Influence of polarizability on metal oxide properties studied by molecular dynamics simulations

Abstract

We have studied the dependence of metal oxide properties in molecular dynamics (MD) simulations on the polarizability of oxygen ions. We present studies of both liquid and crystalline structures of silica (SiO(2)), magnesia (MgO) and alumina (Al(2)O(3)). For each of the three oxides, two separately optimized sets of force fields were used: (i) long-range Coulomb interactions between oxide and metal ions combined with a short-range pair potential; (ii) extension of force field (i) by adding polarizability to the oxygen ions. We show that while an effective potential of type (i) without polarizable oxygen ions can describe radial distributions and lattice constants reasonably well, potentials of type (ii) are required to obtain correct values for bond angles and the equation of state. The importance of polarizability for metal oxide properties decreases with increasing temperature.

Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

“…For instance, in Fig. 4 we show another plausible TLS motif found for the 360-atoms system using the force field II 43 . This motif is characterized by a central tetracoordinated oxygen atom (typical coordination in the crystal) jumping back and forth between two aluminum atoms, labelled as Al1 and Al2 in the Fig.…”

“…In all calculations, we used two force fields, hereafter called, I 42 and II 43 , to check the reproducibility and robustness of our motifs. The first of these force-fields (I) has been extensively tested against experimental properties of crystalline [44][45][46][47] , liquid 48,49 and amorphous alumina 41 .…”

“…The first of these force-fields (I) has been extensively tested against experimental properties of crystalline [44][45][46][47] , liquid 48,49 and amorphous alumina 41 . The second force-field (II) was fitted to ab initio data on crystalline and liquid alumina 43 . The structural properties of our alumina systems have been compared to previous calculations (see Fig.…”

“…(b) Tuning the asymmetry of the energy profile at 0 K by mechanical strain (given as %) along X,Y, and Z axis. Results according to force field II 43 . (c) Estimation of the tunneling splitting by solving the Schrödinger equation numerically using an effective mass m eff = 15 a.m.u.…”

“…Fig. 1(c) shows the reconstructed free energy profiles at 0, 5, and 25 K computed using both force fields I 42 and II 43 . The free energy difference between wells is about 2 meV in both cases and the largest energy barriers for the transition between minima are 4 and 12 meV for force fields I and II, respectively.…”