A new proposal for the structure of platelets in diamond

A new model is proposed for platelets in diamond that is based upon the unambiguously determined molecular structure of the EPR centre known as R1, which comprises two parallel [001] split interstitials at nearest neighbour positions in the (1 (1) over bar 0) plane. The new feature of the model is the structure of the "pillar", which holds apart the two sides of the platelet, which comprises two R1 centres related by a displacement of a(o)/root 2 along [1 (1) over bar 0], linked by two covalent bonds. The two sides of the platelet are pushed apart by a measured distance of 0.40(1) a(o): an approximate calculation for the proposed structure gives 0.40 a(o). The pillar has a rectangular cross-section in the (001) plane, having dimensions 1/2 a(o)/root 2 x a(o)/root 2: that its symmetry is only twofold about [001] could account for the fact that platelets are often elongated along one of the [110] axes. The pillar comprises basically carbon interstitials, but could accommodate randomly up to 50% nitrogen interstitials. In particular, the model indicates a clear method of nucleation and growth. The pillars are most likely to be arranged in a regular array in one of two ways. In units of a(o)/root 2, the unit cell is 3 units in one [110] direction, and 2 or 4 units in the perpendicular direction. In either case, the number of interstitials is 8/3 per a(o)(2). (C) 1998 Elsevier Science S.A.