Electronic absorption spectra of Ni2+in NiCl2and NiBr2. (Phonon and magnon sidebands)

Abstract

The optical absorption spectra of the antiferromagnetic insulator NiCl₂ and NiBr₂ single crystals have been measured between 5*10³ and 33*10³ cm^-1, and from 300 to 4.2 K. The electronic spectrum of Ni²⁺ in these materials has been analysed using the spin-orbit matrices for a (3d)⁸ system and the parameters of the crystal field have been derived. It is found that Dq=692 cm^-1, lambda =:140 cm^-1 and F₄=91 cm^-1 for NiCl₂, and Dq=680 cm^-1, lambda =-390 cm^-1 and F₄=85 cm^-1 for NiBr₂. A detailed discussion of all the bands of the spectra and comments on the interesting structures of vibronic and magnonic origin of the ³T_1g^(a) band are given. The absorption peaks located around 16.6*10³ cm^-1 in NiBr₂ are not easily interpreted but their puzzling origin is discussed.