Nilesh P. Salke

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.

Raman Spectroscopic Investigation of Thorium Dioxide- Uranium Dioxide (ThO2-UO2) Fuel Materials

Raman spectroscopic investigations were carried out on proposed nuclear fuel thorium dioxide–uranium dioxide (ThO2–UO2) solid solutions and simulated fuels based on ThO2–UO2. Raman spectra of ThO2–UO2 solid solutions exhibited two-mode behavior in the entire composition range. Variations in mode frequencies and relative intensities of Raman modes enabled estimation of composition, defects, and oxygen stoichiometry in these compounds that are essential for their application. The present study shows that Raman spectroscopy is a simple, promising analytical tool for nondestructive characterization of this important class of nuclear fuel materials.

Raman and ab initio investigation of negative thermal expansion material TaVO5: Insights into phase stability and anharmonicity

TaVO5 is a framework structured compound that exhibits negative thermal expansion (NTE) above room temperature, upto 1073 K. We report Raman spectroscopic investigation of TaVO5 as a function of temperature in the range 77-873 K, which confirms the reported reversible low temperature transition to monoclinic phase at 259 K. Structural stability of TaVO5 at high pressures investigated using in-situ Raman spectroscopy shows a reversible structural transition at around 0.2 GPa to a phase, which is probably the same monoclinic phase as the low temperature phase, indicating that this structural phase transition may be volume driven. From the pressure and temperature dependence of the Raman modes, some of the zone centre phonon modes, particularly, the librational modes, responsible for the NTE are identified and anharmonicity of the Raman modes is also estimated. We have found that explicit anharmonicity dominates over implicit anharmonicity and the low frequency modes have significant quartic anharmonicity. T...

Phase transitions in delafossite CuLaO2 at high pressures

Structural stability of a transparent conducting oxide CuLaO2 at high pressures is investigated using in-situ Raman spectroscopy, electrical resistance, and x-ray diffraction techniques. The present Raman investigations indicate a sequence of structural phase transitions at 1.8 GPa and 7 GPa. The compound remains in the first high pressure phase when pressure is released. Electrical resistance measurements carried out at high pressures confirm the second phase transition. These observations are further supported by powder x-ray diffraction at high pressures which also showed that a-axis is more compressible than c-axis in this compound. Fitting the pressure dependence of unit cell volume to 3rd order Birch-Murnaghan equation of state, zero pressure bulk modulus of CuLaO2 is determined to be 154(25) GPa. The vibrational properties in the ambient delafossite phase of CuLaO2 are investigated using ab-initio calculations of phonon frequencies to complement the Raman spectroscopic measurements. Temperature dep...

Raman spectroscopic studies of Pr2Ti2O7 at high pressures

Vibrational properties of Pr2Ti2O7 have been studied at high pressures upto 18 GPa using Raman spectroscopy. While all the Raman modes of Pr2Ti2O7 harden gradually with pressure without any anomaly, interesting intensity exchange, frequency variation and mode coupling is observed in Ti-O stretching modes under pressure. The present study indicates good structural stability of Pr2Ti2O7 over the investigated pressures.

Raman spectroscopic investigations on delafossite CuLaO2 at high pressures

Vibrational properties of delafossite CuLaO2 are investigated using Raman spectroscopy at ambient conditions and at high pressures. Changes are observed in the Raman spectrum at around 1.8 GPa which could be related to a phase transition. The changes observed are reversible from 7 GPa.

