High-pressure stability, structure and compressibility of Cmcm-MgAl_2O_4: an ab initio study

摘要

Quantum-mechanical solid-state calculations have been performed on the highest-pressure polymorph of magnesium aluminate (CaTi_2O_4-type structure, Cmcm space group), as well as on the low-pressure (Fd3-barm) spinel phase and on MgO and Al_3O_3. An ab initio all-electron periodic scheme with localized basis functions (Gaussian-type atomic orbitals) has been used, employing density-functional-theory Hamiltonians based on LDA and B3LYP functionals. Least-enthalpy structure optimizations in the pressure range 0 to 60 GPa have allowed us to predict: (1) the full crystal structure, the pV equation of state and the compressibility of Cmcm-MgAl_2O_4 as a function of pressure; (2) the phase diagram of the MgO-Al_2O_3-MgAl_2O_4 system (with exclusion of CaFe_2O_4-type Pmcn-MgAl_2O_4), and the equilibrium pressures for the reactions of formation/ decomposition of the Fd3-barm and Cmcm polymorphs of MgAl_2O_4 from the MgO + Al_2O_3 assemblage. Cmcm-MgAl_2O_4 is predicted to form at 39 and 57 GPa by LDA and B3LYP calculations, with K_0 = 248 (K' = 3.3) and 222 GPa (K' = 3.8), respectively. Results are compared to experimental data, where available, and the performance of different DFT functionals is discussed.