1`When each of two polymorphs is thermodynamically stable in a definite range of temperature and pressure, the pair is said to be enantiotropic...When one of two polymorphs is thermodynamically unstable at all temperatures below the melting point, the two are said to be monotropic' (see Westrum & McCullough, 1963, p. 73). This definition tacitly assumes working at fixed (generally atmospheric) pressure.
2Ubbelohde (1957) writes `The fancied resemblance of such curves to the Greek capital letter (lambda) has led to the description of such phenomena as `lambda-point anomalies”. I have not been able to locate the first use of this term.
3Mnyukh's book also deals with second-order transitions (in passing) and, more substantially, with ferromagnetism and ferroelectricity. However, these topics are outside the bounds of the present review.
4For brevity our reference is generally to `phase transitions', but it must be understood that the full phrase `first-order enantiotropic solid-state phase transitions' is always implied, and we hardly move outside these boundaries.
5The phrase is actually taken from Pippard (1964), p. 136.
6Note the omission of V from this list.
7The nomenclature of Herbstein (2001) is used for the phases (see Appendix A), while crystal data are given through the CSD (Cambridge Structural Database) refcode as well as in the standard manner.
8We quote from Chattaway & Lambert `Tubes containing a mixture of crystals... of the two forms, just covered with acetone, were sealed... and heated... At 55° the pale yellow crystals [monoclinic], and at 55.5° the orange crystals [triclinic] were unmistakably growing, although very slowly. Between these temperatures no definite alteration of either form could be observed. The transition point must therefore be between these temperatures, and lie in the neighbourhood of 55.25°'.
9The (HT) to (LT) transition of resorcinol was used as an illustration of this situation; the two phases are enantiotropically related.
10See p. 187: `The fact that the H phase is more `disordered' than L is merely another difference between them, bearing no relation to the mechanism of molecular rearrangement at the interface'.
11Other measurements on NH4Cl not in the direct line of the present argument are summarized in Appendix 4.
12I am grateful to Dr Marylize Buron-Le Cointe for a copy of her doctoral thesis (University of Rennes I).
13Le Cointe, Lemée-Cailleau et al. (1995; see #2, ionic phase) remark that `only the (070) superstructure reflection was clearly extracted from the background'; however, it is the temperature dependence of (030) that is shown in their Fig. 2(a) (our Fig. 11).
14Possibly triclinic crystal data have not been reported for this phase.
15Atomic coordinates for the EI phase are not available.
16The phase stable below 117 K is generally designated phase III in the literature.
18Chernyshov et al. (2003) mention that crystal structures at 115, 148, 171, 199 and 227 K were determined by Katz & Strouse (1979); these were for the orthorhombic methanol solvate. Katz & Strouse determined the structure of the ethanol solvate only at 115 K; a comparison of the cell volumes suggests that this was the LS structure.
19`Any similarity that may exist between triclinic structures is not necessarily seen to best advantage by transforming the cells to their reduced form' (Richardson et al., 1990, p. 658).
20`Visual observations can sometimes be more sensitive indicators of phase changes than the DSC measurements' (Richardson et al., 1990, p. 655).
21The following quotation from Byrn et al. (1972) for Y to W is typical: `The onset temperature was quite unpredictable and ranged from 383 to 413 K'.
22However, it is not known whether the two phases of NH4Cl are epitaxic.