A size effect on crystal structure has been investigated for barium titanate (BaTiO3) nanoparticles of 40-, 140-, and 430-nm sizes, by means of neutron and high-resolution synchrotron x-ray powder-diffraction and Raman-scattering techniques. These samples were prepared by a modified two-step thermal decomposition method from barium titanyl oxalate, resulting in very few lattice impurities. Rietveld analysis of the neutron-diffraction data for the 430-nm- and 140-nm-sized BaTiO3 particles was performed assuming a single phase of tetragonal (P4mm) structure. The axial ratio c∕a of tetragonal BaTiO3 decreases with a decrease in particle size from 430 to 140 nm. Barium titanate particles with a size of 40 nm consist of (1) tetragonal crystals (83 wt %) with a large cell volume and an axial ratio of unity c∕a=1.000(5) and of (2) a hexagonal phase (P63mmc, 17 wt %) with a large unit-cell volume. Rietveld and maximum-entropy method analyses suggest that there exist atomic displacements from the ideal site of a cubic structure and a spontaneous polarization of the tetragonal phase even in the 40-nm-sized BaTiO3 particles. The nuclear-density distribution of the 140-nm-sized particles with a high dielectric constant does not exhibit a large positional disorder, while the Ba atom of tetragonal BaTiO3 in the 40-nm-sized particles has a smaller atomic displacement parameter.

No abstract

The scattering amplitude distribution of an yttria-doped ceria material (Ce0.93Y0.07O1.96, space group: Fm3m) has been investigated between 23 and 1434 degrees C by the maximum-entropy method (MEM) combined with a Rietveld analysis using neutron powder diffraction data. The refined unit cell and atomic displacement parameters increased with an increase in temperature. The results of the MEM analysis reveal that the oxide ions have a positional disorder spreading over a wide area. One possible diffusion path of the oxide ions lies on the tie line along the (100) directions. The other pathway of the oxide ions can be seen along the (110) directions. The curved feature in the diffusion path would be common in various ionic conductors.

Accurate nuclear density distribution of ceria, CeO2 has been studied between 1005 and 1497 °C by the maximum-entropy method-based pattern fitting combined with the Rietveld method using neutron powder diffraction data. The results reveal that the oxygen ions have a complicated disorder spreading over a wide area and shift to the 〈111〉 directions from the ideal fluorite position. This feature is more significant at higher temperatures, which is consistent with the higher ionic conductivity.