Thallous Halides Research Articles

Supercontinuum generation in mixed thallous halides KRS-5 and KRS-6

We experimentally study supercontinuum generation in mixed thallous halides: thallium bromoiodide (KRS-5) and thallium chlorobromide (KRS-6), as pumped with mid-infrared femtosecond pulses. Almost 2 octave-wide supercontinuum spectra spanning wavelengths from ∼1.5μm to more than 5.5μm were produced by self-focusing of 60 fs pulses with central wavelengths of 3.1μm and 3.6μm in KRS-5 and KRS-6 samples of 6 mm thickness, in the single and multiple filamentation regimes. Our measurements revealed remarkably low energy thresholds (few hundreds of nanojoules) for supercontinuum generation in these materials due to very large values of nonlinear indexes of refraction (n2=10.5×10−15 cm2/W for KRS-5 and n2=5.4×10−15 cm2/W for KRS-6), which were experimentally evaluated from the nonlinear propagation dynamics. A combination of high nonlinearity and wide infrared transparency range make KRS-5 and KRS-6 very attractive bulk materials for SC generation in the mid-infrared spectral range.

Harmonic dynamical behaviour of thallous halides

Harmonic dynamical behaviour of thallous halides (TlCl and TlBr) have been studied using the new van der Waals three-body force shell model (VTSM), which incorporates the effects of the van der Waals interaction along with long-range Coulomb interactions, three-body interactions and short-range second neighbour interactions in the framework of rigid shell model (RSM). Phonon dispersion curves (PDC), variations of Debye temperature with absolute temperature and phonon density of state (PDS) curves have been reported for thallous halides using VTSM. Comparison of experimental values with those of VTSM and TSM are also reported in the paper and a good agreement between experimental and VTSM values has been found, from which it may be inferred that the incorporation of van der Waals interactions is essential for the complete harmonic dynamical behaviour of thallous halides.

Syntheses and crystal structures of Tl 5Se 2Cl and Tl 5Se 2Br

The compounds Tl 5Se 2Cl and Tl 5Se 2Br have been synthesized from stoichiometric amounts of thallium, selenium and thallous halide. Both crystallize with the In 5Bi 3 structure which is a subfamily of the Cr 5B 3 type: Tl 5Se 2Br crystallizes in space group 14/mcm ( a = 859.4(2) pm, c = 1278.8(4) pm, Z = 4, R = 0.047), Tl 5Se 2Cl as a lower symmetric distorted variant in P4/ncc ( a = 856.5(1) pm, c = 1274.1(2) pm, Z = 4, R = 0.069). Around one central thallium atom slightly distorted octahedra of two halogen and four selenium atoms occur. These octahedra are covered by cubes of thallium atoms. The octahedra are interconnected via common vertices and the cubes via common edges to give the three-dimensional structure.

Optical Absorption near the Fundamental Edge in Amorphous Thallous Halides

Optical Absorption near the Fundamental Edge in Amorphous Thallous Halides

Use of Harrison's potential in ZnS, NaCl and CsCl-structure solids

Harrison's overlap repulsive potential has been used for evaluating cohesive energies, bulk modulus and its pressure derivatives, Gruneisen parameter, and the volume derivatives of Gruneisen parameter for cuprous halides, silver halides and thallous halides. This potential has replaced the old and widely used Born-Mayer potential in ionic crystals. The Harrison repulsive potential has been represented by a quantum mechanical analytical potential form derived from the tight binding theory. The parameters involved in this potential form have been determined in terms of valence state energies of outermost d-electrons of Cu+, Ag+ and Tl+ ions. The results calculated in the present study have been found in good agreement with experimental data and better than those predicted from the Born potential model.

Relaxed States of Indirect Excitons in Mixed Crystals of Thallous Halides

By measurements of luminescence and the Urbach tail of indirect absorption, exciton states were investigated in mixed crystals of thallous halides for a wide range of compositions. From the estimation of the steepness coefficients of the Urbach tail in TlCl x Br 1- x mixed crystals, the relaxed exciton state was found to change from the self-trapped state to the free state at x =0.7-0.6, consistent with the results of the emission studied before. On the other hand, the indirect excitons in TlBr x I 1- x (0.3< x <1) were found, upon analysis of the emission band and the steepness coefficient of the Urbach tail, to relax always to the self-trapped states, which become shallow with increasing iodine concentration. Exciton states in mixed thallous halides are discussed together with a theoretical study on a binary mixed crystal and in relation to percolation of minority-halogen ion clusters.

Analysis of compression for cuprous, silver and thallous halides

An analysis of compression, i.e., pressure-volume relationship is presented for CuCl, CuBr, CuI, AgCl, AgBr, AgI, TlCl, TlBr and TlI crystals. An interionic potential model using Harrison's repulsive interaction has been developed for calculating the pressures corresponding to different compression values. The results obtained are found to present close agreement with available experimental data on the equation of state for the crystals under investigation. It is concluded that thallous halides are more compressible as compared to cuprous and silver halides.

