No difference in local structure about a Zn dopant for congruent and stoichiometric ${\mathrm{LiNbO}}_3$

We compare extended x-ray absorption fine structure (EXAFS) data at the Zn K edge for a low concentration of Zn (0.7 mol%) in a stoichiometric crystal with that for higher Zn concentrations (nominally 5 and 9 mol%) in congruent ${\mathrm{LiNbO}}_3$ (LNO). Note that stoichiometric and congruent LNO have significantly different optical properties. We find no significant difference in the local structure about Zn out to 4 Å for the two types of crystals and different dopant levels. Although some earlier theoretical models suggest a self-compensation model with 75% of Zn on a Li site and 25% Zn on Nb, we find no clear evidence for a significant fraction of Zn on the Nb site, and estimate at most 2%–3% of Zn might be ${\mathrm{Zn}}_{Nb}$.

Figure 1

(Top) The $k$-space data for sLNO (0.7 mol% Zn) and cLNO (4.4 mol% Zn) samples; sLNO black line and cLNO red dashed line at 10K. The data for sLNO have more noise at high $k$ because of the much lower Zn concentration, and this limits the Fourier transform (FT) range. (Bottom) Plot of the corresponding fast FT (FFT) of the $k$-space data with an FT window 3.8–13.2 Å${}^{-1}$; the FT window is Gaussian rounded using a width, 0.2 Å${}^{-1}$. There is a tiny difference between the data for sLNO and cLNO—the sLNO $r$-space plot has a slightly larger amplitude $\left(\sim 1\%\right)$ for all Zn-Nb peaks, consistent with a lower dopant concentration. For the $r$-space data (here and in later plots), the fast oscillating function is the real part $R$ of the FFT while the amplitude is $\pm \sqrt{R^2+I^2}$ where $I$ is the imaginary part of the FFT.

Figure 2

The environment about the central Li site (red) in ${\mathrm{LiNbO}}_3$. Bonds are shown for the nearest O atoms; other O shells are shown as small (blue) atoms. The four green atoms (three in a plane) above the central atom are Nb at $\sim 3.06$ Å; a second plane of Nb atoms is below the center at 3.36 Å, while the bottom Nb atom is 3.87 Å below center. There are six Li (red) neighbors (three above and three below center) at 3.76 Å.

Figure 3

A fit (solid line) of the Zn K edge data (squares) for sLNO with 0.7 mol% Zn, to a sum of functions calculated using FEFF [15]. The $r$-space fit range was 1.3–4.3 Å, and the FT range was 3.8–13.2 Å${}^{-1}$; remaining degrees of freedom is 4. The fit is excellent over the entire fit range, particularly near 3.5 Å where a large Zn-Nb peak would exist if significant Zn were on the Nb site. The main individual peaks are plotted beneath the fit; the weak peaks for more distant neighbors—and also two MS peaks—are shown in an expanded view (zoomed by 2.5) in bottom panel.

Figure 4

The effect on the EXAFS plot if a small Zn-Nb peak with an actual distance of 3.76 Å, corresponding to 10% Zn on the Nb site, is added to 90% of the fit to the Li site. This adds a peak near 3.4–3.5 Å on the EXAFS plot, and even for this small amount of Zn on Nb, the shape of the real part of the transform $R$ is changed significantly.