Crystal structure in function of pressure
Seems like homework, so I'll give some hints:
What do you expect to happen to a crystal under pressure?
What does the crystal structure have to do with the response to pressure?
Why would the crystal structure want to change under pressure?
What crystal structures are common for metals? (there aren't many)
Since there aren't many, what distinguishes their responses to pressure that might make the metal want to change crystal structure (a phase change).

That should get you going.