Tungsten (VI) fluoride

Structural formula
General
Surname	Tungsten (VI) fluoride
other names	Tungsten hexafluoride
Molecular formula	WF 6
Brief description	non-flammable gas with a pungent odor below 17 ° C: pale yellow liquid below 2 ° C: white crystals
External identifiers / databases
CAS number 7783-82-6 EC number 232-029-1 ECHA InfoCard 100.029.117 PubChem 522684 Wikidata Q418962
properties
Molar mass	297.84 g mol −1
Physical state	gaseous
density	Gas: 12.4 kg m −3 solid: 4.56 g cm −3 (−9 ° C)
Melting point	2.3 ° C
boiling point	17.1 ° C
Vapor pressure	113.2 k Pa (20 ° C)
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 330-314-280 EUH: 071 P: 260-280-304 + 340-303 + 361 + 353-305 + 351 + 338-315-405-403
MAK	1 mg m −3
Thermodynamic properties
ΔH f 0	411.7 ± 0.5 kcal mol −1
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Tungsten (VI) fluoride (WF ₆ ), often also tungsten hexafluoride , is a colorless, gaseous compound of the elements tungsten and fluorine and belongs to the group of hexafluorides . It is the densest known gas under standard conditions. In humid air, it smokes due to hydrolysis and has a pungent odor. It is most commonly used in the manufacture of semiconductor circuits and printed circuit boards in the chemical vapor deposition process; when it decomposes, a residue of metallic tungsten remains.

presentation

Tungsten hexafluoride is obtained by converting tungsten in a fluorine stream at temperatures between 350 and 400 ° C:

${\displaystyle \mathrm {W+3\ F_{2}\longrightarrow WF_{6}} }$

The gaseous product is condensed and separated from WOF ₄ impurities by distillation.

Instead of fluorine gas, chlorofluoride (ClF), chlorine trifluoride (ClF ₃ ) or bromine trifluoride (BrF ₃ ) can also be used. An alternative process for the production of tungsten hexafluoride is the reaction of tungsten trioxide (WO ₃ ) with hydrogen fluoride (HF), BrF ₃ or sulfur tetrafluoride (SF ₄ ). Tungsten hexafluoride can also be obtained by converting tungsten hexachloride (WCl ₆ ):

${\displaystyle \mathrm {WCl_{6}+6\ HF\longrightarrow WF_{6}+6\ HCl} }$

${\displaystyle \mathrm {WCl_{6}+2\ AsF_{3}\longrightarrow WF_{6}+2\ AsCl_{3}} }$

${\displaystyle \mathrm {WCl_{6}+3\ SbF_{5}\longrightarrow WF_{6}+3\ SbF_{3}Cl_{2}} }$

Due to the chemical similarity of molybdenum and tungsten, molybdenum hexafluoride is contained as an impurity . This can be removed by reducing a WF ₆ -MoF ₆ mixture with any element, including molybdenum, at a moderately elevated temperature.

properties

Physical Properties

At normal pressure and temperatures above 17.1 ° C, tungsten hexafluoride is a colorless diamagnetic gas. In the temperature range from 2.3 to 17.1 ° C it is a pale yellow liquid with a density of 3.44 g / cm ³ at 15 ° C.

At 2.3 ° C it freezes to a white solid in the cubic crystal system with the lattice parameter a  = 628 pm and two formula units per unit cell with a calculated density of 3.99 g · cm ⁻³ . A solid phase transition can be observed at −9 ° C. Below this temperature it crystallizes in the orthorhombic crystal system in the space group Pnma (space group no. 62) and four formula units per unit cell with a calculated density of 4.56 g · cm ⁻³ . The fluorine atoms occupy the hexagonal closest packing of spheres .

