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Ostwald's rule

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(Redirected from Ostwald's step rule) Rule that less stable polymorphs crystallize first

In materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first. Ostwald's rule is not a universal law but a common tendency observed in nature.

This can be explained on the basis of irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature. Unstable polymorphs more closely resemble the state in solution, and thus are kinetically advantaged.

For example, out of hot water, metastable, fibrous crystals of benzamide appear first, only later to spontaneously convert to the more stable rhombic polymorph. A dramatic example is phosphorus, which upon sublimation first forms the less stable white phosphorus, which only slowly polymerizes to the red allotrope. This is notably the case for the anatase polymorph of titanium dioxide, which having a lower surface energy is commonly the first phase to form by crystallisation from amorphous precursors or solutions despite being metastable, with rutile being the equilibrium phase at all temperatures and pressures.

References

  1. Ostwald, W. (1897). "Studien über die Bildung und Umwandlung fester Körper 1. Abhandlung: Übersättigung und Überkaltung". Zeitschrift für Physikalische Chemie. 22U (1): 289–330. doi:10.1515/zpch-1897-2233.
  2. Van Stanten, R. A. (November 1, 1984). "The Ostwald step rule" (PDF). Journal of Physical Chemistry. 88 (24): 5768–6769. doi:10.1021/j150668a002.
  3. Threlfall, T. (2003). "Structural and thermodynamic explanations of Ostwald's Rule". Organic Process Research & Development. 7 (6): 1017–1027. doi:10.1021/op030026l. ISSN 1083-6160.
  4. Hanaor, Dorian A. H.; Sorrell, Charles C. (2011-02-01). "Review of the anatase to rutile phase transformation". Journal of Materials Science. 46 (4): 855–874. Bibcode:2011JMatS..46..855H. doi:10.1007/s10853-010-5113-0. ISSN 1573-4803. S2CID 97190202.


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