Silicon Nitrides

Several nitrides of silicon appear to exist as white amorphous substances. They are:

SiN, density 3.17,
Si₂N₃, density 3.64, insoluble in HF,
Si₃N₄, density 3.44, soluble in HF, forming (NH₄)₂SiF₆,

and are produced together when silicon is strongly heated in nitrogen. When silicon is heated in a crucible packed round with carbon, access of oxygen is almost prevented, and Si₃N₄ is formed together with the carbonitride Si₂C₂N (or Si₃C₃N), the carbide SiC and the carboxide SiCO.

According to Martignon, the nitride Si₃N₄ is formed when a mixture of silica and-carbon is heated in a current of nitrogen at 1400-1500° C., according to the equation:

3SiO₂ + 6C + 2N₂ = Si₃N₄ + 6CO.

From theoretical considerations the heat of formation of this nitride is calculated to be 159,300 calories.