$\mathrm{E}{\mathrm{r}}^{3+}-\mathrm{doped} \mathrm{Ca}{\mathrm{F}}_2$ is a promising laser gain medium for applications requiring mid-infrared radiation. It is well known that the $\mathrm{E}{\mathrm{r}}^{3+}$ ions embedded in the fluorite matrix tend to agglomerate forming so-called clusters thought to enhance the optical properties of the material. However, studies of the structure of these clusters have been limited to high doping concentrations. Furthermore, the experimental techniques used, such as extended x-ray absorption fine structure (EXAFS), provide only orientationally averaged information, which can be an issue for the unambiguous solution of the local disorder structure. Here, three-dimensional difference pair distribution function ($3\mathrm{D}-\mathrm{\Delta }\mathrm{PDF}$) analysis is applied to single-crystal diffuse scattering collected on $\mathrm{Ca}{\mathrm{F}}_2$ crystals with nominal doping levels of 1%, 3%, and 6% $\mathrm{E}{\mathrm{r}}^{3+}$. Despite the low doping concentrations and the weak diffuse scattering resulting from this, clear three-dimensionally resolved local structure information is available from the $3\mathrm{D}-\mathrm{\Delta }\mathrm{PDF}$. Comparing the experimental results to big-box simulations and analytical models, the preferred $\mathrm{E}{\mathrm{r}}^{3+}\mathrm{cluster}$ is observed to be an octahedral hexamer. Additionally, the $3\mathrm{D}-\mathrm{\Delta }\mathrm{PDF}$ and average structure information reveal that the charge compensating fluoride ions form a cuboctahedral structural motif. Combined with the $\mathrm{E}{\mathrm{r}}^{3+}$ hexamer, the structure of the preferred cluster can therefore be described as a (local structure) $\mathrm{E}{\mathrm{r}}^{3+}$ analoge to the ${\mathrm{Y}}_6{\mathrm{F}}_{37}$ clusters known from the average structure of the ordered yttrofluorite mineral tveitite. Comparison of the experimental diffuse scattering to diffuse scattering based on this local structure model shows good agreement speaking to the validity of the model.

Close

PrevNext

View figure in article

• Crystal phenomena
• Defects
• Structural properties