*3.2. Structure of the Samples: XRD, Raman and Optical Reflectance Measurements*

Figure 2 shows the XRD patterns of the ground materials compared with those oxidized and/or reduced at 800 ◦C for 3 h. The broad peaks of the XRD patterns of the as-ground samples indicate small grain sizes as determined numerically using the Williamson-Hall method (see Table 2). Heat-treatment processes resulted in narrower diffraction lines due to increased grain sizes. The reflections of a LiNb3O8 (lithium triniobate) phase appeared in the diffraction patterns of all annealed samples (best seen for oxidized samples, especially for those ground in the stainless steel vial, see Figure 2a). The formation of the LiNb3O8 phase taking place as a result of the combined ball-milling and annealing procedure can be described as

$$\text{13 LiNbO}\_3 = \text{LiNb}\_3\text{O}\_8 + \text{Li}\_2\text{O}\_7 \tag{1}$$

where lithium oxide is a volatile byproduct.

The fact that the lithium triniobate can only be identified in the heat-treated samples indicates that the structural rearrangement of the residual Nb containing oxides is not completed in the as-ground samples. However, the annealing process provides the activation energy required for crystallization of the new phase with sufficiently large crystallites to yield strong enough reflections in the diffraction patterns.

In the XRD pattern of the sample ball-milled in alumina vial the reflections of α-Al2O3 can be clearly identified due to abrasion of the vial and balls during the milling process already before the heat-treatments (see Figure 2b). The fact that alumina was present in a crystalline form already in the as-ground samples indicates that the milling destroyed the ball/vial material and the majority of this impurity does not arise as a result of the side reaction that weakened the alumina structure. The lack of further crystalline compounds of aluminum in the annealed samples shows that the amount of possibly reacted alumina was insignificant as compared to alumina that entered the ground mixture by the mechanical effect of milling.

**Figure 2.** Diffraction patterns of Lithium niobate (LN) ground in different vials, stainless steel (**a**), alumina (**b**), and tungsten carbide (**c**). The unmarked peaks are the reflections of LiNbO3.

Figure 3 shows the Raman spectra of the as-ground and heat-treated samples ball-milled in stainless steel and tungsten carbide vials. The Raman intensities of the as-ground samples are weak, the bands are broad, not showing all characteristic features of LiNbO3 crystals. The heat-treatment process resulted in line narrowing and increased intensity of the bands corresponding to the pure LiNbO3 phase. In addition, in the oxidized samples some weak bands appeared at 59, 79 and 96 cm−<sup>1</sup> corresponding to the LiNb3O8 phase (Figure 3a) [29]. This confirms the XRD results, where the presence of the LiNb3O8 phase predicted by Equation (1) was best seen for oxidized samples, especially for those ground in stainless steel vial.

Equation (1) suggests that at least one new component without any niobium content has to appear during the milling process. Lithium oxide, Li2O, may be present as the primary byproduct and can be transformed in air to another lithium compound (LiOH·*x*H2O, Li2CO3·*x*H2O) by water and/or CO2 uptake. Indeed, the water suspensions of all ball-milled LN powders were found to be alkaline, regardless of the chemical state of the Li-rich segregate, which is an unambiguous confirmation of the decomposition of LN via Li2O separation during the milling process. The as-ground LN particles were structurally disordered in the decomposed region but recrystallized upon annealing, hence both the Raman and XRD lines of LiNb3O8 could manifest themselves. The CO2 uptake of Li2O produced during ball-milling can also be observed, viz. in the Raman spectrum of the LN powder ground in tungsten carbide vial shown in Figure 3b. The bands at about 190 and 1090 cm−<sup>1</sup> present in the as-ground samples are attributed to Li2CO3 generated from Li2O (Figure 3b) [30]. Heat-treatments at 800 ◦C either in air or in vacuum resulted in the loss of CO2 evidenced by the disappearance of those bands from the Raman spectra. The presence of α-Al2O3 contamination in the powder ball-milled in alumina vial was observed in the XRD diffractogram; however, it could not be detected by Raman spectroscopy as the corresponding bands at about 383 and 420 cm−<sup>1</sup> overlap with the larger bands of LN [31].

**Figure 3.** Raman spectra of ground and heat-treated samples ball-milled for five hours in stainless steel vial, shown in the range of 40–460 cm−<sup>1</sup> (**a**), and in tungsten carbide vial, shown between 125–1150 cm−<sup>1</sup> (**b**).

Equation (1) does not account for the redox processes indicated by color changes seen during ball-milling and annealing treatments. The colors of samples ground in alumina, stainless steel and tungsten carbide vials varied from light gray to dark gray (see Scheme 1), evidenced by their optical reflection spectra (Figure 4a)—the darker the sample, the lower its reflection in the whole spectral range. As mentioned above, the samples underwent a change concerning the oxidation state of niobium during the grinding process. This partial reduction could be compensated by oxidizing the sample applying a heat-treatment in air at 800 ◦C. The oxidative annealing resulted in white color for the powder ground in stainless steel vial (Scheme 1). Upon subsequent reduction, the sample became brownish, while the pellet pressed from the as-ground powder became gray when reduced directly. The oxidation process resulted in a white color even in the case of the previously reduced sample. As an example, the optical reflection spectra of the as-ground and annealed samples ball-milled in stainless steel vial are shown in Figure 4b. Similar effects were observed for powders ground in the other two vials: the change of color was less evidenced for alumina vial but was stronger for tungsten carbide vial.

**Figure 4.** Optical reflection spectra of samples ball-milled in different vials (**a**) and of ground and heat-treated samples ball-milled in stainless steel vial (SS-5) (**b**).

In Figure 4a no distinctive feature characteristic for Fe2<sup>+</sup> having an absorption band near 500 nm can be discerned for any of the as-ground samples. The comparison of reduced samples also shows very small differences in this respect. Instead, differences in amplitude of the whole spectrum dominate. Still some change of coloration induced by the redox treatments may be related to the iron contamination at least partly coming from the starting material. For the preparation of LiNbO3 high- purity raw materials with less than 2 ppm, Fe was used. Since the effective distribution coefficient of Fe between the molten and solid lithium niobate of congruent composition is around 1, it is not expected to be enriched in the crucible residue during the growth process. On the other hand, even 10 ppm iron does not induce a sizable increase of the optical absorption. According to Phillips et al. [32] the difference in the absorption coefficient at about 500 nm between oxidized and reduced LN containing about 0.5 mol% Fe is less than 4 cm<sup>−</sup>1, i.e. less than 0.001 cm−1/ppm Fe, which cannot cause dominant coloration changes in our case.
