Prediction and Rationalization of Site Preference of Rare Earth Elements in Fluorapatite from Density Functional Theory
来源：ACS Publications
Fluorapatite (FAp, nominally Ca10(PO4)6F2) has been identified as an important host material for rare earth elements and yttrium (REY) in marine sediments. REY can be accommodated in either the larger 6+3 coordinated Ca1 site or the smaller 6+1 coordinated Ca2 site, yet little is known about the site preference of REY through the lanthanide series despite its importance for understanding REY enrichment processes in FAp. Theoretical investigations based on density functional theory (DFT) predict that all REY intrinsically prefer the smaller and more ionic Ca2 site. The Ca2 site preference is less pronounced when the excess of positive charge resulting from the REY3+ for Ca2+ substitution is compensated by a coupled Na+ for Ca2+ substitution, instead of the energetically more favorable Si4+ for P5+ coupled substitution. The site preference varies quadratically with the ionic radius of REY and linearly with the sum of the first and second ionization energies. The quadratic shape of the site preference is similar to the shape of the Onuma diagrams, which suggests that the local effective elastic constant of the site controls the site preference rather than the nominal size of the site. Despite being smaller, the Ca2 site has a lower effective elastic constant and is, therefore, more flexible than the Ca1 site for accommodating larger and smaller trivalent REY cations. Concentration-dependent computations show that REY clustering is thermodynamically favorable except for Yb and Lu.
Recently, we have studied the incorporation of Ce3+ in FAp using high energy-resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy and DFT. We showed that Ce occupies the Ca2 site and that Na favors the Ca1 site at a doping concentration of ∼12,500 ppm. In the absence of meaningful reference energies for “free” Ca2+, P5+, Si4+, and Na+ cations to balance chemical equations, the most favorable charge-compensation models were obtained by comparing their energies relative to those of reference structures in which Ce and the codopant are well separated and thus minimally interacting. Using this scheme, we found that the charge compensation with a SiO44– for PO43– substitution is energetically more stable than that with a Na for Ca1 substitution. Furthermore, the coupled Ce3+-Na+ and Ce3+-Si4+ substitutions preferentially take place at a short distance in the FAp structure (e.g., edge-sharing CeO7–SiO4 linkage. Site preference and ionization energies were correlated linearly for divalent cations. On the basis of the atomic charge of divalent substituents, we found that the Ca2 site is more ionic and is, therefore, preferred by dopants with low ionization energies (e.g., Ba) over those with higher ionization energies (e.g., Mg).
Here, we extend our previous study on Ce to the whole REY series and explore the effect of REY concentration, focusing on the two main charge-compensation mechanisms, i.e., Na and Si codoping. We reveal an intrinsic preference of all REY for the Ca2 site, which is enhanced by the Si-based charge balance. However, at a high concentration, REY can be stabilized in the Ca1 site if the charge is balanced by Na.