Search results for "Phase Transition"
showing 10 items of 1281 documents
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 …
In situ XAFS study of phase transitions and hydrogen intercalation in WO3-MoO3 system
1995
Abstract The WO 3 -MoO 3 system has several reconstructive and displacive phase transitions which can be induced by temperature and/or by change of stoichiometry or valence state of metal ions under hydrogen intercalation. We have studied in situ the local electronic and structural changes arising in WO 3 -MoO 3 system with hydrogen intercalation and temperature. In situ XAFS measurements at the Mo K and W L 3 edges in H y Mo x W 1− x O 3 compounds, performed at different hydrogen intercalation level and temperatures (from RT to 500°C), allowed us to determine the rearrangement in the first and second coordination shells of Mo and W cations during phase transitions. It was found that hydrog…
High-pressure electronic structure and phase transitions in monoclinic InSe: X-ray diffraction, Raman spectroscopy, and density functional theory
2008
We have studied the crystal and electronic structure of monoclinic (MC) InSe under pressure finding a reversible phase transition to a ${\mathrm{Hg}}_{2}{\mathrm{Cl}}_{2}$-like tetragonal phase. The pressure evolution of the crystal structure was investigated by angle-dispersive x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. From the diffraction experiments, we deduced that MC InSe becomes gradually more symmetric under pressure, transforming the crystal structure into a tetragonal one at $19.4\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This phase transition occurs without any volume change. Ra…
Spectroscopic ellipsometry applied to phase transitions in solids: possibilities and limitations
2009
The possibilities of in situ spectroscopic ellipsometry applied to phase transitions investigation in oxide thin films and crystals are examined in this work, along with the use of various parameters calculated from ellipsometric data (band gap energy Eg, refractive index n and surface roughness) together with the directly measured main ellipsometric angles psi and Delta, for the detection of phase transitions. The efficiency of spectroscopic ellipsometry on "surface" phase transition and its sensitivity to surface defects are also demonstrated.
Kinetics of dielectric response in SBN-75 ceramics at infra-low frequencies under illumination
2014
A study of the photo-dielectric effect in ferroelectric SBN-75 ceramics of a broad phase transition under light of small intensity is reported. Relations determining kinetics of the infra-low frequency dielectric response under light and after illumination are obtained from experimental measurements revealing the features of the behaviour of relaxation time constants of the dielectric polarisation with temperature.
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…
Spatial inhomogeneities and defect structures in CIGS and CIS materials: An ab-initio based Monte Carlo study
2011
The chalcopyrite semiconductors CuIn 1−x Ga x Se 2 (CIGS) and CuInSe 2 (CIS) are excellent materials for high efficiency and low cost thin-film solar cells. This is due to the effective absorption of the solar spectrum and the inherent resilience to defects and composition fluctuations. Although the CIGS and CIS material in solar cells is highly inhomogeneous and exhibits a lot of different defects, the cell efficiencies are exceptionally high. If single crystalline absorbers are used, efficiencies are lower. Therefore, studying spatial inhomogeneities and defect structures is of great importance for understanding what supports and what diminishes the efficiency and robustness of the cells.…
Verwey-type transition in EuNiP
2006
High temperature 151Eu Mossbauer measurements provide proof for inhomogeneous mixed-valent behaviour in EuNiP. We observed that EuNiP undergoes a Verwey-type charge delocalisation transition when heated above 470 K prior to the structural γ-β phase transition at T ≈ 510 K. This finding confirms the results of photoemission spectroscopy in the isostructural compound EuPdP and of TB-LMTO-ASA band structure calculations. We discuss the role of a van Hove singularity associated with a high density of 4f states close to the Fermi energy in inhomogeneous mixed europium valency, and the microscopic mechanism of γ-β phase transition in compounds analogous to EuNiP.
Crystallographic changes and thermal properties of lanthanum?strontium ferromanganites between RT and 700 �C
2005
Abstract X-ray powder diffraction was used to determine the crystallographic changes between room temperature (RT) and 700 °C of the (La 0.8 Sr 0.2 )(Mn 1− y Fe y )O 3± δ perovskites with y =0.2, 0.5, 0.8, 1 which can be considered as possible cathode materials for the ITSOFCs. For y =0.2, the rhombohedral symmetry was preserved in the whole temperature range. For y =1, a transition from orthorhombic to rhombohedral symmetry occurred at 290 °C. The compounds with y =0.5 and 0.8 were found to be diphasic at RT by combination of both rhombohedral and orthorhombic symmetries. They became only rhombohedral at 80 and 160 °C, respectively. This reversible transition is discussed in terms of iron …
Multitechnique Analysis of the Hydration in Three Different Copper Paddle-Wheel Metal-Organic Frameworks
2019
The structural instability in a humid environment of the majority of metal-organic frameworks (MOFs) is a challenging obstacle for their industrial-scale development. Recently, two water-resistant MOFs have been synthetized, STAM-1 and STAM-17-OEt. They both contain copper paddle wheels, like the well-known water-sensitive HKUST-1, but different organic linkers. The crystal lattice of both the MOFs undergoes a phase transition upon interaction with water molecules. Their unusual flexibility allows the controlled breaking of some interpaddle wheel Cu-O interactions in the so-called crumple zones, with a mechanism called hemilability, which is considered to have a crucial role for the stabili…