Raman scattering of rare earth sesquioxide Ho2O3: A pressure and temperature dependent study

Pressure and temperature dependent Raman scattering studies on Ho2O3 have been carried out to investigate the structural transition and the anharmonic behavior of the phonons. Ho2O3 undergoes a transition from cubic to monoclinic phase above 15.5 GPa, which is partially reversible on decompression. The anharmonic behavior of the phonon modes of Ho2O3 from 80K to 440K has been investigated. We find an anomalous line-width change with temperature. The mode Gruneisen parameter of bulk Ho2O3 was estimated from high pressure Raman investigation up to 29 GPa. Furthermore, the anharmonic components were calculated from the temperature dependent Raman scattering

Investigations of anharmonic effects via phonon mode variations in nanocrystalline Dy2O3, Gd2O3 and Y2O3

The nanocrystalline rare earth sesquioxides Dy2O3, Gd2O3 and Y2O3 have been investigated for anharmonic effects in the temperature range 80-440K using Raman spectroscopy. These samples were cubic in structure under ambient conditions with particle sizes in nano-range. The predominant T-g+A(g) phonon modes of the samples primarily exhibited phonon softening in the investigated temperature range. However, the line width variations clearly reflected the variations in the effects of anharmonicity on these samples, and the related anharmonic constants were estimated. The mode Grueneisen parameters required for these studies were deduced from our own high-pressure data for these samples reported earlier. The line width variations for these samples were found to display similar trends; however, their magnitudes were indicative of the factors that are expected to contribute to the variation in phonon behaviour

A comparative approach for the characterization of a pneumatic piston gauge up to 8 MPa using finite element calculations

This paper reports the behavior of a well-characterized pneumatic piston gauge in the pressure range up to 8 MPa through simulation using finite element method (FEM). Experimentally, the effective area of this piston gauge has been estimated by cross-floating to obtain A0 and λ. The FEM technique addresses this problem through simulation and optimization with standard commercial software (ANSYS) where the material properties of the piston and cylinder, dimensional measurements, etc are used as the input parameters. The simulation provides the effective area Ap as a function of pressure in the free deformation mode. From these data, one can estimate Ap versus pressure and thereby Ao and λ. Further, we have carried out a similar theoretical calculation of Ap using the conventional method involving the Dadson’s as well as Johnson–Newhall equations. A comparison of these results with the experimental results has been carried out

Temperature Dependent Variations of Phonon Interactions in Nanocrystalline Cerium Oxide

The temperature dependent anharmonic behavior of the phonon modes of nanocrystalline CeO2 was investigated in the temperature range of 80–440 K. The anharmonic constants have been derived from the shift in phonon modes fitted to account for the anharmonic contributions as well as the thermal expansion contribution using the high pressure parameters derived from our own high pressure experimental data reported previously. The total anharmonicity has also been estimated from the true anharmonicity as well as quasiharmonic component. In the line-width variation analysis, the cubic anharmonic term was found to dominate the quartic term. Finally, the phonon lifetime also reflected the trend so observed

Anharmonic behavior and structural phase transition in Yb2O3

The investigation of structural phase transition and anharmonic behavior of Yb2O3 has been carried out by high-pressure and temperature dependent Raman scattering studies respectively. In situ Raman studies under high pressure were carried out in a diamond anvil cell at room temperature which indicate a structural transition from cubic to hexagonal phase at and above 20.6 GPa. In the decompression cycle, Yb2O3 retained its high pressure phase. We have observed a Stark line in the Raman spectra at 337.5 cm−1 which arises from the electronic transition between 2F5/2 and 2F7/2 multiplates of Yb3+ (4f13) levels. These were followed by temperature dependent Raman studies in the range of 80–440 K, which show an unusual mode hardening with increasing temperature. The hardening of the most dominant mode (Tg + Ag) was analyzed in light of the theory of anharmonic phonon-phonon interaction and thermal expansion of the lattice. Using the mode Grüneisen parameter obtained from high pressure Raman measurements; we have calculated total anharmonicity of the Tg + Ag mode from the temperature dependent Raman data

High pressure behavior of nanocrystalline CeO2 up to 35 GPa: a Raman investigation

