Features of moire patterns in HPHT-grown diamond single crystals

1. Features of moire patternsin HPHT-grown diamond single crystals [The 3rd report.] Summaried by Kim Bumyong 1998007662

2. Features of moire patternsin HPHT-grown diamondsingle crystals Long-Wei Yin a,), Mu-Sen Li a, Dong-Sheng Sunm a, Zeng-Da Zou a, Yu-Xian Liu a, Zhao-Yin Hao b a College of Materials Science and Engineering, Shandong UniOersity, Jinan 250061, PR China b National Key Laboratory for Superhard Materials, Jilin UniOersity, Changchun 130012, PR China Received 24 November 2000; accepted 15 December 2000

3. Abstract • Diamond single crystal particles, which were grown at HPHT in the presence of FeNi catalyst, have been successfully examined by cross-sectional transmission electron microscopy TEM . • Structure defects : moire fringe images. (stacking faults, stacking-fault tetrahedral, twins and concentric dislocation loops in the HPHT-grown diamonds.) • Some stacking faults terminating on intersecting twin boundary suggest that the Shockley partial dislocations react with the twins on the (111) plane. The stacking faults, twins and stacking-fault tetrahedral derive from the supersaturated vacancies in the HPHT-grown diamonds. The concentric dislocation loops and fringe distortions in the moire images may result from the internal stresses associated with the inclusions in the diamonds.

4. 1. Introduction • The structural defects and their formation process in the HPHT grown diamonds may be different from that in natural and CVD diamonds. • One of the important characteristics is that these contain large amounts of supersaturated vacancies and micro-inclusions in the HPHT-grown diamonds. • The defects in the HPHT-grown diamonds should be closely associated with such supersaturated vacancies and inclusions. • The study on the defects may also provide information about the diamond nucleation and growth mechanism.

5. Some structural defects contained in diamond single crystals prepared from Fe–Ni–C system at HPHT. • The cross-sectional TEM method was employed to reveal internal microstructures of HPHT-grown diamonds. The structural defects in the diamonds may be interpreted by moire images. • Four types of defects were successfully observed. 1. stacking faults 2. twins 3. stacking-fault tetrahedral 4. concentric dislocation loops • The formation process of these defects was analyzed.

6. 2. Experimental • A high temperature of 1570 K and a high pressure of 5.5 GPa for 12 min in a FeNi catalyst-graphite system. • The HPHT-grown diamonds of about 0.7 mm in dimension were directly thinned by argonion beam milling machine until they were suitable for TEM observation. • Moire patterns were used to examine stacking faults, twins, stacking-fault tetrahedral and concentric dislocations in the HPHT-grown diamonds.

7. 3. Results and discussion

8. For the HPHT grown cubic diamond, it may be considered to be composed of close-packed (111) planes stacking in a sequence of ABCABCABC. Corresponding selected area electron diffraction pattern SADP shown in Fig. 1b suggests the moire pattern be formed by two overlapping (111) close-packed planes rotated with respect to each other by an angle of 5°. • The simplest type of planar defect to occur in a crystal may be the stacking fault.

9. From available data, the stacking faults in the natural and CVD diamonds are common. • The density of stacking faults in the HPHT grown diamonds may also be higher, and should be of great concern in pursuit of practical applications. • The stacking faults observed are apparently related to the supersaturated vacancies. • Vacancies are stable point defects in diamond at high temperatures as in most metals, and vacancies are expected to aggregate during cooling when diamond is quenched from a high temperature.

10. Larger amounts of vacancies will be trapped in the diamond during rapid cooling from high temperature. The excess amount of supersaturated vacancies generated by rapid quenching from high temperature will agglomerate into discs on the (111) close-packed planes because of the low energy associated with such close-packed planes. • The supersaturated vacancies reaches a critical value, they will collapse. Subsequent collapse ⇒ a fault-formation mechanism, that is equivalent to the removal of a close-packed (111) plane, leading to the change of the crystal regular stacking sequence to ABCBCABC. The removal of a close-packed (111) plane gives rise to the formation of low-energy intrinsic stacking faults. Therefore, the intrinsic (111) stacking fault in the HPHT grown diamond is formed.

11. The stacking fault structure is so closely related to twin that they often have common origins. • The stacking sequence becomes ABCBA where the C plane is mirror plane. Compared with other defects, twinning on the (111) planes of diamond corresponds to a very small amount of twin energy since little structural disturbance is created. • From Fig. 2, stacking faults are found to terminate on intersecting twin boundary suggesting that, during the diamond growth or quenching from high temperature, the bordering partial has propagated by glide up to the twin interface, this can be described by the reaction of the Shockley partial dislocations with twins on the (111) plane.

13. Fig 3 : A bright field rotation moire image which indicates the presence of stacking-fault tetrahedral in the diamond. It may also be formed by two overlapping (111) close-packed planes, as demonstrated by SADP in Fig. 2b. • Stacking-fault tetrahedral is formed by vacancy condensation.

14. The formation of stacking-fault tetrahedral Rapid cooling from high temperature The supersaturated vacancies The formation of vacancy disc and its subsequent collapse form a stacking fault bounded by a Frank partial dislocation. The Frank partial dislocation may dissociate into a low-energy stair-rod dislocation and a Shockley partial dislocation on an intersecting slip plane according to a reaction of the type

15. Then the Shockley partial dislocations on different (111) planes react with each other to form stair-rod dislocation according to the reaction

16. The stacking faults on the (111) planes are bounded (a/6) [011] stair-rod dislocation along the edge of the tetrahedral, so the final result is the formation of stacking-fault tetrahedral, a tetrahedral bounded completely by intrinsic faults and stair-rod dislocations.

17. The distorted fringes : by strain field, which is related to the thermal stress associated with the micro-inclusions. Ex) (FeNi)23C6, Fe3C, amorphous graphite, SiC or Ni3C are trapped in the HPHT-grown diamonds.

18. As the HPHT diamond diamond cooled from high temperature to room temperature, because of the thermal coefficient difference between the inclusions and the diamond the thermal contraction differs between them , thermal internal stress can be generated, and the strain field can be formed near the micro-inclusions. So the fringe distortions can be generated in moire patterns due to the strain field.

19. Because the internal stress associated with the inclusions is introduced, which may exceed the average stress by many tens of a percent near the inclusions, such stress concentrators may be the most probable sites for the nucleation of the dislocations. • Dislocations are nucleated as the stress reaches a critical value of G/30. Therefore, it is believed that the concentric dislocation loops in Fig. 4 might be a result of internal stress caused by the micro-inclusions in the diamond. The classical multiplication mechanism is that proposed by Frank and Read . • Another dislocation multiplication mechanism analogous to Frank–Read type, is Bardeen–Herring type, which operates by climb.

20. 4. Conclusion • Several types of structure defects in the HPHT-grown diamond single crystals were examined by moire images. The structure defects consist mainly of stacking faults, twins, stacking-fault tetrahedral and dislocation loops. • The stacking faults, twins and stacking-fault tetrahedral were derived from supersaturated vacancies in the HPHT-grown diamonds. The concentric dislocation loops and the distorted fringes may be related to the internal stress associated with the inclusions contained in the diamonds.

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