**1. Introduction**

Lithium niobate (LN, LiNbO3) crystals have countless non-linear optical and acoustic applications due to their versatile optical and ferroelectric properties. In recent decades nanocrystalline materials have attracted considerable interest as their properties greatly differ from their coarse-grained counterparts. The reduction of the grain size may cause, e.g., increased mechanical strength, higher specific heat and larger electrical resistivity [1]. Nonlinear optical nanocrystals may have many applications such as building blocks of coherent subwavelength light sources [2–6] and nonresonant markers in second harmonic generation microscopy [4,7,8]. Rare-earth-doped LN nanocrystals could play an important role in coherent quantum optical experiments, e.g., as single photon sources, providing sharply defined wavelengths [9,10].

Many wet chemical synthesis methods are known for the preparation of nanocrystalline materials. In these methods, nanocrystals are assembled from single atoms or molecules (bottom-up methods). The other way to prepare dispersed nanograined material is to reduce the particle size by mechanical grinding (top-down method). High-energy ball-milling is one of these methods. This is a simple, general and easy-handling technique for nanocrystal preparation, while another advantage of this method is the possibility to produce large quantities of nanopowders. However, it is known that mechanical grinding may also induce phase transformations and chemical reactions beside particle and grain size reduction [11–14]. An example for a phase transformation without composition change

was the occurrence of various phases in ball-milled TiO2 (rutile, anatase and srilankite) whereas the starting material contained a single phase only [11,15]. The mechanochemical transformation can also be used for the synthesis of nanocrystals, as it was demonstrated by grinding a (1−*x*)Li2O: *x*B2O3 mixture resulting in the formation of Li2B4O7 [15].

Ball-milling was proved to be a successful method also for the preparation of lithium niobate. De Figueiredo et al. reported that ball-milling of Li2CO3 and Nb2O5 resulted in the formation of LiNbO3, but non-reacted starter materials also remained in the milled powder [16]. Pure LiNbO3 can still be prepared by mechanochemical methods if the milling process is followed by calcination at high temperature [17–19].

High-energy ball-milling was used in several works to produce nano-LN. Spex Mixer Mill 8000 is one of the most commonly used ball mills for this purpose. In general, dry grinding with one ball with a diameter of around 1 cm was used in these processes with grinding times as long as 5–20 h [13,15,20–22] and even longer than 100 h [23]. At the beginning, the grain size rapidly decreased with increasing grinding time as determined by X-ray diffraction measurements. Five and 20 h of grinding resulted in grain sizes of about 60 and 20 nm, respectively, but longer ball-milling did not decrease the grain size considerably. It should be noted that the grain size deduced from X-ray diffraction data is informative for the coherently scattering domains but does not characterize the size of the particles obtained.

Pooley and Chadwick reported that the sample ball-milled for five hours in a stainless steel vial contained a significant amount of amorphous material [13] and traces of iron [20]. Other experiments also proved that contamination from the vial's material may appear: a sample ball-milled for 16 h in an alumina vial contained 5% of alumina [22]. In these reports, however, there is no further information about whether either a phase transformation or any mechanochemical reaction occurred.

The aim of the present work is a systematic characterization of LN nanocrystals prepared by high-energy ball-milling. For the sake of simplicity, small pieces of the residue of Czochralski-grown single crystals with identical purity are used as starting material, since the milling experiments did not require large, perfect single crystalline LN. Besides the determination of both particle and grain sizes and contamination from the vial's material, the possible phase transformation and structural changes induced by the mechanochemical process are also studied using dynamic light scattering (DLS), powder X-ray diffraction (pXRD), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, optical diffuse reflectance measurements and coulometric acid-base titration.

## **2. Materials and Methods**
