PhaseTransformations in the High-Tc Superconducting Compounds, Ba2RCu3O7-delta (R = Nd, Sm, Gd, Y, Ho, and Er)

0.58 MB·English

by  Wong-Ng, W.

Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
[J. Res. Natl. Inst. Stand. Technol. Ill, 41-55 (2006)]
Phase Transformations in the High-T^
Superconducting Compounds,
Ba2RCu307-5 (R=Nd, Sm, Gd, Y, Ho, and Er)
Number 1 January-February 2006
Volume 111
W. Wong-Ng and L. P. Cook
The phase transformation between the
the lanthanide series can be explained
using a quasi-chemical approximation
orthorhombic and tetragonal structures of
National Institute of Standards
(QCA) whereby the strain effect plays an
six high-Jj, superconductors, Ba2RCu307_^
and Technology,
important role on the order-disorder transi-
where R = Nd, Sm, Gd, Y, Ho, and Er, and
Gaithersburg, MD 20899-0001
tion due to the effect of oxygen content on
5= 0 to 1, has been investigated using
techniques of x-ray diffraction, differential the CuO chain sites.
H. B. Su
thermal analysis/thermogravimetric analy-
sis (DTA/TGA) and electron diffraction.
Beckman Institute, California
The transformation from the oxygen-rich
Institute of Technology,
orthorhombic phase to the oxygen-defi-
Pasadena, CA 91125
cient tetragonal phase involves two
orthorhombic phases. A superlattice cell
M. D. Vaudin and C. K. Chiang
Key words: ac magnetic susceptibility;
caused by oxygen ordering, with a' = 2a,
orthorhombic/tetra-gonal transformation;
National Institute of Standards
was observed for materials with smaller
oxygen ordering; phase transformation of
ionic radius (Y, Ho, and Er). For the larger
and Technology,
Ba2RCu307_5 (R=lanthanides); strain
lanthanide samples (Nd, Sm, and Gd), the
Gaithersburg, MD 20899-0001
effect on phase transition; transition tem-
a' = 2a type superlattice cell was not
perature.
observed.
D. R. Welch
The structural phase transition tempera-
Department of Materials Science,
tures, oxygen stoichiometry and character-
Brookhaven National Laboratory,
istics of the T^ plateaus appear to correlate
with the ionic radius, which varies based
Upton, NY 11973
on the number of f electrons. Lanthanide
and
elements with a smaller ionic radius stabi-
Accepted: January 3, 2006
lize the orthorhombic phase to higher
E. R. Fuller, Jr, Z. Yang, and
temperatures and lower oxygen content.
L. H. Bennett
Also, the superconducting temperature is
less sensitive to the oxygen content for
National Institute of Standards
materials with smaller ionic radius. The
and Technology,
trend of dependence of the phase transfor-
Gaithersburg, MD 20899-0001
Available online: http://www.nist.gov/jres
mation temperature on ionic radius across
1. Introduction
nologies for producing textured coated conductors
show promise: Ion Beam Assisted Deposition (IBAD)
Recent advances in coated conductor science and [2-5], Rolling Assisted Bi-axially Textured Substrate
engineering have brought commercial high-r^ super-
(RABiTS) deposition [6-11], and Inclined Substrate
conductor technology closer to reality [1]. It is now Deposition (ISD) technique [12,13]. Good quality tex-
likely that many potential large-scale industrial applica- tured conductors, which are based on the BajYCujOy.^
tions will soon be realized. Three state-of-the-art tech-
superconductor, have been successfully produced with
41
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
2. Experimental
both film deposition and open-air solution techniques
[14-22]. To further optimize the superconducting prop-
erties of long-length coated conductors for practical Since NdjOj reacts with atmospheric moisture to
applications, recent research has also included the use form Nd(OH)3, the NdjOj powder was heat-treated at
600 °C overnight prior to sample preparation. A single
of lanthanide-substituted variants, BajRCujO,.^
(R=lanthanides with stable 3+ oxidation state). Phase phase master batch of BajNdCujOy.^ was prepared from
equilibrium research pertaining to BajRCujO,.^ includ-
a stoichiometric mixture of CuO, NdjOj and BaCOj.
ing a thorough understanding of phase transition phe-
Before firing and annealing, the powder was pressed
nomena, is therefore important for processing.
