  We show that polycrystalline GeSb2Te4 in the fcc phase (f-GST), which is an
insulator at low temperature at ambient pressure, becomes a superconductor at
elevated pressures. Our study of the superconductor to insulator transition
versus pressure at low temperatures reveals a second order quantum phase
transition with linear scaling (critical exponent close to unity) of the
transition temperature with the pressure above the critical zero-temperature
pressure. In addition, we demonstrate that at higher pressures the f-GST goes
through a structural phase transition via amorphization to bcc GST (b-GST),
which also become superconducting. We also find that the pressure regime where
an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous
peak in magnetoresistance, and suggest an explanation for this anomaly.
