Superconductivity ?1000 °C for around 48 h.

Superconductivity was discovered by H,
Kamerlingh Onnes in Leiden in 1911 but full understand of the phenomenon was
not known by scientists until the 1950’s.  The definition of superconductivity is the
phenomenon of exactly zero electrical resistance and expulsion of magnetic flux
fields when cooled to its critical temperature TC. This is different to perfect conductivity where
magnetic flux does not tend to be expelled. (1) Superconductors have the
potential to be very useful however their remarkably low critical temperatures
often limit their ability to be used. Raising the critical temperature or
finding new superconductors with higher critical temperatures would be
particularly beneficial.
Intercalation is the reversible process of inserting a molecule, atom or ion
between the layers in a crystal lattice. This is often done to change the
properties of a molecule or increase/improve the favourable properties it
already possess’, in this case in combination with increased pressure it allows
superconductivity of FeSe at a much higher critical temperature.  Compounds (2)

Structure of FeSe:
Iron based superconductors were only recently discovered in 2008 and are
similar to the better known cuprate superconductors in their
quasi-two-dimensional crystal structure. However unlike cuprates, the FeSe
layers are thought to be the reason the compound has the ability to be a
superconductor as seen in Fig.1. FeSe alone has the TC of   ?8K and it was one of the earlier Iron based
superconductors discovered as a PbO structure type tetrahedral compound.
Research has shown that an excess of Fe in the compound is beneficial to maintaining
the stabilisation of the crystal structure, but the correct stoichiometry is
vital to maintaining superconductivity.One way of preparing Barium intercalated FeSe is by combining Fe
and Se, in powder form, and using a mini mill Pulverisette to mix them. The
pulverisette uses a centrifugal force as well as a grinding wall to combine the
two elements. The powder product from this can then be pressed into a pallet
and vacuum-packed into a quartz tube which is annealed at   ?1000 °C for around 48 h. Following this the
compound is quenched to 410 °C and tempered for a further 100 h. Ice water is
used to quench the compound to form ?-FeSe and a very small amount of ?-FeSe as
a side product. A Schlenk-line can be used to intercalate other elements, in
this case Barium, into the FeSe compound. The reaction can take place in an
ammonia filled line and an Ar occupied glove-box. Dry ice would then be used to
condense NH3 (l) and evacuation and purification techniques are then
used to prepare the apparatus. Two reactions can take place in order to produce
the desired material, one at low temperature and one at room temperature.
The low temperature reaction can occur in a round bottom flask where a stoichiometric
amount of Ba is added to an amount of ?-FeSe in a solvent. Reaction completion and
solvent removal can take up to 12 h then the product can be dried in a vacuum,
after which X-ray diffraction analysis can be used to determine the correct product
was synthesised.
At Room temperature ?-FeSe powder and a stoichiometric amount of Ba can be
placed in a glass container together with a glass stirer  (6)

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