Phase transitions in melt-grown PbI2 crystals

Employing the method of zone- refining single crystals of PbI₂ of different purities were grown. Using X-ray diffraction technique their structure was found to be of the type 12R and in some cases 12R+4H. The crystals were re-examined after prolonged storage of several months at room temperature. Neither the crystals of very high purity nor of relatively less purity transformed. But thecrystals having intermediate purities did transform from 12R to 2H. This observation establishes a clear link between impurities and phase transitions. The results have been explained on the basis of nucleation and growth mechanism of stacking faults. Also it has been concluded that the presence of impurities is a must for the formation of polytypes.     

Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.     

Luminescence from Ti2+ ions in LiF crystals

Luminescence of Ti²⁺: LiF crystals in the wavelength range 650–900 nm excited using laser radiation is reported. The values of 10Dq and the Racah B parameter obtained from the present results indicate that in LiF the ¹ E, ¹ T ₂, and ³ T ₂ energy levels are almost at the cross-over point in the Tanabe-Sugano diagram. In consequence photoluminescence from all three energy levels to the ground state is observed. At low temperature (T 14 K) the emission is mainly due to sharp emission lines with weak vibronic structure due to the ¹ E, ¹ T ₂³ T ₁ transitions of Ti²⁺ ions. The ³ T ₂³ T ₁ broad-band emission is weak at low temperature, becoming stronger at 300 K due to a phonon-assisted tunnelling process, similar to that observed in Cr³⁺: garnets [1]. The excitation spectrum of these lines is a broad peak at 590 nm due to absorption in the ³ T ₁³ T ₂ transition.     

Luminescence and electrical conductivity of KCl-PbCl2 crystals grown from aqueous solutions

Luminescence spectra of KCl-PbCl₂ crystals grown from aqueous solutions are compared with those of KCl-PbCl₂ crystals grown from a melt. It is found that at PbCl₂ concentrations C > 3·10^–3 mole % there appears in the luminescence spectrum of solution-type crystals a new band with a peak at the 365-nm wavelength, which has never been noted in the case of melt-type crystals. The appearance of this band is attributed to (PbCl_n)^–(n–2) complexes entering the crystals when the latter are grown from solutions. The concentration characteristics of ion conductivity are analyzed here and seem to confirm the hypothesis of this complex formation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 31–35, August, 1974.     

Luminescence decay in CdIn2S4 single crystals

Summary An experimental study of the luminescence decay in CdIn₂S₄ at 80 K is reported. Time-resolved spectra and luminescence decay curves have been measured in the time interval (10÷10⁵) ns, using different excitation energies. A preliminary analysis of the obtained results provides new information on the recombination processes of photoexcited carriers in CdIn₂S₄, supporting a previously published model. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Luminescence decay in AgCl crystals

The kinetics of luminescence decay of single crystal plates of AgCl was measured at the temperature of liquid nitrogen. Luminescence decay first takes place (fort≦2·5× ×10^?3 sec) according to a hyperbole and then according to an exponential. The constantsa anda of the hyperbolic andt of the exponential dependence were measured for different intensity of the exciting radiation in normal and deformed samples and in samples irradiated withb-particles during measurement.     

Luminescence of mercuric iodide crystals

Low-temperature (4.2–130 K) photoluminescence spectra of HgI₂ crystals have been measured in the 540–700 nm region. An analysis of the characteristics (intensity vs temperature and excitation power relations, afterglow times, excitation spectra) of the 560, 620, and 635 nm emission bands suggests the following assignments: the 560 nm band is due to radiative annihilation of excitons bound to mercury vacancies, and the “red” emission originates from recombination of free (620 nm) and donor-localized (635 nm) electrons with a hole-filled acceptor level. The energies of the corresponding donor and acceptor levels have been estimated. New emission bands at 540, 545, and 575 nm have been discovered, and their origin discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 67–73 (January 1997)     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)     

Luminescence centers in SnO2 single crystals

The luminescence spectrum observed in SnO₂ has been analyzed based on the vibroelectronic model. However, additional measurements of the photoconductivity and the spectral distribution of recombination probability cannot be explained by this model, which only takes into account strong phonon coupling. Arguments are presented in favor of Birman's model which is more consistent with all the present results.     

Excitonic emission of PbI2 from nitrogen laser excitation

Strong excitonic emission from PbI₂ has been observed to result from high intensity illumination with a nitrogen laser. Spectroscopic examination has shown both a shift and broadening of the excitonic band suggestive of excitonic interactions.