Magnesium Vacancies Research Articles

Trivalent titanium in tetragonal symmetry in MgO and CaO

Observation of 47,49Ti hyperfine structure in a tetragonal EPR spectrum in magnesium oxide has established conclusively that the impurity involved is titanium. The spin-Hamiltonian parameters are g ∥ = 1.95304 ± 0.00010, g ⊥ = 1.89878 ± 0.00010, | 47,49 A ∥| = (26.8 ± 0.2) × 10 −4 cm −1 and |47,49A ⊥⋎ = (10.0 ± 0.5) × 10 −4 cm −1 . These indicate that the titanium is trivalent and is situated in a field of predominantly octahedral symmetry, with a smaller tetragonal component. The spectrum is ascribed to the structure Ti +  O  []. Here deviations from the normal lattice charge are shown; the brackets represent a magnesium vacancy. The parameters for the corresponding center in calcium oxide are found to be g ∥ = 1.9427 ± 0.0001, g ⊥ = 1.9380 ± 0.0001, | 47,49 A ∥| = (27.4 ± 0.1) × 10 −4 cm−1 and | 47.49 A. ⊥| = (9.9 ±1.5) × 10 −4 cm −1. Two further tetragonal centers are reported for CaO. The parameters are g ∥(A) = 1.9425 ± 0.0001, g ⊥(A) = 1.9365 ± 0.0001 and g ∥(B) = 1.9424 ± 0.0001, g ⊥(B) = 1.9359 ± 0.0001. It is tentatively suggested that these centers have the form Ti +  O  []  O  M, where M is a diamagnetic ion with positive charge greater than two.

Optical studies of the thermal dissociation of OH - ion aggregates in MgO

Abstract The thermal dissociation of two OH − ion aggregate centres has been studied through the decay of the associated optical absorption bands. The first of the centres studied, with an absorption band at 3296 cm −1 , has been shown to consist of an OH − ion in association with a magnesium vacancy (1), the (OHV) centre. The second (absorption band at 3310 cm −1 ) is an (OHV) centre in association with a trivalent impurity ( I )[2]. We term this the (OHVI) centre. Over the temperature range 470°–620°C the decays take a simple form, predicted by rate theory. From these measurements, and new data on the equilibrium concentration of the centres as a function of temperature, the dissociation activation energy (1.43 eV), the binding energy (⪆FG 0.23 eV) and the association activation energy (⪅ 1.20 eV) of the (OHV) centre are found, together with the binding energy of the trivalent impurity I to the (OHVI) centre (84 meV).

Interaction between solute magnesium atoms and vacancies in aluminium

A study by resistivity measurements at 20°C has been carried out on the effects of small magnesium concentrations (to a magnesium of 1.68 per cent at. fr. Mg) on the annealing out of vacancies frozen-in by quenching aluminium wires (quenching medium: brine at 2°C). The following results are observed: 1. (1) a concentration as small as about 0.13 per cent at. fr. of Mg atoms is sufficient nearly to suppress the annealing out of vacancies which is observed in pure aluminium at room temperature; 2. (2) in the meantime we have an increase of resistivity of the sample which is sensitive to the quenching speed; 3. (3) this increase is annealed out in isochronal annealing in the range 80–120°C. These facts are easily interpreted if we assume, in agreement with a point of view advanced recently by Ferryman in the case of the examination of other properties of Al-Mg alloys, that magnesium atoms in aluminium are able to trap vacancies at room temperature and that only at about 80–120°C vacancies can free themselves from traps. A preliminary study of the isothermal annealing out of vacancies in the presence of magnesium has shown that the phenomenon is complex and not well understood. Finally, some additional experiments on the isochronal recovery of resistivity of cold-worked Al-Mg alloys have shown that in the same range in which it is possible to observe the recovery after quenching, a larger recovery, the greater the magnesium content, is observed. This result is in agreement with the view of generation of vacancies by cold-working.