Internal Chemistry of the Pure and Chemically Substituted Quadruple Perovskites Ln'Ln''Ba2Cu2Ti2O11 (Ln'Ln'' = LaY or NdDy)
and
Contribution from the Departments of Materials Science and Engineering and Chemistry and Science and Technology Center for Superconductivity, Northwestern University, Evanston, Illinois 60208-3108, and Department of Physics and Science and Technology Center for Superconductivity, Northern Illinois University, DeKalb, Illinois 60115
Received November 12, 1996
Abstract:
Important features of the internal chemistry of the quadruple
perovskite family of layered cuprates, such
as solid solution behavior, stable defect species, and oxidation
behavior, have been investigated by structural and
physical property measurements and related to the inner architecture of
these potential superconductors. In-situ
high-temperature (650-800 
C) electrical conductivity and Seebeck
coefficient measurements have been performed,
in various oxygen partial pressures, on pure and chemically substituted
Ln'Ln''Ba2Cu2Ti2O11
(Ln'Ln'' = LaY or
NdDy) compounds. The electrical properties display transitions
from dual carrier (intrinsic) semiconductivity to
extrinsically doped p-type semiconductivity as a function of chemical
composition, temperature, and oxygen pressure.
The exclusion of interstitial oxygen defects between the
copper-oxygen double layers, as evidenced in the weak
oxygen partial pressure dependence of the electrical properties in the
LaYBa2Cu2Ti2O11
systems, is directly related
to the A-site order and is an important step toward achieving
superconductivity by preserving the structural integrity
of the CuO22- planes. The
electrical properties of the NdDy system, however, are strong functions
of the oxygen
partial pressure, indicating that oxygen defects play an important role
in determining their properties. Oxidation of
the latter compound has been achieved, and underdoped metallic behavior
reminiscent of known superconductors
has been observed at low temperatures (100-300 K). Jonker
analysis of the electrical properties of the highly
oxidized material further illustrates the similarity of the transport
behavior of the quadruple perovskites to
superconducting cuprates. A combination of synthesis and annealing
methods which allow for both increased
substitution levels and carrier concentrations should lead to
superconductivity in the ordered-lanthanide quadruple
perovskites.
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