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The formation of transitory HOH species in hydroxide lattices prior to dehydration was studied by thermally stimulated depolarisation, TSDp, and by high temperature infrared spectroscopy. Adsorbed H2O give regular TSDp maxima. Protonic space charge clouds, assigned to excess protons H* and defect protons H1, give inverted TSDp effects. At the temperature of the maximum of the proton conductivity the charge carrier concentration is of the order of 1013 mol−1 which corresponds to the expected Boltzmann occupancy in the conduction band. Excess protons H* temporarily trapped from the space charge clouds on lattice OH− sites form H2O molecules, visible in the IR spectrum, which redissociate at room temperature.

The thermal decomposition of finely divided Mg(OH)2 does not occur smoothly. Several maxima of the gas evolution rate are observed, when microgram quantities of Mg(OH)2 are heated under ultrahigh vacuum conditions. This phenomenon is attributed to the formation of an overlayer of partially decomposed hydroxide Mg(OH)2-x Ox,2 [[]x,~ with x--+ 2 ([--] : anion-sized neutral vacancy left by H20 molecules in the hydroxide lattice). This defect layer represents a diffusion barrier for further H20 molecules. When a critical thickness is reached, it spontaneously recrystallizes to cubic MgO exposing fresh Mg(OH)2 surface.The differential thermal analysis of magnesium hydroxide, Mg(OH)2, in the usual quantities, namely a few milligrams, yields a smooth endothermic reaction just as one would expect for the simple dehydration reaction Mg(OH)2 ~ MgO + + H20. However, we have repeatedly observed [1,2] that the dehydration of microgram quantities of magnesium hydroxide under ultrahigh vacuum conditions, as measured mass-spectroscopically by the gas evolution rate, does not result in a smooth curve. ExperimentalThe samples consisted of very high purity MgO-powder (total cation impurity content <5 ppm) either superficially hydroxylated or bulk hydroxylated and deuteroxylated to Mg(OH)2 and Mg(OD)2 respectively. The diameter of the individual hexagonal platelets was well below 1 pro, and their thickness in the 0.1 pm range.The samples were heated in a fused-silica capillary tube attached to an oil-free, all-glass ultrahigh vacuum system operating at a base pressure of 10 -~~ mbar. In case A a step mode with 25 ~ intervals and 7 min. isothermal heating periods was used [1 ] whilst in case B a linear heating rate of 2~[2] was used. The gas evolution from the sample was monitored by a memory-free, all platinum Omegatron mass spectrometer [3]. In case A the gases evolved were pumped away rapidly through a wide tube of large conductance, whilst in case B pumping was carried out more slowly through a capillary of known conductance for quantitative evaluation.

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