Structural and optical studies of FeSb2 under high pressure

Nanostructured orthorhombic FeSb2 and an amorphous phase were formed by mechanical alloying starting from a mixture of high purity elemental Fe and Sb powders. The effects of high pressures on structural and optical properties were studied using X-ray diffraction (XRD) and Raman spectroscopy (RS). XRD patterns showed the presence of the orthorhombic FeSb2 phase up to the maximum pressure applied (28.2 GPa). The XRD patterns showed also an increase in the amount of the amorphous phase with increasing pressure up to 23.3 GPa. At 14.3 GPa, together with the former phases, a new phase was observed and indexed to a tetragonal FeSb2 phase, but its volume fraction is small at least up to 23.3 GPa. For the orthorhombic FeSb2 phase, the pressure dependence of the volume fitted to a Birch-Murnaghan equation of state gave a bulk modulus = 74.2 +- 3.0 GPa and its pressure derivative = 7.5 +- 0.6. RS measurements were performed from atmospheric pressure up to 45.2 GPa. For the orthorhombic FeSb2 phase, the Raman active mode was observed up to the maximum pressure applied, while the mode disappeared at 16.6 GPa. For pressures higher than 21 GPa, the Raman active mode of a tetragonal FeSb2 phase was observed, confirming ab initio calculations reported in the literature.Comment: 31 pages, 11 figures and 2 tables. Already submitted for publicatio

Dependencia con la presión del índice de refracción del AgGaS2

En el presente trabajo estudiamos el comportamiento en presión del espectro de absorción (óptica de una muestra monocristalina de AgGaS2, realizando medidas en el rango de energías del infrarrojo, desde 0.30 hasta 0.70 eV, para presiones P inferiores a 4 GPa y temperatura ambiente T, utilizando para ello una celda de yunques de diamantes en combinación con la técnica de micro?espectroscopia infrarroja [1]. Con este estudio determinamos la variación de su índice de refracción n en función de la presión, en el rango de estabilidad de la estructura calcopirita [2?6], así como la variación en presión de las constantes dielectricas estática (?0) y de alta frecuencia (??). Nuestros resultados del índice de refracción a presión y temperatura ambiente en el AgGaS2 pueden compararse con los valores experimentales reportados por Boyd et al.In this work, we study the pressure behavior of the optical absorption spectrum of a single crystal AgGaS2, taking measurements in the infrared energy range from 0.30 up to 0.70 eV for pressures values P below 4GPa and room temperature T, using a diamond anvil cell in combination with infrared micro spectroscopy technique [1]. With this study, we determine the refraction index n variations in terms of pressure within the stability range of the chalcopyrite structure [2?6] as well as the changes under pressure of both the static (?0) and high frequency (??) dielectric constants. These results can be compared with the experimental values reported by Boyd et al

Absorción óptica a altas presiones del TLGaSe2

En el presente estudio hemos medido el espectro de transmisión óptica normal no polarizado en el compuesto semiconductor laminar TlGaSe2, hasta la presión de 27.6 GPa. Nuestros resultados muestran que en el rango del visible existe la contribución de dos brechas de energía directas, que presentan una dependencia lineal con la presión. La transición débil , asignada a la brecha fundamental de energía, decrece hasta los 0.9 GPa con un coeficient lineal de -5.31 × l0?2 eVGPa-1 y la transición muestra un coeficient de -9.95 × 10-2 eVGPa-1 hasta 5.3 GPa (límite de presión en el espectro visible). Los resultados en el infrarrojo no muestran la presencia de la transición EA, dejando ver solamente el comportamiento de la segunda transición justo hasta la presión de metalización 24.6 GPa. En el rango de presión estudiado de 0.0 a 27.6 GPa, la transición EB muestra un comportamiento no lineal con la presión de coeficient cuadrático 1.83 × l0 -3 eVGPa -2.In this paper the spectrum of optical normal transmission not polarized of TlGaSe2 is measured as a function of pressure up to 27.6 GPa at room temperature. Our results show that in the range of the visible exist the contributions of two direct gaps of energy, which present a linear dependence with the pressure. The weak transition assigned to the fundamental gap of energy, decreases up to 1.5 GPa with a linear coefficien of -31 × l0 ?2 eVGPa-1 and the transition shows a coefficien of -9.95 × 10-2 eVGPa-1 up to 5.3 GPa (limit of pressure in the visible spectrum). The results in the infrared do not show the presence of the transition EA allowing to see only the behavior of the second transition rightly up to the pressure of metallization 24.6 GPa. In the range of pressure studied from 0.0 to 27.6 GPa, the transition EB shows a not linear behavior with the pressure of quadratic coefficien 1.83 × l0 -3 eVGPa -2

