EXAFS evidence for a primary ZnLi dopant in LiNbO3

We present extended x-ray-absorption fine structure (EXAFS) data as a function of temperature (10-300 K) at the Zn and Nb K edges for Zn-doped LiNbO3. The focus is on higher Zn concentrations (7-11 mol %) for which there is disagreement in the literature as to the substitution site for Zn. Our data show that Zn substitutes only on the Li site; we find no evidence for Zn on the Nb site. However, uncertainties result in an upper bound of at most 5% of the Zn dopants being Zn-Nb. In addition, the Zn-Li defect produces a significant distortion in the lattice out to at least 4 angstrom; the O atoms are attracted toward Zn while the Nb neighbors are repulsed. The Nb EXAFS agree well with the structure from diffraction for the main Nb-X peaks out to about 3.7 angstrom. However, there appears to be a weak third Nb-O peak in the first O shell, which has a low amplitude and a longer bond length. For Nb, the shortest Nb-O bond is extremely stiff (correlated Debye temperature, theta(cD) similar to 1100 K), while the longer Nb-O bond is a little weaker (theta(cD) similar to 725 K) and of comparable strength to the shortest Zn-O bond (theta(cD) similar to 600 K); consequently, substituting Zn at the Li site will stiffen the structure as the Li-O bonds are weak. We discuss implications of a dominant Zn-Li defect.