A comprehensive analysis of gened relativistic spacetimes which admit a shearfree, irrotational and geodesic time-like coogruence is presented. The equations governing the models for a gened energy-momentum tensor are wrilten down. Coordinates in which the metric of such spacetimes takes on a simplified form are established. The general subwses of 'zero anisotropic stress', 'zero heat-flux vector' and 'two-component Ruids' are investigated. In particular, perfect-fluid Friedmann-Robertson-Walker models and spatially homogeneous models are discussed. Models with a variety of physically relevant energy-momentum tensors are considered. Anisowopic fluid models and viscous fluid models with heat conduction are examined. Also. models with B perfect Ruid plus a magnetic field or with pure radiation. and models with two non-collinw perfect fluids (satisfying a variety of physical conditions) are inwtigated. In particular. models with a (single) perfect Ruid which is tilting with respect to the shem-free, vorticity-free and acceleration-free time-like congruence are discussed.PACS numbers: 0420J. 9880H

Substituting Cd 2؉ ions into the A and B sites in alead, strontium, etc., is located on the A site; the other, including manganese, iron, zinc, cobalt, nickel, magnesium, etc., may Pb(Ni 1/3 Nb 2/3 )O 3 -PbZrO 3 -PbTiO 3 (PNN-PZ-PT) ternary perovskite material made it possible to determine the effects be located on the B site in a complex perovskite. Based on ionic radius, the Cd 2ϩ ion is located on the boundary between the two of the Cd 2؉ ion substitution site on sintering behavior. The substitution site of the Cd 2؉ ion was identified by using groups 7 and, thus, can be substituted for the A-site Pb 2ϩ ion and the B-site Ni 2ϩ ion. Some examples of this phenomenon are X-ray photoelectron spectroscopy spectra. Although Cd 2؉ ions were substituted into the A and B sites in PNN-PZ-PT, CdTiO 3 , Cd(Ni 1/3 Nb 2/3 )O 3 , Pb(Cd 1⁄3 Nb 2/3 )O 3 , and Ba(Cd 1/3 -Nb 2/3 )O 3 , which have a perovskite structure and ferroelectric the Cd 2؉ ions preferred the A site over the B site. When Cd 2؉ ions replaced Pb 2؉ ions, a weight gain was observed properties. Because the ionic radius of the Cd 2ϩ ion is smaller than those of the Pb 2ϩ or Ba 2ϩ ions, CdTiO 3 has low symmetry during sintering. On the other hand, replacing Ni 2؉ ions with Cd 2؉

We have analysed the optical double resonance signal to second order in the modulation of the amplitudes of the two lasers at high intensities. Resonant modulation in the populations of the upper level of the transitions are induced at the effective Rabi frequency a n , whenever the modulation frequencies themselves or a combination of them becomes equal to 0, *a,, *Zen. It is proposed that by performing experiments similar to those done by Miyazaki et al such Rabi resonances in the populations of the levels may be detected by optogalvanic technique-i.e., by monitoring the oscillations in the current or the voltage at the sum or the difference of the modulation frequencies of the two lasers.

have been investigated. Generally, the compounds that contain PbO show p-type conductivity, because of PbO evaporation. Thus, the conductivities are known to be proportional to the PbO evaporation. However, PNN-PZ-PT ceramics exhibit a reciprocal relationship between the conductivities and the evaporation of PbO. Generally, no vacancy change is observed with the substitution of the same-charge valence ion. However, in the PNN-PZ-PT ceramic, lead vacancies could be created or annihilated by replacing Pb 2+ and Ni 2+ ions with Cd 2+ ions, because of incomplete substitution. The electrical conductivities are influenced by this incomplete substitution.

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