hysteresis behavior at RT and the magnetic transition in YCO is due to the presence of Cr 3+ ion. High temperature x - ray diffraction studies reveal that the global Pnma orthorhombic structure throughout the measured temperature range. Thus diffraction studies did not reveal global structural changes with temperature but involve minor changes in lattice parameters and volume. In order to understand the local structural changes in YCO , phonon studies were performed across the dielectric p hase transition temperature ( i.e. 303 - 573K). From phonon studies , the Raman active B 3g (3) ( out - of - phase O2 scissor like) mode (SM) showed anomalous behavior with temperature. The phonon mode B 3g (3) softens across the proposed ferroelectric to paraelectric transition temperature and hardens while at low temperatures . The experime ntal results on disappearance of B 3g (3) mode with temperature reveal that it is not merely from the volume expansion effect but du e to inherent order – disorder process within YCO i.e. possibl e local structural distortions. To get into deeper insights of microscopic origin, Density functional theory (DFT) calculations were performed. The combined experimental and DFT studies reveal tha t disappearance of Raman active B 3g (3) mode above the dielectric transition temperature is due to the transformation of B 3g (3) (out of phase O2 scissors like motion) into Raman inactive mode of B 1u symmetry . The change in B 3g (3) symmetry is associated w ith the local structural distortions such as Y - displacement along with octahedra distortions in the unit cell of YCO . Our calculations also suggest that the isolated and anomalous behavior of this B 3g (3) mode at lower temperatures could be due to the conv ersion of scissor mode to anti - stretching mode at extreme Y cation displacement. Further to confirm the Y - displacement effect , we replaced the Y - site with Bismuth and compared the composition dependent phonon studies with temperature dependent phonon studi es. The observed intensity behavior of B 3g (3) indicates that replacement of Y - site by a small amount of Bismuth exhibit the similar effect of temperature dependent YCO. The composition dependent Raman shift of B 3g (3) from 486 (x=0) to 479 cm - 1 (x=0.15) is comparable to the 486 (303K) to 479 cm - 1 ( 523K ) in temperature dependent YCO. These inherent distortions are seen as a change in morphology with the composition which is direct experimental confirmation of the local structural distortions pres ent in YCO and is indeed due to Y - site distortions/displacements observed from phonon studies . Hence, the out of phase scissor like B 3g (3) mode seems to play a crucial role in the polar nature of YCO ceramics. Magnetic and optical properties were studied to understand the influence of structural distortions on magnetic rare earth chromite, Nd 1 - x Bi x CrO 3 ( NBiCO ) as well as nonmagnetic rare earth chromite, Y 1 - viii x Bi x CrO 3 (YBiCO) ceramics. The diffraction studies on NBiCO and YBiCO reveals an averaged orthorhombic Pnma structure. The solubility limit of Bi is observed to be 15at% and 10at% in YBiCO and NBiCO respectively , when synthesized through sol - gel method . The phonon studies show that local structural distortions are getting stronger in NBiCO samp les whereas the distortions are suppressed in YBiCO samples. T he out o f phase O2 scissor - like mode (B 3g (3)) is shifting towards the blue frequency regime (hardening of this mode) which indicates the structural distortions are getting stronger in NBiCO . Fro m the observations it is inferred that, that the structural distortions are associated with B 3g (3) phonon mode irrespective of magnetic or nonmagnetic nature of A - site in orthochromites. And t he major changes involved in phonon modes of NBiCO suggests that the collective behaviour of CrO 6 rotation al and Nd - displacement modes create the structural distortions . On the other hand, the phonon studies on YBiCO compounds suppress the possible structural distortions in YCO by Bismuth substitution esp ecially the phonon mode B 3g (3) (out of phase scissor mode which is responsible for structural distortions) with the composition. This evidently shows that Bi subs titut ion exhibits an enhanced structural distortion in case of NBiCO and a suppressed distort ion in case of YBiCO. The influence of structural distortions on magnetic properties of NBiCO and YBiCO compounds has also been studied where; Chromites with Cr 3+ configuration, only π bonds of t 3 - o - t 3 are present and eventually lead t o an antiferromagnetic ordering. It’s worth remembering