A transition in which the degree of order of the system changes. Three principal types of disordering transitions may be distinguished: (i) positional disordering in a solid, (ii) orientational disordering which may be static or dynamic and (iii) disordering associated with electronic and nuclear spin states. Examples:
- The transition of LiFeO2, with a tetragonal unit cell, in which the Li+ and Fe3+ cations are perfectly ordered on crystallographically non-equivalent octahedral sites to cubic LiFeO2 in which the Li+ and Fe3+ cations are distributed randomly over all the octahedral sites.
- The transition of orthorhombic KCN to cubic KCN in which the CN− ions become oriented in any of the eight  directions.
- A superconducting transition
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 587 (https://doi.org/10.1351/pac199466030577)