Stacking faults and structural transformations in Zn_xMn_1-xS single crystals grown from vapour

Abstract

This paper reports an X-ray diffraction study of the stacking faults and solid state transformations observed in Zn_xMn_1-xS (0.9 <x <1) single crystals grown from the vapour phase at 1100°C in the presence of H₂S. The crystals were found to contain 2H, 3C, 2H + 3C and polytypes as well as structures with considerable disorder due to random stacking faults. Needle shaped 2H crystals were annealed in vacuum in the temperature range from 300 to 1100°C to induce structural transformations. Nearly 75% of the crystals transformed to a disordered twinned 3C structure on annealing around 600°C. The rest of the crystals transformed first to a disordered 6H structure and then, on further annealing at higher temperatures, to the disordered twinned 3C structure. Most of the transformed 3C structures showed an intensity enhancement near the positions of the 6H reflections on X-ray diffraction photographs. All the 3C crystals transform back to a disordered 2H structure on annealing above 1050° C. To determine the nature of stacking faults involved in the 2H–6H transformation we have investigated the broadening of X-ray diffraction maxima along reciprocal lattice rows with H - K ≠ 3n produced by crystals undergoing the transformation. The point intensity distribution along the 10.L reciprocal lattice row of several partially transformed crystals were recorded on a four circle single crystal diffractometer in steps of ΔL = 0.01. A study of the experimental profiles obtained by plotting the diffractometer record of intensity versus L in reciprocal space shows that the transformation occurs by the non-random insertion of deformation faults in the 2H structure. It is shown that the one-parameter model proposed by Pandey, Lele and Krishna [Proc. Roy. Soc. (London) A369 (1980) 451 is not applicable to 2H–6H solid state transformation in Zn_xMn_1-xS crystals. A two-parameter model involving different fault probabilities for the nucleation (α) and the growth (β) of the 6H phase in the 2H structure is suggested.