TlCl Crystals Research Articles

Preparation of optical media with NIR luminescent Bi+ impurity centers by ion exchange

AbstractSingle crystal of TlCl was doped with NIR photoluminescent univalent bismuth cations by prolonged immersion in liquid bismuth metal. The ion exchange Tl+ + Bi0 ↔ Tl0 + Bi+ at the crystal surface with subsequent Bi+ migration to the bulk are expected to drive the doping process. Contrary with Bi‐doped TlCl crystals, grown by Bridgman method, the ion exchange does not produce the additional nonluminescent bismuth‐containing centers. The investigation of photoluminescence emission and excitation spectra lead to the conclusion, that Bi+ is the main NIR emissive center in Bi‐doped TlCl.

Far-infrared response of acoustically modulated transverse optical-phonon polaritons

The authors propose a scheme to achieve strong modification of the light properties in the terahertz (THz) range and in particular up to 70% changes in the THz reflectivity of CuCl, TlCl and LiNbO3 crystals. This is realised by using transverse optical (TO) phonons as a mediator in the interaction between an acoustic wave (AW) and a THz light field, via the strong anharmonicities of the interatomic potential. Their numerical modeling of CuCl, TlCl and LiNbO3 crystals also predicts that these effects are tunable by applying various coherent AWs from sub-GHz to few GHz frequency. The length of the interaction between electromagnetic and acoustic fields is also greatly reduced compared to conventional acousto-optics. The modifications of the reflectivity spectrum are because of single and multiple intra-branch phonon transitions within the TO-phonon polariton dispersion branches.

Terahertz response of acoustically driven optical phonons

The manipulation of TO-phonon polaritons and the terahertz (THz) light field associated with them by means of an ultra-sound acoustic wave is proposed and illustrated by calculating the TO-phonon-mediated THz response of acoustically-pumped CuCl and TlCl crystals. We show the high-contrast acoustically-induced change of the THz reflectivity alongside with multiple THz Bragg replicas, which are associated with the infrared-active TO-phonon resonance driven by the ultrasonic wave. The effect, which stems from phonon anharmonicity, refers to an operating acoustic intensity I_ac ~ 1-100 kW/cm^2 and frequency nu_ac ~ 0.1 - 1 GHz, with possible applications in THz spectroscopy.

Porphyrin controlled anisotropic and dendritic growth of thallium chloride crystals

In this work, the morphology of TlCl crystals grown at the liquid-liquid interface between Tl2SO4 and HCl solutions was investigated in the presence of various porphyrin molecules. The addition of the tetrakis(4-sulfonatophenyl)porphyrin (TPPS2) molecules changed the TlCl morphology from spherical shape to single-crystalline strips with a diameter of 100-3,000 nm. The addition of meso-tetra(4-pyridyl)porphine (TPPyP) changed morphology of TlCl crystals into branching dendrites. The presence of the different porphyrins molecules in the crystallisation medium significantly influenced the morphology of the crystals. Crystals of TlCl with elongated rod-type or branching superstructures were nucleated with high aspect ratios depending on porphyrin type that was used. The composition and microstructure of dendrites was investigated by X-ray technique, SEM and atomic force microscopy (AFM). The wires were produced by elongation and stacking of planar TlCl crystals exhibiting (100) faces. The anisotropic growth behaviour causing the wire formation is tentatively ascribed to stepwise capping of the specific surfaces of TlCl crystals by porphyrin molecules.

Characteristic electron loss spectra and electronic structure of thallium chloride

The bands, state densities N(E), and permittivity are theoretically found for TlCl crystals. On the basis of the experimental characteristic electron loss spectra, the spectra of the complete set of optical functions including permittivity and bulk electron loss spectra are calculated and decomposed into components. The obtained data are compared with the well-known theoretical results.

Optical and structural properties of thallium halide microcrystals in a porous matrix

Thallium halide microcrystals were grown from water solutions in a porous matrix obtained from alkali borosilicate glass by etching, and their absorption and luminescence spectra were studied. Constraining the crystal size was found to affect the stability of some crystal modifications. The exciton radii were determined, and the dimensions of microcrystals in which size quantization effects become observable were estimated from the absorption spectra of bulk TlBr and TlCl crystals.

ESR of paramagnetic Tl2+-type centres in Rb2ZnCl4 crystals

Tl2+(6s 1)-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4: TlCl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

High Density Excitation Effect of the Indirect Band Luminescence in Thallous Halides*

A broad band luminescence is observed at the indirect gap region in highly purified TlBr and TlCl crystals under high density excitation. From the analysis of the line shape, power dependence, lifetime and some characteristic features of the luminescence, it is discussed the possibility that the luminescence is associated with an LA-phonon assisted indirect recombination (X 6 + -R 6 - ) of electron-hole liquid (EHL). Equilibrium density, binding energy and other parameters of EHL are estimated and it is presumed that the hole-phonon scattering is dominant in their recombination process.

