Crystalline silicon nitride (Si<sub>3</sub>N<sub>4</sub>) is of considerable interest commercially, e.g., as a passivating layer in the microelectronics industry and in the construction of advanced heat engines. The high-temperature, nitriding processes normally used in the manufacture of this ceramic yield materials in which the ratio of the α- and β-Si<sub>3</sub>N<sub>4</sub> polymorphic, crystalline modifications varies considerably depending on the actual synthetic conditions employed. Almost ten years ago, Luongo showed that IR spectroscopy could readily be used for the rapid, quantitative identification of crystalline α- and β-Si<sub>3</sub>N<sub>4</sub> in the presence of each other, despite the very close structural similarities between the two polymorphic phases. The IR method was based on determining the ratio of the absorbances (<i>I</i><sub>685</sub>/<i>I</i><sub>578</sub>) of two characteristic marker bands which appear at 685 (α-Si<sub>3</sub>N<sub>4</sub>) and 578 cm<sup>−1</sup> (β-Si<sub>3</sub>N<sub>4</sub>). A graph for determining the relative amounts of the two polymorphic phases in the presence of each other in an unknown sample was published as part of Luongo's paper.

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