Lanthanide containing apatites: Synthesis and calorimetric studies with focus on materials for solid oxide fuel cells and drug delivery

Abstract

Materials with apatite structure have attracted interest due to their versatile applications from medical diagnostics and drug delivery to energy and fuel cells. Apatites have flexible structures that can accommodate a range of elements and dopants (in terms of both size and charge) in two distinct crystallographic cation sites and consequently, exhibit fascinating defect chemistry. Recently, several different properties have been engineered into apatite structures by varying their doping schemes. However, advancement of these properties for various material applications needs a fundamental understanding of stability and sustainability of apatites under various conditions. A systematic thermodynamic study can provide insights to understanding structure - composition - property - stability relations as well as guidance to predict, prepare, and select optimized materials for specific desired applications. The lack of knowledge of thermodynamic stabilities in apatite systems with different dopants and doping levels was the motivation behind this study on apatite materials with various compositions and stoichiometry. For this study, a unique and powerful high temperature oxide melt solution calorimetry technique was used to obtain energetics of lanthanide silicate/germanate and calcium phosphate materials with non-doped and doped divalent and trivalent cations. Apatites with composition Ln9.33+x(TO4)6O2+3x/2 (Ln = lanthanide and T = Si and/or Ge), have been proposed as a new group of candidate materials for electrolytes at the intermediate temperature (~600 ˚C) in solid oxide fuel cells (SOFCs). The enthalpies of formation of these materials from oxides were determined to investigate a relationship between the stability and the structural defects (cation vacancy and intestinal oxygen). For the lanthanide silicate group, we found that formation of vacancies and oxide interstitials destabilize the apatite structure and vacancy formation appears to be a dominate factor. Both lanthanide cation and tetrahedral anion sizes were found to affect the stability of this system. Substitution of La with Nd and Si with Ge separately in lanthanide silicate apatites shows that the structure becomes more stable with an increase in the lanthanide cation radius and a decrease in the tetrahedral-site cation radius. Another apatite system, calcium phosphate hydroxyapatite doped with lanthanides, has gathered attention for its luminescent properties and its potential as a drug carrier for the delivery of a variety of pharmaceutical molecules. The enthalpies of formation of calcium phosphate hydroxyapatite, Ca10-x-yLny(PO4)6-x(HPO4)x(OH)2-x-yOy.nH2O (Ln = lanthanides), was investigated using high temperature oxide melt solution calorimetry. Energetic results show that the enthalpies of formation become linearly less exothermic with increase in concentration as well as the size of the dopant lanthanide. The calorimetric studies have revealed the existence of distinctive energetic trends with defects formation and interactions in different doped apatite systems. These fundamental thermodynamic studies could help to understand the structure - property - stability relations in apatite-type materials. These results could guide processing, application and sustainability of apatite type materials in both the energy and biotechnology industry.