FTIR Analysis of SiC Crystals

At Anderson Materials Evaluation, Inc., we specialize in solving unusual materials problems using advanced analytical techniques. A Moissanite merchant asked us to distinguish between the 4H and 6H polytypes of artificial silicon carbide crystals. To achieve this, we utilized FTIR (Fourier-transform infrared) spectroscopy, a technique typically used for analyzing organic materials.

SiC Crystals and Their Properties

  • Cost-effective Substitute for Diamonds: SiC crystals, also known as Moissanite, are harder and more chemically inert than cubic zirconia and possess greater brilliance, even more than diamonds.
  • Crystal Structure: The crystals are artificially produced with bilayers of atoms arranged in three different configurations: A, B, and C. The two polytypes, 4H and 6H, differ in the sequence of these bilayer stacks.

FTIR Spectroscopy for Polytype Differentiation

Our report, “FTIR to Distinguish the 4H and 6H Polytypes of SiC Crystals”, details how we used FTIR spectroscopy to differentiate these structures and address issues of disorder. Presented with our customer’s permission, it outlines our process and findings.

Key Findings

  • Spectra Analysis: We show three FTIR transmission spectra of the Princess-cut SiC crystal below. These spectra are very similar to those of the known 4H polytype SiC crystal (Moissanite).
This graph shows the analysis of a sample of Moissanite. Three FTIR transmission spectra of the Princess-cut SiC crystal are shown. The spectra are very similar to those of the known 4H polytype SiC crystal. The There are minor differences, such as the doublet nature of the major absorption minimum near 1148 cm-1, with the slightly deeper minimum of absorption at 1154 cm-1 and another local minimum of 1112 cm-1, in two of the three spectra . The third spectrum in green has a minimum at 1154 cm-1, very close to the minimum of 1148 cm-1 observed in the 4H polytype. The absorption minimum at 978 cm-1 found in the 4H polytype is also found here to be similar, but slightly altered with a small, sharp absorption peak at 974 cm-1 in the midst of the broader minimum. The green spectrum looks very much like the 4H polytype spectra, while there are still other differences in the blue and black spectra, such as the suppression of the local absorption maximum at 1035 cm-1 relative to the one at 1017 cm-1. There is relatively more absorption at about 850 cm-1 in the unknown crystal than in the 4H crystal. All of the spectra of the unknown crystal have absorption maxima at about 570 cm-1 similar to the 4H crystal and a minimum of absorption of about 506 cm-1.

  • Customer Satisfaction: By using FTIR spectroscopy for this application, we were able to provide a solution that exceeded our customer’s expectations.

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If you have any unusual materials problems or need advanced materials analysis services, contact us today to learn how we can assist you.