Sadequa J. Patwe

Observation of a new cryogenic temperature dielectric relaxation in multiferroic Bi7Fe3Ti3O21

In this communication, we report details of structural, magnetic, and dielectric properties of multiferroic Bi7Fe3Ti3O21 at cryogenic temperatures. The low temperature impedance spectroscopic studies showed a relaxor-like dielectric anomaly which follows Vogel-Fulcher relation with freezing temperature (TVF) of 33u2009K. Temperature and field dependent magnetization indicate the onset of a possible antiferromagnetic ordering below 150u2009K. Variable temperature powder neutron diffraction studies indicate no structural change down to 22u2009K. The appearance of the low temperature dielectric anomaly along with evidence of magnetic ordering at low temperatures suggests the presence of magnetic domains of mesoscopic scale within the bulk matrix.

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.

Ferroelectric and proton conducting behavior of a new elpasolite-related vanadium oxyfluoride (NH 4 ,K) 3 VO 2 F 4

A new elpasolite-type (NH 4 ,K) 3 VO 2 F 4 compound was prepared and characterized by x-ray diffraction, differential scanning calorimeter (DSC), impedance analysis, and electrical polarization measurements. It crystallizes in an orthorhombic lattice with unit-cell parameters: a = 8.9584(4), b = 18.6910(14), c = 6.2174(4) A, V = 1041.04(11) A 3 , Z = 6. NH 4 + , and K + ions are distributed statistically over crystallographically four equivalent sites. There are two distinguishable vanadium atoms forming cis- and trans- VO 2 F 4 octahedra present in the unit cell. High-temperature studies by DSC and in situ x-ray diffraction revealed a first-order structural transformation from orthorhombic to cubic lattice around 343 K. Impedance measurements show two different kinds of conductivity behaviors for the two phases. In orthorhombic phase a significant conductivity resulting from involvement of protonic species is observed. In the orthorhombic phase, a clear ferroelectric hysteresis loop is observed.

Ionic conductivity in solid solutions of PbF2 and YF3

Abstract Ionic conductivity of a series of fluorite-type solid solutions Pb1-xYxF2+x (0.007 ≤ x ≤ 0.222) has been measured in the frequency range 100 Hz – 3 MHz and in the temperature range 25–250oC under vacuum. The conductivity decreases and the activation energy increases gradually upon increasing the yttrium content up to ∼ 4.5 mol % of Y3+ whereas, beyond that and up to 22 mol % of Y3+, the ionic conductivity increases and the activation energy decreases. Thus two distinct regions in the general composition Pb1-xYxF2+x could be delineated, in one the polarizability factor predominates over the number of extra-interstitial F anions and vice-versa in the other region. These results indicate the competing effects between the disorder caused by interstitial F anions and the polarizability of the host and guest cations.

Role of phonons in negative thermal expansion and high pressure phase transitions in β-eucryptite: An ab-initio lattice dynamics and inelastic neutron scattering study

β-Eucryptite (LiAlSiO4) shows anisotropic thermal expansion as well as one-dimensional super-ionic conductivity. We have performed the lattice dynamical calculations using ab-initio density functional theory along with inelastic neutron scattering measurements. The anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10u2009meV have large negative values and govern the negative thermal expansion behavior at low temperatures along both the “a”- and “c”-axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30–70u2009meV are responsible for the thermal expansion at high temperatures, which is positive in the a-b plane and negative along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous the...Beta eucryptite (LiAlSiO4) shows one-dimensional super-ionic conductivity as well as anisotropic thermal expansion behavior. We have performed inelastic neutron scattering measurements in beta eucryptite over 300 to 900 K and calculated the phonon spectrum using ab initio density functional theory method. The calculated energy profile for cooperative lithium ion displacements indicates preferential movement of Li ion along the hexagonal c-axis in the high temperature phase. However, the energy barrier for Li ion diffusion is significantly reduced when a Schottky defect is introduced in the crystal. Further, the anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10 meV have large negative values and govern the negative thermal expansion behavior both along the a and c axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30 to 70 meV are responsible for positive thermal expansion in the a-b plane and negative expansion along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous thermal expansion behavior. We extend the study to discuss the relationship of the soft phonons in the Brillouin zone with the observed high-pressure and high-temperature phase transitions as reported in the literature.

High-temperature phonon spectra of multiferroic BiFeO3 from inelastic neutron spectroscopy

We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO3. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Neel (TNu2009∼u2009650u2009K) and Curie (TCu2009∼u20091100u2009K) temperatures. Experimental results are accompanied by ab initio lattice dynamical simulations of phonon density of states to enable microscopic interpretations of the observed data. The calculations reproduce well the observed vibrational features and provide the partial atomic vibrational components. Our results reveal clearly the signature of three different phase transitions both in the diffraction patterns and phonon spectra. The phonon modes are found to be most affected by the transition at the TC. The spectroscopic evidence for the existence of a different structural modification just below the decomposition limit (TDu2009∼u20091240u2009K) is unambiguous indicating strong structural changes that may be related to oxygen vacancies and concomitan...

Fluorination of Sr2CuO3 by NH4HF2

Abstract Different amounts of fluorine can be incorporated in the place of oxygen in Sr2CuO3 by reacting it with NH4HF2 in different stoichiometry. The superconducting (SC) behaviour of the product was found to be critically dependent on the amount of F incorporated into the lattice. Too little fluorine content or too much produced a non-superconducting product. The fact that in all stoichiometries of Sr2CuO3 and NH4HF2 in the mixture, SrF2 is invariably formed, indicates that all the products including the superconducting ones are Sr-deflcient. It is argued that sub-stoichiometry in Sr in the product serves the same function as that of interstitial F in producing superconductivity.