Analysis of Crystal Binding and Thermoelastic Properties of Cuprous Halides, Silver Halides, and Thallous Halides

AbstractCrystals of cuprous halides, silver halides, and thallous halides are found to show significantly different behaviour from that of alkali halides. This difference is mainly due to the presence of outermost d‐electrons present in Cu+, Ag+, and Tl+ ions. In the present paper an analytical potential form is used for the overlap repulsive energy derived by Harrison from quantum mechanical considerations. The parameters in this potential form have direct correlation with the valence state energies for outermost d‐electrons. The potential model is applied to calculate the cohesive energies, pressure derivatives of bulk modulus, Grüneisen parameter and its volume dependence using different existing theories.

Optical Properties of Orthorhombic Thallous Iodide

Reflection spectra of a single crystal of orthorhombic TlI have been measured for the polarization \(\bm{E}//\bm{c}\) and \(\bm{E}//\bm{a}\) over a range of photon energy from 2.6 to 4.1 eV at 2 K. The spectra show pronounced anisotropy between these polarizations. The lowest exciton transition at 2.87 eV is confirmed to occur in a cationic sublattice (Tl + 6 s →6 p ). However, the admixture of the cation s orbital in the top valence band is found to be not so large as in cubic thallous halides. The binding energy of the first exciton is estimated to be 17.1 meV.

Strain Derivatives of Dielectric Constants of MgO and Thallous Halides

AbstractCalculations are made to determine strain derivatives of static and high‐frequency dielectric constants of MgO, TlCl, and TlBr, using the two‐ion polarizable shell model. The calculated values of pressure derivatives of the static dielectric constant of MgO, TlCl, and TlBr are in good agreement with the experimental values. A reasonable agreement is seen between the calculated values and the measured values of strain‐optical constants for MgO. Strain‐optical constants are also calculated for thallous halides. Anisotropy in the polarizability of ions arising from strain is fairly accounted for by the shell model through the large contributions from overlap forces.

The use of pressure to study electronic excitations

Pressure is a very powerful and versatile tool for studying electronic excitations. Examples are given both for delocalized (band to band) and localized excitations. The band-to-band excitations discussed here show the approach to zero energy gap for sulfur and the thallous halides. For localized excitations there is a balance between the effects of pressure on the stabilization of the bonding and destabilization of the antibonding states and, on the other hand, the increase in intensity of the Van der Waals forces. Examples are given of the various effects using a number of organic molecules and metals cluster compounds.

High-pressure studies of the absorption edges of three thallous halides

Visible and near-infrared optical-absorption edges of TlCl, TlBr, and TlI were measured as a function of pressure in a diamond anvil cell. The indirect band gap of the three thallous halides shifts red (to lower energy) with pressure and was followed into the near infrared. The direct band gaps of TlBr and TlCl also shift red at a slower rate. The calculated band structures [J. P. Van Dyke and G. A. Samara, Phys. Rev. B 11, 4935 (1975)] of the thallous halides as a function of pressure are quantitatively compared with the experimental results.

Specific heat and thermal-conductivity measurements on cesium and thallous halide crystals at low temperatures

Specific-heat and thermal-conductivity measurements in the temperature range of 1.7 to 20 K are reported on single crystals of CsBr, CsI, T1Br, and T1C1 and on a polycrystalline sample of T1I. All crystals display minima in the effective Debye temperatures which are reflected in maxima in $\frac{C}{{T}^{3}}$-versus-$T$ curves, and these extrema are especially pronounced for T1C1. Comparisons are made with the predictions of existing lattice-dynamics calculations, and the specific heat of T1Br is in very good agreement with the shell-model calculations of Cowley and Okazaki. A Schottky specific-heat term is resolved in the cesium halides at the lowest temperatures and attributed to hydroxyl ions; by adopting the zero-field splitting for ${\mathrm{OH}}^{\ensuremath{-}}$ in KC1, hydroxyl concentrations \ensuremath{\sim}${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ are estimated in CsBr and CsI. The $\frac{C}{{T}^{3}}$ maxima are fitted with Einstein terms added to the Debye backgrounds, and these fittings (together with the Schottky fits for CsBr and CsI) yield the following Debye temperatures: CsBr, 145 K; CsI, 124K; T1Br, 116 K; T1C1, 120 K; and T1I, 103 K. Thermal-conductivity data for all crystals display maxima at \ensuremath{\sim}4-5 K and an apparent ${T}^{3}$ boundary-scattering regime at the lowest temperatures. Phonon mean free paths at the higher temperatures are analyzed according to the Peierls relation, and for CsBr and T1C1 it is found that the Debye modes are the dominant heat carriers. For the remaining crystals, the Einstein modes also carry heat. (The Einstein modes at these higher temperatures dominate the specific heats, contributing up to 75% in the case of T1C1.) Thermal-conductivity measurements were also made on thin crystals (\ensuremath{\sim}0.05 cm) of CsBr, T1Br, and T1C1 to examine the boundary-scattering region. It is found that in this region the limiting phonon mean free path for CsBr scales with the crystal dimension, and it is suggested that the suppression of the thermal conductivity may be due to the hydroxyl concentration. For T1Br and T1C1, the limiting phonon mean free path is \ensuremath{\sim}0.01 cm and is independent of the crystal thickness; this behavior is attributed to a mosaic structure within these crystals.

