DETERMINATION OF SiC POLYTYPES DISTRIBUTION WITH HIGH ACCURACY. By Hongchao LIU and Changlin KUO, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050.

Quantitative determination of SiC polytypes distribution is very difficult from traditional X-ray powder diffraction quantitative analysis methods. SiC polytypes seem to form as the result of stacking faults introduced into the parent structures, the main reflections will overlap with those of the parent polytype and the differences will be relative small. So many efforts have been concentrated on this topic(see L. K. Frevel, et al, J. Mater. Sci., 27(1992)1913-1925). However, it was still hard to obtain accurate results of SiC polytypes distribution by these traditional methods for various reasons.

The whole pattern fitting Rietveld method(H. M. Rietveld, J. Appl. Crystallogr., 2(1969)65-71) is introduced to determine SiC polytype distribution. With the scale factors and cell parameters given by the Rietveld program, equation

, where S,

[[rho]] and V are scale factor, density and unit cell volume of each polytype, can be used to calculated the relative weight percentages of each polytypes. The simulated data by Warren and Averbach method(B.E. Warren & B.L. Averbach, J. Appl Phys.,, 21(1950): 595-599; 23(1952): 1059) with 4H, 3C, 6H and 15R polytypes and two specimens from the same nano material were used to test this approach.

The results show that the distribution of SiC polytypes can be accurately and reliably determined by this approach. It can be applied in nano-material case.