This work is aimed at a detailed investigation of surface of diamond crystals etched in HPHT cameras to macro-, micro, and nano-sizes by means of optical parabolic goniometry and AFM method.

As an object of investigation we used the diamond powder of two types. The examination of diamond crystal surface in the scope greater than 30 mkm revealed only growth surfaces. However, the crystal forming system going out of the growth mode, there can spontaneously occur conditions for transitory dissolution (etching) of diamond in the process of the residual melt degassing, which was proved by AFM examination of the negative pyramids of dissolution  (etching pits) of micro- and nano-sizes on diamond faces.

The detected pits present sockets shaped as negative pyramids with the basis being a square whose sides are parallel to the directions [011]. This form of pits is conditioned by the symmetry of tetrad axis of diamond structure on the hexahedron face. AFM-images of some relatively large (about 5 mkm) pits have a clear stepped structure of inner side-walls (the height of large steps being 40-60 nm) with visible terraces between the steps. According to the kinematic theory, pits terracing occurs due to disorientation of the etching surface in relation to the real crystallographic plane. It is also influenced by the slope of dislocation line in relation to the open surface.

Etching pits smaller than 4 mkm on synthetic diamonds demonstrate a different structure. |Cross sections of five relatively large pits, 2-3 mkm in size (Figure), showed them to have even slopes and the average angle between the inner sub-face of the pit and crystal surface (100) of 54,3°, which allows to identify the etch pit sidewalls as flat lattices {111}. High resolution investigations have also revealed that faces of synthetic crystals may have square pits of a much smaller size, 30 – 70 nm. They show some competition character. Terraces between the nano-size etch pits can be uneven or, on the contrary, very even.

The standard AFM probes of pyramidal form with the needle curvature radius of 10 nm are known to misrepresent real morphology of nano-size pit slopes. If they don’t reach the pit bottom, they may underrate their real depth. That is why the representations of the bottom and the walls of a 30 nm wide pit are not very reliable. The cuts of six pits of 70 and more nm wide showed that they present somewhat asymmetric near-pyramidal sockets with very mild slopes corresponding to flat lattices from {12.1.1} to {811} (deviation angles in relation to hexahedron face from 6 to 10°).

This used optical goniometry was established, that faceting of synthetic HPHT diamond crystals emphasizes the validity of Hartman and Perdok’s periodic bond chain (PBC) theory. Consecutively considering flat nets (hkl) with small Miller indices belonging to zone within 90° curve, we calculated with a list of all F- and S- diamond faces. Apart from cubic faces (100), octahedron faces (111) and rhombidodecahedron faces (011), this zone contains nets that form faces on diamond. The other diamond faces with large Miller indices, including all hexahedron faces, can be attributed to K-faces. Strictly following the criterion presented in Hartman and Perdok’s theory, there are two forms that belong to diamond F-faces – octahedron and hexahedron, that contain three and two periodic bond chains correspondingly; as for S-faces, apart from rhombidodecahedron they include 13 kinds of trigontrisoctahedrons (hkk)h<k, and 5 kinds of tetragontrisoctahedrons (hkk)h>k.

Faces of hexahedron (two PBC-s) and octahedron (three PBC-s) belonging to F-faces are revealed in all diamond crystals. Rhombidodecahedron faces {110} (S-face – one chain of carbon bond) are present on one third of diamond crystals, and simple forms of tetragontrisoctahedron{311} and {433}, as well as  trigontrisoctahedron {122}, also belonging to S-faces, appear only at the terminal growth stages connected with the off-effects. Theoretically, according to Hartman and Perdok’s theory, apart from the above listed ones, diamond faces may have some other S-faces: three tetragontrisoctahedrons and twelve trigontrisoctahedrons. K-faces, which contain no bond chains, are not present in outer growth faceting of synthetic diamonds.

We think the peculiarities of the negative relief of synthetic diamonds have to do with the processes of thermal etching at the stage of synthesis termination. AFM-investigations showed that large etching pits (greater than 4 mkm) on cubic faces are made up of terraces that form protuberant surface. This surface can be composed of areas of flat nets of tetragontrisoctahedron, pertaining to either S- or K-faces. This is an evidence of non-equilibrium process of diamond etching over screw dislocations.

Numerous pits with a typical size of 2–3 mkm, on the contrary, possess even slopes, strictly corresponding to planes {111}. The appearance of equilibrium octahedron form {111} on the negative diamond relief in the etching process can indicate that there is another mechanism of such etch pits formation, which is not connected with linear defects of the structure. Micro- and nano-size etch pits can be associated with subsurface nano defects of the structure, formed during the terminal growth stage, i.e. be a result of the “off-effect”. Plane {111} formation on microsize pits can be explained thermodynamically as reaching the minimum of surface energy at dying away of the tense defect area of the structure. Since nanosize pits demonstrate an unfinished non-equilibrium etching process, their faceting can be spontaneous.

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1. Influence of mechanical deformations on morphology and kinetics of crystal growth in solution (AFM data)