Nonlinear photoacoustic measurement of laser pulse correlation functions

A simple method is described for the measurement of temporal characteristics of short picosecond-nanosecond laser pulses in the visible and near IR. It relies on the photoacoustic detection of two-photon absorption in thallous halides in an otherwise conventional correlation experiment.

Theory of excitonic molecules in thallous halide: The role of electron-hole exchange interaction

A theoretical calculation which fully takes into account the electron-hole exchange effect is performed for the ground state energy of the excitonic molecule in thallous halides. It is found that the previous variational calculations are insufficient to obtain a stable formation of the excitonic molecule in these materials.

Cohesive Energy of Ionic Crystals within the Framework of Compressible Ion Theory

AbstractCohesive energy for 24 alkali, silver, and thallous halides and 15 alkaline earth chalcogenides crystallizing in NaCl and CsCl structures are calculated within the framework of compressible ion theory proposed by Narayan and Ramaseshan, splitting the short‐range repulsive interaction into its ionic components. The nearest and next‐nearest neighbour interactions as well as van der Waals interactions are taken into account to evaluate the cohesive energy for these diatomic ionic solids. The results obtained are found to be in close agreement with recent experimental data.

Far-Infrared and Raman Scattering Spectra Due to the Two-Phonon Processes in Thallous Halide Crystals

The far-infrared absorption spectra due to the two-phonon difference process have been observed. Peaks are found at about 27 and 35 cm -1 in TlCl and at 35.2 cm -1 in TlBr, which are due to saddle points in the difference energy surface of two phonons. The second order Raman spectra of TlCl and TlBr at low temperature have been measured and the assignments of the spectral peaks to particular combinations of two phonons are made. In the near resonant region, the scattering efficiency of the overtone of the LO phonon shows a more remarkable resonant behavior than that of other phonons. From the analysis of the experimental result by the theory of Ganguly and Birman, it is concluded that the two LO phonons at the \(\varGamma \) point are successively scattered by the exciton, while two phonons of other modes with finite wave vectors are, presumably, scattered simultaneously.

Pressure and temperature dependences of the ionic conductivities of the thallous halides TlCl, TlBr, and TlI

Detailed studies of the pressure and temperature dependences of the ionic conductivities of TlCl and TlBr have allowed determination of the lattice volume relaxations and energies associated with the formation and motion of Schottky defects in these crystals. The volume relaxations deduced from the conductivity are found to be comparable in magnitude with values calculated from the strain energy model and a dynamical model. The association energy of ${\mathrm{Tl}}^{+}$ vacancies and divalent impurities was also determined for TlBr. A particularly important result is the finding that for these CsCl-type crystals the relaxation of the lattice associated with vacancy formation is outward. Earlier studies on ionic crystals having the NaCl structure have yielded a similar result. This outward relaxation thus appears to be a general result for ionic crystals of both the NaCl and CsCl types (and possibly other ionic lattice types), in disagreement with earlier theoretical calculations which show that the relaxation should be inward for all models of ionic vacancies investigated. The conductivity of TlI was studied in both the (low temperature and pressure) orthorhombic phase as well as in the cubic CsCl-type phase. There is a large electronic contribution to the conductivity in the orthorhombic phase. An interesting result for all three materials is the observation in the cubic phase of a pressure-induced transition from ionic to electronic conduction. This is in qualitative agreement with what is known about the pressure dependences of the electronic structure of these materials.

X-ray and UV photoemission studies of the thallous iodide phase transformation

The orthorombic to simple cubic thallous halide phase transformation induced by temperature increase is studied by means of X-ray and UV photoelectron spectroscopy. The structural transformation is well evidenced by an abrupt decrease of the binding energy separation between the iodine and the thallium core levels at the phase transition temperature. Valence band spectra recorded for both crystal forms by using the HeI radiation evidence modification in the band structure.

Nuclear spin relaxation in stoichiometric and thallium-doped thallous halide melts

Relaxation times for 203,205Tl, 79,81Br and 35,37Cl are reported for molten TlBr, TlCl and TlBr : Tl. For Br and Cl, relaxation is predominantly quadrupolar; for Tl the relaxation indicates contact hyperfine interaction with paramagnetic centres produced thermally and by adding Tl.