IT-5-P-3376 Comparison of the silicon/phosphorus ratio in natural and synthetic nagelschmidtite for possible use as standard for microanalysis based on X-ray lines of Si and P

Quantitative chemical microanalysis by energy-dispersive X-ray spectroscopy (EDXS) in a (scanning) transmission electron microscope (STEM) relies on the use of accurate k-factors. The most commonly used reference line is Si K.

For semiconductor research, standards for elemental semiconductors and for III/V compound semiconductors including elements from groups III and V of the periodic table are required. For the arsenides we have published results on X-ray quantification based on standards of InGaAs [1]. InAs and InP can be used to link arsenides and phosphides. However, the nominal k-factor for the P K-line in the ISIS software of k=1.000 indicates that this has probably not been measured at all.

Here, we use a natural and a synthetic sample of the mineral nagelschmidtite, a calcium silico-phosphate (ideal formula Ca₇(SiO₄)₂(PO₄)₂ [2]), to evaluate the Si/P ratio from EDXS.

The natural mineral stems from the Hatrurim formation [3] and was cut from a thin section by a focused ion beam to produce an electron transparent specimen for TEM. Electron probe microanalysis (EPMA) of a larger inclusion of nagelschmidtite yielded an atomic ratio of Si/P=3.15. Results from TEM-EDXS are displayed in green.

The synthetic mineral was prepared in the laboratory of C Wu, Shanghai Institute of Ceramics [4]. Its chemical analysis using a Spectro Cirus Vision ICP-OES spectrometer gave a Si/P ratio of 0.36 (by at%), i.e. an almost inverted ratio. (S)TEM-EDXS results from these particles are displayed in dark blue.

Figure 1 shows that, for the detector setting used the deadtime of the detector is linearly related to the count rate up to a max of ~2500 counts/second or 50%, above which the detector runs into saturation.

Figures 2 and 3 plot atomic ratios as obtained from ISIS without absorption correction. The synthetic compound (blue) clearly reveals a higher P/Si ratio than the natural mineral (green) in Fig.2.

If the chemical concentration x_n of an element n is proportional to the product of X-ray intensity I_n, k-factor k_n,Si (for weight%) and absorption factor a_n, divided by the atomic weight A_n, then we can calculate an effective k-factor [5]:

k^eff_P,Si = k_P,Si a_P,Si = (I_Si x_P A_P) / (I_P x_Si A_Si)

This is plotted in Fig.4. While the data scatter is rather large, a linear fit to the spectra that gave reasonable densities (≤3.5 gcm^-3) as determined by ISIS allows us to determine the thin-film k-factor by extrapolation to zero count rate. The result is k_P,Si=1.16±0.45 (R²=0.287). 

[1] T Walther, Proc EMAG2009, J Phys Conf Ser 241 (2010) 012016

[2] G Nagelschmidt, J Chem Soc 1 (1937) 865

[3] M Fleischer, LJ Cabri, GY Chao, A Pabst, Am Mineral 63 (1978) 424

[4] Y Zhou, C Wu, Y Xiao, Acta Biomaterialia 8 (2012) 2307

[5] Y Qiu et al, Proc EMC2008, 2 (2008) 643