λ-transition (lambda-transition)

A second-order or higher-order transition, in which the heat capacity shows either a discontinuity (second-order) or a vertex (higher-order) at the transition temperature. Example: The transition at $846 K$ , accompanied by a finite change in specific heat capacity at constant pressure $C p$ , of the low-temperature polymorph of quartz to the high-temperature polymorph.

Source:

PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 584