Materials Science and Engineering B54 (1998) 92 – 97
Hole-doping of the CuO
2
planes in high T
c
superconductors
H. Yamauchi
a,
*, M. Karppinen
a,b
a
Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226 -8503, Japan
b
Laboratory of Inorganic and Analytical Chemistry, Helsinki Uniersity of Technology, FIN-02150 Espoo, Finland
Abstract
The concept of ‘net holes’ as calculated from the existing crystallographic data via the bond-valence-sum method is utilized for
discussing the charge transfer balances in layered cuprates. When focusing on the net holes, i.e. summing the excess positive charge
residing either on copper or on oxygen in the CuO
2
planes, one notices that in the resulting p(CuO
2
) parameter all the
contributions from the planar Cu – O
pl
bonds are counteracted and only the vertical bonds from Cu to the apical oxygen (O
api
)
and from O
pl
to the nearest-neighbouring cations have net effects on the hole concentration. Based on this observation, three
different ways of doping the CuO
2
planes are defined and illustrative examples of each doping route are given. Furthermore, in
order to estimate the distribution of holes between inequivalent CuO
2
planes in the n 3 members of homologous series of
superconducting cuprates, expressed by M
m
A
2
Q
n -1
Cu
n
O
m+2 +2n
or M-m2(n -1)n, the p(CuO
2
)’s for the individual CuO
2
planes were calculated. As a surprising result, the holes were found to be confined in the innermost CuO
2
planes in all the systems
analyzed. This observation is discussed in comparison with the results obtained for n-type and p-type doped infinite-layer
compounds. Finally, the roles of the apical anion and the Cu–O bond length in the CuO
2
plane are considered. © 1998 Elsevier
Science S.A. All rights reserved.
Keywords: Net holes; CuO
2
planes; High T
c
superconductors
1. Introduction
Since the discovery of the first high T
c
superconduc-
tor (La,Ba)
2
CuO
4
in 1986 [1] enormous achievements in
terms of material development have been seen. Espe-
cially the recent discoveries facilitated by using ultra-
high oxygen pressures for the synthesis have increased
the total number of known high T
c
superconducting
phases to more than 100 [2]. All of them are layered
cuprates with elongated unit cells along the c -axes.
Pyramidally coordinated (coordination number
CN =5 or 6), mixed ( +II/+III)-valent copper in
CuO
2
planes has been considered essential for p-type
high T
c
superconductivity, while the structures of the
existing n-type cuprate superconductors are based on
square-planar (CN =4) CuO
2
planes without the apical
anions. Consistently, only n-type superconductivity has
been observed in the so-called infinite-layer structure
[3]. During the last few years, however, the intensive
research on mixed copper oxides has resulted in the
successful introduction of carriers, besides the two-di-
mensional CuO
2
planes, also into Cu–O chains [4,5],
Cu
2
O
3
ladder systems [6] and triangular Cu
3
O
6
sand-
witch lattices [7]. Challenging goals for the future re-
search are to learn whether these lower-dimensional
structures could be turned to superconducting and what
are the ultimate conditions for superconductivity in
terms of copper coordination. Also, an interesting ques-
tion is which are the optimum ways to dope the existing
as well as the ‘not-yet-discovered’ cuprate
superconductors.
2. Classification of the superconducting cuprates and
general trends in their properties
In order to describe the structures of the existing
superconducting cuprate phases, the concept of ‘ho-
mologous series’ is most useful, at least for the struc-
tures which consist of perovskite and rock-salt type
blocks, only [2]. Each homologous series is written as
M
m
A
2
Q
n -1
Cu
n
O
m+2 +2n
or M-m 2(n -1)n, with n
being the number of CuO
2
planes in one Q
n -1
Cu
n
O
2n
infinite-layer block. Structures which differ only in n, * Corresponding author.
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