Paramagnetic resonance, magnetic susceptibility, and antiferromagnetic exchange in a Cr5+ paramagnet: Potassium perchromate (K3CrO8)

Abstract

EPR and magnetic susceptibility of potassium perchromate (K3CrO8) have been studied from liquid helium temperatures to 400 K. EPR spectra of dilute aqueous solutions of K3CrO8 yielded giso=1.9712±0.0005, and 53Aiso=(18.6±0.5)×10−4 cm−1. The (positive) sign for 53Aiso, deduced here from the dependence of the linewidths of hyperfine components on nuclear spin quantum numbers, constitutes the first direct experimental measurement of the (negative) sign of the unpaired electron spin density at a Cr5+ nucleus. EPR spectra of powdered K3CrO8 doped in (diamagnetic) K3NbO8 and of K3CrO8 crystals yielded g∥=1.9428±0.0005, g⊥=1.9851±0.0005, A∥(53Cr)=+(36.3±0.5)×10−4 cm−1 and A⊥(53Cr)=+(11±1)×10−4 cm−1, the ∥ direction coinciding with the crystal c axis. These parameters establish that the unpaired electron orbital is the metal dx2–y2 orbital, with considerable charge transfer from the peroxy ligands. In concentrated samples, the EPR line shape is Lorentzian, providing direct evidence for electron spin exchange between Cr5+ ions. Temperature dependence of magnetic susceptibility χ over 1.4 to 343 K follows the law χ(cm3/mol)=[2.7933/(T+2.7)]+χd, where χd=−1.5×10−4(cm3/mol) is estimated to be the diamagnetic contribution. From the Curie–Weiss temperature of 2.7 K, an exchange coupling J=1.35 K between the Cr5+ ions is estimated. The calculated exchange-narrowed EPR linewidth is consistent with the experimental value of 15 G observed for K3CrO8. No magnetic or structural phase transition was detected in the range of 1.4 to 400 K; hence K3CrO8 crystals can serve as useful internal standard for spin concentration measurements over a wide temperature range, especially for free radicals.