The kinetics of dehydration of crystalline powder and individual single crystals of manganow formate dihydrate have been investigated in vacuo from 20 to 75°C. Dehydration takes place by surface nucleation followed by anisotropic penetration of a reaction interface into the hydrate matrix. Dehydration kinetics of approximately parallelopiped single crystals are well described by an isotropic contracting envelope model. Comparison of the data with the Polanyi-Wigner equation gives a normal frequency factor. The heat of dehydration has been measured and is comparable to the activation energy. The dehydration has been studied as a function of water vapour pressure.

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The anhydride resulting from the dehydration of manganous formate dihydrate is crystalline and its skeletal structure is distinct from the dihydrate. Under the proper conditions of temperature and water vapour pressure the freshly dehydrated anhydride rehydrates readily and completely to the dihydrate. Rehydration against time curves have been obtained for anhydrides prepared from the dihydrate which was in the form of individual single crystals, crystalline powder, ground powder and reactor-irradiated powder. The pressure, time and temperature dependences of the rehydration process have been determined. The extent of rehydration is described by a square-root dependence on time. This suggests that the rehydration is diffusion-controlled. The rate of rehydration increases markedly with pressure. Below a supersaturation ratio of approximately 10, rehydration does not occur; rather there is an appreciable sorption. The rate of sorption is diffusion-controlled with an energy of activation for diffusion of 12.0 kcal/mol. Isotherms have been determined for the sorbed water.

Data on two parameters important for judging the deleterious effects of heating on the efficiency of nonlinear optical processes are presented. These parameters are absorption coefficients at 10.6 pm and the temperature variation of the refractive index difference for second harmonic generation of 5.3 um. The crystals examined are GaSe,T13AsSe3, AgGaSez, CdGeAsz, and Ag3AsS3.

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