(ii) Inorganic compounds

SiC has a binary tetrahedral structure in which the Si and C layers are stacked alternately, each carbon layer occupying half the tetrahedral voids between successive close-packed silicon layers. One can regard the structure as consisting of two identical interpenetrating close-packings, one of Si and the other of C, with the one displaced relative to the other along the c -axis through one fourth of the layer spacing. The binding between Si and C atoms in SiC is predominantly covalent. The silicon-carbon bond length of 1.94 Å as calculated from the known covalent tetrahedral radii of C and Si is nearly equal to the observed silicon-carbon bond length of 1.89 Å. The tetrahedral arrangement of Si and C in SiC does not permit either a centre of symmetry ( $\overline{1}$ ) or a plane of symmetry (m ) perpendicular to [00.1]. Silicon carbide can therefore have only four possible space groups -- P 3m 1, R 3m 1, P 6₃mc and F $\overline{4}$ 3m.

**Figure 11:** Tetrahedral arrangement of Si and C atoms in 6H SiC structure.
$\begin{figure} \includegraphics {fig11.ps} \end{figure}$

Commercial SiC crystals are grown at temperatures above 2000 $^{\circ}$ C and are called $\alpha$ -SiC crystals. The more common modifications in the $\alpha$ -SiC crystals are 6H , 15R and 4H . They have stacking sequences /ABCACB/ (=6H ), /ABCBACABACBCACB/(=15R ) and /ABCB/...(4H ). Figure 11 depicts the structure of the most common $\alpha$ -SiC modification 6H with a packing ABCACB. In addition to the common modications (often called the `basic structures` of $\alpha$ -SiC) several polytype structures with stacking sequences of larger repeat periods have been discovered. These have either a hexagonal or a rhombohedral lattice². Table 3 lists the known structures of SiC. In all these structures the h/a ratio is 0.817, which is very close to the value of 0.8165 for an ideal close-packing. The cubic or $\beta$ -SiC, with a packing /ABC/ABC/..., is denoted as 3C and normally forms² at temperatures below 1800 $^{\circ}$ C. It is regarded as the low- temperature modification of SiC and undergoes a solid-state transformation to the 6H structure at temperatures above 1800 $^{\circ}$ C^2,16,17. The wurtzite (2H ) modification of SiC, with a stacking sequence /AB/AB/..., does not occur in commercial SiC and has been synthesized by special methods between the temperatures of 1400 and 1500 $^{\circ}$ C¹⁸. It is regarded as a metastable modification of SiC and undergoes solid-state transformation to the 3C and 6H structures at temperatures above 1400 $^{\circ}$ C¹⁷. The h/a ratio in this structure is 0.8205 which differs considerably from the ideal h/a ratio for perfect close-packing.

**Table 3:** List of SiC polytypes with known structures
Polytype	Structure (Zhdanov sequence)	Polytype	Structure (Zhdanov sequence)
2H	11	54H	(33)₆323334
3C	${\infty}$	57H	(23)₉3333
4H	22	57R	(33)₂34
6H	33	69R ₁	(33)₃32
8H	44	69R ₂	33322334
10H	3322	75R ₁	(33)₃34
14H	(22)₂33	75R ₂	(32)₃(23)₂
15R	23	81H	(33)₅35(33)₆34
16H ₁	(33)₂22	84R	(33)₃(32)₂
16H ₂	332332	87R	(33)₄32
18H	(22)₃33	90R	(23)₄3322
19H	(23)₃22	96R	(33)₃3434
20H	(22)₃44	99R	(33)₄3222
21H	333534	105R	(33)₅32
21R	34	111R	(33)₅34
24R	35	120R	(22)₅23222333
27H	(33)₂(23)₃	123R	(33)₆32
27R	2223	141R	(33)₇32
33R	3332	147R	(3332)₄32
33H ₁	(33)₂353334	159R	(33)₈32
33H ₂	(33)₃(23)₃	168R	(23)₁₀33
36H ₁	(33)₂32(33)₂34	174R	(33)₆6(33)₅4
36H ₂	(33)₄3234	189R	(34)₈43
39H	(33)₂32(33)₃(32)₂	222R	(33)₆34(33)₄34
39R	3334	267R	(23)₁₇22
45R	232332	273R	(23)₁₇33
51R ₁	(33)₂32	303R	(33)₁₆32
51R ₂	(22)₃23	393R	(33)₂₁32

The structure of ZnS is analogous to that of SiC. The bonding in ZnS is known to be partly ionic and partly covalent. The wurtzite and sphalerite modifications of this compound, which occur as minerals, correspond to the /AB/AB/AB/... and /ABC/ABC/,... packings respectively.

The cubic form is known to be the low-temperature modification and undergoes a reversible phase transformation² to the 2H form around 1020 $^{\circ}$ C. In addition to these two common modifications, ZnS is known¹⁹ to display a large variety of polytype structure with larger identity periods. As stated earlier, all the polytype modifications of a material have identical a and b lattice parameters and differ only along c . The h/a ratio for the 2H modification of ZnS is 0.818 which is somewhat different from the ideal value 0.8165 for a perfect close-packing.

(ii) Inorganic compounds

(a) Silicon carbide and zinc sulphide