R. L. Segall

Surface structure and surface states in magnesium oxide powders

The photoluminescent and reflectance spectra of MgO smoke have been compared with the spectra of MgO ex-hydroxide and ex-carbonate. Parallel studies by electron microscopy have shown that a strong correlation exists between the optical spectra and the different morphology of the particles obtained by the different routes. The spectra observed are associated with the surface of the oxide particles and the excitation and absorption bands are associated with oxygen ions in 3- and 4-fold coordination. The same kinds of sites are present on all samples but the ions of lowest coordination are much less abundant on the nearly perfect cubes of MgO smoke. The number of 3-fold coordinated sites on the smoke particles is considerably increased after exposure to water vapour for several h. This picture is consistent with the electron micrographs which show erosion of the edges and corners of the MgO smoke after treatment with water vapour. A model involving the removal of ion pairs from the corners and edges of the cube by water is shown to give the observed increase in the sites of lowest coordination.

Twin Boundaries in Perovskite

The twin behaviour of natural and synthetic perovskite (CaTiO3) was examined by high-resolution electron microscopy (HREM) and selected-area electron diffraction (SAD). Pseudosymmetric twinning yields domains related to each other by either a 180 ° or a 90 ° rotation about an axis perpendicular to { 101}. A third type of twin involving a 180 ° rotation about the normal to ( 121 ) had previously been reported but this was not observed in the present study. The boundaries between twin domains were often complex and consisted of intergrowths on the finest possible scale of the various composition planes. It was confirmed that the most common composition planes were {101}; however, interface surfaces on {010} and {121} were also observed. For the simpler twin boundaries structural models were proposed and described using the Glazer notation.

Ionic oxides: distinction between mechanisms and surface roughening effects in the dissolution of magnesium oxide

The mechanism of the rate determining step (r.d.s.) for dissolution of well characterised, very perfect MgO “smoke” crystals, has been re-evaluated by studies of the dependence of log (rate) on pH. Surface potential barrier modification is the most likely r.d.s. when solution diffusion is not limiting. Electron microscopic studies of changes in surface structure of partly dissolved crystals have been related to rate changes during dissolution. There is a doubling of surface area per unit mass in the first 10% of dissolution, due to initial attack at defect sites, after which the increase in surface area per unit mass (to more than ten times the initial area) is due to both decreasing particle size and surface roughening.

Surface area and the mechanism of hydroxylation of ionic oxide surfaces

The nearly perfect {100} surfaces of MgO smoke cubes formed in air do not show significant v(OH) absorption in infrared spectra from thin (10 mg cm–2), coherent films exposed to H2O vapour for several hours. It is shown that perfect, five-fold-coordinated sites are not protonated and that the proportion of protonated, low-coordination (i.e. less than five-fold) sites is < 5%. These results are in accord with theoretical predictions for H2 adsorption. In contrast, v(OH)[and v(CO3)] absorptions are observed in identical preparations subjected to prior abrasion. The increase in protonated sites is more than ten-fold. Electron microscopy shows that only minor initial alteration of surface structure at edges and corners is caused by abrasion but a major increase in the rate of surface roughening is observed when new, nucleating sites appear and develop in regions across the surface. This process results in a major, time-dependent increase in hydroxylation but it is substantially complete before infrared spectra can normally be obtained. B.E.T. studies, using N2 adsorption, do not measure the change in surface area produced by this surface roughening. Multilayer water adsorption followed by desorption more than doubles the B.E.T. surface area because of the formation of platelets decomposed from the Mg(OH)2 brucite surface layer. The particle-size distribution is altered and the B.E.T. method correctly measures this change.

