Berthollide

Berthelot-Thomsen principle physchem The principle that of all chemical reactions possible, the one developing the greatest amount of heat will take place, with certain obvious exceptions such as changes of state. ber ta lo tam s3n. prin-sa-pal berthollide chem A compound whose solid phase exhibits a range of composition. bar-tha.lTd ... 

BERTHOLLIDE COMPOUNDS. See Chemical Composition Compound (Chemical). 

A nonstoichiometric metal oxide exists as a single phase over a range of chemical composition. Berthollides, as compounds of this kind are sometimes called, form a significant part of the oxide literature which we can summarize here only in the most general way a more detailed account is being published (40). 

The dual evidence of a berthollide, together with ordered and related phases, has also been reported for the oxides of the lighter transition elements, and it is difficult to discuss this except in terms of order and disorder. This problem has been studied for simple cases where, for example, a stoichiometric phase, AB02, with the NaCl structure can exist in two forms, depending upon whether A and B are randomized in one set of sites or resolved into positions of their own. 

Several examples are reported in Table IV. The bismuth compounds consist of single sheets of BiO square pyramids separated regularly by one or more perovskite sheets (Figure 9) (12). Elements of the pervoskite structure may also intergrow with NaCl-type sheets in the phases (ATi03).A0, and ordered phases are now known in a composition region once thought to contain a berthollide. 

While it is to some extent arbitrary, a classification of this kind provides a means of discussing some of the general features now emerging from studies of metallic oxides. We have stressed the evidence that a nonstoichiometric phase is disordered, but may be related to chemically similar phases of fixed composition where an anomaly of structure is ordered and identifiable by x-ray diffraction methods. Where such ordered phases are found, it is possible that features of them are retained as blocks or domains with short range order in the related berthollide. Efforts should be directed towards order-disorder effects, with a view to reconsidering the status of the nonstoichiometric compound with a very wide composition range. 

The compounds discussed above are not nonstoichiometric in the strict sense of the word, since they form well defined, ordered phases which can be assigned precise stoichiometric formulas. Accordingly, they are not berthollides. They may, however, be regarded as nonstoichiometric in the wider sense of the term which has become current, since fractional valence numbers must be assigned to the metal atoms and the ordered structures which they exhibit constitute a preferred alternative to the formation of disordered phases of variable composition. 

The law of definite proportions was a crucial step in the development of modern chemistry, and by 1808, Proust s conclusions had become widely accepted. We now recognize that this law is not strictly true in all cases. Although all gaseous compounds obey Proust s law, certain solids exist with a small range of compositions and are called nonstoichiometric compounds. An example is wiistite, which has the nominal chemical formula FeO (with 77.73% iron by mass), but the composition of which, in fact, ranges continuously from Feo.950 (with 76.8% iron) down to Fco.siO (74.8% iron), depending on the method of preparation. Such compounds are called berthollides, in honor of Berthollet. We now know, on the atomic level, why they are nonstoichiometric (see the discussion in Section 21.6). 

Borides. An X-ray examination of the tetragonal berthollide phases(Bi2)4B2Vi 5 i 9 has shown that the cell constants do not depend significantly on the vanadium content. 

In last century the knowledge of defects in a solid, especially an oxide, has been explored comprehensively. The contribution of Schottky and Wagner successfully put the problem on a quantitative basis and promote the discovery of semiconductor transistor. The idea of non-stoichiometry was developed by Berthollet more than a hundred years ago and the controversy between berthollides, which do not obey the Dalton s law, and daltonides, which follow Dalton s law of constant and multiple proportions based originally upon the study of simple ionic and molecular species, encouraged the scientific debates on how existence of point defect in a compound is is it random statistic distribution or the structure related The experimental data are the best way to explore the truth. Indirect and direct observations of atom... 

While the connection between the structure of the hydrides and their pyrophoric behavior seems to be obscure, it should be pointed out that the compounds of hydrogen fall into three distinctive groups the above-described phosphines, silanes, and boranes, which have covalent bonds the salt-like hydrides of the alkali and alkaline earth metals and the interstitial, nonstoicbiometric or berthollide —type hydrides of the transition metals—e.g. the rare earths, titanium, and zirconium. Pyrophoric compounds are found in all three groups. 

Berthollide compound) A chemical compound in which the elements do not combine in simple ratios. For example, rutile (titanium(IV) oxide) is often deficient in oxygen, typically having a formula TiOi 3. 

To the left of the Be-Po diagonal the metallic elements form intermetallic solid compounds or alloys with each other. Metallic atoms are characterized by a small number of valence electrons and low electronegativity. These compounds and alloys are themselves usually metals but not always, e.g., CsAu is a semiconductor. The stoichiometries in intermetallics (these are the numbers n and m in the compound A BJ do not usually correspond to atomic valencies, as they would in ionic and covalent compounds. The existence regions of intermetallic compounds may be wide, which means that a compound such as A B may have continuous ranges of n and m in the same phase. In other words, intermetallics can be berthollides. 

The number of charge carriers in an insulating compound or a semiconductor is related to the concentration of the impurities, which can be aliovalent ions or vacancies if the compound is nonstoichiometric. Binary nonstoichiometric compounds (many transition metal oxides are berthollides) can be n-type or p-type semiconductors (Table 4.7) even if they are not doped with other ions. The excess of electrons or holes is the result of a higher or lower valency of the lattice ions owing to oxidation or reduction that is caused by an excess or a depletion of oxygen ions. Whether a given oxide is n-type or p-type depends on the valency of the metal ion and not primarily on the structure of the lattice. 

Proust s conclusions were immediately challenged by Berthollet, who maintained that many compounds could have a variable composition. Berthollet quoted the example of the metal copper, which appeared to form a wide range of oxides. Proust pointed out that this was due to the formation of different mixtures of two oxides, each of definite composition. Berthollet s concept of continuously variable composition would have been hard to reconcile with the chemical atomic theory, but by about 1808 Proust s views were generally accepted. However, many years later it was discovered that some compounds, such as the oxides and sulphides of iron, could indeed have a variable composition. In the case of iron(ll) sulphide, a compound corresponding to the formula FeS is rarely encountered, and samples are usually deficient in iron to a variable extent. This is due to some of the lattice sites of iron(ll) ions being vacant, while others are occupied by iron(lll) ions to maintain electrical neutrality. These non-stoichiometric compounds are sometimes called Berthollide compounds. 

The comparison of the computed data with the experimental ones indicates that the former not only reflects qualitatively the phenomenon nature (namely the existence of clathrates in inert gas (except He and Ne) - hydroquinone systems and the dissolving of guests in the host a-modification) but describes the phase diagram quantitatively. Computed P,x- and T,x-sec-tions of the hydroquinone - xenon system diagram (see Fig.2) confirm the berthollide nature of hydroquinone clathrates which helps to clarify the essence of Kurnakov s imaginary compounds [17]. 

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