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Subsections

• 3.1. First field
• 3.2. Second field
• 3.3. Third field
• 3.4. Fourth field
• 3.5. Fifth field
• 3.6. Sixth field

3. Six-field phase-transition nomenclature

The recommended nomenclature consists of a six-field notation for each phase with given chemical composition as follows, with field definitions summarized in §§3.1-3.6. The fields are separated by vertical bars:

 

Usual label in literature ($\alpha$, I, $\dots$)

Temperature range (K), pressure range (Pa)

Space-group symbol and No.

Number of chemical formula units per conventional cell

Ferroic properties

Comments

3.1. First field

The label commonly associated with a given phase should be listed. If no label has been used in the literature for the phase, the field should contain a hyphen; nicknames may still be given to newly discovered phases. In the event that more than one label has been used, retention of the most common label is recommended.

3.2. Second field

If the phase is stable over a thermal range, then the temperature limits should be given in kelvins; if for a pressure range, in pascals. SI prefixes should be used as required. If no pressure range is indicated, the observations correspond to atmospheric pressure; similarly, if no temperature range is indicated, the observations correspond to room temperature. The more complex case of a pressure-temperature range is commented on in §4.9. If more accurate limits are determined later, the earlier values should be replaced.

3.3. Third field

The space-group symbol and number of the phase, as used in International Tables for Crystallography (1996), should be given. When incomplete crystallographic information is available, these data may be replaced by specifying the point group (e.g. 4mm) or the crystal system (e.g. tetragonal).

3.4. Fourth field

The number of chemical formula units per conventional cell should be entered; if undetermined, the field should contain only a hyphen. The (tripled) hexagonal unit cell setting in rhombohedral systems should be used.

3.5. Fifth field

Contains the name of the ferroic property exhibited by the phase, see Clark et al. (1994). If this property has not actually been observed experimentally but is nevertheless probable on crystallographic grounds, a comment indicating this situation should be added in the sixth field. An unknown property is denoted by a hyphen (see §4.6 for example).

3.6. Sixth field

If the phase crystallizes with a standard structure type [for nomenclature and definitions of isotypism, see the IUCr Commission Report by Lima-de-Faria et al. (1990); for a handbook listing inorganic structure types, see TYPIX by Parthé et al. (1993, 1994); for a corresponding PC database, see TYPIX 1995 by Cenzual et al. (1995); and for a recent proposal that applies structure-type formulas to all inorganic materials, see Smith et al. (1998)], then the sixth field should begin with an entry in the format

$$\vert \textrm{Type}~=~\textit{XXXX}.$$

Typical structure-type names are, for instance, XXXX = NaCl, pyrite, sphalerite etc., with the name in italics, or, alternatively, structure-type formulas such as XXXX = GXX$\,^\prime$, T3(T$\,^\prime$/L)X4 etc., with the formula in italics. The format is designed for future computer recognition, the italics indicating a standard structure-type name or formula and the period acting as a signal that further information in the field is unrelated to the structure type; if the name or formula is nonstandard or is not unique, this format should not be used. In the event the full six-field notation requires a terminating period (full stop) and is to be used within a longer sentence, the addition of delimiters around the notation such as square parentheses is recommended.

The remainder of the field is free form and may contain pertinent information such as transition characteristics, e.g. first order, Mott, martensitic, order-disorder etc.; the number of variants corresponding to the ferroic orientations formed in the transition, not including antiphase domains related to the onset of superlattice reflections; additional information relative to incommensurability, if present; etc. Information in this field may be updated as necessary.

Copyright © 1998 International Union of Crystallography