A reaction in a system containing two solid solution phases, α' and α'' in which α' decomposes into α'' and a new phase β: \[\alpha\mathit{'}\rightleftharpoons \alpha\mathit{''}+\beta \]
Source:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 586 (https://doi.org/10.1351/pac199466030577)