STRUCTURE DETERMINATION OF FERROELECTRIC NANO-POWDER BY PDF ANALYSIS

A φ=0.5 mm diameter boron-silicate capillary was filled with BT powder having an average particle size of about 30~50 nm. The sample was then measured by transmission X-ray diffraction. To measure the diffraction profile to a high Q range, we used a high-intensity Mo rotating anode source with a CBO-E focusing mirror designed for Mo radiation. The observed diffraction profile was corrected and normalized to obtain the structure factor S(Q), then Fourier transformed to G(r). Fig. 1(a) shows the G(r) plot and the calculated results using cubic, tetragonal, and cubic+tetragonal (shell) structural models, respectively. In the Rietveld results, the tetragonality of BT is close to 1 when simulated using a tetragonal model. In addition, the reliability factors (Rwp) from the three different models were almost the same (Table 1, Figure 1(b)). In the PDF analysis, Rwp decreased from 23.6% using a cubic model to 12.6 % when simulated by a cubic+tetragonal model. Therefore, it was concluded that the structure of the nano-sized BT powder is best described using a shell model combining cubic and tetragonal structures. These results indicate that PDF analysis is of great advantage in determining the crystal structure of powder materials with extremely small tetragonality or similar crystal structures.

(a)

Fig. 1: (a) PDF results from nano-sized BT powder analyzed with cubic (bottom), tetragonal (middle), and cubic+tetragonal (shell) (top) models

(b)

(b) Rietveld results using structural parameters obtained from the PDF results using the cubic+tetragonal model