Page 23 - Neutrons For Science - Annual Report 2015

SCIENTIFIC HIGHLIGHTS
To go one step further in the analysis of molecular dynamics trajectories, we developed a code to generate positional recurrence maps (PRM), in order to isolate the dynamical contribution of selected non-equivalent atom displacements [3]. Figure 2 shows PRM maps from 40 ps-long molecular dynamics simulations of the three single-phases with an optimised structure. We observed a pronounced shift of the dynamical delocalisation of apical oxygen atoms from the [100] direction (the tilting direction of NiO6 octahedra in d = 0), to the [110] direction (the diffusion pathway direction).
Considering results from both densities of states and positional recurrence maps, we have evidenced that the mobility of oxygen atoms at room temperature
in Nd2NiO4+d is directly correlated to variations of lattice dynamics with oxygen excess, with a shift of displacements from the [100] direction to the diffusion pathway direction toward [110]. The diffusion pathway itself, as shown in figure 3, is similar to the interstitials mechanism calculated at high temperature for related Re2MO4+d.
We can conclude that, if in the high-temperature regime, the densities of states are independent of the excess oxygen content, as excess oxygen does not contribute to lattice dynamics and behaves as single-particle defects, the coupling of lattice dynamics with oxygen mobility is specific to the moderate-temperature regime.
Figure 2
Positional recurrence maps: displacement vectors of apical oxygen atoms in the Nd2O2 rock-salt layer from centre-of-mass positions projected on unitary vectors (ua,ub), with respect to the conventional F-cell. PRMs are calculated from ab initio molecular dynamics at T = 310K, of (a) Nd2NiO4.0, (b) Nd2NiO4.10 and (c) Nd2NiO4.25. The colour scale is logarithmic. Each PRM is cut in space to the conventional F-cell (black border).
Figure 3
Diffusion pathway calculated from molecular dynamics simulations: zoom on a rock-salt layer containing the interstitial sites. Wired and plain red spheres represent respectively the initial position of apical oxygen atoms and excess oxygen in interstitial site. Grey clouds connecting interstitial sites to apical sites mean the oxygen atoms are mobile on this two-sites pathway, highlighted in yellow.
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