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 A) collected at station BL-4B2 in the photon factory. A refinement procedure that adopted a new weight function, w = 1/Yoe (Yo 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) A, V = 292.447 (3) A3, Z = 4; Rp = 5.08, Rwp = 6.50, RB = 3.36, RF = 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 A-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. (orig.)