Observation of hole trapped Tl 2+ centers in PbCl 2:TlCl crystals

Paramagnetic hole trapped Tl 2+ centers with orthorhombic symmetry have been identified by ESR spectroscopy in PbCl 2: TlCl crystals X-ray irradiated at 77 K. Changes in both ESR and optical absorption spectra were observed after pulse-annealing at room temperature. The resulting ESR spectrum and the absorption band at 335 nm are considered to belong to substitutional Tl 2+ ions at cation sites of the PbCl 2 lattice.

Electron-spin-resonance study of Tl0 centers of the laser-active type structure in SrCl2.

The electron-spin-resonance spectra and production properties of two 〈111〉-oriented ${\mathrm{Tl}}^{0}$ centers, called ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) and ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1), have been carefully analyzed in x-ray-irradiated ${\mathrm{SrCl}}_{2}$:TlCl crystals. It is shown that these two centers possess essentially the laser-active ${\mathrm{Tl}}^{0}$(1)-type structure observed in the alkali-metal halides: The structural core is in both cases a ${\mathrm{Tl}}^{0}$ atom perturbed by a nearest-neighbor anion vacancy. In the ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) center, directly produced by x-ray irradiation at liquid-nitrogen temperature, the ${\mathrm{Tl}}^{0}$ occupies the cation site, whereas in ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1) the ${\mathrm{Tl}}^{0}$ occupies the empty interstitial site of the fluorite lattice and is associated with a divacancy in a linear configuration along 〈111〉. Apart from the study of the production properties, the analysis of the isotropic part of the hyperfine interaction has proved to be very valuable in identifying the microscopic defect structures and to differentiate between the models for the two defects. Our measurements establish that at least part of the substitutional ${\mathrm{Tl}}^{+}$ dopant is charge compensated by an adjoining anion vacancy along 〈111〉. It is also found that in ${\mathrm{SrCl}}_{2}$ the anion vacancy becomes mobile above 140 K, and the thermal conversion of ${\mathrm{Tl}}_{\mathit{c}}^{0}$(1) to ${\mathrm{Tl}}_{\mathit{i}}^{0}$(1), which involves site switching from the cation site to the empty interstitial site, may be helped by the incipient mobility of the anion vacancy.

DETECTION OF ACOUSTIC ZONE-BOUNDARY PHONONS BY PHONON DIFFERENCE ABSORPTION

A new method for detection of zone-boundary phonons is reported. Phonons are detected by phonon difference absorption of far-infrared laser radiation. We present experimental results obtained for TlCl and TIBr crystals. Phonons are generated by nonradiative transitions in optically excited crystals. Our experiments indicate long-lived zone-boundary phonons at low crystal temperature and a strong decrease of the phonon lifetime with increasing temperature.

Edge Emissions and Broad Band Emissions in Thallous Halides

The emission spectra are measured in pure TlBr and TlCl crystals as well as in the mixed crystals of TlBr(Cl), TlBr(I), TlCl(Br) and TlCl(I). Sharp emission lines near the absorption edge are observed in TlBr and TlCl. High energy lines among them are examined with the aid of the exciton state calculation and are interpreted as the radiative decay of the indirect forbidden X 6 + × R 6 - excitons. In the halogen-doped crystals, the emissions due to the zero phonon decay of the X 6 + × R 6 - excitons are observed. The broad band emissions observed in the long wavelength region in the halogen-doped crystals are assigned to the decays of the relaxed excitons localized at the added halogen ions.

Optical Absorption in TlCl: Pb Crystal Induced with Reactor and γ-Ray Irradiation at Low Temperatures

TlCl crystals doped with Pb++ ions were irradiated in a reactor at low temperature and their absorption spectra were measured from 4.2 K to room temperature. Two prominent absorption bands were observed at 429 and 478 nm at 4.2 K which shifted to 426 and 465 nm at 77 K. They were quite similar to the bands which were observed in the undoped TlCl crystals irradiated in the reactor at low temperature. In the Pb–doped crystals more absorption bands were observed at 393 and 750 nm on warming to 77 K. The absorption of the undoped and Pb-doped crystals irradiated by Co60–γ rays at 4.2 or 77 K were also measured and were compared with the results of the reactor-irradiated crystals. Only in the Pb–doped crystals γ-irradiated at 77 K, the 426 and 393 nm bands were observed with the intense 750 nm band. An interpretation of the effects of Pb++ ions on the creation of the defects in the crystal by irradiation is presented.

Photomagneto-Current Effect of TlCl and TlBr

Photomagneto-current effect was found in TlCl and TlBr single crystals under the transient condition by the illumination of the light of high absorption coefficient at 1.8K. A current is observed to flow in the direction which depends upon the direction and the magnitude of the applied magnetic field and the absorption coefficient of the exciting light as well as the direction and the magnitude of the applied electric field. The observed result was analyzed in terms of the combination of a diffusion current and a current by the electric field under the magnetic field. Electron and hole mobility at the surface were determined, and their values were found to be smaller than that of the bulk crystal by a factor of 2∼3. The wavelength dependence of the magneto-photoconductivity is explained by the above mechanism.

