Infra-red absorption spectra, and their temperature dependence, were investigated with regard to isolated and impurity-coupled CN- defects. Two types of absorption line were observed, at 18K and above, in the energy region of the CN- stretching vibration. One was a single sharp line, that was due to isolated CN- defects, that appeared at 2098.2, 2093.6 and 2064.2/cm in CdCl2, CdBr2 and CdI2, respectively. With increasing temperature, these lines broadened and shifted to higher energies. On the high-energy side of these isolated CN- defect lines, extra lines were observed at positions which depended sensitively upon the metal ion doping. After monovalent Na+ doping, a single sharp line appeared (at 2133.4/cm in the case of CdBr2, for instance). This exhibited a marked broadening, but a very small energy shift, with increasing temperature. After divalent Hg2+ doping, 2 sharp lines of almost equal strength appeared (at 2127.6 and 2181.2/cm in the case of CdBr2). These broadened markedly and shifted to lower energies with increasing temperature. The lines were interpreted qualitatively as being due to a CN- defect that formed a pair, or complex, with a monovalent or divalent impurity cation. In the case of the Hg2+-NC- and Hg2+-CN- pair centers, the CN- ion was assumed to make a rather strong bond with the Hg2+ ion, due to the extended d orbit of Hg2+ and symmetry breaking due to the presence of Hg2+. The temperature dependence of the peak shift of the absorption lines near to 2127 and 2181/cm could be simply explained in terms of thermal lattice expansion. In the case of the Na+-CN- center, it was pointed out that the monovalent Na+ ions (acting - in effect - as negatively charged defects) introduced a strong Coulomb field into their surroundings, but also required - in order to maintain lattice neutrality - the introduction of charge-compensating positive defects. These were expected to be probably anion vacancies; which also had to be considered as possible CN- partners. A complicated behavior of the peak shift of the 2133/cm line, and its very small temperature coefficient, reflected the participation of the anion vacancy in the Na+-CN- center.

H.Nakagawa, J.Igarashi, F.Luty: Materials Science Forum, 1997, 239-241, 481-4