Page 59 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

S O L I D S IR-11.2

IR-11.1 I N T R O D U C T I O N

IR-11.1.1 General
This chapter deals with some aspects of terminology, nomenclature and notation for solids.
However, in cases where detailed structural information is to be conveyed, fully systematic
names can be difficult to construct. An attempt to deal with this problem is described in Ref. 1.

IR-11.1.2 Stoichiometric a nd non-stoichiometric phases

In binary and multi-component systems, intermediate crystalline phases (stable or
metastable) may occur. T hermodynamically, the composition of any such phase is variable.
In some cases, such as sodium chloride, the possible variation in composition is very small.
Such phases are called stoichiometric. However, in other p hases appreciable v ariations in
composition can occur, for example i n w ustite (nominally FeO).
These are called non-stoichiometric phases. In general, it is possible t o d efine an ideal
composition relative to which the variations occur. This composition, called the
stoichiometric composition, is usually that in which the ratio of the numbers of different
atoms corresponds to the ratio of the numbers of normal crystallographic sites of different
types in the ideal (ordered) crystal.
This concept can be used even when t he stoichiometric composition i s n ot included in the
homogeneity range of the phase. The term ‘non-stoichiometric’ does not mean phases w ith
complex f ormulae, but those with variable composition, f or which the term solid mixture is
an alternative. F ormerly, the term solid solution was used, but this term is recommended to
apply only in the following sense. 2–4 Mixture is used to describe a s olid phase containing
more than one substance, when a ll substances are treated in the same w ay. Solution is used to
describe a l iquid or solid phase containing more than one substance w hen, for convenience,
one (or more) of the substances, called the solvent, i s t reated differently from the other
substances, called solutes. F or non-stoichiometric phases, each atom or group of atoms that
contributes to the non-stoichiometry is treated equally, s o t he term mixture is appropriate.

IR-11.2 N A M E S O F S O L I D P H A S E S

IR-11.2.1 General
Names for stoichiometric phases, such as NaCl, are derived simply as in Chapter IR-5,
whereas f ormulae are derived as presented in Chapter IR-4. Although NaCl in the solid state
consists o f a n i nfinite network o f u nits, (NaCl) 1 , t he compound is named sodium chloride
and represented symbolically a s N aCl.
However, for non-stoichiometric phases a nd solid solutions, formulae are preferable to
names, since s trictly systematic names tend to be inconveniently cumbersome. They should
be used only when unavoidable (e.g. for indexing) a nd should be constructed in the
following style.

Examples:
1. iron(II) sulfide ( iron deficient)
2. molybdenum dicarbide (carbon excess)

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