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

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 C_s 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 ⁶A_1g(⁶S) ←⁴T_1g(⁴G) in the (CH₃)₄NMnCl₃ complex.