into pellets and placed on MgO single crystals. The
The progressive reduction in size of the lanthanide, pellets were reground and annealed several times until
which is known as the lanthanide contraction, allows the presence of a single-phase material was confirmed
by x-ray powder diffraction analysis. Annealing was
systematic study of the trend of crystal chemistry, solid
solution formation, and phase equilibria in the systems carried out at temperatures of 850 °C, 870 °C, and
900 °C each in air for 1 d, followed by firing in
BaO-RjOj-CuO^ as a function of the size of lanthanide
air at 940 °C for 2 d. Analysis by scanning electron
ion, R^^ [23-29]. Numerous investigations pertaining to
the crystal chemistry, and the effect of oxygen stoi- microscopy and energy dispersive x-ray analysis
chiometry on properties of BajRCujOy.^ have been (SEM/EDS) showed absence of substitution of magne-
reported [30-35]. The present paper is part of our con-
sium in these samples.
tinuing effort to understand the effect of lanthanide A total of 10 barium-neodymium-copper-oxide sam-
substitution on the properties and processing parame-
ples were prepared for this investigation from the
ters of the high-r^ superconductors BaRjCujOy.^ [23- single-phase master batch. Batches of the other
29,36-39]. Since our preliminary reports [36-38] on the BajRCujOy.^ phases were prepared in similar fashion,
structural phase transitions of BajRCujOy.a for R = using the appropriate lanthanide oxides [36-38]. To
Sm, Gd, Er, Y, and Ho, we have completed studies of investigate the phase transitions, specimens weighing
the Nd series, including additional characterization of
about 200 mg to 500 mg were annealed in an MgO cru-
some of these materials using electron diffraction tech-
cible for up to 2 d in air at temperatures between 400 °C
niques. We have also improved our understanding of and 900 °C. The temperature was measured using a
the phase transition in BajRCujO,.^ as related to the Pt/PtlORh thermocouple calibrated against the melting
size of ionic radius of R^^ by using a quasi-chemical point of gold. Temperature of the Pt-wound resistance
approximation to describe the effect of oxygen order-
furnace used in these experiments was controlled to
disorder [40].
± 2 °C by using a Wheatstone bridge type controller.
After annealing the BajRCujO,.^ compounds at vari- After annealing, the samples were quenched into a liq-
ous temperatures and then rapidly quenching to liquid uid nitrogen-cooled copper cold well, through which
nitrogen temperature, we have made several observa- liquid nitrogen-cooled helium gas was passed at a rapid
tions concerning the phase transitions in these materi- rate (Fig. 1). Rapid cooling under these conditions pre-
als. Most interesting is the presence of T^ plateaus vented the oxygen gain that would normally occur for
(annealing temperature ranges over which 7"^ remains samples cooled in air.
approximately constant, even as the oxygen content
Samples studied by electron microscopy are listed in
Table 1, together with their processing conditions and
changes). Examples are given of the existence of a 90
oxygen content (all were quenched in liquid nitrogen-
K and a 60 K T^ plateau in the Y, Ho, and Er materials
[36-38,41-42]. Although there are extensive literature cooled helium). Specimens for electron microscopy
reports [43-49] describing the experimental observa- were prepared by crushing small pieces of the sample
tions or theoretical predictions of a single phase 60 K material and then following one of two procedures:
material in the Y system, very little information has
either the powder was dispersed in ethanol and drops of
previously been reported describing the lanthanide- the liquid were placed on a carbon-coated copper grid,
substituted materials. Furthermore, most of the report- or a grid was scooped through the powder to avoid pos-
ed Y materials were prepared using processing routes sible reactions with the alcohol. These two techniques
different from our quenching methods [46-49]. produced samples with no detectable differences
between them. Conventional electron microscopy and
electron diffraction were carried out at 120 keV and
300 keV, and exposure times up to 50 s were used to
record the faint superlattice diffraction spots.
42
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
Digital x-ray data were collected at room tempera-
He Gas
ture on a computer-controlled powder diffractometer
equipped with a focusing graphite crystal monochro-
mator and a theta-compensating slit. Copper radiation
(CuKtti, 1= 1.5405981 A [50]) was employed for all
studies. Two certified (/-spacing standards: silicon,
SRM640b [51], and fluorophlogopite, SRM675 [52]
Dewar - *
were used as internal standards for calibration [53].
