Tellurium subhalides

Figure 7 Structural units of tellurium and the tellurium subhalides Te3Cl2, Te2X, fi-Tel, and a-Tel with bond lengths and bond angles. The upper numbers in Te2X refer to X — Br, the numbers below refer to X — I...

The eight-atom systems provide a compelling illustration of the structural diversity of tellurium polycations. There are examples of (a) structural isomers of the monomeric dication Tcg, (b) a polymeric (Teg ) cation and (c) a tetracation Teg" . Unlike cyclo-Sg and -Seg , the Teg dication cannot be made by oxidising tellurium with MF5 (M = As or Sb) in liquid SO2. However, the reactions of (a) the tellurium subhalide Te3Cl2 with ReC at 200 °C and (b) tellurium with WCl6 at 180°C produce the salts Teg[MCl6] (w=l, M = Re ... 

The optical, electrical, and thermodynamic properties of tellurium subhalides have attracted considerable research interest. For instance, a-Tel has been suggested to find applications as solid electrolytes in galvanic cells. 

Structure and bonding in the tellurium subhalides can be well rationalized, either employing valence and electron-counting rules, in-... 

Tellurium-Halogen Compounds.—The first investigation of the structures of the recently prepared tellurium subhalides Te X (X = Cl, Br, or I, y > 1) has been reported. The building unit of TesClg is shown in Figure 8 an infinite tellurium screw, such as is present with a simpler conformation and higher symmetry in elemental tellurium, is observed. Each third Te atom... 

The tellurium subhalides represent a group of recently discovered compounds which have been investigated by the methods of solid-state and structural chemistry. Because of their high tellurium content and their crystal structures containing homonuclear tellurium connections they have attained significance as modified tellurium structures . As a consequence, tellurium subhalides have been the subject of various investigations in the fields of solid-state and chemical physics. 

In this paper a short survey of the group of tellurium halides is followed by detailed information on phase relations, preparation, crystal growth, and crystal structures of tellurium subhalides. Moss-bauer investigations as well as optical, photoelectric, and electrical properties of the respective compounds are discussed. Sohd-state galvanic cells containing tellurium subhalides have enabled the precise determination of relevant thermodynamic data. 

In 1956, the first tellurium subhalide (characterized as TeJJ was found in X-ray investigations of the Te—Tel system About 10 years later, an additional subiodide (TeJ) was prepared by hydrothermal synthesis d . Systematic reinvestigations of the phase relations in the quasibinary systems Te—TeCl Te—TeBr ... 

The basic principles of hydrothermal synthesis performed in hydrohalic acids are descril in With respect to tellurium subhalides hydrothermal theses with concentrated hydroiodic acid as a solvent represent successful methods for growing single crystals of subiodides Te2l, p-Tel and a-Tel n Since hydrohalic... 

The crystal structures of tellurium subhalides are built up of structmal units presented in Fig. 19, Because of the high tellurium content of these compounds, sinularities with the latter and the structural properties of elemental tellurium could be expected and, in fact, were confirmed 5.47)... 

With respect to the homonuclear linkage of tellurium atoms, the tellurium subhalides may be described as modified tellurium structures . The 3i-chain of the element is modified by steps, going from a 2,-chain in Te3Cl2 to a ribbon of condensed planar zigzag chains in Te2X and a separated planar zigzag chain in 3-Tel down to a four-membered tellurium ring in a-Tel. 

Coordination geometries of tellurium in tellurium subhalides clearly follow from a consideration of the structural units presented in Fig. 19. Even if the conditions of chemical bonding in these units are not quite clear at first sight, they are easily integrated into accepted conc ts using the common valence rules in connection with Gillespie-Nyhohn rules and taking into account the different coordination... 

Table 4. Formal states of oxidation and proposed configurations of tellurium atoms within the crystal structures of tellurium subhalides.

Mossbauer spectroscopy has been used to study a number of halogen-containing compounds of telluritun in which the formal oxidation state of tellurium is II (see also LB), IV or VI The knowledge of the existence of tellurium subhalides with low formal oxidation states of the chalcogen gave rise to Te-125 Mossbauer investigations of the subhalides, and additionally to 1-129 Mdssbauer studies on a-Tel... 

Tellurium subhalides contain the series of isotypes Tc2X (X = Cl, Br, I). Solid solutions of the stable Te2Br and the metastable Te2l have been studied. Further examples of studies of ternary subhalides were based on selenium subhalides, with SeCl and a-/ 3-SeBr being the only known subhalides no intermediate binary compound exists in the Se-I system. The latter fact in i rticular stimulated the investigation of the ternary Se-Te-I system in order to find out whether there are distinct intermediate ternary subiodides and/or regions of ternary solid solutions based on the subiodide a-Tel. 

Detailed optical investigations of tellurium subhalides have been carried out on Te3Cl2 and a-Tel which, from the point of view of, modified tellurium structures are the structural antipodes. 

The first subhalide which was investigated by measurement of the electrical conductivity was tellmium monoiodide with a 1.1 eV band gap calculated from the temperature dependence of the electrical conductivity After the crystal structure determination of tellurium subhalides had shown these compounds to be modified tellurimn structures (see IV), systematic investigations of their electrical conductivities were undertaken Electrical contact with subhalide crystals was... 

FI. 32. Schematic representation of the structural elements of tellurium subhalides with regard to ele-mental Te—Te bonds above and electrical conductivity vs temperature plot of representative tellurium subhalides below The room temperature conductivity of pure iodine as well as the intrinsic conductivity lines of tellurium are indicated... 

Tellurium subhalides form a well-investigated group of compounds which can be expected to have a agnificant impact on current chemistry. This shouU be emphasized because the main interest of the binary chalcogenohalides in the fields of basic research and teaching is still catered on the tetrahalid and in puticular on the binary sulfohalides at present. 

Although there have been a large number of solid-state physical investigations of tellurium subhalides, further research work on the physical properties of these... 

From the chemical point of view it seems to be of particular interest that further (metastable) tellurium subhalides can be obtained by hydrothermal syntheses in acid solutions. In the hydrothermal work on the crystal growth of arsenic telluro-iodides a pure binary tellurium subiodide with the composition Tcsl was obtained as well as a ternary phase with only small amounts of arsenic (AsT Ij The composition Tejl was checked by chemical as well as energy-dispersive X-ray analysis. Crystals of the new tellurium subiodide are shown in Fig. 43 a crystal structure analysis failed because the quality of the crystals was not suitable for single crystal investigation. During annealing of the subiodide a stable phase combination a-Tel -H Te was formed which indicated the metastable character of TesL... 

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