Phonon Spectrum in Hydroxyapatite: Calculations and EPR Study at Low Temperatures

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Density functional theory based calculations within the framework of the plane-wave pseudopotential approach are carried out to define the phonon spectrum of hydroxyapatite Ca10(PO4)6(OH)2 (HAp). It allows to describe the temperature dependence of the electronic spin-lattice relaxation time T1e of the radiation-induced stable radical NO32- in HAp, which was measured in X-band (9 GHz, magnetic field strength of 0.34 T) in the temperature range T = (10-300) K. It is shown that the temperature behavior of T1e at T > 20 K can be fitted via two phonon Raman type processes with the Debye temperature of 280 K evaluated from the phonon spectrum.