Crystallographic studies and X-ray diffraction patterns
of Ba
5
R
8
Zn
4
O
21
by Rietveld refinements
W. Wong-Ng,
a)
J. A. Kaduk,
b)
and J. Dillingham
Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Received 12 October 2000; accepted 2 March 2001
The structure of the Ba
5
R
8
Zn
4
O
21
series (R=lanthanides) was investigated using X-ray Rietveld
refinements. The compounds were successfully prepared for R=Eu, Gd, Dy, Ho, Er, Tm, and Yb.
Ba
5
R
8
Zn
4
O
21
crystallizes in the tetragonal space group I 4/m ; for R=Yb to Eu, a ranges from
13.635 025 to 13.960 629 Å, c from 5.658 463 to 5.784 835 Å, and V from 1051.9878 to
1127.45914 Å
3
. The Zn
2+
ions adopt a fivefold distorted square pyramidal coordination. The
seven-coordinate R
3+
reside in monocapped trigonal prisms. These prisms share edges, and form
layers stacked along the c axis. There are two types of BaO polyhedra: bicapped square prisms
(BaO
10
), and irregular BaO
10
polyhedra. For larger R, Ba
5
R
8
Zn
4
O
21
was not stable, and tetragonal
BaR
2
ZnO
5
La, Nd and orthorhombic BaR
2
ZnO
5
Sm phases were observed instead. © 2001
International Centre for Diffraction Data. DOI: 10.1154/1.1367261
I. INTRODUCTION
Extensive structure-property studies of the substitution
of Cu in Ba
2
RCu
3
O
6 +x
by transition metals—including Ti,
Cr, Mn, Fe, Co, Ni, Au, and Zn—aimed at understanding the
correlations between superconducting properties and doping
have been reported Xiao et al., 1987; Wong-Ng et al., 1990;
Cieplak et al., 1990; and Miceli et al., 1988. When Cu
2+
nine 3 d electrons in Ba
2
YCu
3
O
6 +x
is completely substi-
tuted by Zn
2+
ten 3 d electrons, the material is not super-
conducting, presumably the result of filling of electronic
bands. A strong correlation between superconductivity and
the electronic and magnetic properties of the substituting el-
ement has been reported by Xiao et al. 1987. Therefore,
studies of the structures and properties of the Zn analogs
enhance the general understanding of superconductivity.
Replacement of Cu by Zn in compounds of lanthanide
Ba–R–Cu–O systems has been described Michel et al.,
1981, 1982; Michel and Reveau, 1982, 1983; Wong-Ng
et al., 1998. It was found that the structure of the ortho-
rhombic ‘‘Zn-green phase,’’ BaR
2
ZnO
5
, is similar to that of
the ‘‘Cu-green phase,’’ BaR
2
CuO
5
Michel and Raveau,
1982; Hazen et al., 1987; Watkins and Fronzcek, 1988;
Wong-Ng et al., 1989; Wong-Ng et al., 1998; Kaduk et al.,
1999. BaR
2
ZnO
5
can be prepared for compounds with
R=Sm, Eu, Gd, Dy, and Ho. For smaller R i.e., Er, Tm, Yb,
and Lu, the BaR
2
ZnO
5
structure is not stable. Powder X-ray
diffraction patterns of these latter materials indicated mix-
tures of phases, including the Ba
5
Tm
8
Zn
4
O
21
structure type
Sfreddo et al., 1997; Rabbow and Mu
¨
ller-Buschbaum,
1994. The primary goal of this work was to study the for-
mation and structure of Ba
5
R
8
Zn
4
O
21
as a function the size
of R. Because of the importance of the green-phase type
structure, a comparison of the structure of Ba
5
R
8
Zn
4
O
21
and
of BaR
2
ZnO
5
will also be made.
As the powder X-ray diffraction technique is of primary
importance for phase characterization, extensive coverage of
the superconductor and related phases in the ICDD Powder
Diffraction File™ PDF is essential for the high T
c
super-
conductivity community. Presently, no experimental diffrac-
tion pattern is available in the PDF for Ba
5
R
8
Zn
4
O
21
. The
second goal of this investigation was to supplement the dif-
fraction patterns and crystal structure of the Ba
5
R
8
Zn
4
O
21
series by using the X-ray Rietveld refinement technique Ri-
etveld, 1969; Young, 1995.
II. EXPERIMENT
Certain trade names and company products are men-
tioned in the text or identified in illustrations in order to
adequately specify the experimental procedure and equip-
ment used. In no case does such identification imply recom-
mendation or endorsement by National Institute of Standards
and Technology.
A. Sample preparation
Eleven polycrystalline compositions of the Ba
5
R
8
Zn
4
O
21
series R=Nd, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu
were prepared using the solid state sintering method. Sto-
ichiometric blends of powdered BaCO
3
,R
2
O
3
, and ZnO
were well-mixed, and compacted by pressing the powder in a
pelletizing die to about 0.3 GPa. The compacted powders
were heated in air at 850 °C for 2 days, followed by a further
5 days of heat treatments. Each time after the samples were
removed from the furnace, they were reground and repellet-
ized. Approximately 1/2 g of each material was prepared.
B. X-ray powder diffraction
X-ray powder diffraction was used to identify the phases
synthesized and to confirm phase purity. A computer-
controlled automated diffractometer equipped with a theta-
compensation slit and Cu K radiation was used at 45 kV
and 40 mA. The diffracted beam was detected by a scintilla-
tion counter and a solid state amplifier. The Siemens soft-
ware package and the reference X-ray diffraction patterns in
the ICDD Powder Diffraction File PDF were used for per-
forming phase identification for secondary phases.
For Rietveld studies, samples Ba
5
R
8
Zn
4
O
21
were
mounted in a zero-background quartz cell with double-sided
a
Electronic mail: winnie.wong-ng@nist.gov
b
Also at: BP Amoco p.l.c., Naperville, IL 60566.
131 131 Powder Diffraction 16 (3), September 2001 0885-7156/2001/16(3)/131/13/$18.00 © 2001 JCPDS-ICDD