Optical Studies of Electronic Transitions in Hexa- and Tetracoordinated Mn2+ Crystals

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Abstract
The polarized absorption spectra of Mn2+ in (CH3)4NMnCl3 and [(CH3)4N]2MnCl4 have been studied at 298°, 77°, and 2°K. Marked optical anisotropy was observed in both complexes. Details of the spectra indicate that the surroundings of the hexacoordinated Mn2+ ion are trigonally distorted from octahedral symmetry. Unlike the chemically similar alkali metal manganese halides, (CH3)4NMnCl3 undergoes no paramagnetic—antiferromagnetic transition between 298° and ≈4°K. Abnormal intensities in the spectrum appear to be unrelated to exchange mechanisms. The surroundings of the tetracoordinated Mn2+ ion very nearly approximate tetrahedral symmetry. However, the true Cs site symmetry is evident in the very sharp line absorption near 23 000 cm−1. The polarization behavior of the ion in both coordinations is discussed qualitatively in terms of the selection rules for the appropriate double group. The green luminescence of the tetrahedral complex and the pink luminescence of the octahedral complex have also been measured. The emission is shown conclusively to involve the transition 6A1g(6S) ←4T1g(4G) in the (CH3)4NMnCl3 complex.