Polarized Infrared Absorption of Dipole Centers in Cadmium Halide and PbI₂ Crystals

Abstract

Polarized infrared (IR) absorption measurements on CN⁻ or OH⁻ centers in cadmium halide and PbI₂ crystals were carried out at 6 K with a high spectral resolution of 0.025 cm⁻¹ at 2000 cm⁻¹ by using a FTIR spectrometer. Several sharp absorption lines with widths less than 0.1 cm⁻¹ are observed in the energy region of the stretching vibration, i.e. 2000 to 2250 cm⁻¹ for CN⁻ and 2500 to 4500 cm⁻¹ for OH⁻. These lines are classified into several groups attributed to (1) an isolated center simply substituted for a halogen ion, (2) an interstitial center located between the cadmium and halogen ion sheets and (3) a coupled center with an anion vacancy or a host metal ion. Almost all of the dipole axes (bond axes) of the CN⁻ ions doped in MI₂ (M = Cd or Pb) are parallel to the crystal c-axes, while those of the isolated and coupled CN⁻ centers in CdX₂ (X = Cl or Br) lean away from the direction of the c-axis. The most OH⁻ ions doped in CdX₂ (X = Cl, Br or I) and PbI₂ are arranged in the halogen-ion planes with their dipole axes parallel to the crystal c-axes. The first overtone yields values of χ_e and ω_eχ_e for CN⁻ and OH⁻ in CdX₂ and PbI₂. These values explain well the isotope shift of the main stretching band in CdX₂ and PbI₂.