Pseudo-binary compounds

A great many compounds of ternary or higher order behave as if they were compounds of binary order, with a group of atoms acting as if it was a single atom. Such a group is called a radical . 

A classical example is provided by the cyanides, some of which are described below  

Hydrogen cyanide, HCN. This is manufactured by passing a mixture of methane and ammonia over a platinum catalyst at 1200 °C  

It is a colourless liquid, boiling at 26 °C to a colourless gas. It is a poor conductor of electricity. Like most cyanides, it is extremely poisonous. 

Sodium cyanide, NaCN. This is manufactured by absorbing gaseous hydrogen cyanide in aqueous sodium hydroxide or sodium carbonate. The compound is a colourless solid, melting at 564 C to a colourless liquid. The liquid conducts electricity with the formation of sodium at the cathode and a colourless gas at the anode. This gas is cyanogen, C2N2. 

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Many binary and pseudo-binary compounds of the alkali metals are more conveniently treated within the context of the chemistry of the other element and for this reason discussion Is deferred to later chapters, e.g. borides (p. 145),... 

Compounds made by insertion at room temperature are often metastable - if heated, they change their structure or decompose into other compounds. That does not rule out using thermodynamics it just means that processes happening slowly compared to the duration of an experiment are assumed to be frozen. At room temperature, the ratio of Mo to Se in a host like Mo Seg is fixed. From the point of view of thermodynamics, the constraint that the host remain Mo Seg means that we can regard an intercalation compound like Li -MogSeg as a pseudo-binary compound instead of a ternary one. 

Pseudo-binary compounds An(X1, X2)3 X1,2 = non-transition metals Several quasi-binary An(X1, X2)3 systems (preserving the AuCu3 structure of the parent phases) were studied with the aim to follow the development of electronic properties across the onset of magnetic ordering, the latter pointing always to a certain degree of 5f-electron localization. 

Ternary compounds differ from pseudo-binary compounds in that the different crystallographic sites are occupied by different types of atoms. In general, one might say that the physical properties of pseudo-binaries can be described by a weighted mean of the properties of the parent binary compounds from which they are derived. This is no longer true for ternary compounds. Here one may expect novel properties not found in binary compounds. Very often also the crystal structure and the formula composition have no analogue in the corresponding binary systems. Up to now, systematic experimental studies as to the occurrence of ternary compounds... 

The nature of pseudo-binary compounds follows from our discussion of binary compounds. The bonding between radicals and radicals or radicals and atoms is the same as the bonding between atoms and atoms. For example ... 

Compounds of this type have already been discussed imder pseudo-binary compounds (Chap. 9). They are compounds containing molecular ions obc5dng the valency rules, e.g., CN S04 , CH3CO2". 

As shown in 2.2.1 mixt 2-3 configurations are found in A y crystals. Amorphous phases of binary or pseudo-binary compounds with A = As, Sb, Bi and B = S, Se, Te are obtained either by vacuum deposition (Andreyevski, Nabitovitch, and Voloshchuk (1961)) or by quenching the melt, especially if small quantities of other elements like Ga, Ge or I are added (Kolomiets (1964), Hilton, Jones, and Brau (1966. The thermal properties of Ge-doped As2Se3 (Kolomiets, Pajasova, and Stourac (1965) Stourac, Kolomiets, and Shilo, (1968)) indicate that, in accordance with MyuUer s (1966) views, the vitrification of As2Sc3 by addition of Ge is... 

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