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  • Author or Editor: Y. Kuroiwa x
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Summary The size effect on the crystal structure including the chemical bonding nature has been investigated for several kinds of BaTiO3 nanopowder with the particle sizes down to 50 nm in diameter, by means of powder diffraction using high-energy synchrotron radiation. The Rietveld refinement reveals that the BaTiO3 nanopowder consists of tetragonal and cubic structure components at 300 K. The feature of coexistence can be illustrated by the core/shell model for the particle, in which the shell with a cubic structure covers the core with a tetragonal structure. The thickness of the cubic shell is almost constant irrespective of the particle sizes, and is estimated as approximately 8 nm. Hence, the critical particle-size, where the entire particle is covered with the cubic shell, is suggested as 16 nm. The charge density distributions of the BaTiO3 nanopowder in the cubic phase at 410 K are revealed by the maximum entropy method. Changes in the bonding electron density and the ionic valence expected are not observed clearly even in the 50 nm crystal compared with the bulk crystal.

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The heat capacity of PbMO3 (M=Ti, Zr and Hf) at constant pressure was measured using a differential scanning calorimeter (DSC) from room temperature up to 870 K. Large anomalies were found in the heat capacity curves, corresponding to the ferroelectricparaelectric phase transition in PbTiO3 (PT), the antiferroelectric-paraelectric phase transitions in PbZrO3 (PZ) and PbHfO3 (PH). The transition entropies were estimated as 7.3 J K−1 mol−1 (PT), 9.9 J K−1 mol−1 (PZ) and 9.3 J K−1 mol−1 (PH). These values of transition entropies are much larger than that of a typical displacive-type phase transition.

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A new phase transition (III–IV) was found at 311 K in CsCoPO4 by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO4 was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO4 had the same crystal structure as that of corresponding phases in CsZnPO4. An extremely large particle size effect was found on III–IV phase transition in CsCoPO4; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO4. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO4, while such a critical size was not found in CsCoPO4.

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The crystal structures of BaTiO3 and PbTiO3 fine particles have been investigated by powder diffraction using synchrotron radiation high energy X-rays. It is revealed that a BaTiO3 fine particle essentially consists of tetragonal and cubic structure components at 300 K, whereas a PbTiO3 fine particle consists of a tetragonal structure. Adopting a structure model for the BaTiO3 particle that a cubic shell covers a tetragonal core, the thickness of cubic BaTiO3 shell is estimated at almost constant irrespective of particle sizes. Successive phase transitions are detected in 100 nm particles of BaTiO3 near the phase-transition temperatures of a bulk crystal. The changes in diffraction profiles are small, but they are apparent for a most up-to-date powder diffractometry.

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