ENDOR investigations of Cr⁺ centres in NaF, NaCl, and NaBr

Abstract

Isolated paramagnetic Cr+ centres in sodium halide crystals are studied by E P R and ENDOR. They show cubic symmetry. Superhyperfine and quadrupole interactions with many neighbouring ions (up to (300) in NaF) could be determined. The experimental results are interpreted in terms of two models for the wavefunction of the centre: overlap and covalency. A new way for the experimental determination of the coefficients for B-and x-covalency, using the isotropic superhyperfine interactions of neighbours (1 10) and (200), is proposed. The small anisotropic superhyperfine interaction of the nearest neighbours, which becomes even negative for Br-in NaBr, cannot be understood along these lines.Isolierte paramagnetische Cr+-Zentren in Natriumhalogenideinkristallen wurde mit ESR und ENDOR untersucht. Sie zeigen kubische Symmetrie. Superhyperfein-und Quadrupolwechselwirliungen mit zahlreichen Nachbarkernen (bis zum Kern (300) in NaF) konnten bestimmt werden. Die experimentellen Ergebnisse werden anhand von zwei Modellen fur die Zentrenwellenfunktion diskutiert : tfberlappung und Kovalenz. Daraus ergibt sich eine neue Methode zur experimentellen Bestimmung der Koeffizienten von B-und x-Kovalenz, indem man die isotropen Superhyperfeinwechselwirkungen der Nachbarn (1 10) und (200) heranzieht. Die kleine anisotrope Superhyperfeinwechselwirkung mit den nachsten Nachbarn, die fur Br-in NaBr sogar negativ wird, kann auf diese Weise nicht erklart werden.

ENDOR investigation of cubic Fe3+centres in NaCl single crystals

“…In a purely ionic model we can treat the d orbitals of the F$+ ion as an envelope function to interpret the SHF constants. As shown previously for 3d ions in alkali and alkali earth halides (Ziegler 1972, Studzinski et a1 1984. Cases et al 1987, orthogonalization of this envelope function to the s and p orbitals of the first shell of ligands yields values of the isotropic and anisotropic constants that are far.…”

Optical and Magnetic Resonance Studies of Transition Metal Ions and Complexes in Alkali Halides

“…This was attributed to the fact that the ionic radius of Li+ (0.78 A) is smaller than that of Mn2+ (0.91 A), while those of Na+ (0.98 d) and K+ (1. 33 A) are larger than that of Mn*+ (these are Goldschmidt radii). It was also noticed that the Mn2+-vacancy complexes are less mobile in LiCl than in NaCl and KC1.…”

“…Ziegler [33] made an extensive ENDOR investigation of Cr+ in sodium halides. H e determined superhyperfine and quadrupolar interactions involving many neighboring ions (up to (300) neighbors in NaF).…”

Investigations on the EPR Parameters of KMgF3:Cr+

“…They are normally obtained in terms of the cubic eld parameter Dq and the Racah parameters B and C for the 3d 5 ion in crystals [18,19]:…”