Composition Variations

Gross stoichiometry variations in these modular systems are often acconunodated by the formation of other series members or the incorporation of planar faults, often equivalent to isolated lamellae of other series members, into the structure. However, many individual phases also show composition variation which is sometimes accommodated by random populations of point defects and in other phases by ordering and the generation of new structures. In both cases composition flexibility is important in influencing the physical and chemical properties of these materials. In particular this effect has been well studied in the cuprate superconductors, where changes in composition have repercussions for the superconducting transition temperature, r, of the phase. 

Oxygen loss is common in phases in which the B cations can take on a variable valence. In some phases these are distributed at random over the normal oxygen positions, thereby transforming some BO octahedra into BO square pyramids. This occurs in the n=2 Ruddlesden-Popper phase LajNi O when reduced in hydrogen to a composition of La3Ni20g3j. In the reduced stmcture, almost 2/3 of the nickel atoms are in square pyramidal coordination, which is well suited to the nominal charge state of NF (Ni j), thus generating 

The oxygen vacancies appear to be disordered in this phase. Similarly, in the n=3 Ruddlesden-Popper phase Sr Fep in which 5 lies between 0 and 0.73, the oxygen vacancies appear to be disordered. The oxygen loss is balanced by the Fe /Fe ratio, as nominally StjFe +Og gradually transformations to StjFe Fe Og j. The reaction for the oxidation of La,Fe,0. is 

The holes so formed are believed to be located upon Cu ions (Cu, ) to form Cu  

The compound Sr CuO F, a poor conductor, can take on extra F ions to form Sr CuOjF. The additional F ions occupy similar interstitial sites to those occupied in La CuO. Once again the change in stoichiometry results in the generation of holes  

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Whereas there is no universally accepted specification for marketed natural gas, standards addressed in the United States are Hsted in Table 6 (8). In addition to these specifications, the combustion behavior of natural gases is frequently characteri2ed by several parameters that aid in assessing the influence of compositional variations on the performance of a gas burner or burner configuration. The parameters of flash-back and blow-off limits help to define the operational limits of a burner with respect to flow rates. The yeUow-tip index helps to define the conditions under which components of the natural gas do not undergo complete combustion, and the characteristic blue flame of natural gas burners begins to show yellow at the flame tip. These... 

Polymers and Coatings Advances ia polymer chemistry have resulted ia many successful medical devices, including diagnostic assays (26). Polymers (qv), which can be manufactured ia a wide range of compositions, ate used to enhance speed, sensitivity, and versatiUty of both biosensors and dry chemistry systems to measure vital analytes. Their properties can be regulated by composition variations and modifications. Furthermore, polymers can be configured iato simple to complex shapes. 

Fig. 1. Effect of compositional variations on the dielectric properties of strontium titanate-barium titanate solid solutions. A, BaQ SrQ QTiO B,...

Many more correlations are available for diffusion coefficients in the liquid phase than for the gas phase. Most, however, are restiicied to binary diffusion at infinite dilution D°s of lo self-diffusivity D -. This reflects the much greater complexity of liquids on a molecular level. For example, gas-phase diffusion exhibits neghgible composition effects and deviations from thermodynamic ideahty. Conversely, liquid-phase diffusion almost always involves volumetiic and thermodynamic effects due to composition variations. For concentrations greater than a few mole percent of A and B, corrections are needed to obtain the true diffusivity. Furthermore, there are many conditions that do not fit any of the correlations presented here. Thus, careful consideration is needed to produce a reasonable estimate. Again, if diffusivity data are available at the conditions of interest, then they are strongly preferred over the predictions of any correlations. 

Attack at welds due to bacteria is possible, but it is not nearly so common as is often supposed. Because of residual stresses, microstruc-tural irregularities, compositional variation, and surface irregularities, welds show a predisposition to corrode preferentially by most corrosion mechanisms. Attack is common along incompletely closed weld seams such as at butt welds in light-gauge stainless steel tubing (Fig. 6.9A and B). Attack at carbon steel welds may occur. Figure 6.10 shows a severely corroded carbon steel pipe from a service water sys-... 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

The major artifacts contributing to uncertainties in PDCE results stem from effects caused by bombardment of nonideal specimens, particularly thick specimens. The ideal thick specimen would be a homogeneous, smooth electrical conductor that does not change during bombardment. Except for rather simple, well-defined layered structures (e.g., surface oxide layers), specimens having compositional variations with depth yield spectra whose analyses can have large inaccuracies. 

