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A light scattering experiment has been performed on two SrTi( 18 O x 16 O 1Ϫx ) 3 single crystal plates with x ϭ0.86 ͑STO18-86͒ and xϭ0.53 ͑STO18-53͒. Each of the c 44 pure transverse acoustic mode and the antiferrodistortive E g mode splits into two modes below T c ; T c ϭ27Ϯ1 K for STO18-86 and T c ϭ20Ϯ2 K for STO18-53. These splits strongly imply that the crystal symmetry becomes lower than tetragonal one below T c , and that the phase transition really occurs at T c . Moreover, the ferroelectric E u soft mode is found above T c , but its softening is not complete. This fact indicates that the phase transition mechanism of SrTi( 18 O x 16 O 1Ϫx ) 3 is of displacive type ͑the instability of the E u mode causes the ferroelectric phase transition͒, but is not the ideal one. The displacive character is found to become stronger with decreasing exchange rate x in the range xϾ0.33.

We developed a simulation code to calculate electron orbits with collisions between electrons and argon gas atoms. Simulations are applied to trace electron orbits in a bucket-type ion source. This paper introduces the numerical model. It discusses the lifetime of primary electrons and the effects of collisions.

A comparative Brillouin-scattering study of the broad doublet and acoustic phonons in isotopically substituted SrTi( 18 O 1Ϫx 16 O x ) 3 and ''pure'' SrTi 16 O 3 crystals is presented. The acoustic-phonon frequency is found to decrease as a result of the substitution, but the doublet spectra above 20 K show no obvious differences. The results are examined in connection with the second-sound interpretation of the doublet. Influence of the phase transition on the doublet spectra is discussed.

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