Combined spectroscopic and DFT studies of local defect structures in beta-tricalcium phosphate doped with Nd(III)

Abstract

We investigated the distribution of the Nd3+ impurity centers over five Ca2+ positions in the β-Ca3(PO4)2 lattice (space group R3c) using low-temperature site-selective laser spectroscopy and fluorescence kinetics measured by double spectral selection in combination with the results of periodic density functional theory (DFT) calculations. Site-selective fluorescence excitation spectra of the Nd3+ ion were measured by fine tuning the laser wavelength in the spectral band of the 4I9/2(1) → 4G5/2(1,2) transitions and by fluorescence detection in the spectral band of the 4F3/2(1) → 4I9/2(1,2) transitions. The kinetics of nonradiatiative energy transfer from the 4F3/2(1) crystal field (CF) level was used to probe the local structure of Nd3+ sites. Theoretical modelling of the replacement of Ca2+ ions by Nd3+ ions with different charge compensation schemes was carried out. In the context of heterovalent substitution, hydrothermal synthesis conditions, and charge-balance mechanisms, two structural models of substitution have been proposed and thoroughly studied: substitution by a single Nd3+ ion accompanied by a trapped hydroxyl group, and substitution with a pair of Nd3+ ions. Numerical modeling of possible substitution scenarios calculated for a large number of combinations of cation-exchange sites in the lattice was carried out. The results obtained were classified according to the parameters of the accommodation of impurities and the energies of defect formation. The most favorable configurations of the distribution of impurity Nd3+ cations in the β-Ca3(PO4)2 lattice were predicted. By combining the “energy transfer probe” analysis and the results of the structural modeling, a relationship was found between the local geometry and the spectral and kinetic properties of luminescence of the Nd3+ optical centers. This allowed us to accurately determine the local structural geometries of eight experimentally detected the Nd3+ optical centers among the various possibilities in the distribution of the dopant ions over five cationic positions in β-Ca3(PO4)2. Six optical centers represent different types of Nd3+ - Nd3+ pairs, the formation of which is energetically favorable for charge compensation according to the 3Ca2+ → 2Nd3+ + ⎕ scheme, when one calcium position remains vacant. Two optical centers are single centers of the Nd3+ ion in which the charge is compensated by negatively charged hydroxyl ions intercalated during hydrothermal synthesis according to the scheme Ca2+→ Nd3+ + OH−. It was assumed that these pairs are located in several structural cavities free of PO4 tetrahedra.