Temperature range	Crystal system	a [pm]	b [pm]	c [pm]	Density (g cm −3 )
−9 ° C ... 2.3 ° C	cubic	628			3.99
<−9 ° C	orthorhombic	960.3	871.3	504.4	4.56
−140 ° C	orthorhombic	946.6	860.8	499.8	4.86

With an exceptionally high density of around 12.4 g / l, around 10.3 times denser than air and around half the size of foam polystyrene , WF _{6 is} the heaviest known gaseous substance (density of the heaviest elementary gas, radon : 9, 73 g / l). The density in the solid or liquid state, on the other hand, is in the range typical for covalent or ionic heavy metal compounds.

The critical point is 179.6 ° C, 45.7 bar and 1.28 kg / l; the triple point at 2.4 ° C and 0.5597 bar.

The WF ₆ molecule is octahedral ( O _h ); the W – F bond length is 182.6 pm. This high symmetry can be observed in most related compounds. However, it should be noted that tungsten hexahydride (WH ₆ ) and hexamethyl tungsten ( W (CH ₃ ) ₆ ) adopt a trigonal prismatic structure.

Chemical properties

Tungsten hexafluoride is very poisonous and corrosive as it forms hydrofluoric acid with violent reaction when it comes into contact with water . This creates tungsten oxyfluoride or tungstic acid :

${\displaystyle \mathrm {WF_{6}+4\ H_{2}O\longrightarrow H_{2}WO_{4}+6\ HF} }$

The high tendency to hydrolysis, in contrast to the sulfur hexafluoride , which at first glance appears to be similar, can be explained by the significantly larger covalent radius of the central tungsten atom , which is therefore less sterically hindered.

use

In the semiconductor industry

Tungsten hexafluoride is used in the chemical vapor deposition process in the manufacture of semiconductors . When the WF ₆ molecules decompose , a residue of metallic tungsten remains. This layer serves as a low-resistive metallic interconnect .

The expansion of the semiconductor industry in the 1980s and 1990s led to an increase in the consumption of WF ₆ , which is around 200 tons per year worldwide. Tungsten metal is attractive because of its relatively high thermal and chemical stability as well as its low specific resistance (5.6 μΩ · cm) and low electromigration . The use of WF ₆ is more favorable compared to its related compounds (such as WCl ₆ or WBr ₆ ), because of its higher vapor pressure it leads to higher deposition rates . Two deposition processes have been developed since 1967, thermal decomposition and reduction with hydrogen . The required WF ₆ gas purity is quite high and ranges between 99.98% and 99.9995% depending on the application.

Other uses

As a heavy gas, WF ₆ can be used as a buffer to control gas reactions. For example, it slows down the chemistry of the Ar / O ₂ / H ₂ flame and reduces its temperature.

safety instructions

Tungsten hexafluoride is a very aggressive substance that attacks any tissue. When the gas comes into contact with body fluids, hydrofluoric acid is formed, which burns on the skin and the mucous membranes of the airways. Human exposure to the gas initially affects the eyes and respiratory tract, causing irritation, loss of vision, coughing, and excessive saliva and sputum formation. After prolonged exposure, this leads to pneumonitis and pulmonary edema with possible death. Because of these properties, storage containers are provided with Teflon seals.

