rift valley
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Oxidation state of Cu in the superconductor YBa2Cu3O7
Today I had a snow-day so my chem. teacher put our test online if your interested here is the link http://www.stalux.org/classwork/119/OrganicandMaterialsExam.htm
Anyway the first question "In the superconducting ceramic YBa2Cu3O7, what is the average oxidation state of copper, assuming that Y and Ba are
in their expected oxidation states? Yttrium can be replaced with a rare earth element such as La and Ba can be replaced with similar elements
without fundamentally changing the superconducting properties of the material. However, general replacement of copper by any other element leads
to a loss of superconductivity. In what respects is the electronic structure of copper different from that of the two other metallic elements in
the compound?" I know that Yttrium is +3 Ba +2 and oxygen -2 so does this mean that the copper has an oxidation state between +2 and +3? is this
even possible? Could two of the Cu atoms be in a +2 state with the third oxidized to +3? My merck index doesn't even list a +3 state for copper.
Thanks
Rift Valley
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JohnWW
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Yttrium is exclusively trivalent (like Sc and La, and most other rare earths), Ba is exclusively divalent like other alkaline earths, (and almost all
Y and Ba compounds are ionic), oxygen is nearly always divalent. The O atoms produce a charge of -14, Y produces +3, the 2 Bas produce +4; leaving +7
to be accounted for by the 3 Cus, an average valence of 7/3. So 2 of each 3 Cus must be divalent, and the third trivalent. Actually, there would
probably be "resonance" electron transfer among the Cus, making all three equivalent in oxidation state.
I am surprised that the Merck Index does not list a +3 oxidation state for Cu, as I have seen it described in inorganic chemistry textbooks. The
compounds consist only of the fluoride CuF3 and possibly fluorocuprate(III) ion like CuF4- or CuF6---, obtained by direct reaction with fluorine; and
the oxide Cu2O3 and cuprates(III), CuO2-, obtained by fairly drastic oxidation methods such as by electrolysis or reaction of Cu(II) with atomic
oxygen. They are very powerful oxidizing agents. The oxide and cuprates(III) are stated to be steel-blue-gray in color.
But, as it is not produced by methods involving use of strong oxidants, it is more likely that YBa2Cu3O7, being a solid crystalline compound, is a
"non-stoichiometric" compound, with a deficiency of electrons (stabilized by the crystal structure in which there are "holes" making the Cu appear to have an average oxidation state of 7/3. The superconductivity
of it would be due to these positive "holes" being able to move freely in the solid when an external potential difference is applied across
it.
P.S. If you are "snowed in" in New Hampshire, are you at Dartmouth College, by any chance?
[Edited on 11-2-2005 by JohnWW]
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rift valley
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Thanks for the reply. Altough I am in NH I am not at Dartmouth (I wish) I am still in high school and will be attending a university next year. I
won't be attending Dartmouth as it is a highly exclusive school that only the top ten of a graduating class have the chance to be accepted. Only
one person from my school got in and he is ranked #3 (and both his parents went there). My grades are good but not that good.
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JohnWW
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"I won't be attending Dartmouth as it is a highly exclusive school." And an EXPENSIVE one too, no doubt. A playground for
silver-spooners, which shuts out a lot of potential students, no matter how good they may be innately or how hard they try.
"Only one person from my school got in and he is ranked #3 (and both his parents went there)." Of course. That is how nepotism, i.e.
corruption, works, even in academia. He got in only because of his filthy rich, well-connected parents who went there, and who will probably pay for
everything.
Whatever happened to academic merit? Goes to show - there is no beating the "silver spoon" and corrupt connections in high places,
regardless of how good you are or hard you try, even in restricted-entry universities.
BTW: I just looked up "Cu(III)" on Google, and got 5,770 responses. Apparently Cu(III) compounds are more common than I thought, and even in
the absence of fluorine or oxygen Cu(III) is capable of being stabilized by strong ligands; see e.g. http://www.scielo.cl/scielo.php?pid=S0366-16442000000300020&... .
See also http://dx.doi.org/10.1036/1097-8542.160900 which refers to mixed-valence Cu(II)/Cu(III) oxides in high-temperature (liquid nitrogen
cooled)semiconductors.
There are also some very rare Cu(IV) compounds, mostly fluoro-compounds e.g. Cs2CuF6 ; see
http://150.237.140.43/lectures/sja/notes%205%20df%20block.pdf and http://nautilus.fis.uc.pt/st2.5/scenes-e/elem/e02996.html and http://www.springerlink.com/index/5GLDR6UKRDELHQL1.pdf
[Edited on 11-2-2005 by JohnWW]
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Eclectic
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Recipe for 123 Superconductor
http://www.chemsoc.org/exemplarchem/entries/igrant/making_no...
http://www.metlin.org/squids/y123.html
[Edited on 11-2-2005 by Eclectic]
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Antwain
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Wow. I just noticed this thread and felt I had to write something here.
I am in the process of MAKING YBCO at the moment. I should also add that according to my information it is described as being YBa2Cu3O(7-delta) and
nominally ~6.5. With O7 it is not a superconductor, nor with O6. When I go into uni tomorrow I will be giving it its final grind and sticking it in
the furnace for ~24hrs with an oxygen supply from a cylinder passed over it, otherwise you can't get O6.5
Also, I *suspect* that it is one of those funny compounds where you cannot assign formal oxidation states to each copper atom (ie some (II) and some
(III)) otherwise it wouldn't be very special and wouldn't be a superconductor. Since type II superconductors are not well understood (ie. no one knows
why they superconduct yet) there is probably no easy answer to the question of oxidation states.
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