Page 65 - Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005)

S O L I D S IR-11.6

systems. The use of Greek letters, which do not convey the necessary i nformation, and of the
Strukturbericht designations, which are not self-explanatory, is not acceptable.

IR-11.5.2 Recommended notation

The Pearson symbol consists of three parts: first, a l ower-case italic letter (a, m, o, t, h, c)
designating the crystal system; s econd, an italic capital letter (P, S, F, I, R) d esignating t he
lattice setting and, finally, a n umber designating the number of atoms or ions in the
conventional unit cell. Table IR-3.1 summarizes the system.

Examples:

1. Cu, symbol (cF4), indicates copper of cubic symmetry, w ith face-centred lattice,
containing 4 a toms per unit cell.
2. NaCl, symbol (cF8), indicates a c ubic f ace-centred lattice with 8 i ons per unit
cell.
3. CuS(hP12), indicates a h exagonal primitive lattice with 12 ions per unit cell.

If required, the Pearson symbol can be followed by the space group and a p rototype
formula.

Example:

4. CaMg 0.5 Ag 1.5 (hP12, P6 3 /mmc) ( MgZn 2 type).

IR-11.6 N O N - S T O I C H I O M E T R I C P H A S E S

IR-11.6.1 Introduction

Several special problems of nomenclature for non-stoichiometric phases h ave arisen with
the improvements in the precision with which their structures c an be determined. Thus,
there are references to homologous s eries, n on-commensurate and semi-commensurate
structures, Vernier structures, crystallographic shear phases, Wadsley defects, chemical
twinned phases, infinitely adaptive p hases a nd modulated structures. Many of the phases
that fall into these classes have no observable composition ranges although they
have complex s tructures a nd formulae; an example i s M o 17 O 47 . T hese phases, despite
their complex formulae, are essentially stoichiometric and possession of a c omplex
formula must not be taken as an indication of a n on-stoichiometric compound (cf. S ection
IR-11.1.2).

IR-11.6.2 Modulated structures
Modulated structures possess two or more p eriodicities in the same d irection of space. I f t he
ratio of these p eriodicities is a r ational number, the structures are called commensurate;

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