The crystal structures of Bi4Ti3O12 and Bi3.25La0.75Ti3O12 were refined by neutron powder diffraction. Large structural distortions were revealed, and ferroelectric polarizations along the a and c axes were calculated from the displacements of the constituent ions. In Bi3.25La0.75Ti3O12, La atoms substitute for Bi atoms in a perovskite-type unit only, and the substitution causes less distortion of the structure, resulting in smaller spontaneous polarization and lower ferroelectric Curie temperature. Electronic-structure calculations revealed that covalent interaction, which originates from the strong hybridization between Ti 3d and O 2p orbitals, plays an important role in the structural distortion and ferroelectricity of the materials. Changes in ceramic-sample density with sintering temperature give information concerning device fabrication temperature; that is, substituting La for Bi atoms appears to “increase” the synthesis temperature of the Bi4Ti3O12 and Bi3.25La0.75Ti3O12 systems.
Ferroelectric materials of the SrBi2(Ta1−xNbx)2O9 solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the (Ta,Nb)O6 octahedron significantly increase as x increases, which leads to more structural distortion of the perovskite-type unit. The Bi2O2 layer, in contrast, is less distorted in SrBi2Nb2O9 than in SrBi2Ta2O9. The contribution of the perovskite-type unit to total ferroelectric polarization is greater in the SrBi2Nb2O9 sample, while that of the Bi2O2 layer is less; consequently, the total calculated polarization slightly increases. The ferroelectric Curie temperature also increases from 300 (SrBi2Ta2O9) to 440 °C (SrBi2Nb2O9). Three short (Ta,Nb)–O bonds in the (Ta,Nb)O6 octahedron, whose lengths are less than 2 Å, have a covalent character, and the substitution of Nb for Ta makes the bonds more covalent. The strong covalent interaction of the (Ta,Nb)–O bonds increases the structural distortion, resulting in the higher ferroelectric Curie temperature and the larger contribution of the perovskite-type unit to the total spontaneous ferroelectric polarization.
Isotope effect on H− / D − volume production is studied by measuring both VUV emission and negative ion density in the source. In a double plasma type source, under some discharge conditions, extracted D − currents are nearly the same as H − currents, although VUV emission intensity (corresponding to production of vibrationally excited molecules) in D2 plasmas is slightly lower than that in H2 plasmas. Considering the factor √ 2 due to mass difference, D − ion density in the extraction region of the source is higher than H − ion density. In another experiment with a rectangular arc chamber, axial distributions of H − / D − ion densities in the source are measured directly using a laser photodetachment method. Relationship between H − / D − production and plasma parameter control with using a magnetic filter (MF) is discussed. Furthermore, relative intensities of extracted negative ion currents are discussed compared with the negative ion densities in the source. Production and control of D2 plasmas are well realized with the MF including good combination between the filament position and field intensity of the MF. Extracted H − and D − currents depend directly on negative ion densities in the source.
An asymmetric plasma divided by a magnetic filter is numerically simulated by the one-dimensional particle-in-cell code VSIM1D ͓Koga et al., J. Phys. Soc. Jpn. 68, 1578 ͑1999͔͒. Depending on the asymmetry, the system behavior is static or dynamic. In the static state, the potentials of the main plasma and the subplasma are given by the sheath potentials, M ϳ3T Me /e and S ϳ3T Se /e, respectively, with e being an electron charge and T Me and T Se being electron temperatures (T Me ϾT Se ). In the dynamic state, while M ϳ3T Me /e, S oscillates periodically between S,min ϳ3T Se /e and S,max ϳ3T Me /e. The ions accelerated by the time varying potential gap get into the subplasma and excite the laminar shock waves. The period of the limit cycle is determined by the transit time of the shock wave structure.
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