Exchangers failed after two weeks operation due to high chloride content of coohng water, causing stress-corrosion cracking of 304 stainless steel. lOne application failed due to unanticipated composition variations. Relatively low unprotected rate did not provide incentive for further work. 

Sample analyses by metallography, x-ray diffraction and spectroscopy confirmed the desired solid compositions. These analyses revealed that only the small compositional variations expected due to the sublimation were encountered during the studies. 

Recognition among bone-chemistry researchers that strontium enters bone in proportion to dietary levels has resulted in widely accepted yet erroneous inferences about the relationships among various elements in bone and past diet. One such inference is that more of any element in the diet translates directly to more of that element in bone. If an element is not biogenically incorporated within bone, or if biological levels are metabolically controlled, then that element will not reflect diet. A second erroneous inference is that strontium can be used to measure the dietary plant/meat ratio. Sr/Ca ratios in meat are generally lower than those of plants, but meat is also low in calcium and hence has little effect on the composition of bone. Plants, on the other hand, contribute substantially to bone composition. Variations in the strontium levels of bone thus more likely reflect differential consumption of plants rather than trophic position. Although efforts to determine plant/meat ratios from strontium and to draw dietary inferences from elements other than strontium and barium have not been successful, this failure has been due to inappropriate expectations, not to a failure of bone strontium to reflect diet. 

A vertical CVD reactor (cf. Figure lb) consists of an axlsymmetrlc enclosure with the deposition surface perpendicular to the Incoming gas stream. The reactant gases are typically Introduced at the top and fiow down towards the heated susceptor. Thus, the least dense gas Is closest to the growth Interface which destabilizes the fiow. The result Is recirculation cells which Introduce not only film thickness and composition variations but also broaden Junctions between layers. This Is particularly of... 

Martin, S. S., Langenheim, J. H. and Zavarin, E. 1973. Compositional variation of leaf pocket sesquiterpenes in Trachylobium verrucosum. Biochem. Syst. 1 35-37. 

No polydispersity due to compositional variation can be distinguished. Furthermore, when stan rd deviations were superimposed on the calibration curves (Figure 19) it was very evident that resolution in both GPCs had to be improved, particularly in GPC 1. 

Chemical compositions of tetrahedrite-tennantite from epithermal base-metal vein-type deposits are characterized by (1) wide compositional variations, and (2) higher Zn and Sb contents and Ag and lower Fe, As, and Cu contents, compared with Kuroko deposits (Shikazono and Kouda, 1979). 

Consequently, the composition of chlorite in the discharge zone depends largely on the chemical nature of fluids (factors such as Fe "/Mg, SO /H2S, pH, aj 2+) and temperature. Movement of fluids may also be an important cause for the variability in the ratio of Fe " to Mg in hydrothermal chlorite. Wide compositional variations in chlorite from the hydrothermal ore deposits in Japan, including Kuroko and Neogene Cu-Pb-Zn vein-type deposits, are considered to reflect the variable chemical nature of ascending ore fluids and fluids that mix with ascending ore fluids at discharge zone. 

Shikazono (1984) summarized analytical data of the epidote from geothermal areas to consider the relationship between the composition of epidote and that of the original fresh rocks and to inspect the other factors controlling the compositional variations in epidote. The discussion on the epidote composition by Shikazono (1984) is described below. 

Figures 1.137 and 1.138 show the change in elemental contents (Si02, K2O, CaO, MgO) of altered andesitic rocks away from the quartz vein system. Figure 1.139 shows the relationships between (CaO - - Na20) content and K2O content. These data indicate the following features of compositional variation of altered rocks.

In fluids where precious metal transport is dominated by AgCl and AuCl, the following reaction may be written to consider the compositional variation of native gold or electrum ... 

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