We report a study of simple ABO3 type perovskites BaTiO3,
PbTiO3, KNbO3, SrTiO3 and the relaxor perovskites
PbSc1/2Nb1/2O3 (PSN) in their cubic phase using
hard synchrotron radiation and hot neutrons. Gram-Charlier expansions of the
thermal parameters have been performed and have revealed interesting features
about the probability density function and the one-particle potential of the
different atoms. This description is compared with other descriptions in terms
of the split atom model and in terms of the rotator model. Structural trends
regarding the order-disorder versus displacive character of the phase
transitions have been obtained. It is concluded that SrTiO3 and BaTiO3 are quasi-harmonic systems whereas KNbO3 shows weak
anharmonicity and PbTiO3 and PSN display strong anharmonic features.
Optical second-harmonic generation (SHG) in SrTi 16 O 3 (STO16) and SrTi 18 O 3 (STO18) was investigated using the SHG microscope. While no biased STO16 exhibits weak and almost temperatureindependent SHG signals, a marked SHG is observed under the electric field in the quantum paraelectric region. In STO18, strong SHG signals appear spontaneously below 36 K. However, neutron and X-ray diffraction analyses indicate that no structural change appears at low temperature in STO18, or STO16 under the electric field. By taking into account the fact that the SHG is sensitive to the local polar order, the combined studies reveal that the long-range order of the polar phase does not develop on either crystal and is frozen in local regions.SrTiO 3 is a textbook example of both aspects of the fundamentals and the application of solid state physics and new results and renewal of interests have continuously aisen. The most recent ones were initiated by the discovery by Itoh et al. 1) of 'ferroelectricity' induced by the isotopic substitution of oxygen 18 ( 18 O) for oxygen 16 ( 16 O). Indeed, the progressive substitution of 18 O in SrTi 16 O 3 (STO16) which is a well-known quantum paraelectric with a high dielectric constant (2 Â 10 4 ) saturating at 0 K, 2) induces a broad maximum at a nonzero temperature. Also, the development of weak spontaneous polarization and D vs E hysteresis loops were observed below the peak temperature of the dielectric constant. 3) This situation seems very similar to that encountered in STO16 under an electric field above a critical field of 2 kV/cm, 4) and this phenomenon was also explained by the onset of ferroelectricity. However the existence of the dielectric peak itself is not a sufficient condition for the onset of ferroelectricity, as in the case of the relaxor PbMg 1=3 -Nb 2=3 O 3 , where the development of polarization is restricted to nanoscale regions, whereas a very high permittivity is observed. 5) Furthermore, even near 0 K, the dielectric constant of STO18 is quite high, approximately 2 Â 10 4 . This means that large dipole fluctuations still exist below the temperature of the maximum dielectric constant. These seem to suggest that long-range order is not developed in STO18 even near 0 K, or STO16 under an electric field. This is the motivation of the present research. In order to elucidate the polar state, we have performed optical second-harmonic generation (SHG) experiments and neutron and highresolution X-ray diffraction experiments at low temperature in STO16 under electric field and in 18 O-exchanged STO (STO18). This combination of studies allowed us to perform a comprehensive study of the polar order of these compounds at different length scales.We used an SHG microscope 6,7) which yields images of SH waves produced in a polar specimen. In these experiments, the magnitude and anisotropy of nonlinear optical tensor components d ijk were used in order to produce twodimensional SH images of a specimen with inhomogeneous distribution of the polar region. The fundamental wave of...
Carbyne complexes have been extensively studied but only a few of them contain the M᎐ ᎐ ᎐ C-H moiety (M = transition metal). In order to contribute to the debate about the linearity of this fragment, two neutron diffraction experiments on single crystal have been performed at 100 and 293 K on the trans-W(CH)(dmpe) 2 Br (dmpe = 1,2-bis(dimethylphosphino)ethane) (1). The results point, without ambiguity, to the linearity of the fragment. The specific behaviour of the dmpe ligand is modelled and interpreted.
DALTON
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