Ostwald's rule

{{Short description|Rule that less stable polymorphs crystallize first}}

In materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald,{{cite journal |last1=Ostwald |date=1897 |first1=W. |title=Studien über die Bildung und Umwandlung fester Körper 1. Abhandlung: Übersättigung und Überkaltung |journal=Zeitschrift für Physikalische Chemie |volume=22U |issue=1 |pages=289–330 |doi=10.1515/zpch-1897-2233}} describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first.{{cite journal |last1=Van Stanten |first1=R. A. |title=The Ostwald step rule |journal=Journal of Physical Chemistry |date=November 1, 1984 |volume=88 |issue=24 |pages=5768–6769 |doi=10.1021/j150668a002 |url=https://pure.tue.nl/ws/files/1957949/591388.pdf }} Ostwald's rule is not a universal law but a common tendency observed in nature.

{{Cite journal |last=Threlfall |first=T. |year=2003 |title=Structural and thermodynamic explanations of Ostwald's Rule |journal=Organic Process Research & Development |volume=7 |issue=6 |pages=1017–1027 |doi=10.1021/op030026l |issn=1083-6160}}

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.{{Cite journal|last1=Hanaor|first1=Dorian A. H.|last2=Sorrell|first2=Charles C.|date=2011-02-01|title=Review of the anatase to rutile phase transformation|url=|journal=Journal of Materials Science|language=en|volume=46|issue=4|pages=855–874|doi=10.1007/s10853-010-5113-0|bibcode=2011JMatS..46..855H|s2cid=97190202|issn=1573-4803|doi-access=free}}