67 research outputs found
Spin-phonon coupling in multiferroic RCrO (R-Y, Lu, Gd, Eu, Sm): A Raman study
Raman study on a select few orthochromites, RCrO (R = Y, Lu, Gd, Eu and
Sm) shows that the phonon behavior at TN in compounds with magnetic R-ion (Gd
and Sm) is remarkably different from that of non-magnetic R-ion (Y, Lu and Eu).
While anomalies in most of the observed phonon frequencies in all these
compounds may result from the distortion of CrO octahedra due to size
effect and magnetostriction arising from Cr-ordering, the anomalous behavior of
their linewidths observed at TN for the compounds with only magnetic R-ion
suggests spin-phonon coupling. The presence of spin-phonon coupling and the
anomalies in the low frequency modes related to R-ion motion in orthochromites
(R = Gd and Sm) support the suggestion that the coupling between 4f-3d moments
play important role in inducing switchable electric polarization.Comment: 6 pages (two column format), 7 figures; The updated version of the
manuscript can be found at Euro. Phys. Lett. 101, 17008 (2013
Effect of pressure on octahedral distortions in RCrO3 (R = Lu, Tb, Gd, Eu, Sm): The role of R-ion size and its implications
The effect of rare-earth ion size on the octahedral distortions in rare-earth
chromites (RCrO3, R = Lu, Tb, Gd, Eu, Sm) crystallizing in the orthorhombic
structure has been studied using Raman scattering and synchrotron powder x-ray
diffraction up to 20 GPa. From our studies on RCrO3 we found that the
octahedral tilts (distortions) increase with pressure. This is contrary to the
earlier report which suggests that in LaCrO3, the distortions decrease with
pressure leading to a more ordered phase at high pressure. Here we observe that
the rate of increase in distortion decreases with the increase in R-ion radii.
This occurs due to the reduction in the compression of RO12 polyhedra with a
corresponding increase in the compression of the CrO6 octahedra with increasing
R-ion radii. From the Raman studies, we predict a critical R-ion radii, above
which we expect the distortions in RCrO3 to reduce with increasing pressure
leading to what is observed in the case of LaCrO3. These Raman results are
consistent with our pressure dependent structural studies on RCrO3 (R = Gd, Eu,
Sm). Also, our results suggest that the pressure dependence of N\'eel
temperature, TNCr, (where the Cr3+ spin orders) in RCrO3 is mostly affected by
the compressions of Cr-O bonds rather than the alteration of octahedral tilts.Comment: 17 pages, 8 figures This manuscript has been published in Material
Research Expres
Influence of lattice distortion on the Curie temperature and spin-phonon coupling in LaMnCoO
Two distinct ferromagnetic phases of LaMnCoO having
monoclinic structure with distinct physical properties have been studied. The
ferromagnetic ordering temperature is found to be different
for both the phases. The origin of such contrasting characteristics is assigned
to the changes in the distance(s) and angle(s) between Mn - O - Co resulting
from distortions observed from neutron diffraction studies. Investigations on
the temperature dependent Raman spectroscopy provide evidence for such
structural characteristics, which affects the exchange interaction. The
difference in B-site ordering which is evident from the neutron diffraction is
also responsible for the difference in . Raman scattering
suggests the presence of spin-phonon coupling for both the phases around the
. Electrical transport properties of both the phases have been
investigated based on the lattice distortion.Comment: 9 figure
Local A-Site Layering in Rare-Earth Orthochromite Perovskites by Solution Synthesis
Cation size effects were examined in the mixed A-site perovskites La0.5 Sm0.5 CrO3 and La0.5 Tb0.5 CrO3 prepared through both hydrothermal and solid-state methods. Atomically resolved electron energy loss spectroscopy (EELS) in the transmission electron microscope shows that while the La and Sm cations are randomly distributed, increased cation-radius variance in La0.5 Tb0.5 CrO3 results in regions of localised La and Tb layers, an atomic arrangement exclusive to the hydrothermally prepared material. Solid-state preparation gives lower homogeneity resulting in separate nanoscale regions rich in La3+ and Tb3+ . The A-site layering in hydrothermal La0.5 Tb0.5 CrO3 is randomised upon annealing at high temperature, resulting in magnetic behaviour that is dependent on synthesis route
Preparation of iron(IV) nitridoferrate Ca<sub>4</sub>FeN<sub>4</sub> through azide-mediated oxidation under high-pressure conditions
ChemInform Abstract: High-Pressure Synthesis and Characterization of Incompressible Titanium Pernitride.
