Search results for "Spectroscopy"
showing 10 items of 10293 documents
Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change materialGe2Sb2Te5
2009
Phase change materials involve the rapid and reversible transition between nanoscale amorphous $(a\text{\ensuremath{-}})$ and crystalline $(c\text{\ensuremath{-}})$ spots in a polycrystalline film and play major roles in the multimedia world, including nonvolatile computer memory. The materials of choice are alloys of Ge, Sb, and Te, e.g., ${\text{Ge}}_{2}{\text{Sb}}_{2}{\text{Te}}_{5}$ (GST) in digital versatile disk--random access memory. There has been much speculation about the structure of $a\text{\ensuremath{-}}$ GST, but no model has yet received general acceptance. Here we optimize the structure by combining the results of density-functional calculations with high-energy x-ray diffr…
Pressure-Induced Transformations in PrVO4 and SmVO4 and Isolation of High-Pressure Metastable Phases
2013
Zircon-type PrVO4 and SmVO4 have been studied by high-pressure Raman spectroscopy up to 17 GPa. The occurrence of phase transitions has been detected when compression exceeds 6 GPa. The transformations are not reversible. Raman spectra of the high-pressure phases show similarities with those expected for a monazite-type phase in PrVO4 and a scheelite-type phase in SmVO4.The high-pressure phases have been also synthesized using a large-volume press and recovered at ambient conditions. X-ray diffraction measurements of the metastable products recovered after decompression confirms the monazite (PrVO4) and scheelite (SmVO4) structures of the high-pressure phases. Based upon optical properties …
Mössbauer gamma-ray diffraction from the molecular crystal KCN
1980
Abstract Mossbauer gamma-ray diffraction was applied to separate the elastic and inelastic scattering intensities from the (200), (400) and (600) Bragg reflections of KCN. The energy resolution of our experiment was 60 neV. The Debye-Waller factor extracted from the elastic data and the thermal diffuse inelastic data both increase towards phase transition, theoretically a logarithmic singularity was predicted.
Pressure-Driven Symmetry-Preserving Phase Transitions in Co(IO3)2
2021
[EN] High-pressure synchrotron X-ray diffraction studies of cobalt iodate, Co(IO3)(2), reveal a counterintuitive pressure-induced expansion along certain crystallographic directions. High-pressure Raman and infrared spectroscopy, combined with density-functional theory calculations, reveal that with increasing pressure, it becomes energetically favorable for certain I-O bonds to increase in length over the full range of pressure studied up to 28 GPa. This phenomenon is driven by the high-pressure behavior of iodate ion lone electron pairs. Two pressure-induced isosymmetric monoclinic-monoclinic phase transitions are observed at around 3.0 and 9.0 GPa, which are characterized by increasing o…
Structural and vibrational properties of CdAl2S4 under high pressure: Experimental and theoretical approach
2014
The behavior of defect chalcopyrite CdAl2S4 at high pressures and ambient temperature has been investigated in a joint experimental and theoretical study. High-pressure X-ray diffraction and Raman scattering measurements were complemented with theoretical ab initio calculations. The equation of state and pressure dependences of the structural parameters of CdAl2S4 were determined and compared to those of other AB(2)X(4) ordered-vacancy compounds. The pressure dependence of the Raman-active mode frequencies is reported, as well as the theoretical phonon dispersion curves and phonon density of states at 1 atm. Our measurements suggest that defect chalcopyrite CdAl2S4 undergoes a phase transit…
Experimental and Theoretical Studies on alfa-In2Se3 at High Pressure
2018
[EN] alpha(R)-In2Se3 has been experimentally and theoretically studied under compression at room temperature by means of X-ray diffraction and Raman scattering measurements as well as by ab initio total-energy and lattice-dynamics calculations. Our study has confirmed the alpha (R3m) -> beta' (C2/m) ? beta (R (3) over barm) sequence of pressure-induced phase transitions and has allowed us to understand the mechanism of the monoclinic C2/m to rhombohedral R (3) over barm phase transition. The monoclinic C2/m phase enhances its symmetry gradually until a complete transformation to the rhombohedral R (3) over barm structure is attained above 10-12 GPa. The second-order character of this transi…
Phase transitions in i-butylammonium halogenoantimonate(III) and bismuthate(III) crystals
1997
Abstract Differential scanning calorimetry, dielectric, thermal expansion, infrared and preliminary X-ray diffraction studies on i-butylammonium halogenoantimonate(III) and bismuthate(III) crystals are reported. All crystals: (i-C4H9NH3)2BiCl5, (i-C4H9NH3)2SbBr5, (i-C4H9NH3)3BiCl6, (i-C4H9NH3)3Bi2Br9, (i-C4H9NH3)3Sb2Br9, show one or more structural phase transitions of first order type. The values of the transition entropies suggest that the most of the phase transitions are of the order-disorder type. The infrared studies confirmed the contribution of the i-butylammonium cations in the phase transition mechanism.
Structural characterization, thermal, dielectric and spectroscopic properties of di(n-pentylammonium) pentabromoantimonate(III): [n-C5H11NH3]2[SbBr5]
2008
Abstract The di( n -pentylammonium) pentabromoantimonate(III) compound has been synthesized and studied by means of a single-crystal X-ray diffraction, differential scanning calorimetry, thermal expansion, dielectric and IR techniques. Two solid–solid phase transitions of first order: at 416/388 K and 225/224 K (heating/cooling) have been revealed. The crystal structure of [ n -C 5 H 11 NH 3 ] 2 [SbBr 5 ] has been solved at 298 K, Pna 2 1 (phase II) and at 86 K P 2 1 2 1 2 1 (phase III). The crystal structure is composed of the SbBr 5 2 - anions which form an infinite chain and four independent n -pentylammonium cations. The dielectric studies have been made in the frequency range 500 Hz–1 …
High-pressure vibrational and optical study of Bi2Te3
2011
We report an experimental and theoretical lattice dynamics study of bismuth telluride (Bi2Te 3 )u p to 23 GPa together with an experimental and theoretical study of the optical absorption and reflection up to 10 GPa. The indirect bandgap of the low-pressure rhombohedral (R-3m) phase (α-Bi2Te 3) was observed to decrease with pressure at a rate of − 6m eV/GPa. In regard to lattice dynamics, Raman-active modes of α-Bi2Te 3 were observed up to 7.4 GPa. The pressure dependence of their frequency and width provides evidence of the presence of an electronic-topological transition around 4.0 GPa. Above 7.4 GPa a phase transition is detected to the C2/m structure. On further increasing pressure two …
Giant conductivity enhancement: Pressure-induced semiconductor-metal phase transition in Cd0.90Zn0.1Te
2019
Element doping and pressure compression may change material properties for improved performance in applications. We report pressure-induced metallization in the semiconductor $\mathrm{C}{\mathrm{d}}_{0.90}\mathrm{Z}{\mathrm{n}}_{0.1}\mathrm{Te}$. Transport measurements showed an overall resistivity drop of 11 orders of magnitude under compression up to 12 GPa, which is indicative of a metallization transition. X-ray diffraction measurements revealed that the sample underwent a structural transition from a cubic-$F4\overline{3}m$ phase (zinc blende) to a cubic-$Fm\overline{3}m$ phase (rock salt) at about 5.5 GPa, followed by another transition to an orthorhombic $Cmcm$ structure at 13 GPa. A…