The low-temperature photoluminescence (PL), photodiffusion current, photoconductivity (PC), and nuclear quadrupolar resonance spectra of the layered PbI2 single crystals and Pb1−XCdXI2 alloys were investigated. It was shown that the alloys are formed for any composition ratio and they have monopolar (hole) PC. The exciton PL band energy depends linearly on the composition ratio. It was found that the PbI2 clusters with various sizes are formed in Pb1−XCdXI2 alloys. For X≤0.50 the clusters are large [more than the Bohr radius of exciton in PbI2 (RB)] while for X=0.50 they are both large and small (smaller than RB). It was shown that these clusters are formed within the crystal layer. For Pb1−XCdXI2 alloys with X=0.70 the broad PL band at 2.1 eV is observed. This band is caused by the formation of the self-trapping excitons in PbI2 clusters. It was found that this PL band determines the emission of the alloys at T=300 K.

The critical temperature dependence of the nuclear quadrupole-resonance 79 Br frequencies at the phase transition (71 = 163 K) was found in (CH3)4NCdBr3. It was concluded that bromine nuclei take part in the phase transition not directly but via the interaction with the crystal polarization.PACS numbers: 64.70.Kb Tetramethylammonium cadmium tribromide, (CH3)4NCdBr3 (TMCB), belongs to the compousss of the type (CH3)4NMX3 (M=Mn, Ni, Cd; X=Cl, Br, I), which also includes such widely investigated crystals as (CH3)4NMnCl3 (TMMC) and (CH3)4NCdCl3 (TMCC). The peculiarity of their stucture is the existence of infinite chains (... M-X3-M-X3 ...) separated by tetramethylammonium ions. These ions are orientationally disordered at room temperature [1,2]. At low temperatures these compousss are characterized by the structural phase transition of the order-disorder type, accompanied by ordering of the tetramethylammonium ions [3,4].At room temperature the TMCB crystal is isomorphous to the above-mentioned compounds and the first-order structural phase transition occurs at the temperature T = 163 K [5]. However, the NMR investigations did not reveal any anomalies in the temperature dependence of the proton spin-lattice relaxation time at 163 K [6]. This phenomenon may be explained by the fact that in this temperature range the relaxation mechanism is due to fast reorientation of the CH3-groups which masks slower motions connected with the change of stucture at the phase transition.(271)

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Investigations of temperature dependence of a nuclear quadrupole spinlattice relaxation time of 79 Βr nuclei in ferroelectric (CH3) 4 NCdΒr3 are ρre-sented. It is shown that in paraelectric phase iii the vicinity of phase transition temperature relaxation process is determined h ey critical fluctuations of electric field gradient. In ferroelectric plase relaxation is related to three dicherent mechanisms, namely: lattice vibrations, CH3--group reorientations and TMA cation motions. According to the results of measurements it is concluded that the phase transition is connected with ordering of TMA cations. PACS numbers: 64.70.KbTetramethylammonium tribromocadmate (CH 3)4NCdBr3 (TMCB) belongs to the family of crystals with a general chemical formula (CH3)4NMΧ3 , where Μ = Mn, Ni, Cd; Χ = Cl, Br, I. Among them (CH 3)4NMnCl3 (TMMC) crystal has been widely studied as quasi-one-dimensional magnetic system with infinite chains of -Mn-C13-Mn-Cl3-separated by tetramethylammonium cations (TMA) [1,2]. Similarly as in the case of TMMC isomorphous crystals of (CH 3 )4 NCdC13 (TMCC) and (CH3) 4 ΝΝiCl3 (TMNC) exhibit structural phase transitions from hexagonal high-temperature phase to monoclinic low-temperature phase [3,4]. It was shown that in higl-temperature phase of these crystals TMA cations are disordered [2, 5] and order-disorder phase transitions are related to the ordering of TMA cations and small shifts of the metal-halogen chains [4]. TMCB crystal is another member of this family and undergoes a stuctural phase transition of the first order from hexagonal room temperature phase (space group: P6 3 /m) to another hexagonal phase (space group P61) and disordered states of TMA cations in paraelectric and ordered in ferroelectric phase are found in it [6][7][8]. The low-temperature phase exhibits ferroelectric properties along the c-axis. Spontaneous polarization is equal to 1.2 x 10 -3 C/m2 at 125 K [9]. NMR investigations of molecular dynamics in (357)

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