Crystal structure refinement of α-Si3N4 using synchrotron radiation powder diffraction data:: unbiased refinement strategy

The crystal structure of alpha-silicon nitride (Si3N4) was refined by the Rietveld method using synchrotron radiation powder diffraction data (wavelength = 1.2 Angstrom) collected at station BL-4B2 in the Photon Factory. A refinement procedure that adopted a new weight function, w = 1/Y-o(e) (Y-o is the observed profile intensity and e similar or equal to 2), for the least-squares fitting [Toraya (1998). J. Appl. Cryst. 31, 333-343] was studied. The most reasonable structural parameters were obtained with e = 1.7. Crystal data of alpha-Si3N4: trigonal, P31c, a = 7.75193 (3), c = 5.61949 (4) Angstrom, V= 292.447 (3) Angstrom(3), Z = 4; R-p = 5.08, R-wp = 6.50, R-B = 3.36, R-F = 2.26%. The following five factors are considered equally important for deriving accurate structural parameters from powder diffraction data: (i) sufficiently large sin theta/lambda range of >0.8 Angstrom(-1); (ii) adequate counting statistics; (iii) correct profile model; (iv) proper weighting on observations to give a uniform distribution of the mean weighted squared residuals; (v) high-angular-resolution powder diffraction data.