Initial dissolution Kinetics of Ionic Oxides

It is shown th at factors previously recognized, but not regarded as critical, can dominate dissolution kinetics of ionic oxides. The use of the nearly perfect {100} MgO surfaces of smoke cubes to obtain very precise values of dissolution rates per unit surface area, in dilute HC1, HC1O4 and HNO3, has shown th at rates extrapolated to zero dissolution are almost independent of pH in the range 2.0- 3.5. Dissolution rates were measured by monitoring solution pH as a function of time. This revealed increasing rates with increasing pH up to about 5 % total dissolution, followed ultimately by a return to the linear relation between Ig(rate) and pH (slope ca. — 0.5) normally expected. The initial increase in rate is due to increasing [Mg2+] in solution and is observed with [Mg2+] as low as 1 % of the [H+]. A linear relation between lg(rate) and [Mg2+] is found during the early stages of dissolution. Other cations (Al3+, Na+) also increase the initial rate, to a similar extent. Electron-microscope observations of the cubes show alteration of the surfaces to a castellated structure (of {100}-based projections and intrusions) on wetting before dissolution, and the development of facets having an average (110}-natureduring dissolution. The results are in conflict with current theoretical models, and a qualitative account of the mechanism of the establishment of a ‘ stable ’ solution double layer is given.

Semiconducting oxides. The effect of prior annealing temperature on dissolution kinetics of nickel oxide

The paper describes studies of the dissolution rate of NiO (a p-type semiconductor) in nitric acid solution at 60°C as a function of prior annealing temperature in air. The dissolution rate per unit surface area decreased markedly on increasing the annealing temperature from 500 to 1450°C. This effect is not due to gross structural change or to major changes in dislocation density. The higher dissolution rates (i.e., > 10–12 mol cm–2 s–1) of oxide annealed at temperatures below 700°C is due to an excess concentration over thermodynamic equilibrium of point defects (nickel vacancies), introduced during decomposition of the hydroxide, and maintained as a consequence of limited diffusion. For annealing temperatures above 900°C, the defect concentration is roughly equal in all samples because of rapid equilibration in polycrystalline samples during cooling. The decreasing rates (i.e., < 4 × 10–13 mol cm–2 s–1) may be due to (i) limited conduction of charge due to changes in the space-charge region of the semiconductor and/or (ii) reduction of the density of kink sites on the surface of the more perfect crystallites.

Growth twinning in magnesium oxide smoke crystals

Abstract Growth twins in magnesium oxide are shown to occur when magnesium is burnt in air in the presence of water vapour. The twins are coherent and the external morphology shows mirror symmetry across the {111} composition plane. The significance of the existence and form of these twins in such a highly ionic material is considered in terms of the structure of the boundary layer and the role of the water vapour.

The surface layer formed on zinc-containing glass during aqueous attack

Abstract The glass-water interaction is reviewed via an outline of the processes of bulk dissolution, selective leaching and surface enhancement. Special attention is paid to the nature and effects of surface films. Glasses of three compositions, each containing about one-fifth by weight Zn, are subjected to aqueous attack. Solution analysis confirms the roles of Na, Si and Zn in leaching, dissolution and surface enhancement, respectively. Proton-induced X-ray emission corroborates Zn enhancement in the glass surface and X-ray photoelectron spectroscopy fixes the Zn build-up to the first few nanometres. Powder X-ray diffraction examination identifies the specific structures of hemimorphite and Zn orthosilicate while transmission electron microscopy finds such phases to be distributed in a largely amorphous matrix. Infrared spectra are consistent with the presence of a modified silicate structure and additionally indicate that water is loosely bound in the reacted surface layer. Attack of a non-Zn glass by a Zn-containing solution shows the ion in solution produces a Zn surface layer while attack of a Zn glass by a Zn complexing solution shows a Zn layer forms preferentially to complexation. Hence Zn, in either the solid or solution, induces a thin, protective, solvated and partially crystalline silicate zone on a glass under aqueous attack.

Correspondence: Size distribution of MgO smoke particles

Abstract It has been shown by a combination of scanning and transmission electron microscopy that the size distribution of MgO particles prepared by burning magnesium in air is extremely asymmetric. Less than 1% of the particles account for well over half the mass. This result explains an anomaly observed in the infra‐red spectroscopy of MgO smoke. The size distribution can be used in conjunction with sorptometer measurements of specific surface area to give a measure of surface roughness.

Semiconducting oxides: effects of electronic and surface structure on dissolution kinetics of nickel oxide

The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.