Preparation of the Polytypes

The 6 O and 3 R polytypes are stable below 500°C and are easily transformed into the other polytypes by heating. Generally, transformation into the polytypes stable at higher temperatures is easy, whereas the reverse transformations are difficult [6]. The transformation temperature of the 3 R into 6 O polytype could not be determined In YSeF due to the low transformation rate, but is most likely between 500 and 550°C [11]. 

The structure of the 6 0 polytype may thus be described by a periodic stacking of blocks following the sequence. ..SSSTTTSSS... illustrating the existence of two types of block boundaries (SS or TT) and (ST or TS). 

Other Polytypes. The MSeF polytypes represent various mixtures of the two structural features, the pure forms of which are the 2 0 polytype and the 3 R polytype, see Fig. 230. The 4 M and 8 M stacking are shown in Fig. 231. For patterns of composite and defect structures, see the paper. Within the known polytypes, it holds that their crystal lattices are orthorhombic if there are equal numbers of S and T blocks [4]. See Table 29.6-1, p. 490, for lattice parameters, space group, the stacking sequence, and the annealing temperature of the various YSeF polytypes. 

2 O Polytype. The crystal structure of the transparent, pale yellow orthorhombic 2 0 polytype YSeF was determined by single crystal X-ray methods. The space group is Pnam-Dj (No. 62), Z = 4, a = 9.912(4), b = 6.314(2), c = 4.080(1) A. Measured and calculated density 4.86 g/cm. Atomic positions (R = 0.06 with isotropic temperature factors) are  

8 M Polytype. The crystal structure of transparent, pale yellow, monoclinic 8 M YSeF was determined by the usual single crystal X-ray methods space group P2i/m-Cih (No. 11), lattice parameters a = 9.935(3), b = 25.420(1), c = 4.094(2) A, y = 97.50(2)° Z=16. The densities are Dexp = 4.80 and Dcaic = 4.84 g/cm The atomic positions (special position 2e for all atoms, R = 0.064, isotropic temperature factors, z = 0.25 throughout) are as follows  

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