I was running some structural relaxations under constant pressure, and found out that my system has compressibility which increases when pressure is applied. Of course, this is only up to a point, since at pressure high enough, phase transition will occur and compressibility will drop sharply. But, in the phase where compressibility is rising, I find this behavior quite peculiar. No one in my group has any particular experience with materials under high pressure and I was trying to find some references to materials that behave like this, and possible explanation, but I was unable to get anything from my searches. Can anyone help me with some references on this?