Studies of the state of iron contained in α- and β-silicon nitride by Mössbauer and Raman effects

The Mössbauer spectra of β- and α-Si₃N₄ containing Fe have been measured at 300 and 93 K. The former consists of a singlet and a doublet, whereas the latter exhibits paramagnetic hyperfine patterns and a doublet. The single line and hyperfine patterns arise from isolated high-spin Fe³⁺ ions which are bonded equivalently with 4 nitrogen neighbors. Both doublets arise from high-spin Fe³⁺ ion-pairs each of which is strongly bonded by a kind of superexchange force. In β-Si₃N₄ the Fe³⁺ ions occupy the same sites regardless of whether they are doped into the crystal lattice before or after its preparation, while in α-Si₃N₄ the Fe³⁺ ions are located at different sites depending on the doping procedure. From the Raman spectra, the difference in the Mössbauer spectra for the two compounds is attributable to different relaxation by lattice phonons of the host crystals.The Mössbauer spectral lines for β-Si₃N₄ alone have been found to decrease in intensity with tune even at room temperature, such an ageing being observed to be entirely suppressed by γ-irradiation. From different behaviors of the three lines in the ageing and quenching from 1150°C down to room temperature, the ageing is interpretable based on a model that the Fe³⁺ ions jump from substitutional sites into interestitial sites and take place thermal vibration with a large amplitude there.