a transition in which a crystal structure undergoes a continuous change and in which the first derivatives of the Gibbs energies (or chemical potentials) are continuous but the second derivatives with respect to temperature and pressure (i.e. heat capacity, thermal expansion, compressibility) are discontinuous, üleminek, mille puhul kristallstruktuuris toimub pidev muutus, kus Gibbsi energiate esimesed tuletised (keemilised potent-siaalid) on pidevad, aga teised tuletised temperatuuri ja rõhu suhtes (s.t, soojusmahtuvus, soojusjuhtivus ja kokkusurutavus) on pidetud