Lithium niobate: from single crystals to nanocrystals

LiNbO₃ single crystals were first grown more than 50 years ago. Since that time thousands of papers have been published dealing with their outstanding ferroelectric, acoustic, nonlinear optical, holographic etc. properties and demonstrating their countless realized or potential applications. It was about 25 years ago, when the first stoichiometric LiNbO₃ crystals gave a new impulse to the never-ending investigations. Different applications require different undoped or doped systems of bulk, thin film, or nanocrystal forms. In the present talk I’ll show two examples, (i) incorporation of dopants into stoichiometric crystals, and (ii) properties of LiNbO₃ nanocrystals prepared by high-energy ball-milling.

Dopants are generally used to tailor the crystal properties for a given application. To understand the effect of dopants the incorporation mechanism and/or the substitution site in the crystal have to be known. Our IR absorption studies unambiguously showed that for the di-, tri-, and tetravalent cations a threshold concentration exists above which the dopants partially substitute at Nb sites, while below it they can be found on Li sites [1].

Nano-crystalline LiNbO₃ was prepared from single crystals by the high-energy ball-milling technique [2]. During milling the material suffered partial reduction that lead to the formation of bipolarons and polarons yielding gray color together with Li₂O segregation on the open surfaces. Upon high temperature oxidation, the volatile Li₂O phase and the polarons got eliminated and a more stable LiNb₃O₈ shell was formed. The particle size of the nano-crystals were determined by dynamic light scattering (DLS) and scanning electron microscopic (SEM) methods.

This research was supported by the National Research, Development and Innovation Fund of Hungary within the Quantum Technology National Excellence Program (Project No. 2017-1.2.1-NKP-2017-00001)

[1] L. Kovács, L. Kocsor, É. Tichy-Rács, K. Lengyel, L. Bencs, G. Corradi, Opt. Mat. Express 9, 4506 (2019).

[2] L. Kocsor, L. Péter, G. Corradi, Z. Kis, J. Gubicza, L. Kovács, Crystals 9, 334 (2019).