High-density amorphous phase of silicon carbide obtained under large plastic shear and high pressure

In situ x-ray diffraction study of the hexagonal 6H SiC under pressure and shear in rotational diamond anvil cell is performed that reveals phase transformation to the new high-density amorphous (hda) phase SiC. In contrast to known low-density amorphous SiC, hda-SiC is promoted by pressure and unstable under pressure release. The critical combination of pressure ∼30 GPa and rotation of an anvil of 2160° that causes disordering is determined.

Figure 1

X-ray diffraction pattern for sample 1. Rotation angles in degrees and pressure at the center of the sample after rotation are shown near the two lower curves; pressure after compression is shown near the three upper curves. The top pattern marks the index of the peaks of SiC. All other peaks can be indexed into ruby.

Figure 2

In situ x-ray diffraction patterns for sample 2. (a) X-ray diffraction patterns after compression and shear processes (marked near curves) and quenching to ambient pressure. The pressure at 2360° is not measurable. (b) X-ray diffraction patterns after rotation by 2360° at initial compression to 3.3 GPa, leading to disordering of SiC. (c) Comparison of x-ray diffraction patterns after compression to 3.3 GPa and after quenching to ambient pressure after all operations. The disordered phase transforms back to the crystalline 6H phase after pressure release (P-R).

Figure 3

X-ray diffraction pattern of SiC for sample 3. Numbers above patterns on the left are corresponding pressures. Numbers on the right are rotation angles and offset positions to the rotation center.

Figure 4

Pressure distributions after each operation for sample 3. Pressure increase (PI) marks an increase in load.

Figure 5

Effect of shear on the width of diffraction peaks of SiC for sample 3. Data from Fig. 3 are used. (a) The center of a sample. (b) The point 25 μm from the center.