A novel determination of the structure of an anomalous polytype of silicon carbide

Abstract

This paper describes an unusual silicon carbide crystal of special crystallographic interest. During the growth of this crystal there has occurred a change in the structure, from one region of the crystal to another, without a change of space group (P3m) or of the dimensions of the unit cell. The crystal has been identified as type 36H (Ramadell notation) and its two structures. designated as 36H_a and 36H_b, form respectively the upper and lower portions of a single crystal piece. In addition to tho usual extinctions inherent in all silicon carbide structures, the X-ray diffraction photographs of 36H_b show striking structural extinctions. In tho 10.l row the reflexions l = 6n ± 2 [n any positive or negative integer) have zero intensity; this imposes on the structure, the conditions Σ_xA exp [2πi(6n ± 2)z] = Σ_lB exp [2πi(6n ± 2)z)] = Σ_lC exp [2πi(6n ± 2)z)] = 0. From these, by working backwards in a novel way the structure has been uniquely determined. The polytype has an unexpected structure with the Zhdanov symbol (333334333332). which has been confirmed by the excellent agreement between the observed and calculated relative intensities of X-ray reflexions. The growth of this structure is discussed on the existing theories of polytypism. and it is concluded that none of them is able to provide a completely satisfactory explanation for the growth of the crystal.