Polymorphism is the ability of a material to exist in alternate crystalline forms depending upon external conditions like temperature and pressure i.e. to have more than one structure. If the change in structure is reversible then polymorphic change is refer to as allotropy.
Example– Iron exhibits different structures at different temperatures.
(i) From -273°C to 910°C, Iron has BCC structure.
(ii) From 910°C to 1400°C, Iron has FCC structure.
(iii) From 1400°C to 1539°C, Iron has again BCC structure
These changes are reversed on cooling therefore this polymorphic change is allotropic in nature.
Ceramic materials also show polymorphism. Silica exist in three forms i.e. Quartz, Tridymite, Cristobalite. This transformation requires breaking of –Si-O-Si- bonds and rearranging these in different ways. Quartz changes to Tridymite at 870°C and further this Tridymite changes to Cristobalite at 1470°C. Titanium, Zirconium and Lithium also show the property of polymorphism.