X-ray Powder Rietveld Method Research Articles

Preparation of Pyrochlore Type Na<sub>0.39</sub>Bi<sub>3.47</sub>O<sub>7</sub> Containing Bi<sup>5+</sup> by Low Temperature Hydrothermal Reaction

Pyrochlore type Na0.39Bi3.47O7 containing Bi5+ was prepared by low temperature hydrothermal reaction using hydrated sodium bismuth oxide, NaBiO3⋅nH2O, as a starting material. The mean valence of Bi in the pyrochlore is 3.94. The X-ray powder pattern for the pyrochlore was indexed in the cubic system with a=10.943(8)A. The crystal structure was refined with the X-ray powder Rietveld method. The final R-factors, RWP, RP and RB were 13.3, 9.5 and 12.8%, respectively. The pyrochlore decomposes above 380°C with two steps of weight losses caused by complete reduction from Bi5+ to Bi3+. The Na-pyrochlore exhibited high electric resistivity (-107Ω⋅cm) at room temperature.

Topochemical preparation of LiNbO 3 type Li 1−xCu xNbO 3 from CuNb 2O 6

Chemical insertion of lithium into CuNb 2O 6 with n-butyllithium yielded new compounds, Li 1−xCu xNbO 3 (x≤0.24), where the framework of the starting material was intact but part of Cu ion was extracted. When heated at elevated temperatures in Ar atmosphere, the compounds transformed to the LiNbO 3 type structure. Although the transformation accompanies rearrangement of cations, it was considered to be a topochemical change, because the hexagonal close packed oxygen array was preserved in both structural types and no amorphous state seemed to appear as an intermediate phase. The lattice parameters of Li 1−xCu xNbO 3 with the LiNbO 3 type structure ranged from a=5.155(1) and c=13.86(1) A ̊ for x=0.08 to a=5.168(1) and c=13.89(1) A ̊ for x=0.24. The crystal structure of Li 1−xCu xNbO 3 was refined by the X-ray powder Rietveld method.

Crystal Structures of Ilmenite Type LiNbO<sub>3</sub> and NaNbO<sub>3</sub>

Crystal structures of ilmenite type NaNbO3 and LiNbO3, which were prepared by low temperature hydrothermal and ionexchange reactions respectively, were refined by the X-ray powder Rietveld method. Final R-factors were Rwp=16.9, Rp=12.2, RB=6.1% for NaNbO3 and Rwp=13.9, Rp=11.1, RB=5.7% for LiNbO3. The mean Nb-O distances of both ilmenite type compounds agreed with those of their corresponding stable phases. The mean Na-O distance of the ilmenite type NaNbO3 was as large as that of the spinel type Na2WO4, while, the mean Li-O distance in the ilmenite type LiNbO3 was slightly larger than that of the LiNbO3 type LiNbO3 or other compounds containing LiO6 octahedra. Structural refinement of other ilmenite type compounds of MgTiO3, CdTiO3, NaSbO3, NaBiO3 and KSbO3 was also carried out by the X-ray powder Rietveld method. Stability field of ilmenite type compounds, ABO3, was discussed on the basis of volumes of the AO6 and BO6 octahedra.

Phase Transition of the Ilmenite Type NaNbO<sub>3</sub>

The ilmenite type NaNbO3 was prepared by low temperature hydrothermal reaction and had an endothermic phase transition at about 290°C and transformed to the perovskite type structure at about 485°C. The ionic conductivity, the lattice constants at high temperature and the latent heat were measured in order to investigate the phase transition at about 290°C. The crystal structure at 350°C was refined with the X-ray powder Rietveld method and the final R-factors were Rwp=24.1, Rp=18.4 and RB=14.5%. The refinement indicated that partial Na+ ions moved to the empty octahedral sites along the c-axis. From these results the phase transition at about 290°C was concluded to be the order-disorder transition of Na+ ions.