Configuration of CO2 radicals in γ-irradiated A-type carbonated apatites: Theory and experimental EPR and ENDOR studies

Delson U. Schramm, Joice Terra, Alexandre M. Rossi, D. Ellis

Research output: Contribution to journalArticle

31 Scopus citations

Abstract

Electron nuclear double resonance (ENDOR) spectroscopy and electronic structure calculations were combined in order to study the local environment of CO2 radicals in A-type carbonated apatite. At temperatures lower than 20 K, the ENDOR spectra are composed of hyperfine lines corresponding to the interaction of the CO2 unpaired electron with groups of 1H and 31P nuclei located in the radical neighborhood. Hyperfine coupling constants, nuclear orientation in relation to the radical g-tensor axes, and distance between the electron and nuclear spins were estimated using the “molecular orientation-selection” principle and assuming purely dipolar anisotropic hyperfine interactions. It was verified that the CO2 radicals are not located on OH sites as is frequently suggested in the literature, but lie between two oxygen planes (z = 0.426) and z = 0.574). The radical’s O-O direction is tilted relative to the apatite hexagonal c axis. The vacancy inferred on the nearest OH site confirms the CO2−3 → 2OH substitution mechanism. The determination of atomic positions and molecular orientation from ENDOR spectra relies upon assumptions about localization of electron spin density. Self-consistent field electronic structure calculations can provide the necessary check on these assumptions, and at the same time reveal details of bonding interactions which go beyond simple ionic models. The chemical environment induced by CO3 and CO2 in the OH channel is studied by calculating the electronic structure of embedded clusters employing the first-principles self-consistent discrete variational method based on density-functional theory. Mulliken atomic-orbital populations, densities of states, magnetic moments, and charge and spin-density maps are obtained in order to corroborate the location of the CO2 radical inside the OH channel with that implied by experiment.

Original languageEnglish (US)
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume63
Issue number2
DOIs
StatePublished - Jan 1 2001

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics

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