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1. Introduction

The International Union of Pure and Applied Chemistry (IUPAC) published its Definitions of Terms Relating to Phase Transitions in the Solid State (Clark et al.) in 1994, with over 140 terms carefully elucidated. The document defines various types of transition including structural transitions of the solid state, metallurgical transitions, liquid-crystal transitions, solid-liquid or liquid-liquid transitions, transitions related to changes in composition, to changes in electronic structure and, in addition, describes many properties pertaining to these transitions. A systematic nomenclature for a sequence of several solid-state phase transitions was, however, not considered in the document. It is well known that such sequences occur almost as a rule in real materials and that the occurrence of a single transition only is relatively infrequent.

A multiplicity of terminologies may be found in the crystallographic and other literature of the condensed state for distinguishing individual members of a sequence of crystalline phases that form as a function of temperature and/or pressure. Common examples of notations are I, II, III; $\alpha$, $\beta$,$\gamma$; high, medium, low. Another widespread notation uses the point groups of the successive phases (e.g. m$\bar 3$m$\rightarrow$4mm$\rightarrow$2mm$\rightarrow$3m for BaTiO₃). Confusion caused by the lack of a unified notation procedure led the Commission on Crystallographic Nomenclature to establish a Working Group on Phase Transition Nomenclature. The Working Group was charged with studying the multiple nomenclature in current use for naming the sequence of phases that a material may form as a function of temperature and/or pressure and with making such recommendations as may be appropriate. Researchers with experience in a broad range of systems undergoing solid-solid transitions were selected for membership in the Working Group, including representatives from IUPAC and the International Union of Pure and Applied Physics.

Copyright © 1998 International Union of Crystallography