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I have a feeling that til you ask n metallurgists you'll end up either n+1 slightly different explanations... You can argue in several different general directions and they're all right in their own way. Whether one phase is stable or not depends on the relationship between lots of different properties and half of these will be competing against the other half. It's all a balancing act!
Your best bet to finding a satisfactory answer (as with a lot of empirical science like this) is to hear as many answers as possible and understand that they probably all contribute to some degree.RE your specific hypothesis, what is your meaning of spacing? Inter-electron spacing, or electron-nucleus spacing? Why would it decrease with increasing (thermal) energy?
In other news, autocorrect was not designed for this kind of conversation...
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hamrack
Hadn't seen this.
Yeah. So like a simillar affect to varying grain pattern would have.
Back to the ongoing discussion: I had asked my teacher about this and he gave me a roundabout his explanation (dumbed down a lot I can imagine):
At the different energies the electrons are more likely to be present with differing spacing (smaller spacing at higher energies). If the electrons can overlap with the metal ions they'd be electrostatically more stable. Hence at low energy a wider spacing is favoured (lower packing fraction hence BCC) and when the energy increases smaller gaps are favoured hence FCC. That may explain why it returns to the BCC at an even higher temperature.
Does this sound plausible? I know I must have missed/brushed over complications but as an idea...