Der Lamellare Bau von CaF2‐Schichten und die Cs‐ und J2‐Adsorption

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Abstract
CaF2‐layers, obtained by sublimation in vacuo, consist of very thin laminae, which are only a few molecules thick. By heating these laminae become attached to each other, without growing together to form one crystal lattice, so that a remarkable decrease in the apparent surface may take place by this sintering process. On changing the amount of CaF2 or the velocity of subliming the salt, the apparent surface, as measured by the adsorption of iodine may vary more than 100% and the question arises whether this is due to a variation in the thickness d of the primary laminae or to a more or less pronounced degree of sintering, giving rise to an apparent thickness sd (s = sintering factor) (§ 1). This apparent thickness is directly given by the ratio [CaF2]/[J2]max., whereas in studying the adsorption of caesium the ratio [CaF2]/[Cs)max. gives the value of d/n, where n is the number of Cs‐layers which can be adsorbed one over the other. This latter ratio is not affected by the sintering factor s, as the adsorption of Cs completely destroys the effect of the sintering (§ 2). It is found that with increasing amount of CaF2 the value of d/n is practically constant. From this fact we may deduce that the average thickness d of the primary laminae does not depend of the amount of CaF2. s.d., on the other hand, increases at first, whereas it decreases with greater amounts of CaF3 (§ 4), table I). From the dependence of s.d. upon the amount of CaF2 (fig. 1) one may deduce that s nearly = 1 with the smallest amounts of CaF2. d is found to be 6‐7 molecules of CaF2 (§4), whereas n = 3 (§ 5). On decreasing the velocity of subliming, the thickness d remains constant in this case too, but the sintering factor s varies again and may increase to nearly 2 (§ 6).

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