Verum est index sui et falsi

Various compounds may be used as standards in high-frequency EPR spectroscopy:

  • Mn2+ in MgO was proposed as standard suitable for the g-factor scaling at high-field EPR experiments [1] and remains the favorite standard in many labs using millimeter waveband EPR technique. Besides, it can be used as effective standard for calibration of an external magnetic field. In contrast to low fields the ubiquitous six line spectrum has very sharp lines at high fields due to the reduction of second order effects and effectively provides a field calibration over a 400 G range (5 distances between lines splitted by 87 G as hyperfine constant). In practice, a surprisingly large number of samples contain Mn so it is frequently possible to use it as a natural insite standard at high fields.

  • A,A-Diphenyl-B-picrylhydrazyl (DPPH) seems to be the best known standard due to its extremely intense line. However, its use at high fields [2] has been hampered by the fact that the g-factor depends on the solvent used in the preparation and, even then, different crystals within the same preparation are sometimes found to have g-factors varying within 2.0030 - 2.0043 range. Nevertheless, it is possible to found small (nano) DPPH crystals with narrow single line and to use them to calibrate other standards.

  • Organic conducting ion-radical-based single crystals of lowed dimensionality [3]. One of them, (fluoranthene)2PF6, have extremely narrow peak-to-peak linewidth (ΔBpp ≤ 50 mG at 95 - 140 GHz) and the value of its g-factor is known to a high degree of accuracy, gxx = 2.00226, gyy = 2.00258, gzz = 2.00222 (needle axis). The other ion-radical-based single crystals, (naphthalene)2PF6 and (perylene)2PF6, demonstrate solitary EPR spectrum with respectively ΔBpp = 1.8 G, g = 2.00316 and ΔBpp = 1.5 G, g = 2.00321. This allows them to be easily recognized and differentiated by adjusting the magnitude of the modulating field in the study of most systems. Besides, it can be used for determination of relaxation parameters using CW saturation method. Note, however, that it cannot be used at temperatures much below 200 K due to a phase transition. If one keeps the sample at high temperature and air presence it is also slowly deteriorates during with time (over periods of many days. More stable (dibenzotetrathiafulvalene)3PtBr6 is characterized by strong EPR line with ΔBpp = 4.4 G and g = 2.01628. It was shown that this samples being placed in condensed system can be reoriented in high magnetic field, so then its line is shifted by around 280 G. This effect may be successfully used for the study of physical properties of a matrix [3].

  • Isolated paramagnetic vacancy VSi- in 3C-SiC exhibits a narrow, near 0.5 G, strong and stable resonance EPR signal observable within 1.2 - 300 K temperature region and does not show any ageing effect [4]. The samples can be provided by Prof. P.G. Baranov and Dr. E.N. Mokhov of the A.F. Ioffe Institute in St. Petersburg, Russia.

  • 1,3-bis-Diphenylene-2-Phenyl Allyl (BDPA) free radicals embedded to polyethylene [5]. High spin concentration and, therefore, fast spin relaxation make it as suitable standard at relatively low temperatures. However, it is characterized relatively broad (near 10 G) EPR spectrum which may interfere with g = 2 spectra.

  • KC60 metallic fullerene crystalline polymer has the advantage of a temperature independent intensity, linewidth near 2 G and resonance position within wide temperature region [6]. It was stated that this material is a suitable standard in a large temperature range (from 3 to 300 K) and frequency (tested up to 225 GHz) ranges. It has temperature independent magnetic spin susceptibility and only small variations in the linewidth with temperature and frequency. The material is inert; it does not degrade under atmospheric conditions. It has a large magnetic susceptibility and can thus be used in low sensitivity apparatus. The spin-lattice relaxation rate is fast at low temperatures thus saturation problems are not important in continuous wave (CW) study of solid-state samples, especially organic and classic metals. Its usefulness by measuring the susceptibility of a newly synthesized two dimensional fulleride polymer, Mg4+xC60 was demonstrated.

  • P@C60 or N@C60 (C60 fullerene molecule with encapsulated 31P or 14N atom) embedded in C60 crystalline matrix is characterized by extremely narrow (less than 0.1 G) linewidth [7]. It demonstrates a characteristic P dublet (aiso = 49.4 G) or N triplet (aiso = 5.665 G) spectrum with no additional fine structure splitting at ambient temperatures due to fast rotation of the molecule. At lower temperatures a rotation is freezed and a spin-lattice relaxation becomes longer but fine structure splitting remains small. It is not sensitive to exposure in air. It may be used as a g-factor standard as well. It was announced that at the request of the Ecole Pollytechnique Federale de Lausanne (Switzerland) a high precision g-factor standard, endohedral fullerene, N@C60 can be synthesized and delivered.

  • Polyaniline (PANI), polypirrole (PP), and some other organic conducting polymers with lowed dimensionality [8]. Some samples are not sensitive air and also may be used as a g-factor standard at millimeter waveband EPR.

  • Li:LiF characterized by narrow (less than 1 G) solitary EPR spectrum with g = 2.002293 [9] may also be used as high-field EPR standard. Major disadvantage is line shape distortions dependent on sample quality. In case of a tunable source and resonator calibration of the interesting field range is often possible, e.g. with the tuning range of about 800 MHz of the BRUKER E680 94 GHz spectrometer a field range of about 280 G can be covered which is well sufficient for typical organic radicals. There is a very good reproducibility of the field measurements on repetitive removal of the Li:LiF sample and reinsertion into the probe head with a typical deviation of the line position in the range of 0.1 G. This allows the use of this standard for calibration before and/or after the measurement of interest as long as the reproducibility of the field sweep is sufficient. So, Li:LiF may also be used for g-calibrations at millimeter waveband EPR.

[1]  V.I. Krinichnyi, J. Biochem. Biophys. Meths., 23 (1991) 1 Abstract [  Full text 520 kb].

[2]  S.V. Kolaczkowski, J.T. Cardin, and D.E. Budil, Appl. Magn. Reson., 16 (1999) 293.

[3]  V.I. Krinichnyi, Ya.S. Lebedev, and O.Y. Grinberg, Appl. Magn. Reson., 13 (1997) 259 Abstract [  Full text 125 kb].

[4]  S.B. Orlinski, J. Schmidt, E.N. Mokhov, and P. G. Baranov, Phys. Rev. B, 67 (2003) 125207.

[5]  R.A. Wind, et al., J. Magn. Reson., 143 (2000) 233.

[6]  P.R. Surjan, Int. J. Quantum Chem., 63 (1997) 425.

[7]  W. Harneit, Phys. Rev. A, 65 (2002) 032322.

[8]  V.I. Krinichnyi, Synth. Met., 108 (2000) 173 Abstract  [  Full text 1060 kB].

[9]  A. Stesmans and G. van Gorp, Rev. Sci. Instr., 60 (1989) 2949.