Sample preparation, mounting methods and data pro-
Liquid
cessing followed those described by McMurdie et al.
Nitrogen Bath
[54].
The flux exclusion of these powders was studied by
Copper
MgO
Crucible
Cold Well
using a computerized ac magnetometer. A sample pow-
der of 10 mg to 20 mg was packed in a small, non-mag-
Fig. 1. An experimental setup showing a liquid nitrogen cooled cop-
netic holder and mounted on a stage containing a cali-
per cold well into which liquid helium was rapidly flowing through
brated silicon diode thermometer. The ac susceptibility
as an annealed sample that was contained in a MgO crucible was
quenched.
was measured as a fiinction of temperature from 300 K
to 20 K for most samples in a Hartshorn type bridge cir-
cuit at a frequency of 1.68 kHz. The magnitude of the
Table 1. Samples investigated by electron difiraction techniques in
the transmission electron microscope
applied ac field was about 0.5 x 10"* T. The relative
Meissner effect was detected by observing the real part
Annealing Oxygen
Sample Chemical Time
of the signal arising from the diamagnetism of the sam-
notation content
formula temperature
ple. We define the superconducting onset temperature,
(7-5)
as the temperature at which the ac susceptibility
TQQ,
20 h
Nd6.90 BajNdCujO,.^ 620 °C 6.90
deviates from the near zero value of the normal state.
40 h
Sm6.75 BajSmCujOy.^ 580 °C 6.75
20 h
Gd6.79 BajGdCujO,,^ 620 °C 6.79
36 h
Y6.85 BajYCujO,.^ 502 °C 6.85
3. Results and Discussion
23 h
Y6.63 BajYCujO,.^ 635 °C 6.63
21 h
Y6.55 BajYCujO,.^ 675 °C 6.55
The x-ray spectra of Ba2RCu307_5(5from 0 to 1) are,
in general, similar to the yttrium analogs [24]. The pro-
The oxygen content of the BajRCujO,.^ compounds gressive changes of shape and the indexing of peaks in
was determined by thermogravimetric analysis (TGA) the five main regions of the Bragg angle, 26, around
from measurements of the weight change as a fiinction 32° to 33°, 38° to 39°, 45° to 49°, 57° to 60°, and 68° to
of temperature in both air and in an oxygen atmosphere. 70° reveal the crystallographic phase transition from
An MgO sample holder was used for the yttrium com-
orthorhombic to tetragonal for all compounds. Figure 2
pound and a platinum sample holder was used for the illustrates the diffraction patterns of the BajNdCujOy.^
other four samples, since the temperatures of the TGA samples quenched from various temperatures at
curves were below those at which any reactions with 401 °C, 545 °C, 570 °C, 578 °C and 592 °C. These dif-
platinum were observed. The ground powder was heat- fraction patterns demonstrate a gradual reduction in
ed from 50 °C to 850 °C at a rate of 2 °C per min to orthorhombicity as evidenced, for example, by the peak
measure the change of weight, and thereby oxygen shape changes for the 200 and 020 reflections at 46° to
uptake and loss. The maximum observed weight was 48° 26 and the 213 and 123 reflections at 57° to 59° 2ft
assumed to correspond to full oxygenation of seven
oxygen atoms, or BajRCujOy. The weight loss at a 3.1 Lattice Parameters
given temperature was then used to compute the oxy-
gen content. The correspondence between oxygen con-
Using the least-squares refinement results [55], the
tent and quenching temperatures was thus established. transition from orthorhombic to tetragonal symmetry
Due to the difficulty of applying a precise correction
was estimated to take place between 570 °C and
for the buoyancy effect, oxygen content established in
578 °C. No doubling of cell parameters along either the
this way may have an estimated relative uncertainty of a ox b axes has been observed for BajNdCujO,.^ by x-
± 5 % {k=2). ray powder diffraction. Based on a similar analysis, the
43
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
ion The a and b axis dimensions {a„, b„) in the
R'^.
orthorhombic structure and the a axis dimension (a,) in
the tetragonal structure remain the largest for
Ba^NdCu307_5 across the entire annealed temperature
range. Correspondingly, those of Ba2ErCu307_5 are the
smallest.
