**Laura Kocsor 1,2, László Péter 1, Gábor Corradi 1, Zsolt Kis 1, Jen ˝o Gubicza <sup>3</sup> and László Kovács 1,\***


Received: 24 May 2019; Accepted: 26 June 2019; Published: 28 June 2019

**Abstract:** Lithium niobate (LiNbO3, LN) nanocrystals were prepared by ball-milling of the crucible residue of a Czochralski grown congruent single crystal, using a Spex 8000 Mixer Mill with different types of vials (stainless steel, alumina, tungsten carbide) and various milling parameters. Dynamic light scattering and powder X-ray diffraction were used to determine the achieved particle and grain sizes, respectively. Possible contamination from the vials was checked by energy-dispersive X-ray spectroscopy measurements. Milling resulted in sample darkening due to mechanochemical reduction of Nb (V) via polaron and bipolaron formation, oxygen release and Li2O segregation, while subsequent oxidizing heat-treatments recovered the white color with the evaporation of Li2O and crystallization of a LiNb3O8 phase instead. The phase transformations occurring during both the grinding and the post-grinding heat treatments were studied by Raman spectroscopy, X-ray diffraction and optical reflection measurement, while the Li2O content of the as-ground samples was quantitatively measured by coulometric titration.

**Keywords:** lithium niobate; high-energy ball-milling; nanocrystals; mechanochemical reaction
