Nanocrystalline Nd(2-y)Gd(y)Zr(2)O(7) pyrochlore: Facile synthesis and electrical characterization

Three solid solutions of pyrochlores in the series Nd(2-years)Gd(y)Zr(2)O(7) (y = 0.8, 1.0, 1.2) were synthesized by the gel combustion method using citric acid as fuel. This results in a soft agglomerate powder as verified by dynamic light scattering. The single-phase nature of the products has been confirmed by x-ray diffraction. The increase in full width at half-maxima in the Raman spectra with an increase in Gd(3+) content indicates that disorder increases with Gd(3+) content. The morphology and particle size of the products were investigated by transmission electron microscopy. Scanning electron microscopy study reveals that the sintered pellets have a density higher than 92% of theoretical densities. The total ionic conductivity measurements in the temperature range 375-800 degrees C show that with the increase of disorder (Gd(3+) content) in the system the activation energy of conduction increases from 0.98 to 1.06 eV and the preexponential factor, which is proportional to the number of mobile species, also follow the same trend of increase. The total conductivity measured in reducing atmosphere shows no change in electrical conductivity, which verifies a negligible contribution of electronic contribution in this system

Quest for lead free relaxors in YIn<SUB>1–x</SUB>Fe<SUB>x</SUB>O<SUB>3</SUB>(0.0 ≤x≤ 1.0) system: role of synthesis and structure

The B-site tailored YIn1–xFexO3 (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 Fe3+-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 In3+-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, YIn1–xFexO3 (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 YIn0.7Fe0.3O3 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 YIn0.7Fe0.3O3 showed a classical relaxor ferroelectric like behavior which is an important observation in context of the search for new lead free relaxor materials

Role of annealing atmosphere on structure, dielectric and magnetic properties of La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>

Polycrystalline samples of La<SUB>2</SUB>MMnO<SUB>6</SUB> (M = Co and Mg) were prepared by a combined gel-combustion and high temperature reaction method. The samples were annealed in different oxygen partial pressure (p<SUB>O2</SUB>) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P2<SUB>1</SUB>/n) and rhombohedral (Requation image) lattices were observed for La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>, respectively. On annealing in inert atmosphere, La<SUB>2</SUB>MgMnO<SUB>6</SUB> partially converted to monoclinic La<SUB>2</SUB>CoMnO<SUB>6</SUB> type structure, whereas no structural change was observed in La<SUB>2</SUB>CoMnO<SUB>6</SUB>. Dielectric studies of La2CoMnO6 indicated relaxor like behavior with polaronic conduction, which systematically decreased with the increase in p<SUB>2</SUB> of the annealing atmosphere. Magnetic studies indicated multiple ferromagnetic phase transitions in La<SUB>2</SUB>CoMnO<SUB>6</SUB> and a spin-glass like phase transition in La<SUB>2</SUB>MgMnO<SUB>6</SUB>. The fraction of ferromagnetic phases of La<SUB>2</SUB>CoMnO<SUB>6</SUB> was significantly dependent on the annealing environments. The variations of magnetic and dielectric properties of samples were related to the fluctuation of oxidation state of transition metal ions and oxygen vacancies in the samples

Multifunctional nanocrystalline CeCrO<SUB>3</SUB>: antiferromagnetic, relaxor, and optical properties

Nanocrystalline phase pure CeCrO<SUB>3</SUB> was synthesized by a two-step synthesis. The compound was investigated by a host of characterization techniques such as X-ray diffraction, high-temperature X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric−differential thermal analysis, magnetic and specific heat capacity data, ac impedance spectroscopy, diffuse reflectance DR UV−visible spectrophotometer, and dynamic light scattering. The magnetic structure of CeCrO<SUB>3</SUB> was established using variable-temperature neutron diffraction data. On the basis of the detailed studies, this compound was found to exhibit multifunctionalities such as antiferromagnetism, relaxor behavior, and an optical band gap in the visible region. This newly developed synthesis route opens the immense possibilities of preparation of the hitherto unknown Ce<SUP>3+</SUP>-based mixed oxides, analogous to other rare earth (RE<SUP>3+</SUP>) counterparts

FeTiTaO6FeTiTaO_6: A Lead-Free Relaxor Ferroelectric Based on the Rutile Structure

Several materials properties of rutile $TiO_2$ (together with its other polymorphs) have been widely investigated in view of both fundamental and practical interest. It is a wide band gap $(E_g=3.05 eV)$ semiconductor that finds application, among others, as a photocatalyst for splitting water[1] into $H_2$ and $O_2$ and remediation of organic pollutants.[2] Among the lesswell-known properties of $TiO_2$ is its abnormally large static dielectric permittivity that shows strong frequency dependence as well as the associated soft $A_{2u}$ mode that decreases with decreasing temperature.[3,4] The latter, however, never becomes completely soft, even at 0 K. Accordingly, rutile is classified as an incipient ferroelectric.[4] The soft $A_{2u}$ mode involves displacement of the positively charged $Ti^{4+}$ against negatively charged $O^{2-}$ that manifests in a large static dielectric permittivity (165–250) along the tetragonal c axis. The special dielectric behavior of $TiO_2$ is different from that of the other rutile oxides such as $GeO_2$ and $SnO_2$, clearly indicating the role of $d^0$ electronic configuration of $Ti^{4+}$ on the dielectric properties. It is known that transition metal oxides with $d^0$ electronic configuration undergo a second-order Jahn–Teller (SOJT) distortion of the $MO_6$ (M = metal) octahedra arising from a mixing of empty $d_0$ states of the transition metal with the O 2p states.[5] The distortion seems to be at the heart of several of the interesting dielectric/ferroelectric properties of $d^0$ transition-metal-containing perovskite oxides[6] such as $BaTiO_3$, $PbTiO_3$, $PbZr_{1-x}Ti_xO_3$, $Pb_3MgNb_2O_9$, and so on

Structural and electrical properties of layered perovskite type Pr<SUB>2</SUB>Ti<SUB>2</SUB>O<SUB>7</SUB>: 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

Mechanistic Insights into CO₂ Methanation over Ru-Substituted CeO₂

CO₂ methanation is an important probe reaction to understand CO₂ interactions with catalytic surfaces. The importance of this reaction is further increased by its association with CO₂ utilization. This study reports the mechanistic aspects of CO₂ methanation over combustion synthesized Ru-substituted CeO₂ catalyst. Temperature-programmed reaction experiments were carried out to understand the interaction of CO₂, H₂, and their stoichiometric mixture with the catalyst surface. In situ FTIR spectroscopy was used to identify the intermediates of the reaction. It was observed that CO₂ adsorption took place on the surface of Ce_0.95Ru_0.05O₂ and the formation of surface carbonate intermediates took place only when H₂ was present in the gas phase. In the absence of H₂, CO₂ did not show any indication for chemisorption. This behavior was explained in terms of the reaction between CO₂ and the surface hydroxyls leading to the formation of a vacancy. Upon dissociation, carbonates led to chemisorbed CO which eventually formed methane upon reaction with gas-phase H₂. The exact identity of carbonate species and the pathway for the methanation step were ambiguous following purely experimental studies. Density functional theory calculations were carried out to augment the experimental observations. Complete energy landscapes developed on the basis of differentiation of oxidized and reduced forms of the catalyst showed that the reaction followed a pathway consisting of surface carbonate species formed by the interaction of oxide surface and chemisorbed CO, and a sequential methanation via the surface methoxy species formation. The study provides physical insights into the role of the oxidation state of the catalyst and the surface anionic vacancies in governing the reaction pathway