Table 3. Least-squares cell parameters for Ba2NdCu307_^as a func-
tion of quenched temperatures (°C). Number in parenthesis indicates
one standard deviation from results of least-square refinements [37]
c(A)
a(A) b(A) V(A')
Quenched
temperatures
401
3.8681(11) 3.9180(3) 11.762(3) 178.26(7)
464
3.8676(14) 11.779(2)
3.9223(8) 178.69(6)
501
3.9237(12) 11.781(2)
3.8724(7) 179.00(5)
26 n
556
3.882(2) 3.9180(12) 11.792(4)
179.30(9)
570
3.8870(2) 11.797(2)
3.9115(5) 179.35(8)
Fig. 2. X-ray diffraction patterns (CuKtti) from Ba2NclCu307_5Sam-
578
3.9034(a) 11.799(1) 179.78(3)
ples quenclied from 401 °C, 545 °C, 570 °C, 578 °C, and 592 °C,
592
11.8103(14)
3.9029(6) 179.90(5)
showing progressive changes of peak shapes. Selected Miller indices
697
3.9051(4)
11.8336(11) 180.46(3)
(hkl values) of the diffraction peaks for the sample quenched from
800
11.8511(12)
3.9025(3) 180.48(3)
401 °C are indicated.
900
11.8564(12)
3.9000(3) 180.34(3)
Structural transition temperature for the other lan-
The variation with annealing temperature of the c
thanide analogues were found to be as follows: Sm:
625 °C to 650 °C, Gd: 650 °C to 660 °C, Y: 708 °C to axis cell dimension and the cell volume of these six
720 °C, Ho: 748 °C to 761 °C, and Er: 750 °C to compounds is illustrated in Fig. 4. Although these
770 °C [36-39,41-49]. The tetragonal-orthorhombic curves show, in general, the expected trend of increas-
structural transition temperatures are summarized in ing value as the temperature increases, the
Table 2 along with ionic radii of the lanthanide ions, R^^ Ba2GdCu307_^ compounds behave somewhat different-
[56] and the estimated oxygen composition. ly. For example, the c-dimension did not fall in the
expected order relative to the other compounds. By
3+
analogy with the yttrium system, the elongations along
Table 2. Shannon's ionic radii [56] for R (Vlll-coordination),
tetragonal orthorhombic phase transition temperature and oxygen
the c axis of these compounds are considered to be due
content, (x = 7-5), for selected Ba2RCu307_g
to the increased number of oxygen vacancies. At
400 °C the relative c axis cell dimensions of these com-
Shannon's
pounds are as expected: > Cs„ > > Cy- > c^r
CN^ CQ^ CHO
Oxygen
Compound ionic radius Temperature
At higher temperature the trend alters and becomes
content
(°C)
R3+(A)[56]
> Cs„ > > Cy > > The volume plots in
CNCJ CHO CEJ CQJJ-
6.82-6.84
Ba2NdCu307_5 1.109 570 to 578
Fig. 5 illustrate the expected trends in volume expan-
Ba2SmCu307_5 1.079 625 to 650 6.65-6.69
sion as the annealing temperatures increase, namely,
Ba2GdCu307_^ 1.053 650 to 660 6.75-6.76
V^,>Vs,^>V^,>Vy-V„,>V^,.
Ba2YCu307_5 1.019 708 to 720 6.47-6.49
Figure 6 shows the first derivative of the TGA curves
Ba2HoCu307_5 1.015 740 to 760 6.46-6.47
1.004 6.38-6.41
Ba2ErCu307_^ 750 to 770
against temperature for the six compounds. While the
weight change curves (not shown) are continuous, the
first derivatives show relatively abrupt changes in
Table 3 gives the cell parameters of Ba2NdCu307_5 slope. The temperature at which this abrupt change in
calculated from the x-ray patterns. Figures 3(a) to 3(f) temperature takes place can be considered as due to a
depict the convergence of the a and b axis dimensions phase transition, presumably the orthorhombic/tetrago-
as the annealing temperature rises for these six com- nal transition, and the trend of this behavior therefore
pounds. While the merging curves exhibit similar shape parallels that listed in Table 2, namely, the lanthanide
and form, the different positions of the convergence of elements with a smaller ionic radius stabilize the
these curves can be related to the size of the lanthanide orthorhombic phase to higher temperatures.
