Tl-Te Alloys Research Articles

The miscibility gap region and liquid–liquid equilibrium in immiscible In–Tl–Te alloys

Electroconductivity measurements were performed for liquid immiscible (In x Tl 1− x ) 0.80Te 0.20 alloys over a wide temperature range between 1200 K and the temperature of crystallization under ambient pressures of argon gas (up to 50 MPa). The electrical conductivity shows the same behaviour as for binary liquid alloys during the mixing–demixing process. It was revealed that variation of the In-to-Tl ratio at constant Te content changes the properties of the coexisting liquids and affects the phase separation temperature T c in a nonlinear manner. T c varies between 855 K for Tl 0.80Te 0.20 and 817 K for In 0.80Te 0.20 with a maximum temperature T c=874 K for In 0.37Tl 0.43Te 0.20. The results are analyzed in comparison with available data for binary and ternary immiscible alloys and the possible influence of the ion subsystem on the formation of the critical point parameters is discussed.

Velocity of sound in liquid Tl-Te alloys

The velocity of sound in liquid Tl-Te alloys has been measured in order to investigate the composition dependence of the compressibility. The isothermal compressibility deduced from the velocity of sound, density and specific heat shows a characteristic cusp at Tl2Te, which supports stable Tl2Te association persisting in the liquid. In addition a logarithmic singularity in the adiabatic compressibility has been found at the critical point of the two-melt phase separation between Tl and Tl2Te.

Heat Capacity of Liquid Tl-Te Alloys

The measurements of heat capacity were made for liquid Tl-Te alloys using an adiabatic scanning calorimeter over the whole concentration range. The excess heat capacity for liquid Te-rich Tl-Te alloys can be attributed to the change in the bonding configuration around Te. The concentration dependence of the heat capacity suggests that the bonding nature changes gradually with increasing Tl content and drastically changes around the composition of Tl 2 Te. The heat capacity for liquid Tl 2 Te alloys has a small temperature coefficient at temperature where a nonmetal-metal transition occurs. It means that the excess heat capacity for liquid Tl 2 Te alloy may not be due to the dissociation of the chemical complex.

Reflectances and densities of states for liquid Ga-Te, In-Te and Tl-Te alloys

Normal reflectances of these alloys have been measured over the 0.65 to 4.5 eV range at temperatures above the liquidus. The spectra are generally smooth and featureless, yet show the transition from metallic to semiconducting behaviour as concentration is changed. The results are interpreted in terms of a model density of states. The concentration dependence of the band gap is obtained as is the band edge shape.

The structure of liquid In - Te and Tl - Te alloys

The atomic structure of liquid In—Te and Tl—Te alloys have been investigated as a function of composition and temperature by means of X-ray diffraction. The following features emerge from this experiment. The structure of liquid In—Te alloys is similar to that of pure Te in the range of composition from 35 to 100 at % Te. This implies that the structure of pure Te plays a significant role in these alloys. At the composition of In2Te3, the second peak near the region ofQ=3.2A−1 disappears with the increase of temperature. This corresponds to the breakdown of the electron localization based on the measurement of magnetic susceptibility. On the other hand, there is no substantial change in the structure of liquid Tl—Te alloys as the delocalization temperature of the electrons is approached.

On the Miscibility Gap in Liquid Tl–Te Alloy

We report a detailed study of the phase-separation in binary liquid Tl–Te alloys. We have determined the location of the miscibility gap in the concentration-temperature ( x - T ) plane from the measurement of the electrical conductivity σ. It is found that (i) the upper consolute point is at 80.7 at.% Tl and 583.3°C, (ii) the law of rectilinear diameters holds and (iii) the critical exponent β for the x - T phase boundary is 0.33.