High Pressure Phases and Amorphization of a Negative Thermal Expansion Compound TaVO5

Negative thermal expansion material TaVO5 is recently reported to have pressure induced structural phase transition and irreversible amorphization at 0.2 and above 8 GPa, respectively. Here, we have investigated the high pressure phase of TaVO5 using in situ neutron diffraction studies. The first high pressure phase is identified to be monoclinic P21/ c phase, same as the low temperature phase of TaVO5. On heating, amorphous TaVO5 transformed to a new crystalline phase, which showed signatures of higher coordination of vanadium indicating pressure induced amorphization (PIA). PIA observed in TaVO5 might be due to the kinetic hindrance of pressure induced decomposition (PID) into a compound with higher coordination of vanadium. Mechanism of PIA observed in TaVO5 is investigated by carrying out ex situ Raman, XRD, XPS, and XAS measurements. We have also proposed a pressure-temperature phase diagram of TaVO5 qualitatively delineating the phase boundaries between the ambient orthorhombic, monoclinic, and amorphous phases.

Anharmonicity of Raman modes of Bi12SiO20

Bi12SiO20 is a technologically important material reported to be extremely stable under high pressure. Here, we report the results of Raman spectroscopic investigations on Bi12SiO20 in the temperature range 300-770 K. The compound remains in the ambient cubic phase in the entire temperature range. Using pressure-and temperaturedependence of Raman modes, intrinsic anharmonicity of each of the Raman modes has been calculated.

Structural, vibrational, and electronic topological transitions of Bi1.5Sb0.5Te1.8Se1.2 under pressure

Topological insulators have been the subject of intense research interest due to their unique surface states that are topologically protected against scattering or defects. However, the relationship between the crystal structure and topological insulator state remains to be clarified. Here, we show the effects of hydrostatic pressure on the structural, vibrational, and topological properties of the topological insulator Bi1.5Sb0.5Te1.8Se1.2 up to 45 GPa using X-ray diffraction and Raman spectroscopy in a diamond anvil cell, together with first-principles theoretical calculations. Two pressure-induced structural phase transitions were observed: from ambient rhombohedral R 3¯m phase to a monoclinic C2/m phase at ∼13 GPa, and to a disordered I4/mmm phase at ∼22 GPa. In addition, the alloy undergoes several electronic transitions within the R 3¯m phase: indirect to direct bulk band gap transition at ∼5.8 GPa, bulk gap closing with an appearance of Dirac semimetal (DSM) state at ∼8.2 GPa, and to a trivial semimetal state at ∼12.1 GPa. Anomalies in c/a ratio and Raman full width at half maximum that coincide with the DSM phase suggest the contribution of electron-phonon coupling to the transition. Compared to binary end members Bi2Te3, Bi2Se3, and Sb2Te3, the structural phase transition and anomaly were observed at higher pressures in Bi1.5Sb0.5Te1.8Se1.2. These results suggest that the topological transitions are precursors to the structural phase transitions.Topological insulators have been the subject of intense research interest due to their unique surface states that are topologically protected against scattering or defects. However, the relationship between the crystal structure and topological insulator state remains to be clarified. Here, we show the effects of hydrostatic pressure on the structural, vibrational, and topological properties of the topological insulator Bi1.5Sb0.5Te1.8Se1.2 up to 45 GPa using X-ray diffraction and Raman spectroscopy in a diamond anvil cell, together with first-principles theoretical calculations. Two pressure-induced structural phase transitions were observed: from ambient rhombohedral R 3¯m phase to a monoclinic C2/m phase at ∼13 GPa, and to a disordered I4/mmm phase at ∼22 GPa. In addition, the alloy undergoes several electronic transitions within the R 3¯m phase: indirect to direct bulk band gap transition at ∼5.8 GPa, bulk gap closing with an appearance of Dirac semimetal (DSM) state at ∼8.2 GPa, and to a trivial semi...

Raman spectroscopic investigation of CuGaTe2 at high pressures

Vibrational properties of ternary chalcopyrite semiconductor CuGaTe2 have been studied using Raman spectroscopy at high pressures upto 24 GPa. Pressure dependent softening is observed for two of the observed modes, which has been correlated with instability in the system leading to structural transition around 9 GPa to a disordered simple cubic phase. Beyond 11.5 GPa, no Raman spectra could be observed. This is possibly due to the transition could be also accompanied by reduction in band gap. On release of pressure, structurally disordered chalcopyrite phase is recovered.