Lattice distortions in hydroxyapatites with size as follows from the electronic relaxation time measurements

Abstract

Pure and substituted hydroxyapatites (Ca₁₀(PO₄)₆(OH)₂, HAp) have been recently gaining special importance in catalysis for selective reactions where the synergetic functioning of acid–base active site pair facilitates the molecular transformations. Transition to the nanostructured HAp materials and ion incorporation can cause lattice distortions and influence physicochemical properties of HAp. We have investigated powders of HAp synthesized by wet techniques from the nitrogen-containing solutions with the average particle sizes of 30 (nanosized) and 1000 nm (microsized) with pulsed electron paramagnetic resonance (EPR). Electronic relaxation times (T_1e) for the stable impurity paramagnetic NO₃ ²⁻ centers were measured in the temperature range of 10-300 K. It is revealed that T_1e shortens with the size reduction of factor two in the whole temperature range. The results are discussed in the framework of the modified Debye model taking into account non-Debye HAp phonon density. The obtained results show the feasibility of pulsed EPR techniques for characterization HAp based materials.