# Reversed freezing point

Helium-3's Phase diagram shows that at the right temperature and pressure combination, the solid region dips downward as temperature increases. That means that you can heat it up and it will freeze. What is going on?

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Amazingly, below 0.3K the latent heat of fusion of $^3$He is negative i.e. you need to inject heat to make the solid freeze. This is the basis of Pomeranchuk cooling.