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The diffusion of interstitial Li in rutile (Ti02) was measured over a temperature range of 80-360°C, utilizing the optical absorption associated with the Li impurity as a measure of its concentration. Diffusion was found to be strongly anisotropic with diffusion coefficient D perpendicular to the C axis smaller than D parallel by a factor of at least 10 8 up to 550°C. D parallel was accurately described by D=D Q e~Q ,KT with ()=0.330±0.003 eV and Z> 0 =0.295d=0.028 cm 2 /sec (95% confidence limits). Concentrations and solubility of Li in rutile were measured approximately using chemical analysis; a solubility limit of 2.5X10 19 /cm 3 at room temperature was obtained. Lattice distortion and other impurities strongly inhibit Li diffusion. Possible explanations for apparent deviations from Fick's law are suggested, based on the assumption that the optical absorption associated with Li doping is caused by conduction electrons,

Measurements of Hand D diffusion in Ti0 2 • using the isotope exchange technique described in the preceding paper. are reported. Use of this technique resulted in diffusion which was accurately described by Fick's law with a constant diffusion coefficient, as predicted theoretically. in sharp contrast to single ion diffusion, where dramatic departures from classical diffusion theory were observed. The measured diffusion coefficients for H were 1.8X 10-3 exp( -O.5geV/kT) and 3.8X 10-1 exp( -1.28eV/kT) cm 2 /sec for diffusion II and 1 to the c axis, respectively. Ionic conductivity measurements are reported. which agree well with the bulk diffusion measurements. and permitted us to extend the temperature range of the measurements for c -axis diffusion from 125 to 750"C, corresponding to a range of more than four orders of magnitude in D. The measured diffusion parameters were found to be essentially independent of sample purity. although it was observed that significant concentrations of lattice defects sharply inhibited H diffusion.

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1 Although (-)-cytisine is a rigid structure, it occurs in the crystal in two distinct but very similar conformations in which the pyridone ring is tilted relative to the charged nitrogen atom at much the same angle as the pyridine ring is in (-)nicotine hydrogen iodide. The carbonyl group in the pyridone ring of (--cytisine, however, is on the side of the ring opposite to the pyridine nitrogen in (-)-nicotine. 2 The pKa of (-)-lobeline HCI at 250C is 8.6 (approx), indicating that (-)-lobeline is at least 90% in the protonated form at physiological pH (7.6). It is probably the phenyl 2-keto-ethyl part of (-)-lobeline, rather than the phenyl 2-hydroxy-ethyl part, which interacts with the receptor. 3 The combination within one molecule of a charged ('onium') nitrogen atom lying out of the plane of, and some distance (4.5-6.5 A) from, an aromatic ring is common to many compounds with nicotine-like activity (e.g. nicotine, cytisine, choline phenyl ether bromide, dimethyl-phenyl-piperazinium (DMPP) iodide, coryneine iodide and m-hydroxyphenylpropyl trimethyl ammonium iodide). In some molecules the aromatic ring can be replaced by an unsaturated group, such as carbonyl (e.g. acetylcholine) or double-bonds (e.g. anatoxin). 4 Activity at nicotinic receptors appears to involve interactions between the positively charged nitrogen atom and a negatively charged group, probably close to cysteine residues 192 and 193 in the receptor. It is suggested that rather than specific groups in the molecule also being involved, activity at nicotinic receptors depends on interactions between a flat part of the drug containing double-bonds, or systems of double bonds, and a planar area in the receptor, possibly tyrosine or phenylalanine residues.