# Modelling the Defect Processes of Materials for Energy Applications
## 4 Oxide Interfaces  
Nuclear wasteform elements can be encapsulated and immobilised in designed glass-ceramic composites (refer to [58] and references therein). In that respect, understanding the structure-property relations is worthwhile. For these systems, molecular dynamics calculations provide a solid starting point to investigate the impact of the glass-crystal interface on the glass composition [58]. For example, in the study by [PERSON] et al. (refer to [58] and references therein), molecular dynamics calculations were used to model the interface between MgO and CaO crystals and a mixed alkali borosilicate glass (glass used for the immobilisation of high-level wasteform from the reprocessing of Magnox fuel). A typical system is illustrated in Figure 4[58]. These studies demonstrated that the crystal remained practically unchanged; however, the glass exhibited considerable structural changes [58]. The alkali concentration increased at the layer closest to the glass-crystal interface, whereas the in the glass network, the crystal induced a partial ordering (with depth up to 10 A into the glass) [58]. These changes (ordering decreases the entropy in the glass) need to be understood in depth using thermodynamic analysis, and in this respect the cB\(\Omega\) model may be important. Figure 3: Surface of the constant charge density on seven different SQS configurations in Si\({}_{1-x}\)Ge\({}_{x}\) alloys (x = 0.125, 0.25, 0.375, 0.5. 0.625, 0.75 and 0.875) showing the interaction of substitutional Al [34]. Interfaces are also important for energy applications. For example, [PERSON] et al. [96] observed colossal conductivity in very thin (\(\sim\)1 nm) yttria-stabilised zirconia (YSZ, here ZrO\({}_{2}\) with 8 mol% Y\({}_{2}\)O\({}_{3}\)) layers that were enclosed epitaxially between layers of SrTiO\({}_{3}\). These SrTiO\({}_{3}\)/YSZ/SrTiO\({}_{3}\) heterostructures were determined to have conductivities higher than bulk YSZ by 8 orders of magnitude [96]. They were deemed by [PERSON] et al. [96] to be ionic conductivities due to the 7% tensile strain in the YSZ thin films and the disorder of oxygen at the interface. The existence of colossal ionic conductivity can lead to very efficient oxygen-ion conducting electrolytes in solid oxide fuel cells (SOFCs) and in effect has the potential to revolutionise the way we generate and store electricity. The findings of [PERSON] et al. [96] have, however, raised a debate in the community about whether the determined conductivity is due to ionic charge carriers (i.e., oxygen diffusion) or electronic charge carriers in the SrTiO\({}_{3}\) layers [97; 98; 99; 100; 101; 102; 103; 104]. A recent experimental study by [PERSON] et al. [102] in a SrTiO\({}_{3}\)/YSZ/SrTiO\({}_{3}\) heterostructure determined that the total conductivity dependence is inconsistent with ionic transport in YSZ. [PERSON] and [PERSON] [100] employed DFT in conjunction with the nudged elastic band (NEB) method to calculate the migration enthalpy barrier in biaxially strained bulk YSZ. This study is consistent with an ionic enhancement of up to 4 orders of magnitude. The DFT calculations of [PERSON] and [PERSON] [100] were confined to small supercells at 0 K. [PERSON] et al. [103] used quantum molecular dynamics to simulate oxygen-deficient strained ZrO\({}_{2}\), calculating a migration energy of 0.4 eV. This was compared with the experimentally determined activation enthalpy for oxygen vacancy migration in unstrained YSZ (refer [96]), resulting in an enhancement in ionic conductivity of 4 \(\times\) 10\({}^{6}\). [PERSON] et al. [104] criticised the inconsistency of comparing strained ZrO\({}_{2}\) with unstrained YSZ and suggested that the enhancement of the ionic conductivity would be far smaller. Figure 4: Typical model of a glass–crystal interface. The centre is the (1 0 0) surface of MgO and is surrounded by lithium sodium borosilicate glass [58].