Tungsten oxygen system

FIGURE 8.1. A summary of the tungsten-oxygen system. From Lassner and Schubert. 

A H CWOg, cr, 298.15 K) -201.46 kcal mol. This result is less certain because of the Incomplete characterization of the final products. Ackermann and Rauh (2 ) have investigated the tungsten-oxygen system by mass effusion, mass spectroscopy, and x-ray diffraction. In the temperature range from 1300 to 1600 K, they have derived indirectly two values of A H (W02 qq, cr) = -190.9 and -188.0 kcal mol" (corresponding to -196.1 and -193.3 kcal mol", respectively, at 298.15 K, using all JANAF functions). 

The adopted value, A H (W20g, g, 298.15 K) = -278.2 kcal mol", was reduced from A H (1450 K) = -272.5 kcal mol", reported by Ackeraann and Rauh ( ). They have studied the sublimation behavior of the tungsten-oxygen system over the temperature range... 

Ackermann and Rauh (1 ) have studied the sublimation behavior of the tungsten-oxygen system over the temperature range from... 

The adopted value, A H (Wg0g, g, 298.15 K) = -483.6 kcal mol", was reduced from A H (Wg0g, g, 1450 K) = -474.1 kcal mol" of Ackermann and Rauh ( ), who studied the sublimation behavior of the tungsten-oxygen system over the temperature range from 1300 to 16000 K by mass spectrometry, mass effusion and x-ray diffraction, and determined A G (W20g, g) = -474,100 + 110.26T cal K" mol" from partial pressure measurements over V-VO (cr). They reviewed the previous determinations (, 2, 3, 4) of the thermodynamic... 

The tungsten-oxygen system is rather complex. Besides the stable stoichiometric binary oxides (WO3, WO2.9, WO2.72, and WO2), and the stoichiomettic tungstates and acids, a variety of nonstoichiometric, fiilly oxidized and reduced compounds exists, according to the scheme in Fig. 4.2. 

As previously mentioned, the hydrated species W02(0H)2 is the primary volatile species in the tungsten-oxygen-hydrogen system. This species can be formed from most forms of tungsten and its oxides. For example ... 

The existence of at least nine phases in the molybdenum-oxygen system is well established. Their crystal structures are briefly described and it is shown that they can be classified into four main families dependent on whether they possess a basic structure of rutile type, ReOs type, or MoOs type, or have complex structures where polygonal networks can be distinguished. The known tungsten and mixed molybdenum tungsten oxides fit into this scheme. Because of their complicated formulas many of these compounds may be termed "nonstoichiometric," but variance in composition has not been observed for any of them. 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Important differences distinguish the molybdenum and tungsten systems. In aqueous solution, equilibration of the molybdenum species is complete within a matter of minutes whereas for tungsten this may take several weeks it also transpires that whereas the basic unit of most isopolymolybdates is an MOg octahedron with a pair of cis-terminal oxygens, that of the isopolytungstates is more commonly an MOg octahedron with only one terminal oxygen. The two must therefore be considered separately. 

In one of the earliest reports of the use of clean evaporated alloy films in surface studies, Stephens described the preparation and characterization of Pd-Au films and presented some results for the adsorption of oxygen on them 46). Films of pure Pd and 60% Au were evaporated directly from wires, while films of 80% Au and pure Au were evaporated from a pre-outgassed tungsten support wire. The films were evaporated in a UHV system and the pressure was kept below PC8 Torr during evaporation. After evaporation, the films were stabilized by cycling between —195° and 30°C four times. They w ere characterized by X-ray diffraction and chemical analysis surface areas were measured by the BET method using krypton adsorption. 

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