Comment on "Assignment of the Raman active vibration modes of beta-Si3N4 using micro-Raman scattering" [J-Appl. Phys. 85, 7380 (1999)]

In this comment to a recent experimental paper by Honda [J. Appl. Phys. 85, 7380 (1999)], we report first-principles calculations of phonon modes of b eta-Si3N4, which theoretically confirm the experimental micro-Raman assignm ents. In addition, the theory is able to locate the "missing" A(g) mode, an d we show that this mode is a bond-stretching mode. Hence, it is not in the low-frequency band as previously believed, but rather at intermediate freq uency. We discuss the possible reasons this mode is "missing" in the spectr a, and show that the application of high pressure will separate this mode f rom others. (C) 2000 American Institute of Physics. [S0021-8979(99)00123-1] .