Crystal structure refinement of α‐Si 3 N 4 using synchrotron radiation powder diffraction data: unbiased refinement strategy

The crystal structure of α‐silicon nitride (Si 3 N 4 ) 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‐Si 3 N 4 : trigonal, P 31 c , a = 7.75193 (3), c = 5.61949 (4) Å, V = 292.447 (3) Å 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 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.