ACTIVITIES OF SPIRAL GROWTH HILLOCKS ON THE (111) FACES OF BARIUM NITRATE CRYSTALS GROWING IN AN AQUEOUS-SOLUTION

In situ measurements of normal growth rates R, slopes p, and step advancing rates v = R / p of the spiral growth hillocks generated from two types of dislocations, screw (b = [111]) and mixed (b = 〈110〉), were made on the (111) faces of Ba(NO3)2 crystals growing from an aqueous solution at different flow velocity, u. These two types of dislocations were identified also in situ by stress birefringence microscopy. It was found that: (1) The growth hillocks originated from the screw dislocations grow faster than those from the mixed dislocations, and the tendency is more pronounced as u increases (Rs/Rm = 1.3 when u = 40 cm/s). (2) As u increases, the step advancing rates become equal for the two types, in spite of the difference of step heights (hs/hm = 1.5). (3) The growth hillocks generated from the screw dislocations are steeper under any growth conditions than those from the mixed dislocations. It was also demonstrated that the dislocations outcropped near the periphery of a growing face were twice more active as growth centres than those at the centre of the face, due to the higher supersaturation along the periphery of the face. It is concluded that the direct integration model explains the result on the step movement better than the surface diffusion model. It is also concluded that the increase of the interstep spacing and therefore the edge free energy of steps are not proportional to the increase of the step height. © 1990.