Search results for "stoichiometry"

Gamma-X-ray coincidence Mössbauer emission spectroscopy on57Co/CoO

1994

The time integral Mossbauer emission spectrum of a57Co/Co1−xO source (x ≈ 10−5) at RT consists of two single Lorentzian lines of an Fe2+ (76%) charge state and an Fe3+ (24%) aliovalent charge state. The spectrum measured by γ-X-ray coincidence spectrpscopy shows the same fraction of the aliovalent charge state, contrary to the expectation derived from the competing acceptor model as successfully applied by Tejada and Parak [1], who could explain the dependence of the formation of aliovalent charge states after the nuclear transformation on the stoichiometric parameterx. The consequences of this unexpected behaviour for the competing acceptor model are discussed.

Cathodoluminescence and IR absorption of oxygen deficient silica – influence of hydrogen treatment

1999

Abstract The cathodoluminescence (CL) and IR absorption of silica samples with normal stoichiometry as well as with an extremely high level of oxygen deficit were studied. Additionally, some samples have been treated in hydrogen at 800°C. Crystalline quartz was used for reference measurements and the CL data have been compared with those of X-ray excited luminescence (XL). The luminescence spectra of silica have a band at 1.85 eV due to non-bridging oxygens and the two bands at 2.7 and 4.4 eV due to twofold-coordinated silicons. The energetic yield for CL is about 0.1%, for XL it approaches 0.15%. Cathodoluminescence of quartz at temperatures >130 K exhibits the self-trapped exciton lumines…

Stoichiometry-related Auger lineshapes in titanium oxides: Influence of valence-band profile and of Coster-Kronig processes

2004

International audience; The ability to determine the nature and the occurrence of defects is a central need of ceramic surface chemistry. In titanium oxides, the Ti-LMV Auger decays line shape is very sensitive to the titanium degree of oxidation, and has long been empirically used as a qualitative probe of the stoichiometry. In the present work, resonant Auger and resonant valence-band measurements at the Ti-L2,3 edges in TiO2, TiO2–x and metallic titanium provide a clear evidence that the evolutions of the Ti-LMV Auger line shape are due to drastic changes in the valence-band profile and in the probability of L2L3V Coster-Kronig decay processes when a fraction of titanium ions is reduced.…

Complex formation of Tb3+ with glycolate, D-gluconate and α-isosaccharinate in neutral aqueous perchlorate solutions

2003

Summary An electromigration technique was used for measurements of metal-ligand formation constants of non-carrier-free 160Tb3+ with glycolate, D-gluconate and α-isosaccharinate ligands. The overall ion mobilities of Tb at different concentrations of the ligands were measured in chemically inert perchlorate solutions (pH 7 and T= 298.1K) with an overall ionic strength μ = 0.1. The stepwise stoichiometric stability constants are: Tb3+/glycolate: log K 1=2.72(18), log K 2=1.73(19), log K 3= 1.12(17), Tb3+/D-gluconate: log K 1=2.96(11), log K 2=2.60(11), log K 3=1.13(9), Tb3+/α-ISA: log K 1=3.07(8), log K 2 = 2.69(11), log K 3 = 1.80(12).

2021

FTIR spectroscopy was combined with the matrix isolation technique and quantum chemical calculations with the aim of studying complexes of isocyanic acid with sulfur dioxide. The structures of the HNCO⋯SO2 complexes of 1:1, 1:2 and 2:1 stoichiometry were optimized at the MP2, B3LYPD3, B2PLYPD3 levels of theory with the 6-311++G(3df,3pd) basis set. Five stable 1:1 HNCO⋯SO2 complexes were found. Three of them contain a weak N-H⋯O hydrogen bond, whereas two other structures are stabilized by van der Waals interactions. The analysis of the HNCO/SO2/Ar spectra after deposition indicates that mostly the 1:1 hydrogen-bonded complexes are present in argon matrices, with a small amount of the van de…

Formation of Layers of Diverse Stoichiometric and Phase Composition in Lithium Tantalate Crystals at Treatment by Vapour Transport Equilibration

2012

Layers from tens to hundreds of microns thick of a different phase composition and stoichiometry are shown to form in lithium tantalate crystals subjected to treatment by vapor transport equilibration in lithium vapour. The 500 microns thick stoichiometric (Li/Ta ≈ 1) layers have ten times lower values of the coercive field compared with congruent lithium tantalate. As electrical measurements suggest, a new polar structure of phase transition at ∼120°C is possibly formed within a thin (30 μm) surface layer of the lithium tantalate sample after vapour transport equilibration treatment.

Influence of BaTiO<inf>3</inf> on synthesis and structure of lead-free ceramics based on KNN

2012

In the present work lead-free compositions with a stoichiometric formula (1−x)(K 0.5 Na 0.5 )Nb 1−y Sb y O 3 -xBaTiO 3 (x=0.01, 0.02, 0.04; y=0.04, 0.07) were produced by solid state sintering method. Manganese oxide MnO 2 was used as a dopant in order to lower the sintering temperatures. The influence of BaTiO 3 on the microstructure, density, mechanical and electrical properties of Sb-substituted KNN ceramics was investigated. X-ray diffraction analysis revieled pure perovskite structure. Microstructural investigation of the ceramics samples showed homogeneous structure with a slightly rounded cubical grain shape. Phase transition temperature T c is shifted to lower temperatures in compar…

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…

Oxygen vacancy formation energies in Sr-doped complex perovskites: ab initio thermodynamic study

2014

Abstract La 1 − x Sr x Co 0.25 Fe 0.75 O 3 − δ is known as one of the best cathode materials for permeation membranes and solid oxide fuel cells. Optimization of its chemical composition is a challenging problem. One of the key properties is concentration of oxygen vacancies, which is controlled by their formation energies. Ab initio calculations were employed in order to study the formation of oxygen vacancies in La 1 − x Sr x Co 0.25 Fe 0.75 O 3 − δ perovskites by varying the Sr content from x = 12.5% to 50%. The formation energies were obtained for different stoichiometries as functions of temperature and oxygen partial pressure. For this purpose we calculated the phonon frequencies in t…

Ordered helium trapping and bonding in compressed arsenolite: Synthesis ofAs4O6·2He

2016

Compression of arsenolite has been studied from a joint experimental and theoretical point of view. Experiments on this molecular solid at high pressures with different pressure-transmitting media have been interpreted thanks to state-of-the-art ab initio calculations. Our results confirm arsenolite as one of the most compressible minerals and provide evidence for ordered helium trapping above 3 GPa between adamantane-type $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}$ cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry $\mathrm{A}{\mathrm{s}}_{4}{\mathrm{O}}_{6}\ifmmode\c…