Search results for "Ambient pressure"
showing 10 items of 40 documents
Stability of the fergusonite phase in GdNbO 4 by high pressure XRD and Raman experiments
2017
Abstract We describe the results of high pressure x-ray diffraction and Raman measurements on gadolinium orthoniobate. The ambient pressure monoclinic fergusonite phase remains stable in a remarkable large pressure range. There is no significative evolution of the monoclinic distortion up to 25 GPa , the maximum pressure achieved. Instead, the anisotropic compressibility is associated to the stiffness of NbO 4 tetrahedra in respect to the GdO 8 polyhedra. The high pressure evolution of external modes parallels the wavenumber dependence on ionic radius along the lanthanide series. The chemical pressure analogy is attributed to the compression of GdO 8 polyhedra. There is no evidence of any p…
Compressibility and structural behavior of pure and Fe-doped SnO2 nanocrystals
2017
We have performed high-pressure synchrotron X-ray diffraction experiments on nanoparticles of pure tin dioxide (particle size ~30nm) and 10 mol % Fe-doped tin dioxide (particle size ~18nm). The structural behavior of undoped tin dioxide nanoparticles has been studied up to 32 GPa, while the Fe-doped tin dioxide nanoparticles have been studied only up to 19 GPa. We have found that both samples present at ~13 GPa a second-order structural phase transition from the ambient pressure tetragonal rutile-type structure (P42/mnm) to an orthorhombic CaCl2-type structure (space group Pnnm). No phase coexistence was observed for this transition. Additionally, pure SnO2 presents a phase transition to a …
Investigation of conduction-band structure, electron-scattering mechanisms, and phase transitions in indium selenide by means of transport measuremen…
1996
In this work we report on Hall effect, resistivity and thermopower measurements in n-type indium selenide at room temperature under either hydrostatic and quasi-hydrostatic pressure. Up to 40 kbar (= 4 GPa), the decrease of carrier concentration as the pressure increases is explained through the existence of a subsidiary minimum in the conduction band. This minimum shifts towards lower energies under pressure, with a pressure coefficient of about -105 meV/GPa, and its related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. The pressure value at which the electron trapping starts is shown to depend on the electron concentration at ambient pressure an…
Pressure-induced hysteresis in the high spin \leftrightarrow low spin transition in bis(2,4-bis(pyridin-2-yl)thiazole) iron(II) tetrafluoroborate
2008
Studies of the spin transition behavior of the mononuclear compound [Fe(pythiaz)2](BF4)2 have been carried out under hydrostatic pressures up to 9.13 kbar in the 5–300 K temperature range. Under ambient pressure this compound exhibits an approximately half-step (incomplete) HS ↔ LS transition with T1/2 = 146 K without any thermal hysteresis. At pressures up to 4.5 kbar the behavior remains similar but with an upward displacement of T1/2 and a slight decrease in the residual high spin fraction at low T . Application of higher pressures resulted in an almost complete two-step spin transition with several unusual pressure effects. Along with the expected pressure dependence of T1/2 the surpris…
Synergy of Miniemulsion and Solvothermal Conditions for the Low-Temperature Crystallization of Magnetic Nanostructured Transition-Metal Ferrites
2017
Crystalline first-row transition-metal (Mn, Fe, Co, Ni, Cu, and Zn) ferrites were prepared by an unprecedented synergetic combination of miniemulsion synthesis and solvothermal route, pursuing unconventional conditions in terms of space confinement, temperature, and pressure. This synergy allowed for obtaining six different crystalline ferrites at much lower temperature (i.e., 80 °C) than usually required and without any postsynthesis thermal treatment. X-ray diffraction (XRD) revealed that analogous ferrites synthesized by miniemulsion at ambient pressure or in bulk (i.e., from an aqueous bulk solution and not in the confined space of the miniemulsion droplets) either at ambient pressure o…
Pressure Effect Studies on the Spin‐Transition Behavior of a Dinuclear Iron(II) Compound
2013
Magnetic studies into the effect of different hydrostatic pressures between ambient and 1.03 GPa on the high-spin (HS) i low-spin (LS) transition behavior of the dinuclear iron(II) compound [Fe II 2(PMAT)2](BF4)4·DMF (1, PMAT = 4-amino3,5-bis{[(2-pyridylmethyl)amino]methyl}-4H-1,2,4-triazole, DMF = N,N-dimethylformamide) have been carried out at 2– 300 K. Under ambient pressure, the sample studied exhibits a [HS–HS] to [HS–LS] half spin transition (ST) at T½ = 208 K without any thermal hysteresis. Increasing the pressure above 0.2 GPa causes an increase (initially rapid but above 0.5 GPa more gradual) of T½ as well as a matching reduction in the residual high-spin fraction at room temperatu…
In-situ high-pressure Raman scattering studies in PbWO4 up to 48 GPa
2016
The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphization. All structural changes are reversible, being the scheelite phase recovered at ambient pressure.…
ChemInform Abstract: New Developments in Nitrogen Fixation.
2010
The production of ammonia from atmospheric dinitrogen at room temperature and ambient pressure in analogy to nature is a long-term goal for coordination chemists. Novel reactions of N2 -containing transition metal complexes with H2 , the first side-on N2 -bridged structure of an actinide complex, and an interesting variation of synthetic N2 fixation are the key points addressed in this contribution. The results are related to the known chemistry of N2 complexes, and their significance is discussed with respect to enzymatic N2 fixation.
Direct to Indirect Crossover in III-VI Layered Compounds and Alloys under Pressure
1999
The pressure dependence of the optical absorption edge of In1± xGaxSe (0 < x < 0.2) and GaTe has been investigated in order to determine the direct to indirect crossover pressure and the energy difference between the absolute and subsidiary minima of the conduction band at ambient pressure. In the In1± xGaxSe alloy, the crossover pressure decreases with increasing Ga proportion. For InSe, from the extrapolation to x = 0 the band crossover is found to occur at 4.3 GPa and the subsidiary minimum of the conduction band is located, at ambient pressure, (0.32 0.02) eV above the absolute minimum. In addition, the energy difference between the conduction band minima is shown to decrease linearly w…
ScVO4 under non-hydrostatic compression:a new metastable polymorph
2016
Ustedes se ocupan e ver si se puede hacer de acceso público. Podria buscra el preprint al ser algo reciente. Se estudia el comportamiento bajo alta presión del vanadato de scandio, ScVO4, bajo compresión no hidrostática. El estudio se realiza mediante difracción de rayos X en polvo usando radiación sincrotrón. Se detecta una transición no reversible desde la fase zircon a la fase fergusonita alrededor de 6 GPa con una discontinuidad en el volumen de un 10%. La fase fergusonota se puede recuperar como metaestable confirmandose mediante XRD. Las simulaciones ab intio confirman los resultados experimentales. Las propiedades ópticas y la propiedades vibracionales de la fase fergusonita son disc…