Electron spin resonance studies of 53Cr3+ and VO2+ ions in AlCl3∙6H2O

Abstract

The electron-nuclear double resonance (ENDOR) of 53Cr3+ ions and the electron spin resonance (ESR) of VO2+ ions are studied in single crystals of AlCl3∙6H2O. The ENDOR measurements are done at 4.2 K while the resonance of VO2+ is studied at room temperature. The sign of the Cr3+ spin Hamiltonian zero field splitting parameter D is found to be negative while the anisotropic hyperfine parameter has the relation A < B. It is concluded that the octahedron of waters surrounding the chromium ion is trigonally compressed. The value of the 53Cr3+ quadrupole interaction parameter Q′ is nearly the same as that found previously in the cesium alums even though the D value is considerably smaller in AlCl3∙6H2O. This indicates that Q′ is independent of D at 4.2 K in these salts and the expected proportionality [Formula: see text], which is based on a purely static distortion for the octahedron of waters surrounding the Cr3+ ion, does not hold. The resonance results for VO2+ show the nature of the two inequivalent but otherwise similar Al∙6H2O complexes in the unit cell. The VO2+∙5H2O complexes associated with the two types of aluminum sites are found to be rotated relative to each other by an angle of 33 ± 3° in the plane perpendicular to the crystal c axis. A comparison of the spin Hamiltonian parameters of VO2+ and 53Cr3+ in AlCl3∙6H2O with those obtained previously in CsAl alum indicates that the Al∙6H2O complexes in CsAl alum are more susceptible to distortion upon impurity doping than are those in AlCl3∙6H2O.