*2.1. Synthesis*

High purity lithium carbonate (Li2CO3) and niobium pentoxide (Nb2O5), from Alpha Aesar, were used as starting reagents in a 1:1 molar ratio. The respective masses of the precursors were determined such that 1 g of lithium niobate (LiNbO3; LN) was produced from the following balanced chemical equation:

$$\rm Li\_2CO\_3 + Nb\_2O\_5 \to 2LiNO\_3 + CO\_2 \tag{1}$$

The resultant product was labeled—and hereafter referred to—as LN-STm (ST: stoichiometric, m: mixture) because, in principle, a LN mixture was obtained after milling with a 1:1 molar ratio in terms of Li and Nb. Variations in the chemical composition (CC) of the final resultant powders were sought by adding, at the milling stage, 1–5% of the mass in one of the precursors (with steps of 1% with

resolution of 10−<sup>4</sup> g) while keeping the mass of the other precursor constant, in both cases with respect to the masses measured for sample LN-STm (see Appendix A for table). In this way, 10 more samples were synthesized and labeled as LN + 1%LiP, LN + 1%NbP, LN + 2%LiP, and so on up to LN + 5%NbP (P stands for precursor). It must be clarified that the percentages that appear on these labels are not in terms of the ion species solely, but in terms of the whole mass of the precursors that contain them.

The high-energy milling was carried out in an MSK-SFM-3 mill (MTI Corporation) using nylon vials with YSZ balls; a powder:ball ratio of 0.1 was used for each sample preparation. The milling was performed in 30 min cycles, with 30 min pauses to avoid excessive heat inside the milling chamber, until 200 min of effective milling time was reached. Calcination of the resultant materials (amorphous) was done with a Thermo Scientific F21135 furnace in an air atmosphere. All samples were simultaneously calcined with the following programmed routine: 10 ◦C/min → 600 ◦C for 30 min → 2 ◦C/min → 850 ◦C for 120 min → cooling down slowly to room temperature.
