  Layered transition-metal trichalcogenides have become one of the research
frontiers as two-dimensional magnets and candidate materials used for
phase-change memory devices. Herein we report the high-pressure synchrotron
X-ray diffraction and resistivity measurements on Cr2Ge2Te6 (CGT) single
crystal by using diamond anvil cell techniques, which reveal a mixture of
crystalline-to-crystalline and crystalline-to-amorphous transitions taking
place concurrently at 18.3-29.2 GPa. The polymorphic transition could be
interpreted by atomic layer reconstruction and the amorphization could be
understood in connection with randomly flipping atoms into van der Waals gaps.
The amorphous (AM) phase is quenchable to ambient conditions. The electrical
resistance of CGT shows a bouncing point at ~ 18 GPa, consistent with the
polymorphism phase transition. Interestingly, the high-pressure AM phase
exhibits metallic resistance with the magnitude comparable to that of
high-pressure crystalline phases, whereas the resistance of the AM phase at
ambient pressure fails to exceed that of the crystalline phase, indicating that
the AM phase of CGT appeared under high pressure is quite unique and similar
behavior has never been observed in other phase-change materials. The results
definitely would have significant implications for the design of new functional
materials.
