Temperature independent isotropic EPR spectra of [(CH3)(4)N](2)MnCl4 and [(CH3)(4)N](2)FeCl4 single crystals

Electron paramagnetic resonance of [(CH3)(4)N](2)MnCl4 and [(CH3)(4)N](2)Fe Cl4 Single crystals was studied between 20 and 400 K. The peak-to-peak deri vative linewidths of these crystals seem not to change in this temperature interval and approximately 100 mT for [(CH3)(4)N](2)MnCl4 and similar to 20 mT for [(CH3)(4)N](2) FeCl4. The spectra were found to be isotropic, with g = 2.0039 for [(CH3)(4)N](2)MnCl4 and g = 2.0042 for [(CH3)(4)N](2)FeCl4. This temperature independence is attributed to isotropic strong exchange in teractions of Mn2+ and Fe2+ nuclei, and it seems that hindered rotation of the MnCl42- and FeCl42- tetrahedra does not occur in this temperature inter val.