Crystal Vacancy Evidence from Electron Spin Resonance

Three distinct patterns have been observed in the electron spin resonance spectrum of chromium (+3) ions in single crystals of magnesium oxide. The chromium (present in naturally-occurring amounts or augmented) replaces magnesium (+2) ions substitutionally. The Cr(+3) spectrum depends upon whether compensation for the extra charge occurs in the immediate vicinity, and if so, upon the position of the compensating charge. The first pattern is a single line surrounded by a hyperfine quartet, representing that expected for a ${}_{}{}^4{}_{}{}^{}F$ ion in a purely cubic electric field if most of its isotopes have zero nuclear spin $I$, and one isotope has $I=\frac{3}{2}$. This means that charge compensation cannot occur in a position as close as that of the next-nearest magnesium ion. The second pattern has nine lines (each with a hyperfine quartet), the angular dependence of which shows axial symmetry about the principal crystal axes. It is explained in terms of a vacancy in the magnesium position next-nearest the chromium ion. The third pattern arises from ions with axes of symmetry in face-diagonal ([110]-type) direction. This symmetry is ascribed to the effects of a magnesium ion vacancy in a position nearest the central ion.