Theoretical and experimental investigation of the electron spin resonance of Co2+ in Zn-2(OH)PO4 and Mg-2(OH)AsO4

Abstract

Electron spin-resonance experiments were performed on Co2+ substituting for Zn2+ or Mg2+ in powder samples of Zn-2(OH)PO4 and Mg-2(OH)AsO4. These two compounds are iso-structural and contain the Co2+ in two environments with approximately octahedral and trigonal bipyramidal structures. The observed resonances are described using a theoretical model that considers the departures from the two perfect structures. It is shown that resonance in the penta-coordinated complex is allowed, and the crystal fields that would reproduce the resonance of the Co2+ in the two environments are calculated. The low intensity of the resonance in the penta-coordinated complex is explained by assuming that this site is much less populated than the octahedral one; this assumption was verified by a molecular calculation of the energies of the two environments, with both Co and Zn as central ions in Zn-2(OH)PO4.