A transition of either of two types: (a) a @M04037@ (single-valent systems). (b) a small-@PT07416@ to itinerant-electron transition (mixed-valent systems).
Source: PAC, 1994, 66, 577. (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 585 [Terms] [Paper]