Search results for "Metas"
showing 10 items of 1693 documents
Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites
2019
Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films…
Polymorphism of praseodymium orthovanadate under high pressure
2021
Zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ has been studied at high pressures and room temperature by means of synchrotron powder x-ray diffraction. At room temperature, we observed the previously known zircon-to-monazite phase transition at 5.5(4) GPa and a second phase transition from monazite to a monoclinic structure at 12.7(8) GPa, which we identified as a $\mathrm{PbW}{\mathrm{O}}_{4}$-III-type phase. This conclusion is supported by our ab initio calculations, which also predict a scheelite-type phase to be stable at high pressure. Motivated by this finding, we subjected zircon-type $\mathrm{PrV}{\mathrm{O}}_{4}$ samples to high pressure (7 GPa) and temperature (600, 800, and 1000 \if…
Pressure-induced order–disorder transitions in β-In2S3: an experimental and theoretical study of structural and vibrational properties
2021
This joint experimental and theoretical study of the structural and vibrational properties of β-In2S3 upon compression shows that this tetragonal defect spinel undergoes two reversible pressure-induced order-disorder transitions up to 20 GPa. We propose that the first high-pressure phase above 5.0 GPa has the cubic defect spinel structure of α-In2S3 and the second high-pressure phase (ϕ-In2S3) above 10.5 GPa has a defect α-NaFeO2-type (R3m) structure. This phase, related to the NaCl structure, has not been previously observed in spinels under compression and is related to both the tetradymite structure of topological insulators and to the defect LiTiO2 phase observed at high pressure in oth…
Cavitation in 3He-4he Liquid Mixtures
2002
Phase transitions under equilibrium conditions are experimentally well determined and take place in the coexistence regime. However, phase transitions do not always occur under equilibrium conditions. As the new phase forms, the free energy of the system is lowered, but the original phase can be held in a metastable state close to the equilibrium transition point. Although they are internally stable, in each case there exists another configuration that has a lower thermodynamical potential. The metastable state is separated from the stable state by a thermodynamic barrier. Due to statistical fluctuations in density or concentration, this barrier can be overcome as the result of the formatio…
Lattice dynamics of zircon-type NdVO4 and scheelite-type PrVO4 under high-pressure
2021
Abstract Zircon-type NdVO4 and scheelite-type PrVO4 have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO4, a reversible phase transition to a PbWO4-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported by ab initio calculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase …
Kinetic behaviour of a structural phase transition in Langmuir-Blodgett multilayers studied by energy dispersive x-ray reflectivity
1992
Various multilayers of Cd stearate were prepared by means of the Langmuir-Blodgett technique and investigated at T = Ts - ΔT (Ts, melting point of LB film). The x-ray scattering spectrum was recorded in several time intervals using an energy dispersive set up. For ΔT = 3 ... 9 K the observed intensity reduction of Bragg peaks, which corresponds to the multilayer periodicity, is recorded as a function of time. In addition new peaks appear. This is interpreted in terms of a kinetically driven phase separation of the metastable LB-phase. The time needed for phase disorder and the nucleation of the new phase decreases for smaller ΔT. The measured layer spacing of the new phase is larger than th…
Pressure dependence of photoluminescence of InAs/InP self-assembled quantum wires
2007
6 páginas, 4 figuras, 1 tabla.-- PACS 62.50.+ p, 73.21.Hb, 78.55.Cr, 78.67.Lt, 81.15.Hi, 81.16.Dn
High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping—Dynamics of Spin Transition Versus Dynamics o…
1996
The iron(II) complex [Fe(bpp)2]-(CF3SO3)2 H2O (bpp = 2,6-bis(pyrazolyl-3-yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse-LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mossbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal …
Double-well thermodynamic potentials and spinodal curves: how real are they?
2007
The concept of double-well thermodynamic potentials, ubiquitous since the van der Waals description of the vapour-to-liquid transition and the Landau theory of phase transitions, is critically re-examined. Particular emphasis is put on the extent to which spinodal curves (separating ‘metastable’ from ‘unstable’ states) are meaningful. It is argued that in full thermodynamic equilibrium spinodals are well-defined when one either considers finite subsystems of an infinitely large system, or systems with all linear dimensions finite. Evidence is given that in a finite (cubic) d-dimensional box the spinodals correspond (in a fluid) to the rounded ‘droplet evaporation’ or ‘bubble condensation’ t…
Luminescence-detected EPR of oxygen-fluorine vacancy complexes in CaF2
2009
In several CaF2 single crystals grown by the Bridgman method and doped with CaO or in addition with SrF2 or NaF a luminescence band between 470 nm and 600 nm could be excited at 212 nm, its peak wavelength depending on the doping. With photo-luminescence (PL) detected EPR five spin triplet centres were identified. Their axial fine structure constants D varied from 87 mT to 690 mT whereby the most intense spectra had the smallest D value. Theoretical calculations of the fine structure tensors and superhyperfine interactions show that the most intense and probable triplet centre consists of a pair of an OF– on a F– site next to a nearest neighbour F– vacancy which, compared to the well-known …