We study the effect of a single excluded site on the diffusion of a particle undergoing a random walk in a d-dimensional lattice. The determination of the characteristic function allows us to find explicitly the asymptotical behaviour of physical quantities such as the particle average position (drift) x (t) and the mean square deviation x 2 (t) − x 2 (t). In contrast to the one-dimensional case, where x (t) diverges at infinite times ( x (t) ∼ t 1/2 ) and where the diffusion constant D is changed due to the impurity, the effects of the latter are shown to be much less important in higher dimensions: for d 2, x (t) is simply shifted by a constant and the diffusion constant remains unaltered although dynamical corrections (logarithmic for d = 2) still occur. Finally, the continuum space version of the model is analysed; it is shown that d = 1 is the lower dimensionality above which all the effects of the forbidden site are irrelevant.

AbslracL Raman xattering investigations of lithium polasium sulphate at high pressures have revealed l h a l the sulphate tetrahedra are orientationally disordered in the highpressure phases. I n order la understand the nature of lhe disorder, the lemperature dependence of the relative intensities of Raman lines associaled with lhe sulphale inlernal mnds are investigated. l l~l h e resulls suggest a frozen-in static disorder rather lhan dynamic equilibrium between the acupancics of different orientations The behaviour of lhe Raman linewidths up to a preuure of 2. 50 kbar indicates sucassively broader distributions of the sulphate orientations. A mechanism of amorphization involving disorder in cation sublattices induced the orientational disorder of sulphate is proposed.

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A laser deposition arrangement for the preparation of smooth YBa 2 Cu 3 O y and EuBa 2 Cu 3 O y films is presented. An excimer laser beam is split by a knife-edge prism into two beams, which are focused onto two rotating targets oriented perpendicular to each other. The two laser-induced molecular streams collide and form a new stream containing mainly light molecules and almost no droplets. The molecules from this stream deposit on a substrate that is placed in the shadow of the original molecular streams. The use of copper-poor targets prevented the occurrence of precipitates.

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