K. K. Mishra

Impedance and Raman spectroscopic studies of (Na0.5Bi0.5)TiO3

Polycrystalline powder of (Na0.5Bi0.5)TiO3 (NBT) was prepared by a high-temperature solid-state reaction route. Preliminary x-ray diffraction analysis carried out at room temperature showed the formation of a single phase compound with a rhombohedral crystal system. Scanning electron micrograph reveals the polycrystalline nature of the material with size anisotropy. Dielectric study showed an existence of diffuse phase transition around 300 °C. The ac conductivity spectrum obeyed the Jonscher power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature. The bulk conductivity of the material indicates an Arrhenius type of thermally activated process with three different conduction mechanisms as different activation energies are observed. The hopping charge carriers dominate at low temperature, small polaron and oxygen vacancy dominates at intermediate temperature and ionic conduction at higher temperatures. Studies of impedance spectroscopy indicate that the dielectric relaxation is of non-Debye type. In situ high-temperature Raman spectroscopy shows discontinuous changes in the phonon frequencies across the rhombohedral–tetragonal transition. In addition, anomalous changes in the intensity and the linewidth of a lattice mode are found around 350 °C.

Phase transition and magneto-electric coupling of BiFeO3–YMnO3 multiferroic nanoceramics

We report the crystal structure, dielectric, magnetic, and magneto-electric properties of (1−x) BiFeO3-xYMnO3 (0.00 ≤ x ≤ 0.2) multiferroic nanoceramics prepared by auto-combustion technique. YMnO3 substitution is found to induce a structural phase transition from R3c to R3c+Pbnm after x ≈ 0.1 using Rietveld refinement technique. Field emission scanning electron micrographs show decrease in grain size with increase in YMnO3 content. The dielectric permittivity and loss tangent are found to be increased with composition x. The anomalies noticed from the temperature dependent dielectric analysis reveal the signature of magneto-electric coupling in the system. A decrease in magnetic ordering temperature as a function of composition is found from dielectric study. At room temperature, the dielectric permittivity of all the YMnO3 modified samples decrease with increasing magnetic field. The maximum value of magneto-electric coupling coefficient (e(H)-e(0))/e(0) is found to be ∼ −5.5% at H = 2 T for x = 0.2. Th...

Phase transition and enhanced magneto-dielectric response in BiFeO3-DyMnO3 multiferroics

We report systematic studies on crystal structure and magneto-dielectric properties of (1 − x) BiFeO3-x DyMnO3 (0.0 ≤ x ≤ 0.2) nanoceramics synthesized by auto-combustion method. Rietveld refinement of X-ray diffraction data indicates a structural transition from R3c to R3c + Pn21a at x = 0.1. Field emission scanning electron micrographs display a decrease in grain size with increase in x. The presence of dielectric anomalies around antiferromagnetic transition temperature implies the magnetoelectric coupling. Dielectric measurements showed decrease in magnetic ordering temperature with increasing x in agreement with differential scanning calorimetry results. A significant increase in magnetization has been found with increasing DyMnO3 substitution. Magneto-impedance spectroscopy reveals a significant change (∼18%) in dielectric permittivity at H = 2 T for x = 0.2.

Dielectric and polarized Raman spectroscopic studies on 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal

Tetragonal-cubic phase transition has been investigated in relaxor-ferroelectric 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal using dielectric and Raman spectroscopy. A detailed analysis of the dielectric data suggests that the transition is of second order. In the tetragonal phase (P4mm), all the modes predicted by group theory were found in the Raman spectra and were assigned based on the symmetry and polarization configuration. Frequencies of several modes were found to disappear while a few modes exhibited discontinuous change across the phase transition temperature TC ∼473 K. While in the high temperature cubic phase (Pm3¯m) no first order Raman spectrum is expected, the presence of several Raman peaks at elevated temperature suggests substitutional disorder causing the appearance of symmetry-forbidden Raman bands. The line-width of A1(TO) mode at 273 cm−1 shows anomalies across TC and the intermediate temperature T*. Furthermore, based on the temperature dependence of total integrated intensities ...

Phonons in La‐Substituted BiFeO3‐PbTiO3

(Bi1−xLax)0.5Pb0.5Fe0.5Ti0.5O3 (BF‐PT) ceramics were prepared for x = 0.0, 0.2, 0.3, 0.4 and 0.5 using solid state reaction method. x = 0.0 samples were found to have tetragonal perovskite structure, same as that of PbTiO3 (PT), while in La‐substituted samples tetragonal distortion reduced and system turned cubic at 40% La‐concentration. Raman spectroscopic investigations reveal 10 modes in pure BF‐PT which were assigned by comparing with those in PT. Although in the cubic phase no Raman active phonons are expected, 7 modes are found that have correspondence with those of the tetragonal phase. The modes in the cubic phase are activated due to substitutional disorder at cation site.

Phase Transformation, Vibrational and Electronic Properties of K2Ce(PO4)2: A Combined Experimental and Theoretical Study