Reflectance of liquid Ga-Te, In-Te, and Tl-Te alloys

The liquid alloys ${\mathrm{Ga}}_{2}$${\mathrm{Te}}_{3}$, ${\mathrm{In}}_{2}$${\mathrm{Te}}_{3}$, and ${\mathrm{Tl}}_{2}$Te are well-known examples of liquid semiconductors. More than 20 alloys in these systems, including the stoichiometries listed above, have been studied for photon energies between 0.65 and 3.00 eV. Reflectances lie in the range 25%-65%, relative to a quartz window. The normal reflectances show smooth transitions from behavior characteristic of a small-gap semiconductor to that of a large-gap semiconductor to that of a Drude metal as the compositions are varied from pure Te to pure metal. The data have been fit with oscillator models. The data are consistent with the bonding scheme proposed by Cutler for Tl-Te alloys and a similar model appears to fit the other data as well.

Bond equilibrium theory for Te-rich liquid Tl–Te alloys

Recent work has provided independent information about the behavior of the hole concentration c in TlxTe1−x as a function of temperature T and composition x in the range 0.2 ≤ x ≤ 0.6. This makes possible a critical reexamination of a molecular bond model for the structure of the alloy, in which holes are generated by broken Te—Te bonds. The earlier theory is revised to formulate an unrestricted independent bond model (ibm), for which the equations are simple and have obvious physical interpretations. This provides a good description of c(T) but only a qualitatively correct c(x). Using a Thomas–Fermi model for the screening interaction between holes and the acceptor ions, it is shown that the equilibrium constant can be expected to increase rapidly with c at large enough values. A modification in which the free energy of a dangling bond is decreased by proximity to a Tl—Te bond is found to significantly improve the result for c(x). The thermochemical behavior is derived. The entropy of mixing is in fair agreement with experiment, but the enthalpy of mixing is grossly wrong. This reflects the neglect of intermolecular interactions in the theory, which, it seems, can easily account for the remaining discrepancies in the predicted behavior of c.

X-ray Diffraction Study of Molten Te and Tl-Te Alloys

Abstract X-ray diffraction patterns have been obtained from molten Te at 470, 520 and 570 °C. The heights of the peak maxima in the structure factor were much the same in contrast with those of typical molten metals such as sodium. Molten Tl-Te alloys have been studied by X-ray diffraction for the alloy compositions 25, 33.3, 50, 60 and 75 at% Te at 500 °C and at about 20 °C above the liquidus. The total structure factors for the 25 and 33.3 at% Te alloys were almost the same as that of pure Tl. This implies that the atomic arrangement of these molten alloys is very close to that of pure Tl. Although a drastic change is not found in the general form of the structure factor, the parameter of the range of local atomic order abruptly increases on passing from Tl2Te to more Te-rich alloys. The three partial structures were also evaluated from the observed X-ray intensities assuming that each partial structure is independent of the relative abundance of the constituent elements in the alloys.

Acoustic properties of some liquid Tl-Te alloys

Measurements of the velocity of sound in three Tl-Te alloys (30, 50 and 66.67 at.% Tl respectively) suggest that the loose three-fold coordination in pure liquid Te at temperatures below 700 degrees C gives place to a typically metallic coordination on the addition of only a small amount of Tl. Some measurements of acoustic attenuation as a function of frequency for the 50 and 66.67 at.% alloys show no clear signs of structural relaxation and give ratios of bulk to shear viscosity typical of those found in covalently bonded liquids, in contrast with the high values usually found in ionic liquids.

Concentration fluctuations and the cluster model in liquid binary alloys

The cluster model of Cohen and Sak (1972) for anomalous liquid alloys is examined. The existence of clusters within the alloy large enough to be used in this model implies a concentration fluctuation, which can be evaluated exactly for dilute alloys. This calculated concentration fluctuation is in general much larger than that deduced thermodynamically from experiment, which strongly suggests that real alloys do not contain such clusters. Data are presented for Mg-Bi and Tl-Te alloys.

Thermoelectric behaviour of molten TlTe alloys at compositions approaching pure thallium

Thermoelectric behaviour of molten TlTe alloys at compositions approaching pure thallium