Polycrystalline 2H a Phase of TaS

The synthesis of 2f/(fl)-TaS2 powder is carried out in fused quartz or Vycor ampules—typically 10 cm long and 1.7 cm in diameter, with a wall thickness of 1.0-2.0 mm (Fig. 2a). About 1.41 g (0.008 mol) of 0.020-in.-diameter tantalum wire (99.8% purity) is cut in the form of 3.8-cm lengths, washed in hot dilute HCl to remove iron contamination introduced in the cutting process, rinsed in distilled water, and dried. Sulfur powder, 0.500 g (0.016 mol, 99.9999% purity),t is then added, and the ampule is sealed under a vacuum of about 10 torr. The sealed ampule is placed into a cold laboratory tube furnace with the tantalum wire charge placed in the end of the ampule towards the center of the oven and elevated about 1 cm (see Fig. 2b). Thus, as the sample is heated, the sulfur melts and remains in the lower, cooler end of the ampule. A thermocouple is placed near this end to ensure that the temperature is at or below 450° (the pressure of sulfur vapor in equilibrium 

The use of tantalum wire along with the two-zone technique ensures a safe reaction rate. When the sulfur has reacted completely with the tantalum wire (usually within 7-10 days) the ampule is removed from the furnace, shaken vigorously to break up clumps of unreacted material, and placed in the center of the furnace. It may be necessary to repeat this procedure several times to ensure that all the wire has reacted. After 1 or 2 days at a 850-900°, the cooling process may be started. The sample is eooled initially to 750° and annealed for 1 day. It is then annealed at 650° for about a day and at 550° for another 2-3 days. The furnace is then shut off the sample is allowed to cool for 5-6 h to room temperature. 

The polycrystaliine 2W(a)-TaS2 should be free-flowing and in the form of black platelets. Any gold-colored material indicates the presence of the 1T phase and results from improper annealing. Further, the presence of a fibrous or needle-like material indicates incomplete reaction of the tantalum. This fibrous material is most likely TaSj, which decomposes above 650°.  

This material may be removed by reheating the sample to 850°, followed by the same annealing procedure outlined above. The X-ray diffraction pattern for 2H(a)-TaS2 may be used for identification. The following d values have been obtained for major low-angle X-ray diffraction lines (and intensities) 6.05(1) 3.025(0.06) 2.8709(0.32) 2.7933(0.07) 2.3937(0.80) 2.3389(0.04) A. Note that it is difficult to obtain the ideal intensities because of preferred orientation of the crystallites. This material is a superconducting metal with 2n = 0.8 0.05 K.  

Several techniques exist for loading iodine into the reaction vessel, and some are described in Reference 13. The simplest method is to load iodine quickly in air followed by pumping down to a few micrometers of pressure and sealing. The ampule is then heated to 9(X)° (with appropriate precautions 

The synthesis of 2H(a)-TaS2 powder is carried out in fused quartz or Vycor ampules-typically 10 cm long and 1.7 cm in diameter, with a wall thickness of 1.0-2.0 mm (Fig. 2a). About 1.41 g (0.008 mole) of 0.020-in. diameter tantalum 

Available from United Mineral and Chemical Corp., 129 Hudson St., New York, NY 10013. 

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