T. R. Ravindran

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.

Charged vacancy induced enhanced piezoelectric response of reactive assistive IBSD grown AlN thin films

Piezoelectric response of AlN thin films was investigated in a AlN/Ti/Si(1 0 0) layer structure prepared by ion beam sputter deposition (IBSD) in reactive assistance of N+/ ions. The samples were characterized for their microstructure, piezoelectric response and charged defects using high resolution x-ray diffraction (HR-XRD), piezo force microscopy (PFM) and photoluminescence (PL) spectroscopy respectively. Our results show that the films are highly textured along the a-axis and charged native point defects are present in the microstructure. Phase images of these samples obtained from PFM show that the films are predominantly N-polar. The measured values of piezoelectric coefficient d 33(eff) for these samples are as high as 206 ± 20 pm V−1 and 668 ± 60 pm V−1 calculated by piezo response loop for AlN films of a thickness of 235 nm and 294 nm respectively. A mechanism for high d 33(eff) values is proposed with a suitable model based on the charged defects induced enhanced polarization in the dielectric continuum of AlN.

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.

High-temperature phase transformation and low friction behaviour in highly disordered turbostratic graphite

Microstructure of turbostratic graphite consists of a large amount of pores and voids which eventually convert to plates and flakes constituting layered microfolding upon high-temperature heat treatment. This treatment results in apparent phase transformation from 3D to 2D structured graphite where stacking order between adjacent layers is absent. Interestingly, low friction coefficients 0.08 and 0.06 were measured in 2D graphite in ambient atmosphere. This is explained in terms of passivation of carbon dangling bonds by the formation of C–O and C–OH where both physisorbed and chemisorbed oxygen species are present. In contrast, these values were as high as 0.17 and 0.19 in 3D graphite. At significantly less humid atmosphere, progressive increase of friction coefficient (up to the value of 0.8) with sliding distance is observed in both 3D and 2D graphites. This consistent increase in friction coefficient is ascribed to gradual loss of residual passivating chemical species such as oxygen and H2O molecules due to tribochemical reaction. Interestingly, the structure of graphite remains similar while Raman spectra obtained from the locations of wear track, where ultra-low and high friction coefficient is measured.

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 ...

Twin chamber sample assembly in DAC and HPHT studies on GaN nano-particles

In this paper, we have suggested a novel idea of twin chamber sample assembly for separating ruby from the sample to overcome certain problems during high pressure?high temperature experiments using diamond anvil cell. Two holes of diameter 70?m were drilled symmetrically about the centre of the preindented area (500 ?m diameter) in a stainless steel gasket using EDM. Using ruby pressure calibration, good pressure correlation between these two holes was established up to about 15GPa. Also, high pressure ? high temperature (HPHT) experiments on the III-V compound semiconductor GaN were performed up to ~ 3GPa and 2000K using the two chamber sample assembly and a laser heated diamond anvil cell facility. NaCl, both hydrous and anhydrous, were used as the pressure transmitting media in two separate experiments. Micro-Raman spectroscopy was used to characterise the HPHT treated samples. While GaN remained stable in its wurtzite phase when heated in anhydrous NaCl medium, it transformed to Ga2O3 with rocksalt structure in the hydrous NaCl medium.

High pressure Raman spectroscopy of layered matlockite, PbFCl

The pressure dependence of various inter- and intra-layer Raman modes has been studied on pristine matlockite compound, PbFCl, up to ~41 GPa. The low-frequency interlayer vibrational modes, A1g(1) and Eg(1), identified as rigid layer modes, exhibit non-monotonic behavior with increasing pressure. They exhibit points of inflexion at ~24 GPa and ~31 GPa respectively, indicating the onset of a subtle instability. The emergence of a new Raman mode (~181 cm(-1)) at ~24 GPa and a sudden large increase in the intensity of the A1g(1) mode signify the occurrence of a symmetry lowering structural transition of the parent tetragonal phase with enhanced interlayer coupling. Two more modes appear at higher pressures (~33 GPa) at frequencies below the A1g(1) mode and are ascribed to a monoclinically distorted phase (space group P21/m). High pressure x-ray diffraction studies performed up to ~47 GPa confirm the occurrence of the structural transitions with decreasing crystal symmetry. These observations are consistent with a picture in which the structural distortion involves destabilization of the tetragonal unit cell following a gradual change in the bonding nature from layer-like (2D) to non-layer like (3D) involving the Cl-bilayers along the c direction.

Tribomechanical studies on PLD grown DLC/Ni multilayers

Tribomechanical properties of pulsed laser deposited diamond like carbon (DLC) with Ni interlayers are studied. Low coefficient of friction is found for DLC/Ni multilayer when compared to DLC/Ni bilayer. The behavior of low friction coefficient of DLC/Ni multilayer is attributed to the formation of graphitized tribofilm.

Evidence of disorder in cubic zirconium tungstate from the temperature dependence of Raman spectra

The cubic phase of zirconium tungstate exhibits a larger number of vibrational Raman modes in the frequency region of the symmetric stretching mode of tungstate ions than permitted by group-theoretical analysis for a perfectly ordered lattice. This suggests the existence of disorder in the tungstate sublattice. The additional modes are identified and assigned on the basis of their relative intensities and temperature dependences of line-widths. Using a real-space model for misoriented tetrahedral ions, the occurrence of four disorder modes is explained. From the evolution of the intensities of different components of the symmetric stretching mode as a function of temperature, the disorder is found to be largely static (frozen-in) in nature. As each tungstate ion carries a net dipole moment, randomly misoriented tetrahedral units can result in a dipole glass, explaining a recently reported glass-like behaviour of thermal conductivity. The proposed disordered configurations of tungstate ions are indeed found to be energetically stable from first-principles simulation studies.