Stoichiometric compounds, and

The gas is washed with water to remove any hydrogen chloride. Since iron(II) sulphide is a non-stoichiometric compound and always contains some free iron, the hydrogen sulphide always contains some hydrogen, liberated by the action of the iron on the acid. A sample of hydrogen sulphide of better purity can be obtained if antimony(III) sulphide, (stibnite) SbjSj, is warmed with concentrated hydrochloric acid ... 

Many of these sulphides occur naturally, for example iron(ll) sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb S, (stibnite). They can usually be prepared by the direct combination of the elements, effected by heating, but this rarely produces a pure stoichiometric compound and the product often contains a slight excess of the metal, or of sulphur. 

The results of such experiments, carried out over a range of temperatures, are summarized on a phase diagram (Fig. 4.1 b), which can be used to predict the outcome of any preparation in the NaF-ZnF2 system. In such diagrams, each phase is drawn as a line, as in the example of NaZnF3 above, because they show no composition range. Such compounds are referred to as stoichiometric compounds and, sometimes, with the appearance of the phase diagram in mind, line phases. 

Let us now discuss some recent work by Sano and myself on completely characterizing barium stannate, a material first proposed by Plotnikova, Mitrofanov, and Shpinel (21), as a source for tin Mossbauer spectroscopy. It is easily prepared, is a stoichiometric compound and has all the properties one desires in a Mossbauer matrix. The recoil-free fraction at room temperature is about 0.55 with about a 10% error. The line width extrapolated to zero absorber thickness is about 6% larger than natural—i.e., the line width observed is ca. 0.318 mm./sec. at zero ab-... 

Chapter S examines various models used to describe solution and compmmd phases, including those based on random substitution, the sub-lattice model, stoichiometric and non-stoichiometric compounds and models applicable to ionic liquids and aqueous solutions. Tbermodynamic models are a central issue to CALPHAD, but it should be emphasised that their success depends on the input of suitable coefficients which are usually derived empirically. An important question is, therefore, how far it is possible to eliminate the empirical element of phase diagram calculations by substituting a treatment based on first principles, using only wave-mecbanics and atomic properties. This becomes especially important when there is an absence of experimental data, which is frequently the case for the metastable phases that have also to be considered within the framework of CALPHAD methods. 

The experimental correlations, a) through e), obtained for the stoichiometric compounds and their theoretical justifications can now be extended to non-stoichiometric compounds. We shall start by examining AB2 compounds based on available data [4, 5,6,17],... 

In this account we extend the definition to include the concept of systematic, synthetic proliferation within an abiotic molecular structure starting from an origin core initiator and continuing through a succession of well-defined, stoichiometric compounds and structures. In general, this synthetic strategy involves the following criteria ... 

Russian workers have advanced some apparently related ideas. Ariya and his colleagues (1958, 1961) (3, 17) have pointed out that the standard heat of formation of a compound across its stoichiometric range frequently falls rather smoothly on the curve joining the value for the stoichiometric compound and that of the next phase in the equilibrium diagram (Figure 2). Furthermore, there is... 

Direct interaction of the metal and other elements such as S, Se, Te, P, As, Sn, or Pb under selected conditions produces dark, often semi-metallic solids that are resistant to acids other than HN03. These products may be stoichiometric compounds and/or nonstoichiometric phases depending on the conditions of preparation. 

Figure 1. Schematic of the isothermal ternary B-C-N phase diagram at temperatures below the decomposition of the stoichiometric compounds and the regimes of composition of some soKd solutions discussed. For explanation of the compounds see text. First published in [152] and reproduced with permission.

The CD -ferrocene complexes were characterized by elemental analysis, IR, UV, and H-NMR spectra. Stoichiometries were determined by elemental analysis, and especially the iron content measured by atomic absorption analysis and from the H-NMR spectra, which show all the complexes obtained here are stoichiometric compounds and have definite ratios of CD/guest depending on the combinations of host and guest. Table I shows the results of the preparation of inclusion compounds of CDs with ferrocene. 3 CD formed 1 1 inclusion compounds with ferrocene. 

As mentioned in Ch. 3, Sec. 8.1, interstitial carbides are essentially non-stoichiometric compounds and the variations in the reported property values often found in the literature reflect this characteristic. The values given here are an average,... 

In particular it allows us to consider in a simple way possible irregularities of structure, the existence of non stoichiometric compounds, and the possibility of replacing one atom by another. 

In addition to the vacancy and ions, the stoichiometric of the cation and anion between crunodes can be damaged. That is generation of a non-stoichiometric compound, and the typical example is wiistite. The character of non-stoichiometric oxide is a function of their composition the common natme is their composition and can be changed by the surrounding gas atmosphere and temperature. 

First, the chemical shifts for Pt oxides and hydroxides are typically about 3 eV (for the stoichiometric compounds) and therefore significantly larger than the measured maximum shift (1.3eV for the Pt atom). Second, comparing the results to studies on commercial Pt catalysts, these systems exhibit an oxide core-level shift of about 2 eV, again larger than the observed values. 

In previous studies (Kato et al., 2009a, 2011b 2011c), the stoichiometric compositions in (U,Pu)02 have been determined based on defect chemistry. The relationship between oxygen partial pressure and deviation x from stoichiometric composition has been analyzed in non-stoichiometric oxides. Kosuge (1993) used statistical thermodynamics considerations for description of non-stoichiometric compounds, and Karen (2006) reported a point-defect scheme for them. Recently their methods have been applied for nonstoichiometric (U,Pu)02, and experimental data, accurately measured in the near stoichiometric region, were analyzed as a function of temperature. In this report the measurement data and the measurement technique were reviewed and analysis results based on defect chemistry were summarized. 

In practice, even the most pure substance contains impurities. Therefore a theory of impure substances may be more essential than that of pure substances, even if the theory can benefit from defining pure substance as an ideal limit. On the one (theoretical) hand, it would seem that the boundaries between stoichiometric compounds, non-stoichiometric compounds and solutions seem to become more blurred, giving way to a phase-discourse instead of a compound discourse. On the other (practice-related) hand, there is no doubt that there are non-stoichiometric compounds with their own unique identification conditions, the classic example... 

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