Electronic Excitations in NiCl$_{2}$ Hydrates

We measured the temperature-dependent optical absorption spectra of pure polycrystalline NiCl$_{2}\cdot $2H$_{2}$O between 4,000 and 35,000 cm$^{-1}$ and of powdered NiCl$_{2}\cdot $2H$_{2}$O and NiCl$_{2}\cdot $H$_{2}$O (pressed into KBr pellets) in more limited frequency regions. NiCl$_{2}\cdot $2H$_{2}$O has a known complex phase diagram with distinct antiferromagnetic (AF) phases in successively lower temperature regions. It has a rich low temperature spectrum consisting of parity forbidden spin allowed and spin forbidden $d-d$ excitations of the pseudo-octahedrally coordinated Ni$^{2+}$ ion with fine structure due to vibronic sidebands, magnetic dipole allowed transitions, and possible electron-magnon coupling. Although most NiCl$_{2}\cdot $2H$_{2}$O bands show a typical gradually increasing definition with decreasing temperature, a narrow absorption at 12,600 cm$^{-1}$ is strongly suppressed below the lowest AF transition at 6.3 K. The magnetic properties of NiCl$_{2}\cdot $H$_{2}$O are more poorly understood, involving unconventional low dimensional behavior. Although the pressed pellet spectra reveal a loss of detailed spectral information, a downshift of $\sim $ 300 cm$^{-1}$ in the second spin allowed band and associated spin forbidden transition can be discerned for the monohydrate compared to the dihydrate.