Sudhanshu K. Deshpande

Structural and electrical properties of layered perovskite type Pr2Ti2O7: experimental and theoretical investigations

In this communication we report the details of the structural and thermal properties of monoclinic layered perovskite type Pr2Ti2O7 (PTO) using ambient to higher temperature XRD and Raman spectroscopic studies. The monoclinic (P21) structure is found to be the stable structure of PTO compared to the orthorhombic Pna21, Cmc21 or Cmcm and monoclinic P21/m structures. The crystal structure is further supported by the ab initio total energy calculations using density functional theory (DFT) formalism. The total energy calculation and structural relationship favour the ferroelectric (P21) to paraelectric (P21/m) displacive transition. The calculated electric polarization as observed from the displacement of ions is ∼8.3 μC cm−2. The calculated electron density of states indicated a band gap of about 2.7 eV, which closely agrees with that measured by UV-Vis diffuse reflectance spectroscopy. Variable temperature XRD and differential thermal analysis studies revealed no structural transition to Cmc21 in the temperature range from ambient to 1473 K as reported for analogous rare-earth titanates, like La2Ti2O7 and Nd2Ti2O7. A partial decomposition of PTO to cubic perovskite type structure is observed at around 1673 K. The measurement of field dependent electric polarization indicates the ferroelectric nature of PTO. The electrical properties of PTO have also been investigated by ac impedance spectroscopic studies from 173 to 1073 K. The low temperature dielectric data indicate two different types of relaxations, one at a lower frequency region and strongly temperature dependent while the other at a higher frequency region (>1 kHz) and nearly temperature independent. The low and high frequency relaxations have been attributed to the thermally activated polarization process arising from the grain boundaries and dipolar orientations, respectively. The activation energy for a thermally activated low frequency relaxation process is 0.38 eV, which is similar to the interfacial polarizations due to ionic movements. An appreciable contribution of ionic conductivity in PTO is observed at still higher temperature (∼700 K). The activation energy for ionic conductivity is about 0.60 eV.

Quest for Lead Free Relaxors in YIn1–xFexO3 (0.0 ≤ x ≤ 1.0) System: Role of Synthesis and Structure

The B-site tailored YIn(1-x)Fe(x)O3 (0.0≤ x≤ 1.0) series was synthesized by glycine-aided gel-combustion technique and subjected to extensive structural and electrical investigations. The temperature had tremendous bearing on the phase evolution exhibited by the system. The entire system crystallized as C-type metastable polymorph in the as-synthesized form. Hexagonal polymorphs of Fe(3+)-rich compositions could be isolated by controlled heat treatment at 750 °C. Raman spectroscopic investigations showed that, while there is a general shrinkage of the lattice due to substitution of a smaller ion at In(3+)-site, there is an apparent dilation of the Y-O bond, and this anomaly reflects in the electrical behavior exhibited by the system. The single-phasic hexagonal nominal compositions, YIn(1-x)Fe(x)O3 (0.0 ≤ x ≤ 0.3), were also studied by impedance spectroscopy. The dielectric constant was found to drastically increase from 10 for YInO3 to 1000 for YIn(0.7)Fe(0.3)O3 at room temperature stressing the role of B-site tailoring on electrical behavior. More interestingly, careful substitution of Fe into YInO3 could tune the electrical behavior from a dielectric to relaxor ferroelectric in the temperature range studied. The nominal composition YIn(0.7)Fe(0.3)O3 showed a classical relaxor ferroelectric like behavior which is an important observation in context of the search for new lead free relaxor materials.

Synthesis and Structural and Electrical Investigations of a Hexagonal Y1–xGdxInO3 (0.0 ≤ x ≤ 1.0) System Obtained via Metastable C-Type Intermediates

Detailed structural and electrical investigations were carried out on an A-site disordered hexagonal Y(1-x)Gd(x)InO3 (0.0 ≤ x ≤ 1.0) series synthesized by a self-assisted gel-combustion route. The phase relations show profound temperature dependence. The metastable C-type modification could be stabilized for all the compositions, which on further heating get converted to stable hexagonal polymorphs. The conversion temperature (C-type to hexagonal) was found to increase with an increase in Y(3+) content. The system was observed to be single-phasic hexagonal at 1250 °C throughout the composition range. Interestingly, the increase in planar bonds of InO5 polyhedra was found to be twice that of the apical bonds on Gd(3+) substitution. Careful Raman spectroscopic studies highlighted a definitive though subtle structural change from x = 0.7 onward. The same observation is also corroborated by the dielectric studies. Electric field-dependent polarization measurements showed the ferroelectric hysteresis loop for pure YInO3. The system transforms from ferroelectric in YInO3 to almost paraelectric for GdInO3. In the present study, XRD, Raman, and electrical characterizations in conjunction reveal that to tune the electrical properties of the hexagonal rare earth indates, the variation in tilting of InO5 polyhedra has to be influenced, which could not be brought about by isovalent A-site substitution.

Role Of Annealing Atmosphere On The Dielectric Properties Of La2NiMnO6

La2NiMnO6 (LNMO) was prepared by gel combustion method from nitrates using glycine as fuel followed by high temperature annealing. The prepared sample was annealed in O2, air and N2 and characterized by powder X‐ray and neutron diffraction and dielectric measurements. Cation ordered monoclinic (P21/n) and rhombohedral (R‐3) phases are observed in the as prepared sample and fraction of monoclinic phase increases on annealing in oxygen atmosphere. On annealing in inert atmosphere decomposition of the sample to manganese rich perovskite, La2NiO4 and NiO is observed. Temperature and frequency dependent permittivity (∼103) is observed in the sample annealed in air. The relative permittivity decreases significantly on annealing the sample in oxygen or N2.atmospher. The formation and annihilation of defects is attributed to the observed large permittivity of La2NiMnO6.