Ab initio thermodynamic and thermophysical properties of sapphirine end-members in the join Mg4Al8Si2O20-Mg3Al10SiO20

Abstract

Using the hybrid B3LYP density functional method, we computed the ab initio thermodynamic and thermophysical properties of two sapphirine end-members, Mg 4 Al 8 Si 2 O 20 (sapphirine-442) and Mg 3 Al 10 SiO 20 (sapphirine-351), in the join Mg 3 (Mg 1-X Al X )Al 8 (Al X Si 1-X )SiO 20 with X = 0-1. Static and vibrational calculations performed in the framework of the quasi-harmonic approximation allowed to define the equation of state (EOS), elastic constant tensor, seismic velocities, IR spectra, mode Grüneisen parameters, and thermodynamic properties of both sapphirine end-members. A modified Kieffer's model was adopted to evaluate the optic and acoustic mode contributions to thermodynamic functions stemming from ab initio phonon frequencies and directionally averaged seismic velocities, respectively. The extrinsic stability and liquidus phase relations of sapphirine were investigated in the model system MgO-Al 2 O 3 -SiO 2 (MAS) at different pressure conditions by coupling first-principles calculations with the hybrid polymeric approach (HPA) for multicomponent liquids and minimizing the Gibbs free energy of liquid and solid phases through the convex-hull analysis of equipotential surfaces. According to our thermodynamic modeling, sapphirine turns out to have a small field of primary crystallization in the MAS ternary diagram at 1-bar pressure, which becomes larger due to pressure effects up to 10 kbar, then progressively shrinks and disappears above 21 kbar.

Lower mantle hydrogen partitioning between periclase and perovskite: A quantum chemical modelling

“…In the case of Fe, the basis set of Valerio et al (1995) was adopted, corresponding to an 86-41G * contraction scheme. A variational re-optimisation of the exponents of the most diffuse Gaussian functions of all three elements was also tested in pe and pvk, with no significant enhancement compared to the original values (see Belmonte et al, 2013Belmonte et al, , 2014 regarding the use of optimised basis sets compared to those in the present work). Further calculations were carried out by semi-empirical potentials and lattice dynamics using the GULP code, in order to estimate the effect of the vibration contribution on DG 0 (P,T,x), as stated in the previous section.…”

Development of Mg/Al/Si/O ReaxFF Parameters for Magnesium Aluminosilicate Glass Using an Artificial Neural Network-Assisted Genetic Algorithm

“…Calculations for the energy response for different phases of crystal structures except sapphirine crystals were performed using the Quantum Espresso package . For the calculation of EOS for the sapphirine system, the CRYSTAL package was applied. − …”

Ab Initio Thermodynamic and Thermophysical Properties of Sodium Metasilicate, Na2SiO3, and Their Electron-Density and Electron-Pair-Density Counterparts

“…For example, the 8-511G basis set adopted for Na returns at the DFT/B3LYP theory level an electronic energy of E 0 = −162.023898 hartree, which is intermediate between the B3LYP/STO-3G low limit of −160.082859 hartree and the B3LYP/Aug-cc-pVQZ value of −162.298993 hartree. Adopting optimized electronic energies for Si and O, 35 the electronic energy of Na returning the experimental value of H f 0 is E 0 = −162.224203 hartree (Table 5). This computation gives the correct dissociation energy of the substance (Table 6) if one assumes that the basis set superposition error (BSSE) is negligible.…”