In the present paper, relating to MgO(Al2O3)n, we describe results obtained by isothermal internal friction experiments in plasma-sprayed polycrystals and single crystals grown by the Verneuil's method. In some cases, a very large internal friction peak has been observed, probably the highest ever found in crystalline materials. Measurements have been done between room temperature and 1100 K, indicating that an Arrhenius law was very well obeyed over more than five decades in frequency, corresponding to a single relaxation mechanism.
The composition dependence of the relaxation strength was also investigated in both single crystals and polycrystals for n, the molar ratio of Al2O3 and MgO, ranging between 1 to 10. Highest relaxation strengths were observed for molar ratios between 2 and 3, corresponding to compounds with large oxygen vacancy concentrations and thus the elastic dipole was identified as a Mg++-O vacancy complex.
The dipole has been found to have monoclinic symmetry from tests of single crystals with ⟨100⟩, ⟨110⟩, and ⟨111⟩ orientations, indicating an elastic distortion lying along a ⟨110⟩ direction.