In materials science, Ostwald's rule or Ostwald's step rule, conceived by

Wilhelm Ostwald Friedrich Wilhelm Ostwald (; 4 April 1932) was a Baltic German chemist and German philosophy, philosopher. Ostwald is credited with being one of the founders of the field of physical chemistry, with Jacobus Henricus van 't Hoff, Walther Nernst, ...

, describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first. Unstable polymorphs more closely resemble the state in solution, and thus are kinetically advantaged. From hot water, metastable, fibrous crystals of benzamide appear first, later to spontaneously convert to the more stable rhombic polymorph. Another example is magnesium carbonate, which more readily forms

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

. A dramatic example is

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

, which upon sublimation first forms the less stable white phosphorus, which only slowly polymerizes to the red

allotrope Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the ...

. This is notably the case for the anatase polymorph of

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...

, which having a lower

surface energy In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...

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. Ostwald suggested that the solid first formed on crystallisation of a solution or a melt is the least stable polymorph. This can be explained on the basis of irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature. 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 , issn = 1083-6160 , doi = 10.1021/op030026l

References

Mineralogy Gemology Crystallography