306
Chapter 8: Crystal Structures of High-T~ Superconducting Cuprates
diffraction data allow a precise determination of the positional and occupational
parameters of the oxygen sites, whereas, in some cases, X-ray diffraction data are
more reliable for the refinement of the metal-atom distribution on the cation sites.
Crystallographic data sets included in the present work refer to complete
structural refinements carried out on superconducting cuprates that are widely
recognized and contain the main structural features. Structures with short in-
plane cell parameters corresponding to the perovskite cell (~3.85 A) or its
diagonal (,~ 5.4 A) are given in the data sets, whereas superstructures reported
in the literature are mentioned in remarks. When no structural refinement on a
superconducting cuprate was available, or when no superconducting representa-
tives were known so far, a crystallographic data set for a related nonsupercon-
ducting cuprate was chosen. Several data sets, presenting deformations typical for
particular chemical families or different kinds of
additional
layer
(A, AO, AO',
AO" or AO2), are sometimes given for the same four-digit code.
Each data set is preceded by a framed header containing the four-digit code
(see Section C,f) and a chemical formula of the compound for which complete
data are given, where the elements are ordered according to the four-digit code.
The first line lists the generalized formula of the structure type (see Introduction),
the Pearson code (Bravais lattice and number of atoms in the unit cell, ignoring
extra, partly occupied oxygen sites), the number and Hermann-Mauguin symbol
of the space group, and the Wyckoff sequence (Wyckoff letters of occupied atom
sites, Wyckoff letters of extra, partly occupied, oxygen sites in
additional A
layers
being placed within parentheses) for the ideal structure. On the second line, the
sequence of layers in the stacking unit is indicated. The next two lines contain the
refined composition of the representative compound, its superconducting transi-
tion temperature (Tc), a code for the diffraction method used, the temperature (T)
for the data collection, the reliability factor of the structural refinement, the
literature reference, the number and Hermann-Mauguin symbol of the space
group, the cell parameters (a, b, c), and the number of formula units in the cell
(Z). The refined composition as given here, multiplied by Z, gives the actual
number of atoms in the unit cell. By default, the superconducting transition
temperature is given as onset of the diamagnetic signal (n.s., nonsuperconduct-
ing). The diffraction data can be single-crystal neutron (SN), single-crystal X-ray
(SX), powder neutron (PN), or powder X-ray (PX). Preference has been given to
crystallographic parameters refined on data collected at room temperature (RT).
The reliability factor reported here is either conventional (R) or weighted
(Rw)
for
refinements on a single crystal, Bragg (Re) or weighted profile
(Rwp)
for
refinements on powder. The space group is given in a setting with the c axis
parallel to the stacking direction, the Hermann-Mauguin symbol for the standard
setting as defined in the International Tables for Crystallography (Hahn, 1983)
being added within parentheses when different. Note that the space group used
for the refinement of the real, often considerably distorted structure, may differ
from the space group of the structure type (see Tables 8.2 and 8.4). In this case
the cell transformation from the ideal structure (new axes and, when relevant, a