A transition in which the molar Gibbs energies or molar Helmholtz energies of the two phases (or chemical potentials of all components in the two phases) are equal at the transition temperature, but their first derivatives with respect to temperature and pressure (for example, specific @E02141@ of transition and @S05807@) are discontinuous at the transition point, as for two dissimilar phases that coexist and that can be transformed into one another by a change in a field @V06600@ such as pressure, temperature, magnetic or electric field. Example: The transition on heating CsCl to \(752\ \text{K}\) at which it changes from the low-temperature, CsCl-type structure to the high-temperature NaCl-type structure. Synonymous with discontinuous phase transition.