Herein we report the high-temperature crystal chemistry of K2Ce(PO4)2 as observed from a joint in situ variable-temperature X-ray diffraction (XRD) and Raman spectroscopy as well as ab initio density functional theory (DFT) calculations. These studies revealed that the ambient-temperature monoclinic (P21/n) phase reversibly transforms to a tetragonal (I41/amd) structure at higher temperature. Also, from the experimental and theoretical calculations, a possible existence of an orthorhombic (Imma) structure with almost zero orthorhombicity is predicted which is closely related to tetragonal K2Ce(PO4)2. The high-temperature tetragonal phase reverts back to ambient monoclinic phase at much lower temperature in the cooling cycle compared to that observed at the heating cycle. XRD studies revealed the transition is accompanied by volume expansion of about 14.4%. The lower packing density of the high-temperature phase is reflected in its significantly lower thermal expansion coefficient (αV = 3.83 × 10-6 K-1) compared to that in ambient monoclinic phase (αV = 41.30 × 10-6 K-1). The coexistences of low- and high-temperature phases, large volume discontinuity in transition, and large hysteresis of transition temperature in heating and cooling cycles, as well as drastically different structural arrangement are in accordance with the first-order reconstructive nature of the transition. Temperature-dependent Raman spectra indicate significant changes around 783 K attributable to the phase transition. In situ low-temperature XRD, neutron diffraction, and Raman spectroscopic studies revealed no structural transition below ambient temperature. Raman mode frequencies, temperature coefficients, and reduced temperature coefficients for both monoclinic and tetragonal phases of K2Ce(PO4)2 have been obtained. Several lattice and external modes of rigid PO4 units are found to be strongly anharmonic. The observed phase transition and structures as well as vibrational properties of both ambient- and high-temperature phases were complimented by DFT calculations. The optical absorption studies on monoclinic phase indicated a band gap of about 2.46 eV. The electronic structure calculations on ambient-temperature monoclinic and high-temperature phases were also carried out.

Structural and Thermal Properties of BaTe2O6: Combined Variable-Temperature Synchrotron X-ray Diffraction, Raman Spectroscopy, and ab Initio Calculations

Variable-temperature Raman spectroscopic and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to understand structural stability and anharmonicity of phonons. The structural and vibrational studies indicate no phase transition in it over a wider range of temperature (20 to 853 K). The structure shows anisotropic expansion with coefficients of thermal expansion in the order αb ≫ αa > αc, which was attributed to the anisotropy in bonding and structure of BaTe2O6. Temperature evolution of Raman modes of BaTe2O6 indicated a smooth decreasing trend in mode frequencies with increasing temperature, while the full width at half-maximum (fwhm) of all modes systematically increases due to a rise in phonon scattering processes. With the use of our earlier reported isothermal mode Grüneisen parameters, thermal properties such as thermal expansion coefficient and molar specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic (implicit) contribution to the total anharmonicity is delineated and compared. The temperature dependence of phonon mode frequencies and their fwhm values are analyzed by anharmonicity models, and the dominating anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external modes of TeOn (n = 5, 6) are found to be strongly anharmonic. The ab initio electronic structure calculations indicated BaTe2O6 is a direct band gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals, namely, O-2p states in the valence band maximum and the sp-hybridized states in the conduction band minimum, are mainly involved in the electronic transitions. In addition a number of electronic transitions are predicted by the electronic structure calculations. Experimental photoluminescence results are adequately explained by the ab initio calculations. Further details of the structural and vibrational properties are explained in the manuscript.

Anomalous behavior of acoustic phonon mode and central peak in Pb(Zn1/3Nb2/3)0.85Ti0.15O3 single crystal studied using Brillouin scattering

Brillouin spectroscopic measurements have been carried out on relaxor ferroelectric Pb(Zn1/3Nb2/3)0.85Ti0.15O3 single crystal over the temperature range 300–873 K. The frequency of longitudinal acoustic phonon (LA) begins to exhibit a strong decrease below the Burns temperature TB followed by an increase in the line-width below the characteristic intermediate temperature T* ∼ 525 K. The line-width of the LA phonon mode exhibits a sharp Landau-Khalatnikov-like maximum at 463 K, the tetragonal-cubic phase transition temperature (Ttc). In addition, a broad central peak, found below T* exhibits critical slowing down upon approaching Ttc indicating an order-disorder nature of the phase transition. The relaxation time of polar nanoregions estimated from the broad central peak is found to be same as that obtained from LA phonon mode suggesting a strong coupling between strain and polarization fluctuations.

Studies of ferroelectric properties and leakage current behaviour of microwave sintered ferroelectric Na0.5Bi0.5TiO3 ceramic

ABSTRACT Single phase lead-free ferroelectric ceramic Na0.5Bi0.5TiO3 (NBT) is synthesized using sol-gel method and sintered using microwave sintering technique. The structural, microstructural, and electrical properties are investigated in detail. Rietveld refinement technique is used to analyse XRD pattern. SEM micrograph shows the densely packed micrometre sized grains. Ferroelectric hysteresis loops for unpoled and poled samples confirm its ferroelectric nature. The characteristic ferroelectric properties like remanent polarization and coercive field are found to be 84.8 µC/cm2, 71.8 µC/cm2 and 58.8 kV/cm, 58.6 kV/cm respectively for the unpoled and poled samples. The current versus voltage curve of the NBT follow the bulk-limited space-charge conduction mechanism.

Study of Phase Transformation in BaTe2O6 by in Situ High-Pressure X-ray Diffraction, Raman Spectroscopy, and First-Principles Calculations

Structural and vibrational properties of orthorhombic BaTe2O6, a mixed valence tellurium compound, have been investigated by in situ synchrotron X-ray diffraction (XRD) studies up to 16 GPa and Raman spectroscopy up to 37 GPa using a diamond-anvil cell. The structure of orthorhombic BaTe2O6 has layers of [Te2O6]2–, formed by TeO6 octahedra and TeO5 square pyramids and Ba2+ ions stacked alternately along the ⟨010⟩ direction. A reversible pressure-induced structural transformation from the ambient orthorhombic (Cmcm) to a monoclinic (P21/m) structure is observed in both XRD and Raman spectroscopic investigations around 10 GPa. Ab initio calculations using density functional theory (DFT) corroborate this phase transition as well as the transition pressure. Both XRD and DFT calculations reveal that the high-pressure monoclinic structure is closely related to the ambient pressure orthorhombic structure, and the transformation is accompanied by a slight rearrangement of the structural units. Pressure evolution ...