Polarized Infrared Absorption in CdI2:CN-Crystals

Abstract

Cadmium halides (CdCl2, CdBr2 and CdI2) crystallize in a layered structure in which a Cd2þ-ion plane is sandwiched between two halogen ion planes. The fundamental layers pile up along the crystal c-axis to make up cadmium halide crystals. When doping a small amount of metal cyanide into these cadmium halide crystals, cyanic ions are supposed to be substituted for the halogen ions. In the present work, polarized infrared (IR) absorption has been investigated on CN centers doped in CdI2 crystals. For interpretation of these IR absorption, interaction of the CN elastic dipole with the local electric field in the crystal must be taken into account: A strong local electric uniaxial crystal field along the c-axis could arrange the CN molecular axis along this axis of the crystal. On the other hand, IR absorption and luminescence on CN centers in alkali halide crystals have been studied extensively without any indication of such arrangement of the dipole centers. The crystals were grown from melt in a vacuum sealed quartz ampule filled with CdI2 powder containing impurity Hg(CN)2 of 0.1mol% as an impurity. The CN -ion would substitute for a host I -ion. The specimens were cleaved in the size of 10 10 5mm from the CdI2 crystal ingot. The concentration of Hg(CN)2 in the specimens was not determined in the present study. The values of concentration in the following are those in the melt. The crystal samples were mounted on a copper holder of a closed-cycle He refrigerator and were cooled down to 6K. The present experiments were performed at the absorption and reflection spectroscopy station on the beamline BL43IR of the synchrotron radiation facility SPring-8, Hyogo, Japan. Polarized IR absorption spectra were measured by using a FT-IR spectrometer (Bruker IFS 120HR). The spectral resolution of this spectrometer was set for 0.025 cm 1 at 2000 cm . An internal globar lamp of the FT-IR spectrometer was used as the light source. Figure 1 shows the IR absorption spectra of CdI2:Hg(CN)2 (0.1mol%) in the range of 2060–2160 cm 1 for the polarization parallel (E k c) and perpendicular (E ? c) to the c-axis. Polarized light with E k c or E ? c was led on the polished crystal surface along the direction perpendicular to the crystal c-axis (E is the electric field vector of polarized light). The interference pattern due to BaF2 windows of the cryostat appears on the background of both spectra. Dichroism between the E k c and E ? c spectra is remarkable: All absorption peaks are observed only in the polarization of E k c. As described above, the CdI2 crystal has the uniaxial crystal field in the direction along the c-axis. The dichroic absorption results show that the CN -ions in the I -ion planes are arranged with their electric dipole axes parallel to the crystal c-axis. In addition, the full width at half-maximum (0.07 cm ) of the absorption lines is much narrower than those ( 5 cm ) in alkali halides. The rotational motion of CN molecules is restrained by the local uniaxial electric field along the c-axis in the CdI2 crystal. The local uniaxial electric field at halogen ion sites comes from Cd2þ-ion planes with positive charge and I -ion planes with negative charge. The absorption peaks are classified into two groups. The one is of sharp doublet peaks (2082.1, 2084.8 cm ) in the low energy region. These peaks are attributed to CN -ions simply substituted for host I -ions, which are called Isolated-CN centers [see Figs. 2(a) and 2(a0)]. They correspond to the absorption peak due to substitutional CN ions for I -ions in alkali-iodides, e.g., the peak observed at Fig. 1. The IR absorption spectra of CdI2 doped CN measured at 6K. Upper and lower spectrum is E k c and E ? c, respectively.