High pressure as tool to study structural properties of thiolate heme proteins

A membrane driven high-pressure cell with sapphire anvils is used to measure the CO ligand stretch mode of various substrate complexes of cytochrome P450cam. In situ pressure measurement is performed using the ruby R1 luminescence band using a Raman spectrometer coupled via fiber optics to the sample compartment of the FT infrared spectrometer. It was found for all substrate complexes that the CO stretch mode shifts to lower frequencies with increasing pressure. The slope of the shift is interpretated as compressibility and correlates with the high-spin state content produced by the substrates in the oxidized form of the enzyme

Brillouin study of liquid and solid ammonia up to 20 GPa

International audienc

Constantes optiques de InSe entre 10 500 cm-1 (1,30 eV) et 22 500 cm-1 (2,78 eV)

We present measurements of the optical constants (refractive index and absorption coefficient) of InSe at 10 K and 300 K. The experiments were performed with polarizations both parallel and perpendicular to the sheet planes of this layer compound. With Elliott's theory, we give a quantitative evaluation of the absorption coefficient at 10 K, the light being polarized perpendicular to the c-axis between 10 500 cm-1 (1.30 eV) and 22 500 cm -1 (2.78 eV). There is a good fit for the two excitonic series bound to the Eo (10 885 cm-1) and E1 (20 682 cm -1) transitions, using a three-dimensional hydrogenoid model.Nous présentons les mesures des constantes optiques (indice de réfraction et coefficient d'absorption) de InSe à 10 K et à 300 K. Les expériences ont été faites en polarisations perpendiculaire et parallèle aux plans des couches de ce composé lamellaire. L'application de la théorie d'Elliott nous permet de rendre compte de manière quantitative du coefficient d'absorption à 10 K, en polarisation perpendiculaire à l'axe c entre 10 500 cm-1 (1,30 eV) et 22 500 cm-1 (2,78 eV). L'accord est obtenu pour les deux séries excitoniques liées aux transitions Eo (10 885 cm -1) et E1 (20 682 cm-1) en prenant un modèle hydrogénoïde à trois dimensions

A thermodynamic model for MgSiO3-perovskite derived from pressure, temperature and volume dependence of the Raman mode frequencies

Raman spectra of MgSiO3-perovskite (Mg-pv) were recorded at simultaneous high-pressure and low-temperature conditions. This allowed to estimate characteristic frequencies (nu(i)) and other mode parameters as a function of both pressure and temperature. The cross derivatives partial derivative(2)nu(i)/partial derivative T partial derivative P were measured for the first time, thus providing new insights into the lattice dynamics of Mg-pv. These parameters are negative for the two lowest frequency modes at 250 and 255 cm(-1) (approximate to-6 X 10(-4) cm(-1) GPa(-1) K-1) and positive for the other modes (+3 X 10(-4) to +5 X 10(-4) cm(-1) GPa(-1) K-1). These measurements were combined with previously published vibrational density of states (VDoS) for deriving entropy, specific heat, thermal pressure, equation of state (EoS), and various thermoelastic parameters of Mg-pv at mantle P-T conditions. The calculations were performed using a general anharmonic formulation including the spectroscopically measured parameters, It is shown that anharmonic effects are relatively small in this compound under geophysically relevant conditions especially for the EoS. The model is used to discuss the discrepancies in the pressure and temperature calibrations in diamond-anvil cells and multianvil presses. Finally, a complete thermodynamic data set for (Mg0.9Fe0.1)SiO3-perovskite is proposed along lower mantle geotherms, It is shown that a pure perovskite lower mantle is unlikely to exist. (C) 2000 Elsevier Science B.V, All rights reserved

Compressibility and uncoupling of cytochrome P450cam: high pressure FTIR and activity studies

The effect of the hydrostatic pressure on the CO ligand stretch vibration in cytochrome P450cam-CO bound with various substrates is studied by FTIR. The vibration frequency is linearily shifted to lower values with increasing pressure. The slope of the shift gives the isothermal compressibility of the heme pocket and is found to be related to the high-spin state content in an opposite direction to that previously observed from the pressure-induced shift of the Soret band. This opposite behaviour is explained by the dual effect of heme pocket water molecules both on the CO ligand and on electrostatic potentials produced by the protein at the distal side. The latter effect disturbs ligand-distal side contacts which are needed for a specific proton transfer in oxygen activation when dioxygen is the ligand. Their loss results in uncoupled H2O2 formation

Amorphous germanium under high-pressure conditions

International audienceIn this work, we address the question of pressure-induced phase transitions in amorphous germanium.We discuss both the pressurization and depressurization cycles on an amorphous film, showing the appearance of various stable and metastable (LDA–HDA) phases by Raman scattering and X-ray absorption spectroscopy. In particular, we report evidence of recrystallization in the metastable ST12 phase driven by the density of voids of the film