Electron–Nuclear Double Resonance of 53Cr3+ in Aluminum Alums

Electron–nuclear double resonance (ENDOR) studies are reported for the isotope 53Cr3+ enriched to 96% in the aluminum alums RbAl(SO4)2∙12H2O and CsAl(SO4)2∙12H2O. The measurements were made at 4.2 K and X-band microwave frequencies (~9.4 GHz). The values of the spin Hamiltonian hyperfine parameters A, B, and Q′ are found to be quite similar to those obtained previously in the corresponding gallium alums even though the zero field splitting parameter D is significantly different. A plot of A − B vs. Q′ indicates that the amount of distortion existing at a chromium site can be related to the type of monovalent ion present, i.e. Rb or Cs. It is possible to draw straight line curves relating the parameters Q′ vs. D, A − B vs. D, and Vzz vs. D which account quite well for all the ENDOR data obtained so far in the alums and GAISH and which could be generally true for chromium magnetic complexes in crystals with D values not too close to zero.