Stoichiometry, deviation from

Nickel oxide, NiO, which is the only oxide formed by nickel during oxidation in air, has a very naiTow range of iroir-stoichiomen y, the maximum oxygeir/nickel ratio probably being 1.001. The oxygen dependence of the deviation from non-stoichiometry is and hence dre oxidation rate... 

Transition elements, for which variable valency is energetically feasible, frequently show non-stoichiometric behaviour (variable composition) in their oxides, sulfides and related binary compounds. For small deviations from stoichiometry a thermodynamic approach is instructive, but for larger deviations structural considerations supervene, and the possibility of thermodynamically unstable but kinetically isolable phases must be considered. These ideas will be expanded in the following paragraphs but more detailed treatment must be sought elsewhere. " ... 

The homogeneity ranges and defect structures of the hexaborides lead to deviations from stoichiometry through the cation defects (see Table 1). 

Extensive structural, optical, and electronic studies on the chalcopyrite semiconductors have been stimulated by the promising photovoltaic and photoelectrochem-ical properties of the copper-indium diselenide, CuInSe2, having a direct gap of about 1.0 eV, viz. close to optimal for terrestrial photovoltaics, and a high absorption coefficient which exceeds 10 cm . The physical properties of this and the other compounds of the family can be modulated to some extent by a slight deviation from stoichiometry. Thus, both anion and cation deficiencies may be tolerated, inducing, respectively, n- and p-type conductivities a p-type behavior would associate to either selenium excess or copper deficiency. 

The photovoltaic implications of the solid-state chemistry of CuInSe2, particularly where deviations from the 1 1 2 stoichiometry along the Cu2Se-In2Sc3 tie line are concerned, have been studied, and results of measurements of some photoelectrochemical properties along this tie line (e.g., for 283) have been given [169]. 

When both Vx and Xj coexist in the lattice, the deviation from stoichiometry (from 3.7.10.) becomes ... 

Since Br2 (gas) is the driving force for defect formation, we need also to consider deviation from stoichiometry, 8. Thus, we also set a Agi=6 Br balance ... 

This result then leads us to a plot of the effect of partial pressure of Bt2 on the deviation from stoichiometry, 6, for the AgBr crystal. (This work is due to Greenwood - 1968). 

Perhaps the most discouraging type of deviation from linearity is random scatter of the data points. Such results indicate that something is seriously wrong with the experiment. The method of analysis may be at fault or the reaction may not be following the expected stoichiometry. Side reactions may be interfering with the analytical procedures used to follow the progress of the reaction, or they may render the mathematical analysis employed invalid. When such plots are obtained, it is wise to reevaluate the entire experimental procedure and the method used to evaluate the data before carrying out additional experiments in the laboratory. 

The first sample has the greatest deviation from stoichiometry. Samples JV°1 - JV°4 have been found to be nonstoichiometric oxide -hydroxide type. They have a mixed conductivity - ionic (o,) and electronic (ae). The ionic one is due to the presence of OH" - groups. Namely, they stabilize the defects of chemical nature in such compounds. These defects are determined by the presence of Mn4+ and Mn3+ in the same crystallographic position. 

Oxide compounds are widely used as cathodic materials in the power sources and electrochemical generators. Some literature data indicates that cathodic materials based on nonstoichiometric oxide compounds make it possible to increase the solid-phase reduction process. The kinetics of electrochemical reactions and consequently the current density are the higher, the greater the degree of deviation from stoichiometry, and the lager the number of the defects in the compounds structure [1,2]. 

Unlike donor-based FRET methods like FLIM, filterFRET also yields spatial information on the acceptor population. This means that in addition to querying donor-FRET (by solving for Ed or / )), we can also assess the relationship between sensitized emission and the acceptor population. At 1 1 stoichiometry obviously Ed should equal the acceptor-normalized efficiency EA. In other cases, EA deviates from E but sometimes can yield biologically more relevant information than Ed or E. For example, dislocation of 50% of the... 

In theory, the III-V compound semiconductors and their alloys are made from a one to one proportion of elements of the III and V columns of the periodic table. Most of them crystallize in the sphalerite (zinc-blende ZnS) structure. This structure is very similar to that of diamond but in the III-V compounds, the two cfc sublattices are different the anion sublattice contains the group V atoms and the cation sublattice the group III atoms. An excess of one of the constituents in the melt or in the growing atmosphere can induce excess atoms of one type (group V for instance) to occupy sites of the opposite sublattice (cation sublattice). Such atoms are said to be in an antisite configuration. Other possibilities related with deviations from stoichiometry are the existence of vacancies (absence of atoms on atomic sites) on the sublattice of the less abundant constituent and/or of interstitial atoms of the most abundant one. 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

The chemical composition of the IF phase deviates only very slightly, if at all, from the composition of the bulk layered compound. Deviations from stoichiometry can only occur in the cap of the nanotube. In fact, even the most modem analytical techniques, like scanning probe techniques and high (spatial) resolution electron energy loss spectroscopy, are unable to resolve such a tiny deviation from the stoichiometry, like the excess or absence of a single Mo (W) or S (Se) atom in the nanotube cap. 

Along these fines, Liebermeister and Klipp [161] suggested the use of a rapid-equilibrium random-order binding scheme as a generic mechanism for all enzymes, independent of the actual reaction stoichiometry. While there will be deviations from the (unknown) actual kinetics, such a choice, still outperforms power-law or lin-log approximations [161]. 

---