Pressure-Induced Structural Phase Transition and a Special Amorphization Phase of Two-Dimensional Ferromagnetic Semiconductor Cr₂Ge₂Te₆

Abstract

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 Cr 2 Ge 2 Te 6 (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 highpressure 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.

Pressure control of the magnetic anisotropy of the quasi-two-dimensional van der Waals ferromagnetCr2Ge2Te6

“…[32], can be achieved. However, a sizable effect on the magnetic anisotropy which we observed at hydrostatic pressures up to 2.39 GPa corresponds to an even smaller compressive in-plane strain ∼ 0.7% [31]. Such straining seems plausible to achieve since, in general, 2D van der Waals materials are known to sustain large strain.…”

“…Finally, it is worth mentioning that in Ref. [31] a pressure-induced difference in the unit-cell volume of Cr 2 Ge 2 Te 6 between applied pressures of 0 and 3 GPa of about 4% was reported. As the unit-cell volume enters into the calculation of the saturation magnetization, such a reduction of volume with increasing pressure could change the absolute value determined for K U .…”

Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material

“…Additionally, reversible phase-change between the amorphous and crystalline phases has been demonstrated for non-volatile phase change memory applications 14 – 17 . Furthermore, pressure-induced amorphization of Cr 2 Ge 2 Te 6 has been recently reported 18 . In particular, understanding the crystallization mechanism is crucially important for phase change memory applications as the amorphous-crystal transition is used more than 10 7 cycles in typical devices 19 .…”

Nitrogen doping-induced local structure change in a Cr2Ge2Te6 inverse resistance phase-change material

“…Here, the formation mechanism of Ge-Ge bonding from the distorted octahedron has been successfully demonstrated by a Ge flipping model. 32,33 Notably, peak C still exists in the 270 1C-annealed sample, indicating that some amorphous local structures still remained (Fig. 4(a)).…”