that the structural distortions due to the Bi substitution in NCO lattice was evident from the raman studies such as change in phonon behavior related to CrO 6 rotations as well as Nd - O modes . In add ition, p resence of such distortion (decrease the Cr - O - Cr bond angle and consequently two facing oxygens are now close enough to each other) facilitates hybridization of t - e orbitals of Cr 3+ which provides the path for transfer of electrons from filled orbi tal ( t 3 ) to empty orbital ( e 0 ) i.e. t 3 - O - e 0 , effectively leads to observed weak ferromagnetic coupling. Hence , with Bi substitution which is similar ionic radii, but with a 6s 2 lone pair configuration plausibly creates the local structural distortions due to rotations of CrO 6 further leads to t - e hybridization which in turn changes the values of T N in our chromites. The observed increase of 40 Oe to 830Oe above spin reorientatio n temperature ( T SR ) and 184 Oe(x=0) to 445 Oe(x=0.1) below T SR gives a clear indication that Bi 3+ can influence greater in Cr 3+ - Cr 3+ interaction than that of Nd 3+ - Cr 3+ interaction . In case of YBiCO the structural distortions are suppressed as is evident from the phonon studies and from increase of Cr - O - Cr bond angles towards 180 º . This further will inhibit the virtual charge transfer of electrons across t 3 - O - e 0 in YBiCO samples and results the observed decrease of coercive and remanent magnetic values. Th e decrease in band gap value s from 3.1eV to 2.6 eV in case of N Bi CO and from 3.3 to 2.9 eV in case of YBiCO was observed with increase of Bi content . The variation of optical absorption edge evidently suggests that Bi ( 6s 2 ) substituted in the magnetic rare earth Nd +3 and nonmagnetic Y 3+ influences the Cr - O overlap integral. ix The electrical studies such as dielectric studies are performed on Y 1 - x Bi x CrO 3 (YBiCO) samples. The dielectric studies reveals that YCrO 3 (YCO) possess single dielectric anomaly and YBi CO samples exhibiting two dielectric anomalies. The observed dielectric anomalie s in YBiCO samples are analyzed through structural as well as conductive mechanisms. YCO shows local structural distortions across the dielectric transition whereas higher losses in the sample might mask the dipolar/structural transitions. Thus the inherent combined effect of dipolar and conductivity in YCO sample mislead the existence of ferroelectricity. However, the conductivity due to grain and grain boundary contributio ns and hopping of localized electron transitions might lead to the existence of Maxwell Wagner effect and eventually concludes the non - ferroelectric nature in YCO. In YBiCO, t he observed two dielectric anomalies are not related to the structural phase tran sitions which are confirmed from high temperature diffraction as well as phonon studies. Hence, the observed dielectric anomalies in YBiCO might be due to the Maxwell Wagner relaxations and/or due to the hopping mechanism. The loss behavior at different fr equencies and temperatures reveal that dielectric relaxations are of different conductive origin . The impedance measurements reveal that the grain and grain boundary contributions are very prominent in YBiCO samples . The huge difference of total resistance compared with the grain resistance in YBiCO samples reveals the electrical heterogeneity across the grain and grain boundary might cause low temper ature dielectric relaxation. Moreover, the frequency dependent as well as temperature depende nt conductivity studies confirm the observed low temperature dielectric relaxation is due to hopping of charge carriers (Cr 3+ /Cr 4+ ) across the grain and grain boundary interface in YBiCO samples . In YBiCO, t he high temperature dielectric anomaly understood from the frequ ency and temperature dependent ac conductivity measurements . The conductivity studies reveal that polaron hopping is the main conductive mechanism that is responsible for high temperature dielectric anomaly . dc - conductivity measurements were used to under stand the polaron hopping model in YBiCO samples . The nonlinear variation of the resistivity with temperature reveals that chromites can be used as potential candidates for the negative thermistor applications. The observed activation energies 0.2 - 0.35eV, the thermistor constant (2872.7K and 3102K for YCO and YBiCO - 10 at 755K) and sensitivity values ( - 5x10 - 3 K - 1 and - 5.44x10 - 3 K - 1 for YCO and YBiCO - 10 at 755K) are in the comparable range for the presently employed oxide materials for NTC thermistor applicati ons

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