The distribution of iron impurity in single-crystal magnesium oxide and some effects on mechanical properties

Abstract

MgO containing ⩽5000 wt ppm Fe was heat-treated in various ways, so that the Fe was present in solution as Fe²⁺ or Fe³⁺, as precipitates of MgO.Fe₂O₃, or as metallic bcc Fe. Crystals were studied by optical absorption spectroscopy and microscopy. Heating in air at 1400‡ C converts most of the Fe to Fe³⁺ and some of this is associated with vacancies, particularly at high impurity levels. In crystals containing Fe³⁺, the magnitude of the hardening is relatively about four times less than that in alkali halides containing divalent metallic impurities, where all the impurities are associated with charge-compensating vacancies. Greater hardening is obtained when precipitates of MgO.Fe₂O₃ are present. Precipitates of metallic bcc Fe are formed on heating in hydrogen at temperatures >1000‡ C; these have the orientation relationships (001)_MgO ∥ (001)_Fe and [110]_MgO ∥ [100]_Fe.