Raman and X-ray investigations of ferroelectric phase transition in NH<sub>4</sub>HSO<sub>4</sub>
Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH<sub>4</sub>HSO<sub>4</sub> single crystals in the temperature range 77–298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as Pc, which has been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO<sub>4</sub><sup>–</sup> ions and that below 143 K is driven by the change in both HSO<sub>4</sub><sup>–</sup> and NH<sub>4</sub><sup>+</sup> ions
Brillouin Scattering Investigation of Solvation Dynamics in Succinonitrile-Lithium Salt Plastic Crystalline Electrolytes
Temperature dependent Brillouin scattering studies have been performed to ascertain the influence of solvent dynamics on ion-transport in succinonitrile-lithium salt plastic crystalline electrolytes. Though very rarely employed, we observe that Brillouin spectroscopy is an invaluable tool for investigation of solvent dynamics. Analysis of various acoustic (long wavelength) phonon modes observed in the Brillouin scattering spectra reveal the influence of trans-gauche isomerism and as well as ion-association effects on ion transport. Although pristine SN and dilute SN-LiClO(4) samples show only the bulk longitudinal-acoustic (LA) mode, concentrated SN-LiClO(4) (similar to 0.3-1 M) electrolytes display both the bulk LA mode as well as salt induced brillouin modes at ambient temperature. The appearance of more than one brillouin mode is attributed to the scattering of light from regions with different compressibilities (''compactness''). Correspondingly, these modes show a large decrease in the full width at half-maximum (abbreviated as nu(f)) as the temperature decreases. Anomalous temperature dependent behavior of nu(f) with addition of salt could be attributed to the presence of disorder or strong coupling with a neighbor. The shape of the spectrum was evaluated using a Lorentzian and Fano line shape function depending on the nature and behavior of the Brillouin modes
Ion transport mechanism in glasses: non-arrhenius conductivity and nonuniversal features
In this article, we report non-Arrhenius behavior in the temperature-dependent dc conductivity of alkali ion conducting silicate glasses well below their glass transition temperature. In contrast to the several fast ion-conducting and binary potassium silicate glasses, these glasses show a positive deviation in the Arrhenius plot. The observed non-Arrhenius behavior is completely reproducible in nature even after prolonged annealing close to the glass transition temperature of the respective glass sample. These results are the manifestation of local structural changes of the silicate network with temperature and give rise to different local environments into which the alkali ions hop, revealed by in situ high-temperature Raman spectroscopy. Furthermore, the present study provides new insights into the strong link between the dynamics of the alkali ions and different sites associated with it in the glasses
White light generation by carbonyl based indole derivatives due to proton transfer: an efficient fluorescence sensor
The motivation of the present work is to understand the optical, chemical, and electrical aspects of the proton transfer mechanism of indole (I) and some carbonyl based indole derivatives: indole-3-carboxaldehyde (I3C) and indole-7-carboxaldehyde (I7C) for both powder form and their liquid solution. Structural information for indole derivatives (isolated molecule and in solution) is obtained with Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT) methods. Calculated transition energies are used to generate UV–vis, FTIR, Raman and NMR spectra which are later verified with the experimental spectra. The occurrence of different conformers [cis (Nc), trans (Nt) and zwitterion (Z*)] have been interpreted by Mulliken charge, Natural Bond Orbital (NBO) analysis and polarization versus electric field (P-E loop) studies. 1H and 13C NMR and molecular vibrational frequencies of the fundamental modes established the stability of Nc due to the presence of Intramolecular Hydrogen Bonding (IHB) in the ground state (S0). Computed/experimental UV–vis absorption/emission studies reveal the creation of new species: zwitterion (Z*) and anion (A*) in the excited state (S1) due to excited state intramolecular and intermolecular proton transfer (ESIraPT and ESIerPT). Increased electrical conductivity (σac) with temperature and increased ferroelectric polarization at higher field verifies proton conduction in I7C
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