3 Rubidium (Rb) adopts a body-centred cubic, BCC, structure at a pressure of 1 atm. Upon application of high pressure (of the order of 30,000 atm), Rb undergoes a structural phase transition to a face-centred-cubic, FCC, structure. (i) Describe what could be the main factor that induces the phase transformation. (ii) Describe how the diffraction patterns of the two forms would differ. [6 marks]