Why is the pure tungsten ($\ce{W}$) melting point way higher than that of its oxides $\ce{WO3}$ or $\ce{WO2}$? And why does pure $\ce{Al}$ react differently? I.e. the oxides have a higher melting point than pure $\ce{Al}$.

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  • $\begingroup$ I think along with these, $\ce{W87O13}$ exists as well. $\endgroup$ – M.A.R. Feb 5 '15 at 13:38
  • $\begingroup$ The phase diagrams available at the ASM Alloy Phase Diagram Database (if your institution has access) show varying numbers of 'stable' phases. Most are likely various orderings on the oxygen sublattice (my guess). I can find no recent (last 20 years) work on the thermodynamics of the oxides. $\endgroup$ – Jon Custer Feb 5 '15 at 14:31
  • $\begingroup$ There is no reason to think that the oxide shold have a higher melting point. The bonding mechanism in the metal is totally different, you cannot reasonably compare their physical properties. $\endgroup$ – Karl Jan 10 '17 at 0:44

As far as I know,

$\ce{WO2}$ and $\ce{WO3}$ have a covalent characteristic, and thus lower melting point than the metal.

while, $\ce{Al2O3}$ have more ionic characteristic, and thus higher melting point than the metal.

$\ce{W}$ is a d-block metal from the $\ce{3^{rd}}$ row which make it possible to have covalent materials rather than ionic materials.

this article might prove this point http://pubs.acs.org/doi/abs/10.1021/jp963724z?journalCode=jpcbfk

  • 4
    $\begingroup$ Well, except that it doesn't explain why the titanium oxides, cobalt oxides, and chrome oxides, amongst others, have higher melting temperatures than their metals. Bottom line is: thermodynamics of solid phases may not be easy to predict based on simple heuristics. $\endgroup$ – Jon Custer Feb 5 '15 at 18:06

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