# Phase transformation

## Main Question or Discussion Point

hi,
can someone explain to me how to distinguish (experimentally or theoretically) between 1st or 2nd (3rd?) order phase transformation in crystal structures.

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Mute
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A first order phase transition involves a release (or intake) of latent heat. A second order transition does not.

There are no third or high order transitions. The name comes from a classification scheme by Ehrenfest that was in error. Though the terminology has stuck around, better names would be discontinuous transitions (which are "first order") and continuous transitions (which are "second order").

In a discontinuous transition the order parameter becomes zero discontinuously at the critical point, whereas in a continuous transition the order parameter changes to zero continuously up to the critical point, and then remains zero afterwards. An order parameter, in case you are unaware of the terminology, is a measurable quantity which is zero in one phase of the material (the 'disordered' phase) and is non-zero in the other phase (the 'ordered' phase).

An example of an order parameter is magnetization in a magnet. At H = 0, there is a continuous transition from a ferromagnetic state (a net fraction of spins are aligned) to a paramagnetic state (M = 0) at a transition temperature Tc as T is increased. At H nonzero, but T less than Tc, the sign of M is equal to the sign of H (but the magnitudes are unrelated). If you flip the sign of H, M will change discontinuously.

Similar considerations will apply to crystal phase transitions.

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Thank you for your great explanation. It really helps.

hi,
I still want to ask you the following: are only the 2nd order transitions reversible? What about the 1st order? I am trying to get some physical feel for how to distinguish between the transition orders and their significance, i.e. why is it important to know whether the transition was 1st or 2nd order?