Term: order–disorder transition
https://doi.org/10.1351/goldbook.O04321

Definition:
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 

Related Terms:
1) unit cell (http://doi.org/10.1351/goldbook.U06562).
2) superconducting transition (http://doi.org/10.1351/goldbook.S06137).

Source: 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)

Citation: 'order–disorder transition' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.O04321

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