P. S. Turner

Statistical analysis of electron channelling microanalytical data for the determination of site occupancies of impurities

Abstract A novel method for determining the concentration and distribution across sites of substitutional impurities in crystals is described, as an alternative to the usual ratio method of atom location by the channelling enhanced microanalysis (ALCHEMI) technique. The integrated X-ray counts for emission from impurity and host atom species under strong zone-axis diffraction conditions are analysed using standard multivariate statistical procedures. This enables both the total concentration and the fractional site occupancy of the impurity species to be determined, together with quantitative estimates of the uncertainties in these quantities. This method of analysis is illustrated through the determination of the site occupancy of Fe in a Cr2MnO4 spinel. Advantages of this technique include reduced sensitivity both to the effects of delocalization and to the effects of experimental errors compared with standard ALCHEMI analyses.

Transmission electron microscopy and high-resolution transmission electron microscopy studies of the early stages in the degradation of yba2cu3o7-δ superconductor in water vapour

Abstract The initial stages of water vapour attack of the high-temperature superconductor YBa2Cu3O7-δ have been studied by transmission electron microscopy (TEM). The planar defects both of thickness 0·2 nm and of thickness 0·4 nm reported previously have been observed. The defects are sometimes associated with preexisting sites of strain in the material, but such strain is certainly not required for their nucleation, since often they occur in initially perfect material. A model for the thinner defect has been developed, which fits well with the TEM observations, as well as with bond valence calculations and previous work on the degradation of YBa2Cu3O7-δ by water. The implications of the model for that degradation process are discussed.

Advances in ALCHEMI analysis

The ALCHEMI method for locating the sites of foreign atoms within crystals is known to be sensitive to the delocalized emission of X‐rays. This can result in large errors in some cases through differences in delocalization for different excitations or by error amplification in the ratio method of analysis. An alternative approach to the analysis of ALCHEMI data, using multivariate statistical analysis, is extended to the case of multiple impurities. Initial results from zone‐axis channelling experiments for a Yb‐doped zirconolite (CaZrTi2O7) are shown to confirm the improved accuracy of this method, especially for axial orientations. Data were collected using a 400‐keV analytical electron microscope fitted with an intrinsic Ge X‐ray detector. The potential advantages for ALCHEMI analysis of Ge detectors are considered.

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.

Axial electron-channelling analysis of perovskite: I. Theory and experiment for CaTiO3

Abstract Crystallographic site location of atoms, using the Borrmann effect in electron-induced characteristic X-ray emission, has usually been undertaken using systematic-row diffraction conditions. In this paper, the variation ofcharacteristic X-ray fluorescence from calcium and titanium in perovskite (CaTiO3) is reported for three zone-axis orientations, and correlated with dynamical n-beam calculations. The axial electron-channelling effect is found to be large for these zone-axis orientations, and may be understood by referring to calculations that account for the variation in the fast-electron probability density ψψ* on calcium and titanium sites as a function of orientation. Zone-axis diffraction conditions enable a two-dimensional compression of ψψ* in the unit cell, instead of the usual one-dimensional variation under systematic-row diffraction conditions, and in general leads to an enhancement in sensitivity. Effects due to delocalization of the inner-shell ionization event are also considered.

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.

Axial electron-channelling analysis of perovskite: II. Site identification of Sr, Zr and U impurities

Abstract The orientation dependence of characteristic X-ray emission (the Borrmann effect) under near-zone-axis diffraction conditions has been used to identify the site preferences of strontium, zirconium and uranium impurities within a CaTiO3 (perovskite) host structure. As characteristic emission lines from these impurities occur at both higher and lower energies than the calcium or titanium K-shell excitations, effects of delocalization are clearly measureable, and are used as a tool in the ALCHEMI analysis. It is found that strontium and uranium strongly partition into calcium sites, whereas zirconium occupies titanium sites.