44
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
w
(b)
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■
- ^ - ^ > ' > -
■ ■ ■
400 BOO «00 1DC0
400 wo eOO ^ 1000
ArreBfrig Temperature {C)
Aniaalina Tenverahre {C)
Fig. 3. Crystallographic dependence of the a and b axes of Ba2RCu307_j on the annealing temperature. Lanthanide ion R:
(a) Nd (b) Sm (c) Gd (d) Y (e) Ho, and (f) Er.
45
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
-
1.185 -
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sy.
Nd
10 (Cn* ^---—
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~~-~~- Sm
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0 100 EOO ^00 ^00 500 600 TOO WO 900 1000
Temperatire ('C)
1000
400 600 800
Fig. 6. Thermogravimetric analysis of Ba2RCu307_g showing the
slope of the heating curve.
Annealing Temperature {"C)
Fig. 4. The c axis cell parameter as a function of annealing tempera-
C
ture for BajRCujO,,^ with R = Nd, Sm, Gd, Y, Ho, and Er.
1^
A A A
-
leo
lA
'^9-~''-v*7T
lA
A
179
A
Ndj^
m^
176
■
■
177
- Sm^
o
176
o
r Gdo
0
0
17E
A
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^^
174
A
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□
Y*
a
173
- Er°
17?
1 —1_
III'
300 400 SOO 900 TOO BOO 900 lOOO
Cua
Anrwai>g Tediparature (°C]
Fig. 5. Unit cell volume as a function of annealing temperature for
BajRCujO,.^, with R = Nd, Sm, Gd, Y, Ho, and Er.
3.2 Chain-Oxygen Order-Disorder Transition
Figure 7 shows the structure of BajRCujOy.^ with the
Cul
labehng of atoms and the oxygen sublattice site. Curve
(a) of Fig. 8 shows a plot of these experimental transi-
tion temperatures as a function of the ionic radius of the
R^^ ions. An obvious trend is observed. Lanthanide ele-
ments of smaller ionic size stabilize the orthorhombic
phase to a higher temperature as well as to lower oxy-
Fig. 7. Crystal structure and atom labels for Ba2RCu307_j. The
gen content. This trend can be understood in terms of
repulsion energy, v, between the oxygen atoms on two sublattice sites
order-disorder theory.
is represented as "v".
46
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
Eq. (1), k is the Boltzmann constant. The long-range
. 'Er
1050
—EKpeimental Data
■
Ho
order parameter is defined such that the fractional site
—Theorefical Results
1000
occupancy of oxygen on one of the two sublattice site
^
(3 is c{\+S), while that on sublattice a is c{\-S). In
Gd
^ 950
Sm
BajYCujO,.^, c is assumed to be 0.5 when 5 is zero, and
X
^900
0)
d= \ -2c. The order-disorder transition temperature
■
vhJd
(orthorhombic to tetragonal), is therefore related
TQ_J,
•" 350
to the oxygen-oxygen repulsion energy, v, on the two
-
800
sublattice sites and the value of the average site occu-
V.
■
pancy c by
750
■
Jl —
■
J 1
1.10 1.12
1.00 1.02 1.04 1.06 1,08
A:7;,_j. =vln{16(l-c)/[l-4(l-2c)']}.
(2)
Radius of R^^ (A)
The trend of dependence of the ionic radius across
Fig. 8. Chain-oxygen order-disorder experimental (curve (a)) and
the lanthanide series using the QCA is summarized in
theoretical (curve (b) [40]) phase transition temperatures vs.
Shannon's ionic radius of R ^(Vlll-coordination) [55]. The theoreti-
Fig. 8. The upper curve (a) represents the experimental
cal data are calculated by using the transition temperature of
data taken from this work while the lower one (b) rep-
Ba2YCu307_j as a reference.
resents the theoretical results using the current data
(i.e., experimental phase transition temperatures) [40].
Theoretical studies aimed at understanding the phase The theoretical data are calculated by using the transi-
transformation in the BajYCujO,.^ system have been
tion temperature of BajYCujOy.^ as a reference (transi-
carried out extensively by Wille et al. [44], Bakker et al. tion takes place at an oxygen content of 6.5). The cal-
[57], and Herman [58]. Recently, the similar approach
culated results agree with experimental data in that the
has been applied by Su et al. to the lanthanide-substitut-
larger the ionic size of R, the lower the
ed systems [40]. In brief, the formation energies of orthorhombic/tetragonal phase transition temperature.
