Theoretical and experimental investigation of the electron spin resonance of Co²⁺in Zn₂(OH)PO₄and Mg₂(OH)AsO₄

Abstract

Electron spin-resonance experiments were performed on Co 2+ substituting for Zn 2+ or Mg 2+ in powder samples of Zn 2 (OH)PO 4 and Mg 2 (OH)AsO 4 . These two compounds are iso-structural and contain the Co 2+ 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 pentacoordinated complex is allowed, and the crystal fields that would reproduce the resonance of the Co 2+ 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)PO 4 .

Study of Co2+ in different crystal field environments

“…We found that for the system parameters obtained from the optical spectra we should expect that the ground doublet be M J = ±1/2, corresponding to an allowed spectrum. The rather small intensity, of this type of spectra seems to indicate a preference of Co 2+ for the octahedral sites in the crystal structure, a conjecture that was advanced in a preliminary report [32] on the ESR of impurities of this ion in Mg 2 (OH)AsO 4 , and was confirmed in [3]. Employing a molecular calculation we have verified in the last reference that the formation energies of the two type of complexes, with both Co and Zn as the central ions, are compatible with this hypothesis.…”

“…The average of the g factors of the intense spectra is 4.15 in the two compounds, a value close to the 4.33 expected for Co 2+ in moderately distorted octahedral symmetry [29], and it seems reasonable to assign these spectra to that environment and apply the same approach employed in the previous section in the study of the ESR of Co 2+ in NH 4 NiPO 4 .6H 2 O (see also [2]). The remaining lines are very weak and we assign them to the penta-coordinated symmetry (the possible reason for their small intensity has been discussed in [3]). …”

“…The expression given in reference [2] for ∆ is not correct, and should be calculated as the difference between the level E( 4 T 2 ) and the ground level E( 4 T 1 ). The ground E( 4 T 1 ) level is the lowest eigenvalue of the matrix −15B − 6Dq 4Dq 4Dq 0 (3) and the coefficients of the corresponding eigenvector give the a and b employed above to calculate the coefficient α of the angular momentum. The level 4 T 2 is given by…”

“…A similar treatment of the Co 2+ in sites with deformed octahedral symmetry of Co 2 (OH)PO 4 or Co 2 (OH)AsO 4 has been considered in [3], but it is less interesting because only the powder samples of these compounds have been measured. The treatment of this problem shall not be further discussed in the present work.…”

“…Here we shall only briefly consider the deformed octahedral symmetry, and give more details of the theory describing the ESR of Co 2+ in the penta-coordinated environment, that was partially discussed in [3]. This last complex is a distorted trigonal bipyramid, and we first calculated the crystal fields of the perfect trigonal bipyramid following the existing literature [30,31].…”