Introduction to Superconductivity

1
Sr2CuO2F2?

• Introduction to Superconductivity
• Introduction to Sr2CuO2F2?
• Preparation of Sr2CuO2F2?
• Structural rearrangement
• Superconducting properties
• Conclusions

2
SUPERCONDUCTIVITY

• A superconductor is a material characterised by
  unique electrical and magnetic properties which
  appear below a critical temperature Tc

Two technologically important properties
3
Zero Electrical Resistance
Superconductors carry current without energy loss
4
Perfect Diamagnetism
Superconductors levitate above magnetic fields
5
Applications of Superconductors
Medical Industry
MRI Exploits the high magnetic fields
expelled by superconducting wires for medical
applications
Transportation Industry
Maglev trains hover above a magnetic field
without any contact with a track
6
High Tc Superconductors
Ternary or multinary cuprates
Superconductivity is induced by cationic
substitution
Ba2/La3
Cu2/Cu3
La2CuO4
La2-xBaxCuO4
Ce4/Nd3
Nd2CuO4
Nd2-xCexCuO4
Cu2/Cu1
Cu2 partially oxidises / reduces in order to
maintain charge neutrality in the ionic compound
7
Sr2CuO2F2?
The first oxide-fluoride to show high Tc
superconductivity
Superconductivity is induced by anionic
substitution / insertion
Discovered in Birmingham in 1994 (M. Al-Mamouri,
P.P. Edwards, C. Greaves and M. Slaski, Nature,
1994, 369, 382)
After the discovery of Sr2CuO2F2?, many research
groups all over the world investigated
oxide-fluorides and similar compounds
The scientific interest of Sr2CuO2F2? lies
in Preparation Structure Superconductivity
(Tc46K)
8
Preparation of Sr2CuO2F2?
Sr2CuO2F2? is a metastable compounds
Sr2CuO2F2? cannot be made by high temperature
solid-state reaction
9
Preparation of Sr2CuO2F2?
Sr2CuO2F2? is prepared by soft chemistry
synthetic routes (low T)
Preparation of Sr2CuO2F2? involves a two-step
reaction
1.
Preparation of precursor Sr2CuO3 by high
temperature reaction
Fluorination of Sr2CuO3 by low temperature
reaction
2.
Three fluorination routes were used
10
Fluorination Routes
Solid-gas reaction
Disadvantages toxicity of F2, small amount of
product
Sr2CuO3 F2 (gas)
Solid-state reactions (fluorinating agents)
Sr2CuO3 NH4F
Disadvantage formation of SrF2
Sr2CuO3 MF2
Disadvantage formation of MO
11
Reaction Mechanisms
NH4F
MF2
12
Structural Rearrangement
F-
Sr2CuO2F2?
Sr2CuO3
Cu2 is coordinated by four O2- (geometrysquare
planar)
Cu2 is coordinated by four O2- and two F-
(geometryoctahedral)
13
Structural Rearrangement
14
Superconducting Properties
In Sr2CuO2F2?, ? represents the excess fluorine
in interstitial sites
Excess fluorine means excess of negative charge
within the compound
Cu2
Cu3
to balance the excess (-)
Cu2/Cu3 mixed valence in Sr2CuO2F2?
superconductivity
15
Conclusions

• Sr2CuO2F2? was the first mixed-anion compound to
  show superconducting properties
• Sr2CuO2F2? was the first compound in which
  superconductivity had been induced by anionic
  substitution