Anelasticity in MgAl₂O₄spinel due to cation order-disorder

The dynamic shear modulus and anelastic properties of MgAl₂O₄spinel have been measured using a forced torsion pendulum between 600 K and 1400 K at frequencies of 0.01-10 Hz. A Debye-like peak in the internal frictionQ^-1is observed at ∼1057 K at 1 Hz, accompanied by a corresponding modulus defect in the shear modulusGof order 4 GPa (∼20% of the unrelaxed modulus). Both the loss peak and the modulus dispersion are well described by a single thermally activated relaxation time, and the loss peak shifts systematically to higher temperature with increasing frequency, with no hysteresis between heating and cooling. Arrhenius analysis of the peak positions yields an activation energy of 331 kJ mol^-1. The peak is approximately 1.8 times broader than an ideal single-relaxation-time Debye peak, and this excess width is independent of frequency, reflecting the distribution of local Mg-Al exchange environments. We attribute the relaxation to stress-induced, vacancy-mediated Mg-Al exchange between tetrahedral and octahedral sites, the direct anelastic analogue of Zener relaxation in substitutional alloys. The result provides a mechanical-spectroscopic demonstration of cation-exchange anelasticity in MgAl₂O₄, and shows that non-convergent order-disorder generates a distinct dynamical signature within the seismic frequency band.