Crystal structure refinement of α-Si3N4using synchrotron radiation powder diffraction data: unbiased refinement strategy

Abstract

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^e (Yois the observed profile intensity ande≃ 2), for the least-squares fitting [Toraya (1998).J. Appl. Cryst.31, 333–343] was studied. The most reasonable structural parameters were obtained withe= 1.7. Crystal data of α-Si3N4: trigonal,P31c,a= 7.75193 (3),c= 5.61949 (4) Å,V= 292.447 (3) Å3,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 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.