Zn atoms in lithium niobate and mechanism of their insertion into crystals

Precision X-ray structural studies were carried out for LiNbO$_3$:Zn$_x$ single crystals with $x=0.0$, $2.87$, $5.20$, and $7.60$ at. $\%$. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic Nb$_{\rm Li}$ defects. At $x=7.6$ $\%$, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the «threshold» Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO$_3$. A change in the intrinsic defect structure of the LiNbO$_3$ crystals with different Zn concentrations is discussed.