Proton Conductivity of Simple Ionic Hydroxides Part II: In Situ Formation of Water Molecules Prior to Dehydration

Abstract

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.

Dehydration-related proton conductivity in kaolinite

“…We conclude that the actual dehydration of kaolinite is delayed until 275~ when proton delocalization starts via the H' mechanism in the closely packed OH-array of the octahedral sheet. The subsequent steps leading to bulk dehydration probably resemble those noted during a study of the simple hydroxides (Wengeler et al, 1980): highly mobile excess protons, H', became trapped on OH lattice sites and thus form H20 molecules. If these H20 molecules are close to the surface, they can leave the lattice.…”

“…When the sample is subsequently allowed to warm up, these space charge clouds become mobile and cause depolarization currents on the capacitor plates which can be measured by an electrometer. The positive charge carrier density of Mg(OH)2 at 200~ determined by this method is of the order of 10 ~3 mole -I (Wengeler et al, 1980).…”

“…To complete the description of the conductivity behaviour of Mg(OH)2, the absolute value of the charge carrier concentration is still needed. Provided the relative mobilities of the positive and negative protonic species are widely different--which is the case for Mg(OH)2 and related hydroxides--this important value can be obtained from thermally stimulated depolarization (Wengeler et al, 1980). Until very recently this method had been applied only to electronic semiconductors and to insulators (Brfiunlich, 1979), but it has now proved also very suitable for protonic conductors.…”

“…The difficulties are aggravated if the samples are not high purity, synthetic compounds like the ionic hydroxides to which the above-mentioned methods have been successfully applied Wengeler et al, 1980). Natural clay minerals have in most cases a fairly high alkali content and some transition metal cation impurities, which may cause undesired cationic and electronic contributions, respectively.…”

In situ synchrotron XRD investigation of the dehydration and high temperature carbonation of Ca(OH)2

“…21,22 Freund et al established that the process of water formation started at 60-80°C with the delocalization of protons (H + ) from OH − sites 19,23 leading as well to the formation of proton defect sites (O 2− ). 20 Chaix-Pluchery et al expanded these observations with in situ XRD and neutron diffraction experiments 21,22 and reported that the water formation process was accompanied by an increase in lattice strain perpendicular to the 00l planes (i.e. 20 Chaix-Pluchery et al expanded these observations with in situ XRD and neutron diffraction experiments 21,22 and reported that the water formation process was accompanied by an increase in lattice strain perpendicular to the 00l planes (i.e.…”