K3CrO8 in K2NbO8 as a proposed standard for g-factor, spin concentration, and field calibration in high-field EPR spectroscopy

Currently there is a lack of an internal standard for EPR spectroscopy empl oying magnetic fields higher than about 3 T. It is here suggested that the solid solution of 0.5% K3CrO8 into K3NbO8 fulfills many of the requirements of such a standard. Its main EPR signal is due to a 3d(1) electron and con sists of a single sharp peak for single-crystal samples. The peak width ran ges from 1.5 G (0.15 mT) at the X-band (B-0 = 0.34 T) to 4 G (0.4 mT) at 33 0 GHz (B-0 = 11.8 T). The g-values have been determined as g(perpendicular to) = 1.98508 +/- 0.00005 and g(parallel to) = 1.94272 +/- 0.00005. This hi gh degree of precision is necessary because EPR work at high frequencies ca n resolve g-tensors to six or seven significant figures. Additionally, K3Cr O8 in K3NbO8 provides a field calibration standard via a weak but well-reso lved hyperfine structure from its Cr-53 isotope (I = 3/2, natural abundance 9.5%): A(parallel to) = 39.55 +/- 0.08 G, A(perpendicular to) = 11.49 +/- 0.05 G. Variable-temperature and variable-frequency studies demonstrate tha t this sample can serve as a reliable standard for EPR measurements over th e range of at least 5-300 K and 9-330 GHz, The sample can be prepared in a simple, reproducible way and can serve also as a standard for absolute spin count, thus making it unique among the other standards in current usage.