Individual evidence

1. ↑ ^{a b c d e f g h i j k l m} Entry on tungsten hexafluoride in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
2. ^ ^{A b c} Stanley Siegel, David A. Northrop: "X-Ray Diffraction Studies of Some Transition Metal Hexafluorides", in: Inorg. Chem. , 1966 , 5  (12), pp. 2187-2188; doi : 10.1021 / ic50046a025 .
3. ↑ Johann Schröder, Franz Josef Sieben: "Enthalpy of formation of tungsten hexafluoride and tungsten pentafluoride", in: Chemical Reports , 1969 , 103  (1), pp. 76-81; doi : 10.1002 / cber.19701030113 .
4. ↑ G. Brauer (ed.), Handbook of Preparative Inorganic Chemistry , 2nd ed., Vol. 1, Academic Press 1963, pp. 260-261.
5. ↑ Homer F. Priest, Carl F. Swinehert: "Anhydrous Metal Fluorides", in: Inorganic Syntheses , Volume 3, pp. 171-183, 1950, Wiley-Interscience, ISBN 978-0-470-13162-6 ; doi : 10.1002 / 9780470132340.ch47 .
6. ↑ Patent US6544889 : Method for tungsten chemical vapor deposition on a semiconductor substrate. Published on April 8, 2003 , inventors: Hans Vercamnen, Joris Baele. ‌
7. ↑ ^{a b c d e f} Erik Lassner, Wolf-Dieter Schubert: "Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds", Springer 1999, ISBN 0-306-45053-4 , p. 111, 168 ( limited preview in Google Book search).
8. ↑ Patent US5234679 : Method of Refining Tungsten Hexafluoride Containing Molybdenum Hexafluoride as an Impurity. Published August 10, 1993 , Inventors: SUENAGA TAKASHI, OHASHI MITSUYA, YONEDA TAKASHI, KOBAYASHI YOSHIYUKI. ‌
9. ↑ Patent US6896866 : Method for Purification of Tungsten Hexafluoride. Published on May 24, 2005 , Inventors: KIKUYAMA HIROHISA, WAKI MASAHIDE, FUJIMOTO KAZUYUK, NAKAGAWA YOSHINORI. ‌
10. ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Properties of the Elements and Inorganic Compounds, pp. 4-97.
11. ^ JH Levy, JC Taylor, AB Waugh: "Neutron Powder Structural Studies of UF ₆ , MoF ₆ and WF ₆ at 77 K", in: Journal of Fluorine Chemistry , 1983 , 23  (1), pp. 29-36; doi : 10.1016 / S0022-1139 (00) 81276-2 .
12. ^ ^{A b} T. Drews, J. Supeł, A. Hagenbach, K. Seppelt: "Solid State Molecular Structures of Transition Metal Hexafluorides", in: Inorganic Chemistry , 2006 , 45  (9), pp. 3782-3788; doi : 10.1021 / ic052029f ; PMID 16634614 .
13. ^ J. Levy: "The Structures of Fluorides XIII: The Orthorhombic Form of Tungsten Hexafluoride at 193 K by Neutron Diffraction", in: Journal of Solid State Chemistry , 1975 , 15  (4), pp. 360-365; doi : 10.1016 / 0022-4596 (75) 90292-3
14. ^ Arne Haaland, Andreas Hammel, Kristin Rypdal, Hans V. Volden: "The Coordination Geometry of Gaseous Hexamethyltungsten is not Octahedral", in: Journal of the American Chemical Society , 1990 , 112  (11), pp. 4547-4549; doi : 10.1021 / ja00167a065 .
15. ↑ Frank Weinhold, Clark R. Landis: "Valency and bonding: a natural bond orbital donor-acceptor perspective", Cambridge University Press 2005, ISBN 0-521-83128-8 , p. 427 ( limited preview in the Google book search) .
16. ^ "Tungsten and Tungsten Silicide Chemical Vapor Deposition" .
17. ^ Jean Aigueperse, Paul Mollard, Didier Devilliers, Marius Chemla, Robert Faron, Renée Romano, Jean Pierre Cuer: "Fluorine Compounds, Inorganic", in: Ullmann's Encyclopedia of Industrial Chemistry , Wiley-VCH, Weinheim 2005.
18. ↑ Semi-conducting metal oxide nanoparticles from a low-pressure premixed H ₂ / O ₂ / Ar flame: Synthesis and Characterization , Cuvillier Verlag, ISBN 3-86727-816-4 , p. 52 ( limited preview in the Google book search) .
19. ↑ Tungsten fluoride MSDS (PDF; 24 kB), Linde Gas.

literature

• Otto Ruff, Fritz Eisner: "Ueber das Wolframhexafluorid", in: Reports of the German Chemical Society , 1905 , 38  (1), pp. 742-747; doi : 10.1002 / cber.190503801120 .
• Gmelin's Handbook of Inorganic Chemistry , System No. 54, Wolfram, Part A, pp. 155-157.

Web links

• Tungsten (VI) fluoride at Air Liquide .