Synthesis of a new cubic phase of silicon nitride: A promising material for technical applications

July 21, 1999

A new phase of the technologically important ceramic material silicon nitride, Si3N4, was sythesized and identified by scientists of the High Pressure Group at the Max Planck Institute for Chemistry in Mainz, from the Technische Universität Darmstadt and the Cornell University, Ithaca, New York.

The synthesis of the novel Si3N4 phase with a cubic spinel structure was performed under high pressure and high temperature conditions using the technique of laser heating in a diamond anvil cell. First results suggest an extraordinary hardness of the new phase (Nature, 22 July 1999).

Silicon nitride is a ceramic material of high fracture toughness, hardness, wear and temperature resistance. Because of these properties it is being used, for example, as cutting tools and antifriction bearings as well as an insulating, masking and passivating layer in electronics. Two hexagonal polymorphs of Si3N4 were known so far, a -Si3N4 and ß -Si3N4, both with a density of about 3.2 g/cm3. a -Si3N4 can be synthesised from silicon and nitrogen at ambient pressure and temperatures below 1800 K, while the ß phase requires higher temperatures. ß -Si3N4 is also obtained from a -Si3N4 at about 5 GPa and 2270 K using an internally-heated high pressure vessel. Traces of Si3N4 were recently discovered as a presolar material in meteorites.

For the synthesis of the novel Si3N4 phase, silicon single crystals were embedded in a nitrogen pressure medium in a diamond cell. A Nd:yttrium-lithium-fluoride (YLF) laser was used to heat the silicon probe up to 2200 K at a pressure of 15 GPa. The reaction product was examined by Raman spectroscopy and by transmission electron microscope and electron dispersion X-ray analysis. The investigations showed a new cubic Si3N4 phase of space group Fd-3m (Z = 8) with a spinel structure where both tetrahedrally and octahedrally coordinated silicon atoms with a 1 : 2 ratio are contained. Thus, this cubic phase may be better described as Si[Si2N4] instead of Si3N4. It is the first known example of a pure-nitride, non-oxide solid silicon compound containing six-fold coordinated silicon atoms. c-Si3N4 is stable at high temperatures and at pressures between 15 and 30 GPa; at ambient pressure in air it persists metastably to at least 700 K. It was also successfully synthesised from amorphous Si3N4 as well as a - and b -Si3N4 in a diamond cell at 15 and 30 GPa on heating up to 2800 K with a CO2 laser.

For c-Si3N4 with spinel structure the calculated density is 3.93 ± 0.12 g/cm3, a value 23% higher than that of a - or b -Si3N4. First-principles calculations of the compressibility and the elastic c44 modulus of c-Si3N4 which are related to its hardness yielded 300 and 340 GPa. Because these values are comparable to those of stishovite (281 - 313 GPa and 252 GPa, respectively), a hardness of c-Si3N4 close to that of stishovite can be expected. Stishovite, a high pressure modification of SiO2, where the silicon atoms are octahedrally coordinated to oxygen, is considered to be the third hardest material (Knoop hardness of 33 GPa) after diamond and cubic bor nitride. Thus, the expected hardness and the metastability of c-Si3N4 make it a promising material for technological applications, especially in the cases where diamond can not be used.
Additional Contacts:
Ralf Riedel
Technische Universität Darmstadt
Phone: 49-61-51-1-66-3-47

Reinhard Boehler
Max Planck Institute for Chemistry
Phone: 49-61-31-3-05-2-52


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