The present paper reports the results of in situ Raman studies carried out on nano-crystalline CeO2 up to a pressure of 35 GPa at room temperature. The material was characterized at ambient conditions using X-ray diffraction and Raman spectroscopy and was found to have a cubic structure. We observed the Raman peak at ambient at 465 cm−1, which is characteristic of the cubic structure of the material. The sample was pressurized using a diamond anvil cell using ruby fluorescence as the pressure monitor, and the phase evolution was tracked by Raman spectroscopy. With an increase in the applied pressure, the cubic band was seen to steadily shift to higher wavenumbers. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa. CeO2 was found to undergo a phase transition to an orthorhombic α -PbCl2-type structure at this pressure. With the release of the applied pressure, the observed peaks steadily shift to lower wavenumbers. On decompression, the high pressure phase existed down to a total release of pressure

Phase progression via phonon modes in lanthanide dioxides under pressure

The present paper reports the phase progression in nano-crystalline oxides PrO2 and CeO2 up to pressures of 49 GPa and 35 GPa, respectively, investigated via in situ Raman spectroscopy at room temperature. The samples were characterized at ambient conditions using X-ray diffraction (XRD), AFM, and Raman spectroscopy and were found to be cubic with fluorite structure. With an increase in applied pressure the cubic bands were seen to steadily shift to higher wavenumbers for both the samples. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa in CeO2 and 33 GPa in PrO2 which were characteristic of an orthorhombic alpha-PbCl2 type structure. The mode Gruneisen parameters for both the phases were obtained from the pressure dependence of frequency shifts. On decompression, the high pressure phase existed down to a total release of pressure

Computer simulation of a 1.0 CPa piston-cylinder assembly using finite element analysis (FEA)

The paper reports a preliminary study of the behavior of a high performance controlledclearance piston gauge (CCPG) in the pressure range up to 1 GPa through finite elemental analysis (FEA). The details of the experimental haracterization of this CCPG has already been published (Yadav et al., 2007 [1]). We have already pointed out that the use of Heydemann–Welch (HW) model for the characterization of any CCPG, has some limitation due to the fact that the linear extrapolation of the cube root of the fall rate versus jacket pressure (v1/3–pj) curve is assumed to be independent of the rheological properties of the pressure transmitting fluids. The FEA technique addresses this problem through simulation and optimization with a standard ANSYS program where the material properties of the piston and cylinder, pressure dependent density and viscosity of the pressure transmitting fluid etc. are to be used as the input parameters. Thus it provides characterization of a pressure balance in terms of effective area and distortion coefficient of the piston and cylinder.The present paper describes the results obtained on systematic studies carried out on the effect of gap profile between piston and cylinder of this controlled-clearance piston gauge, under the influence of applied pressure (p) from 100 MPa to 1000 MPa, on the pressure distortion coefficient (k) of the assembly. The gap profile is also studied at different applied jacket pressure (pj) such that pj/p varied from 0.3, 0.4 and 0.5

Pressure-induced anomalous phase transformation in nano-crystalline dysprosium sesquioxide

The phase transformation in nano-crystalline dysprosium sesquioxide (Dy<SUB>2</SUB>O<SUB>3</SUB>) under high pressures is investigated using in situ Raman spectroscopy. The material at ambient was found to be cubic in structure using X-ray diffraction (XRD) and Raman spectroscopy, while atomic force microscope (AFM) showed the nano-crystalline nature of the material which was further confirmed using XRD. Under ambient conditions the Raman spectrum showed a predominant cubic phase peak at 374 cm<SUP>-1</SUP>, identified as F<SUB>g</SUB> mode. With increase in the applied pressure this band steadily shifts to higher wavenumbers. However, around a pressure of about 14.6 GPa, another broad band is seen to be developing around 530 cm<SUP>-1</SUP> which splits into two distinct peaks as the pressure is further increased. In addition, the cubic phase peak also starts losing intensity significantly, and above a pressure of 17.81 GPa this peak almost completely disappears and is replaced by two strong peaks at about 517 and 553 cm<SUP>-1</SUP>. These peaks have been identified as occurring due to the development of hexagonal phase at the expense of cubic phase. Further increase in pressure up to about 25.5 GPa does not lead to any new peaks apart from slight shifting of the hexagonal phase peaks to higher wavenumbers. With release of the applied pressure, these peaks shift to lower wavenumbers and lose their doublet nature. However, the starting cubic phase is not recovered at total release but rather ends up in monoclinic structure. The factors contributing to this anomalous phase evolution would be discussed in detail