**2. LiNbO3 Preparation Techniques**

Lithium niobate nanocrystals were prepared by high-energy ball-milling of the residue of a Czochralski grown congruent single crystal which depend on different types of vials, milling parameters as described in paper [3]. Characterisation of LN nanocrystals and mechanochemical reactions of lithium niobate such as decomposition and the redox processes induced by high-energy ball-milling were studied. During the milling process, the formation of the LiNb3O8 phase taking place and the reaction can be described as

$$\text{Li}3\text{ LiNO}\_3 \rightarrow \text{LiN} \text{b}\_3\text{O}\_8 + \text{Li}\_2\text{O} \tag{1}$$

where lithium oxide is a volatile by-product. The material undergoes partial reduction that leads to a balanced formation of bipolarons and polarons yielding a grey colour together with Li2O segregation on the open surfaces.

In paper [4], determination of chemical composition between congruent and stoichiometric LiNbO3 powders was worked out by four analytical techniques. Sample preparations were done by mechanosynthesis.

In paper [5], Ø2" LN crystals doped with 0.3 mol% and 5 mol% Mg concentrations with high homogeneity were grown by the Bridgman method using a systematically optimised scheme with careful thermal field design. LN:Mg polycrystalline powders were synthesised by a wet chemistry method to avoid scattering particles and inclusions in the crystal. The homogeneity of LN:Mg crystals was also checked. The extraordinary refractive index gradient was as small as 2.5 <sup>×</sup> <sup>10</sup>−5/cm.
