polymorphism

Polymorphism is the ability of one compound, with a single fixed composition, to crystallise into more than one distinct structure. The different structures are called polymorphs (for pure elements the word is allotropes, as with graphite and diamond, both pure carbon). Which polymorph is stable depends on temperature and pressure, so heating or cooling a ceramic can make its very atoms rearrange into a new pattern.

There are two flavours of transformation between polymorphs. A displacive transformation only nudges the atoms a little, bending or shifting bonds without breaking them, so it is fast, reversible, and happens at a sharp temperature (the alpha-to-beta quartz change at 573 degrees C is one). A reconstructive transformation actually breaks and remakes bonds, so it is sluggish and often does not go to completion, which is why high-temperature polymorphs can survive, metastable, all the way down to room temperature. Many transformations carry a volume change, and that volume change is a hazard: if a fired body expands or contracts suddenly as a polymorph flips, it can crack itself apart.

Polymorphism runs right through practical ceramics. Zirconia (ZrO2) has monoclinic, tetragonal and cubic forms whose transformations both threaten and, when harnessed, toughen it; silica (SiO2) has quartz, tridymite and cristobalite forms that plague silica refractories with disruptive volume changes; titania, alumina and boron nitride are all polymorphic. Honest caveat: a phase diagram tells you which polymorph is stable at equilibrium, but real firing is a race against kinetics, so metastable polymorphs and even glass routinely survive where the diagram says they should not.