*3.1. Derivation of Interatomic Potential Parameters*

It was necessary to derive potential parameters for the dopant oxide structures: VO, V2O3, VO2 and V2O5 as well as LiMoO2, Li2MoO3, Li3MoO4 and Li2MoO4. For V<sup>2</sup><sup>+</sup>-O2<sup>−</sup>, V3<sup>+</sup>-O2<sup>−</sup>, V4+-O2<sup>−</sup> and V5<sup>+</sup>-O2<sup>−</sup> as well as Mo3<sup>+</sup>-Li<sup>+</sup>, Mo4<sup>+</sup>-Li<sup>+</sup>, Mo5<sup>+</sup>-Li<sup>+</sup>, Mo6<sup>+</sup>-Li<sup>+</sup>, Mo3<sup>+</sup>-O2−, Mo4<sup>+</sup>-O2−, Mo5<sup>+</sup>-O2<sup>−</sup> and Mo6<sup>+</sup>-O2<sup>−</sup> interactions, a new set of potentials was derived empirically by fitting to the observed structures as shown in Table 1. The O2−-O2<sup>−</sup> potential was obtained by Sanders et al. [29] and uses the shell model for O [30], which is a representation of ionic polarisability, in which each ion is represented by a core and a shell, coupled by a harmonic spring, and the Li-O potential was taken from [22]. In all cases, the dopant-oxide potentials were obtained by fitting to parent oxide structures.


**Table 1.** Interionic potentials obtained from a fit to the VO, V2O3, VO2, V2O5, LiMoO2, Li2MoO3, Li3MoO4 and Li2MoO4 structures.

Table 2 compares experimental and calculated structures of VO [31], V2O3 [32], VO2 [33] and V2O5 [34] oxides as well as LiMoO2 [35], Li2MoO3 [36], Li3MoO4 [37] and Li2MoO4 [38] lithium molybdate structures, using the potentials in Table 1. It is seen that the experimental and calculated lattice parameters differ by less than 1%, confirming that the potentials can be used in further simulations of defect properties. The calculations were carried at 0 K (the default for the modelling code and used in most other theoretical studies) and at 293 K for comparison with room temperature results. In this way, we can see how the structure and energies vary with temperature.

**Table 2.** Comparison of calculated (calc.) and experimental (expt.) lattice parameters.


## *3.2. Defect Calculations*

In this section, calculated energies for dopant ions in LiNbO3 are reported. The divalent, trivalent, tetravalent, pentavalent and hexavalent dopants can substitute at Li and Nb sites in the LiNbO3 matrix with charge compensation taking place in a number of ways. The proposed schemes described in

the following subsections are written as solid state reactions using the Kroger–Vink notation [39]. This notation appears in the tables in Sections 3.2.1–3.2.5 where the dot/bullet (·) means a net positive charge and the dash/prime ( ) means a net negative charge.
