An established feature of
superconductivity in alkali fullerides is the existence of a simple
relationship between Tc
and the cubic lattice constant, a, namely: Tc increases monotonically with a.
This simple empirical rule has motivated efforts to synthesise fulleride salts
with expanded lattice dimensions via co-intercalation of small molecules like NH3
in order to drive Tc even higher. Ammoniation of
superconducting alkali fullerides of stoichiometry A2A'C60
has led to diverse responses in their structural, conducting and magnetic
properties.
Upon
ammoniation, the Tc of Na2CsC60
increases significantly from 10.5 to 29.6 K whereas K3C60
superconductor becomes and antiferromanetic insulator with a metal-insulator
transition at 40 K. The ultimate goal will be to prepare compositions of the
general type (NH3)xNa2AC60
(A= K, Rb). By varying
both the ammonia content x and selecting the alkali metal A
we can adjust the lattice size in order to explore the phase diagram of the
alkali fullerides close to the boundary between the superconducting to metal to
insulator regions.
Selected Publications:
[1] A. Lappas, K. Vavekis,
and K. Prassides,
"C70 Fulleryl Radicals"
J. Chem. Soc., Chem. Commun. 1994, 2743-2744.
[2] A. Lappas, K. Prassides, K. Vavekis, D. Arcón,
R. Blinc, P. Cévc, A. Amato, R. Feyerherm, F.N. Gygax, and A. Schenck,
"Spontaneous Magnetic Ordering in the Fullerene
Charge-Transfer Salt (TDAE)C60"
Science 1995, 267, 1799-1802.
[3] A. Lappas, M. Kosaka, K.
Tanigaki, and K. Prasssides,
"An Orientationally-Ordered Primitive-Cubic Form of
the Fulleride CsC60"
J. Am. Chem. Soc. 1995, 117, 7560-7561.
[4] M. Kosaka, K. Tanigaki, K.
Prassides, S. Margadonna, A. Lappas, C.M. Brown, and A.N. Fitch,
"Superconductivity in LixCs1C60 Fullerides"
Phys. Rev. B 1999, 59, R6628-6630.
[5] A. Lappas, C.M. Brown, K.
Kordatos, E. Suard, K. Tanigaki, and K. Prassides,
"Neutron Diffraction Study of the Polymeric Structure
of Na2RbC60"
J. Phys.: Condens. Matter 1999, 11, 371-381.
[6] S. Margadonna, C.M. Brown, A.
Lappas, K. Prassides, K. Tanigaki, K.D. Knudsen, T.L. Bihan, and M.
Mezouar,
"Pressure and Temperature Evolution of the Structure
of the Superconducting Na2CsC60
Fulleride "
J. Solid State Chem. 1999, 145, 471-478.