# How do phase transitions occur in the solid phase? [closed]

For example, how does graphite transform into diamond as pressure increases at a certain (adequate) temperature? I'm not sure if this is a great example to look at the bigger picture, so another example would be pure iron. At atmospheric pressure, as liquid iron cools down it solidifies to $\delta\text{-}\ce{Fe}$, with a bcc crystal structure. Further cooling down brings it to $\gamma\text{-}\ce{Fe}$, which has a fcc. Then, it keeps cooling down and it solidifies to $\alpha\text{-}\ce{Fe}$, again with a bcc structure. I don't really see how do the atoms physically rearrange to form the different crystal structures, neither do I see why would they jump back and forth between these. To add to this, I believe the difference between $\alpha$ and $\delta$ are their paramagnetic properties.

I guess the general question is, how do the atoms in the solid phase rearrange themselves by changes in pressure or temperature? (in contrast to, for example, a liquid-gas phase transition). And of course, why? What drives this change?

## closed as too broad by Mithoron, airhuff, Todd Minehardt, M.A.R., aventurinMar 17 '18 at 14:47

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• Magnetic properties have nothing to do with all this and are not the cause of difference between $\alpha$ and $\delta$. Indeed, $\alpha$ iron may or may not be ferromagnetic (dependent on temperature), while its crystal structure remains the same. – Ivan Neretin Mar 15 '18 at 8:26
• So they're both bcc, yet different? – ralk912 Mar 15 '18 at 8:30
• In a way, they can be thought of as not different at all. Imagine one phase which is stable until certain temperature, then ceases to be stable, then becomes stable again, then melts. – Ivan Neretin Mar 15 '18 at 8:57
• Then why call them differently? And so, why would this happen? – ralk912 Mar 15 '18 at 9:00
• Because one exists in one range of temperatures, and another exist in a different range, and never the twain shall meet. As to why, that's a very broad question, about as broad as chemistry itself. Solid phase structures are notoriously hard to predict. – Ivan Neretin Mar 15 '18 at 9:47