Correlation between Raman Peak Shifts and Isotopic Compositions of Sub - Micron Presolar Sic Grains

Introduction: Silicon carbide is a particularly interesting phase because more than a hundred different polytypes can be formed in the laboratory. The formation of these polytypes depends strongly on growth conditions (e.g., temperature, pressure). Daulton et al. [1] used TEM to study the microstructures of ~500 presolar SiC grains in an acid residue of the Murchison meteorite (KJB fraction of [2], mean diameter of 0.49 μm). They found that cubic SiC (β-SiC) is the dominant polytype for presolar SiCs (~80%). Previous Raman studies of large SiC grains (>3 μm) showed that Raman spectra can be diagnostic for distinguishing between non-cubic (hexagonal or rhombohedral, αSiC) and cubic SiC structures [3,4]. However, the grains in the previous Raman studies were of unusually large size and it is therefore of interest to study the SiC microstructural distributions in smaller, more typical size fractions with Raman microscopy. Furthermore, Ivanov et al. [5] showed that the Raman peak positions of synthetic isotope-enriched SiCs are shifted with respect to those of the same polytype but with normal isotope ratios. It is therefore intriguing to investigate whether Raman peak positions correlate with the corresponding C and Si isotope ratios in presolar SiCs, which could potentially provide a new, nondestructive, method to find rare type SiC grains with extremely anomalous C and/or Si isotope ratios (e.g., highly C-enriched A+B grains [6]). Samples and Methods: The SiC grains in this study were extracted from the Murchison meteorite using the isolation method described in [7] and dispersed on a high purity Au mount. Raman spectra were acquired first with a WiTeC multi-function scanning probe microscope, which includes near-field optical microscopy and confocal imaging Raman microscopy (532 nm frequency-doubled Nd:YAG laser). Its typical spatial resolution is ~400 nm at low power. Spectral images were obtained in four areas on the presolar SiC mount (each area ~50×50 μm), where each pixel contains a full Raman spectrum (~0.1Δcm spectral resolution). After Raman measurements, we further verified the chemical compositions of Raman-identified SiC grains by EDS analysis with a JEOL 6500F fieldemission SEM. High-resolution Raman spectra were then acquired on single SiC grains for comparison with previous Raman scanning spectra to see if SiC Raman peaks are shifted due to the electron beam damage. Finally, the C, N and Si isotopic compositions of these SiC grains were simultaneously measured with the Carnegie NanoSIMS 50L ion microprobe using a Cs beam and standard methods. Results: We performed Raman and isotopic measurements on 30 presolar SiC grains. The grain sizes range from 0.2 to 2.2 μm (mean ~0.8 μm), which is similar to the size distribution of the KJD fraction of [2] (mean 0.81 μm). We succeeded in obtaining correlated data for 13 mainstream SiC grains. The C, N and Si isotopic compositions of the 30 presolar SiC grains are in good agreement with the literature data [8]. We did not see any evidence of peak shifts due to electron beam damage arising from the EDS analysis. Two of the 13 grains are hexagonal and the remainder are all cubic. This is consistent with the SiC microstructural distribution found in [1]. All the Raman and isotopic data are summarized in Table 1. Figure 1 shows high-resolution Raman spectra of one 6Hand one 3C-SiC standard, and two presolar grains. The β-SiC (e.g., 3C) has two first-order Raman active phonon modes, a TO mode at 796 cm and an LO mode at 972 cm. The LO mode is absent in the 3C-SiC STD spectrum, probably due to the crystal orientation. In contrast, the TO mode of the α-SiC (e.g., 4H, 6H) splits into several modes. For instance, in Figure 1, the 6H-SiC standard has two TO modes at 764 and 785 cm [9]. Thus, different SiC polytypes can be easily distinguished based on their Raman spectra.