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

The crystal structure of α-silicon nitride (Si₃N₄) was refined by the Rietveld method using synchrotron radiation powder diffraction data (wavelength = 1.2 Å) collected at station BL-4B2 in the Photon Factory. A refinement procedure that adopted a new weight function, w = 1/ (Y_o is the observed profile intensity and e ≃ 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 α-Si₃N₄: trigonal, P31c, a = 7.75193 (3), c = 5.61949 (4) Å, V = 292.447 (3) ų, 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 θ/λ range of >0.8 Å⁻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.