Neodymium Cluster Evolution in Fluorite Laser Crystal: A Combined DFT and Synchrotron X-ray Absorption Fine Structure Study

The neodymium cluster evolution in fluorite crystals has been unknown for nearly 60 years, which hindered further exploration of rare-earth-doped fluorite materials. In this work, the first-principles calculation is coupled with the synchrotron X-ray absorption near-edge structure and extended X-ray absorption fine structure (EXAFS), as well as the dielectric spectra to study the neodymium-doped and yttrium-codoped strontium fluorite crystals. The results reveal that the monomers will emerge first and work as basic units. Then, the units aggregate to become the cubic sublattice centers and the cubic sublattice centers further transform to become square antiprism structure clusters as the neodymium concentration rises. The evolution from the monomers to high order clusters will be accelerated by the codopant of yttrium ions. There are about 100% of square antiprism centers that could be obtained when tailored with yttrium ions. The EXAFS spectra further show that the square antiprism centers consist of about 74.2% of SAP-9CN and 25.8% of SAP-8CN polyhedrons, which is in good agreement with the calculated pentamer and hexamer cluster structures. The spectroscopic parameters of neodymium-doped fluorites could be tuned in large scale by the evolution, which presents us a methodology for designing new rare-earth-doped fluorite materials.