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

TL;DR

This study combines density functional theory calculations of hydroxyapatite's phonon spectrum with experimental EPR measurements to understand temperature-dependent spin relaxation, revealing two-phonon Raman processes above 20 K.

Contribution

It provides the first combined theoretical and experimental analysis of hydroxyapatite's phonon spectrum and its impact on electron spin relaxation at low temperatures.

Findings

Phonon spectrum of hydroxyapatite was successfully calculated using DFT.

Temperature dependence of T1e fits two-phonon Raman processes.

Debye temperature of 280 K was determined from phonon data.

Abstract

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.