The temperature dependences of the NQR frequencies of halides in layer-type crystals of CdBr,, Cd12(4H), Pb12(2H, 4H) are studied within 10 to 300 K temperature range. The NQR frequency grows weakly with temperature in CdI, and CdBr,, whereas it decreases in PbI,.For PbI, the frequency depends linearly on the temperature beginning at T 2 20 K. The difference of the NQR frequencies of nonequivalent nuclei in (4H) crystals corresponds to the calculated differences of the ion contributions to the gradient, while the sum of the calculated ion and covalent contributions does not account for the observed values of the NQR frequencies. This discrepancy is suggested to arise from the strong polarization of the covalent bonds by the internal electric field in the layer crystals. The growth of the NQR frequency with increasing temperature in CdI, and CdBr, is shown to be due to the fact that in these crystals the ionic and covalent contributions t o the gradient are of opposite sign. I n PbI, the temperature dependence of the NQR frequency is associated with the peculiarities of the phonon spectrum of the layer crystals. Formulation of the Problem and Experimental ResultsNuclear quadrupole resonance (NQR) spectra have been reported in a number of layer-t,ype crystals and, in particular, in CdBr, and Cd12(4H) [l, 21 (hereafter, the abbreviation in brackets stands for the crystalline modification of compounds showing polytypism [3]). Although there is an essential ionic bonding in such crystals, the point charge model was shown to be inadequate for calculating the observed NQR frequency values and the covalent contribution to the intracrystalline field gradient was estimated [ 11. The temperature dependence of NQR frequency was not investigated previously in detail for layer-type compounds.I n paper [4] it was shown that layer-type crystals feature a specific spectrum of acoustic phonons, in particular, "bending" vibrations causing some peculiar-1) Prospekt Nauki 144, 252650 Kiev-28, USSR.

The luminescent properties of Mn-activated CdCl 2 , CdBr 2 , CdI 2 , and CaI 2 crystals are studied in the range 4.2-200 K. The photo-and roentgenoluminescence spectra of the crystals show red emission bands due to the Mn activator. The excitation spectra and Mn luminescence kinetics are interpreted in terms of the crystal-field theory with consideration for electron paramagnetic resonance data. The red luminescence of the crystals at 4.2 K is attributed to the 4 T 1 g ( 4 G ) 6 A 1 g ( 6 S ) electron-phonon transitions of Mn 2+ . The Racah parameters B and C are determined. The origin of the 600-nm luminescence in CdBr 2 :Mn 2+ under excitation with a nitrogen laser and xenon lamp at 320-360 nm in the range 4.2-90 K is discussed.

Results of comprehensive research into optical and luminescent-kinetic characteristics of europium-doped cadmium iodide crystals excited by nitrogen laser radiation, α-particles, and x-rays are presented. Crystals under study have been grown by the Bridgman-Stockbarger method. The doping EuCl 3 admixture was introduced into the charge in quantities of about 0.05 and 1.0 mol%. Impurity absorption detected in the near-edge region of the crystals is interpreted as part of the Eu 2+ ion long-wavelength band associated with f-d-transitions. The cation impurity and matrix defects in CdI 2 :Eu 2+ crystals create complex centers responsible for emission with a maximum in the 580-600-nm region. The short component in the luminescence decay kinetics of weakly-doped crystal excited by α-particles and x-ray photons is due to the exciton emission characteristic of CdI 2 . The slow component in the scintillation pulse results from recombination of charge carriers followed by creation of exciton-like states on the defect-impurity centers. Laser or x-ray excitation induces light-sum accumulation on the trapping levels at a depth of 0.2-0.6 eV that is mainly related to matrix microdefects. Trapping centers associated with the chlorine impurity are observed in the heavily-doped crystal. Photostimulated luminescence at 85 K arising at the electron stage of the recombination process is caused by recombination of electrons released from F-type centers with holes localized near the activator.

No abstract