*2.1. Sample Preparation*

Lithium niobate was prepared by solid-state reaction at 800 ◦C from Li2CO3 (Merck, Darmstadt, Germany, Suprapur, 99.99%) and Nb2O5 (Starck, Goslar, Germany, LN grade, 99.99%) raw materials by mixing them in the congruent 48.6:51.4 molar ratio. These high-purity materials contain impurities such as Fe, Al, W, relevant in our studies, in concentrations less than 1–2 ppm. Water-clear single crystals of congruent composition have been grown from platinum crucible by the Czochralski method, leaving the crucible residue also congruent and of single phase. LiNbO3 nanocrystals were prepared from such polycrystalline crucible residue slowly cooled down to room temperature. Pieces of sizes of several millimeters were selected and ball-milled, using a Spex 8000 Mixer Mill (Metuchen, NJ, USA). Dry ball-milling was carried out using different types of vials and balls (SS: stainless steel, ALO: alumina, WC: tungsten carbide). The grinding time was 5 or 20 h with interruptions of 30 min after every hour of grinding to avoid overheating (the ball mill was not equipped with a temperature sensor). Further milling parameters are given in Table 1. During the milling process and thereafter the samples were kept in air (no protecting atmosphere was used).

After the milling process, some samples showed greyish coloration usually observed for LN single crystals after reduction (i.e., decomposition with oxygen release and a concomitant formation of Nb (IV) in the lattice). This indicates that the samples underwent a change concerning the oxidation state of niobium during the grinding process. For this reason, annealing was applied for restoring or modifying the oxidation state of niobium in the ground samples. Annealing treatments in either oxidative or non-oxidative atmospheres were performed for 3 h at 800 ◦C in either air or in vacuum of about 10−<sup>4</sup> mbar, respectively. Samples were put into the furnace in a form so that any possible reaction between the material and its containment could be avoided. Powder samples were held in a platinum crucible for the oxidative treatments, while pellets were placed on a platinum plate inside the quartz tube used for evacuation in the case of the application of non-oxidative conditions. Samples subjected to one of these annealings will be called hereafter oxidized and reduced samples. Oxidized-reduced and reduced-oxidized samples underwent two subsequent heat treatment processes separated by periods long enough to let the samples cool down to room temperature. This way, a series of five differently treated samples was obtained for each vial material (see Scheme 1).


**Table 1.** Grinding parameters used for high-energy ball-milling.

**Scheme 1.** The series of samples ground in different vials (see Table 1) with subsequent heat treatments. Pellets were pressed for optical reflectance measurements.

## *2.2. Sample Characterization*

The phase analysis of the samples was carried out by X-ray diffraction (XRD) using a Rigaku (Tokyo, Japan) Smartlab X-ray diffractometer with CuKα radiation (wavelength: λ = 0.15418 nm). The XRD patterns were measured in the Bragg-Brentano diffraction geometry. The crystalline phases were identified from the peak positions and intensities using an ICDD PDF-2 database. The grain sizes were determined by using the Williamson-Hall method.

The Raman spectra of ground and heat-treated samples were collected at room temperature using a Renishaw (Wotton-under-edge, UK) inVia Raman Microscope in backscattering geometry. A 633 nm laser beam was used as excitation source using a 50× lens. The excitation spot size was 2 μm at the sample surface. The Raman data were recorded in the range of 20−460 cm−<sup>1</sup> with a low-wavenumber filter and in the range of 100−1200 cm−<sup>1</sup> with a notch filter.

Dynamic light scattering measurements in the range of 0.1–10000 nm (Malvern Zetasizer Nano S, Worcestershire, UK) were performed to determine the particle size distribution. The ground samples were suspended in water assuming the nanoparticles to be perfect spheres leaving the viscosity of water unchanged. The refractive index of the particles was taken as ~2.2 corresponding to that of bulk LiNbO3 in the given wavelength region.

Since the LN samples were discolored after the grinding procedure, possible contamination from the vials was suspected. This was checked by EDS measurements in a Zeiss Leo scanning electron microscope (Jena, Germany). Depending on the vial used, the presence of Fe, Al and W was scrutinized for the stainless steel, alumina and tungsten carbide vials, respectively. The sensitivity of the EDS method for the above listed elements is 0.1 at%, while the relative error of the EDS measurement in the x < 0.5 at% concentration range can be as high as 20%.

To characterize the optical reflectance properties of the samples, an Avantes (Apeldorn, The Netherlands) HS-1024X122 TEC UV-VIS fiber-optic modular spectrophotometer was used. The measurements were carried out on pellets pressed from the ground and heat-treated samples. The samples were illuminated in the 190–1100 nm wavelength range using an Avantes AvaLight-D(H)-S deuterium-halogen light source. To determine the reflectance spectra, we used an Avantes WS-2 diffuse white tile as a reference. The spectral data recorded were used without any further processing.

The secondary phase occurring during the grinding process (Li2O or any other compound it may be transformed to) was quantitatively measured by semi-micro coulometric acid-base titration with current control. For this procedure, portions of the powder samples were weighted in a microbalance with a precision of 2 μg and suspended in a few milliliters of 0.3 mol/dm3 Na2SO4 solution in the anode compartment of a diaphragm-divided two-chamber electrochemical cell. The pH in the anode compartment was measured with a combined glass pH electrode and a Consort 860 pH tester. A platinum anode was used for acid generation by the following electrode reaction: 2H2O = O2+ 4H<sup>+</sup> + 4e−. The Na2SO4 solution was boiled prior to the measurement in order to remove any dissolved CO2 that may interfere with the measurements, then cooled to room temperature for the titration. The solution in the anode compartment was stirred with a magnetic stirrer bar. Blank measurements and known quantities of both Na2HPO4 and NaOH were used to verify that no solution intermixing took place between the anode and cathode chambers.