Frenkel pair defects as a function of volumetric strain The observable difference in these two curves is partly
for Ba2RCu307_a and for BajRCujO, under hydrostatic because the orthorhombic to tetragonal transition in the
R-systems takes place at an oxygen content different
pressure were calculated. These theoretical calculations
from that of the reference Y-system, namely, 6.4 (Er) to
show good agreement with experimental observations
in that increased pressure favors ordering of the CuO 6.83 of Nd. The formation energy of Frenkel pairs is
chains. For example, the Frenkel pair formation energy altered because of the difference of oxygen content
indeed increases significantly (around -0.25 eV/0.01 (which affect lattice parameters and atomic positions).
volumetric strain) under compression.
This formation energy of Frenkel pairs decreases as
Based on a quasi-chemical approach (QCA) by anisotropy in the ab plane [{b-a)ld\ decreases.
From a simple point of view, if an assumption is
Bakker et al. [57], the orthorhombic/tetragonal transi-
tion temperatures for BajRCujOy.^ have been computed made that the orthorhombic phase (absence of oxygen
by scaling the effective oxygen-oxygen short-range on the a axis) is favored at lower temperatures, then as
repulsive energy in the CuO chain. At a given tempera- the size of the lanthanide ion decreases, so does the dis-
ture T, a simplified relation [Eq. (1)] can be obtained to tance between neighboring basal oxygens; the repul-
express the oxygen-oxygen repulsion energy, v, on two
sion energy, v (Fig. 7), between these oxygens increas-
es correspondingly. The transformation temperature,
sublattice sites a and (3, as a function of the long range
according to Eq. (2), is directly proportional to the
parameter (S), the short range order expressed by the
fraction (p) of near-neighbor pair sites occupied by repulsion energy, and is therefore higher the smaller the
oxygen-oxygen pairs, and the fractional site occupancy size of R.
averaged over both sublattices (c).
3.3 T^ Dependence of Oxygen Content
/
{c(l + S)-p){c(l-S}-p)
v/kT = ln
(1)
Figure 9 shows a typical plot of the rationalized ac
p(\-2c + p)
susceptibihty of BajRCujOy.^ [31] as a function of tem-
The fraction p is equal to NJ4N, where A'^ is the num- perature. The annealing temperatures are indicated
ber of sites on each of the sublattice, and A'^oo is the num- from 400 °C to 708 °C. A bulk sample exhibiting
ber of oxygen-oxygen near neighbor pairs. In 100 % flux exclusion would have an ac susceptibility
47
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
0-1
t °
7^—f-
^
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60 80
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2Q
Temperature, K
Fig. 9. A plot of the rationalized ac susceptibility as a fiinction of temperature. The
annealing temperatures are indicated. All samples were fully packed fine powders.
Samples annealed at 750 °C and above did not exhibit any flux exclusion.
of-1 (dimensionless). Finnemore et al. [59] has shown 2 and 10 [59]. The decrease in the magnitude of the flux
that 100 % flux exclusion is not expected for fine pow- exclusion seen in Fig. 9 for the samples annealed at
ders of a completely superconducting material. It is higher temperatures can be due either to a decrease in
the fraction of the materials that is superconducting or
thus not possible to determine exactly the fraction of
the sample that is superconducting from the curves of to an increase in the superconducting penetration depth,
or likely, to a combination of both.
Fig. 9. However, these curves have the approximate
shape and magnitude expected for a ratio of particle Figure 10 shows plots of the superconducting tem-
diameter to superconducting penetration depth between perature onset, obtained from flux exclusion meas-
TQQ,
90
.fc^^^^^^
_X^x^^^4\..
80 -
\\
■
'\\M
60
■
8
40
y —•-
-
Nd --*"
Sm - •-
X ^ 1 1
■
^
20
\ T 4 '
■
Qd -€>-
Ho -&-
\ - 1
Er -a-
*t h L1
. i) .' A ^°:
■
1
1
800
400 500 600 700
Annealing Temperature (^C)
Fig. 10. Dependence of the superconducting transition temperature, as determined by ac
magnetic susceptibility, on the annealing temperature in Ba2RCu307_^ with R = Nd, Sm,
Gd, Y, Ho, and Er.
