High pressure modifications, silicon nitrides

Silicon nitride crystallizes in two hexagonal forms (a- and fl-SisN and also a cubic y-modification with spinel structure that is stable at high pressure and temperature (>15GPa, >2000 K Zerr et al, 1999 Leitch et al, 2004). The a-phase occurs in nature as a rare mineral (nierite) in ordinary and enstatite chondritic meteorites (Lee et al, 1995). 

It is believed that the reaction of nitrogen and solid silicon gives predominantly P-silicon nitride, while the reaction of nitrogen with volatile silicon gives a-silicon nitride. Cubic y-modification of silicon nitride may be synthesized only at high pressures. The parameters of a- and p-modifications of silicon nitride are quite similar Table 2.14). 

The cubic y-modification has been recently observed under a pressure of 15 GPa and temperatures above 2000 K by the laser heating technique in a diamond cell [23] and in shock-wave compression experiments with pressures >33 GPa at 1800 K and >50 GPa at 2400 K [29]. This modification is often designated as the c-modification in the literature in analogy to the cubic boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms are octahedrally coordinated by six nitrogen atoms, one silicon atom is coordinated tetrahedrally by four nitrogen atoms (Fig. 3c). The atomic coordinates for the cubic modification are given in Table 2. From calculations it is shown that this structure should have a high hardness similar to that of diamond and c-BN [23]. 

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