Quantitative analysis of element substitution in mixed-vanadate LuxGd1-xVO4 single crystals at the atomic scale

In mixed crystals, two or more elements exhibit comparable properties and can be substituted freely. As a special type of “substitutional doping,” the properties of materials can be adjusted by altering the doping ratio of elements. Mixed-vanadate crystal LuxGd1-xVO4 is a promising laser host material with excellent chemical stability, mechanical property and thermal conductivity. The substitutional doping ratio of the two rare-earth elements determines the value of x. The thermal, optical, and laser properties of the material vary with x. However, the two types of atoms showing little difference in the same location in mixed crystals form substitutional solid solutions with alike atomic radii as well as electronic structures. Therefore, distinguishing comparable elements in mixed crystals at the atomic scale and quantitatively analyzing the doping ratio in mixed crystals are challenging. In this study, combining high angle annular dark field (HAADF) and electron energy loss spectroscopy (EELS), we accurately characterize the atomic structure of mixed-vanadate single crystal LuxGd1-xVO4 along the [111] orientation as well as the distribution of two rare-earth elements at the atomic scale. The ratio of Lu and Gd and the doping rules in the crystals are quantitatively analyzed. In addition, combined with the energy dispersive X-ray spectroscopy (EDS) results of the [100] crystal orientation, a uniform random distribution of Lu and Gd in the LuxGd1-xVO4 crystal is confirmed. This study provides a new means of understanding mixed-crystal structures and solves the problem of quantitative analysis of elemental doping in materials at the atomic scale, thus laying the foundation for exploring the relationships among composition, structure, and function in materials.