Edge Absorption Stimulated by Disorder in Mixed Crystals of Thallous Halides

Low level absorption spectra at the edges of TlCl(I), TlBr(I), TlCl(Br) and TlBr(Cl) mixed crystals are measured. At the low concentration of minority halogen, the absorption peak orginated in the localized level due to iodine is found in TlCl(I) while the dicorder-assisted zerophonon X 6 + ×R 6 - exciton absorptions are observed in TlBr(I),TlCl(Br) and TlBr(Cl). Above the critical concentration of the minority halogen, a long tail associated with the bound states due to the cluster formation appears. The absorption is analyzed and the growth of the long tail is discussed.

Photochromism of Thallium Chloride

Abstract The ultraviolet-induced darkening and subsequent bleaching of crystalline thallium chloride was investigated in order to study the processes occurring in a photochromic glass containing TlCl crystalline particles. Doping with a small amount of copper, silver or indium ion was found to be effective in promoting darkenability of TlCl crystals. Exposure to visible light causes bleaching of UV-induced coloring of TlCl crystals. Almost no fading occurs, however, when TlCl crystalline pieces are kept in darkness at room temperature or higher temperatures up to about 70°C, where very quick fading in color is observed in the case of TlCl crystals embedded in a glassy matrix.

Phototropic Glasses Activated by TlCl

Phototropic glasses in which TlCl crystallites play an active role were developed. When glasses containing Tl and Cl were reheated at 420° to 480°C for 3 to 20 h, phototropic glasses were obtained which were transparent, translucent, or opaque, depending on the temperature and time of heat treatment. X‐ray powder diffraction patterns of these glasses indicated the presence of TlCl crystallites. The optical absorption in the visible region which developed in the glass after uv exposure was similar to that developed in TlCl crystals prepared by melting TlCl powder. These facts suggest that the phototropy is caused by TlCl crystallites precipitated in the glass by heat treatment. Optical absorption was measured during both darkening and bleaching of the glass.

Response of TlCl(I,Be) Crystals to Energetic Ionizing Radiation

Studies of scintillation TlCl(I,Be) crystals have been conducted using 8-GeV negative pions and 50- to 130-MeV positrons. Pulse shapes and resolutions of these crystals have been measured. Pulses from TlCl(I,Be) are observed to have a fast rise time (≤2 nsec) and a complicated decay scheme consisting of at least three separate components. Attempts were also made to observe the Cerenkov radiation from these crystals due to highly relativistic particles. The resolution of the pulse height spectrum for a 2.9-cm diameter by 3.8-cm length TlCl(I,Be) scintillator was measured for 8-GeV negative pions. For comparison, the resolution of the same size CsI(Tl) scintillator was measured using the same experimental configuration. The resolution of CsI(Tl) is only slightly greater than theoretical prediction. The resolution of TlCl(I,Be) is greater, being limited by photon statistics even for 35 MeV of deposited energy. The resolution of a 7.6-cm diameter by 7.6-cm length TlCl(I,Be) crystal used as a Total Absorption Shower Counter for 50-130 MeV positrons has also been measured. This resolution is in reasonable agreement with previously reported data.

Direct observation of excitons in some thallous halide single crystals

A new technique for the preparation of sufficiently thin single crystals has allowed the absorption spectra of TlBr and TlCl crystals to be measured at temperatures down to 4.2°K. New n = 2 and n = 3 exciton states for the (100) plane of TlBr have been found at 4011 Å and 3986 Å respectively which with the n = 1 state may be fitted to a Mott hydrogenic series. A band gap of 3.115 ± 0.005 eV at k = 0 is determined together with an exciton reduced effective mass of 0.18 m. The orbital diameter of the n = 3 state is found to be comparable with a typical crystallite size of an evaporated film of these substances; it is shown that the n = 3 line is not resolved and only an embryonic n = 2 line obtained with evaporated film specimens. The TlCl spectrum shows the n = 1 state at 3575 Å together with another line at 3450 Å which is probably due to a spin orbit splitting of the valence band.

Zur absoluten Präzisionsbestimmung von Gitterkonstanten mit Elektroneninterferenzen am Beispiel von Thallium-(I)-Chlorid

A technique for the absolute determination of lattice parameters by electron diffraction is described and extensively investigated. The accuracy of the measurement of the BRAGG angle ϑ is tested with DEBYE-SCHERRER diffraction patterns. An improved technique of high-voltage measurement is used in order to obtain very accurate values of the electron wavelength λ. The discussion shows that systematic errors of Δϑ/ϑ = ± 2,5 · 10-5 and Δλ/λ = ± 1,3 ·10-5 result. Thus an accuracy of Δα/α= ± 3 · 10-5 for the lattic constant a can be claimed. This precision is comparable with that of absolute X-ray methods. The reported technique is used for absolute measurements with TlCl crystals prepared by vacuum evaporation, having lateral dimensions of about 1000 A. The lattice constant derived from (100) -, (200) - and (310) -interplanar spacings is in full agreement with the value obtained by X-ray diffraction. The remaining interplanar spacings which can be evaluated are impaired by lattice distortions and thus yield anomalous values for the lattice constant.