Phonon spectrum ofZnAl2O4spinel from inelastic neutron scattering and first-principles calculations

The phonon spectrum of ${\mathrm{ZnAl}}_2{\mathrm{O}}_4$ spinel was investigated jointly by inelastic neutron-scattering and first-principles calculations. The results permit an assessment of important mechanical and thermodynamical properties such as the bulk modulus, elastic constants, lattice specific heat, vibration energy, and Debye temperature. The observed generalized phonon density of states shows a gapless spectrum extending to a cutoff energy of ∼840 ${\mathrm{cm}}^{\mathrm{-}1}$. The theoretical results reproduce all of the features of the phonon density of states. The calculated Raman-and infrared-active phonon frequencies agree well with the data in the literature. A comparison of the lattice dynamics of ${\mathrm{ZnAl}}_2{\mathrm{O}}_4$ and ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ spinels was carried out using a simple rigid-ion model, which shows that the major difference in the phonon frequencies of the two materials can be accounted for by the mass effects between the Zn and Mg ions.