48
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
urements as a function of the annealing temperature for broad plateaus at 60 K and 90 K to complete absence of
all six compounds. Two apparent plateaus in were plateaus depending on how samples were prepared
TQQ
observed for the materials with yttrium, holmium and [60], our samples were all prepared under similar con-
erbium substitution: one at 83 K to 92 K and the other
ditions, therefore it is possible that one should be able
at 58 K to 60 K. Narrower and somewhat lower
to correlate the features of these plateaus with size of R.
plateaus were detected for the gadolinium, samarium Figure 11 depicts the oxygen content dependence of
and neodymium compounds. Although, in general, the the transition temperatures of these six compounds as
orthorhombic structure is superconducting whereas the derived from thermogravitmetric analysis/differential
thermal analysis (TGA/DTA) data. It is noteworthy that
tetragonal is not, this structure correlation does not
appear to be exact. For example, the tetragonal yttrium a correlation exists between the size of the R^^ ion and
both the phase transition temperatures (or oxygen com-
material annealed at 719 °C is superconducting, where-
as the orthorhombic Er compound annealed at 750 °C positions) and T^ values for these plateaus, as is sum-
is a non-superconductor. Furthermore, despite reports marized in Table 4. Compounds with a smaller size lan-
from literature that even in the Ba2YCu307_5 system thanide 3+ ion have a tendency to have both a wider
alone, one can achieve plateau features varying from 90 K plateau in T^ and a wider and relatively higher T^
.8
BA
6 5
7.0 6.9 6.8 6,7 6.6
Oxygen Composition
Fig. 11. Dependence of superconducting transition temperature, as determined by ac
magnetic susceptibility, on the oxygen content in Ba2RCu307_§ with R = Nd, Sm, Gd, Y,
Ho and Er.
Table 4. Characteristics of the low temperature T^ plateaus for Ba2RCu307_5 with R = Nd, Sm,
Gd, Y, Ho, and Er
Lanthanide Approximate Approximate anneal
R
compositional range temperature range
To
in air
{X = 1-8)
55 K
Nd 6.83 <x< 6.88 40 °C (460 °C to 500 °C)
Sm 6.82 < X < 6.83 52 K
70 °C (550 °C to 620 °C)
38 K
Gd 6.77 <x< 6.81 70 °C (580 °C to 650 °C)
Y 6.62 < X < 6.80 58 K
100 °C (600 °C to 700 °C)
6.62 < X < 6.77 60 K
Ho 80 °C (580 °C to 660 °C)
Er 60 K
6.58 <x< 6.80 120 °C (600 °C to 720 °C)
49
Volume HI, Number 1, January-February 2006
Journal of Research of the National Institute of Standards and Technology
observed, there was variation in both the intensity and
in the 50 K to 60 K range. The Nd, Sm and Gd samples
length of the streaks between different grains from the
behave otherwise. They lack any obvious 90 K plateau,
same specimen, indicating that the oxygen content was
and they also play a narrow low T^ plateau (55 K, 52 K
not constant throughout the specimen and that the dif-
and 38 K, respectively). The observed trends appear to
fusion of oxygen through the lattice is slow at these
differentiate the early and later members of the lan-
thanide series. temperatures. However, on average, the lengths of the
streaks for specimens Y6.63 and Y6.55 corresponded to
3.4 Structural Features of Ba2RCu307_g short range order on the 5 nm to 10 nm (50 A to 100 A)
scale.
Although short range ordering was observed in the Y
The behavior of the curves of T^ versus annealing
sample using electron diffraction, long range ordering
temperature (Fig. 10) and T^ versus oxygen content
was not observed using either electron diffraction or
(Fig. 11) suggests the presence of more than one struc-
powder x-ray diffraction. Neutron scattering studies on
tural phase. X-ray results indicated both to be
orthorhombic and they are designated here as 0(A) and a 60 K yttrium material annealed at 640 °C also showed
0(B). Other studies such as that of Cava et al. [41] have
no evidence of long range ordering. The results agreed
suggested that the second plateau region indicates the with those of the x-ray powder diffraction and indicate
presence of a second orthorhombic phase. The presence the absence of any doubling of the cell parameters
of an 0(B) phase in the yttrium sample has been con-
along either the a ox b axis. The nature of supercell
firmed by electron diffraction studies, which gave
ordering in Y-123 BajYCujO,.^ have been studied
information about the degree to which ordering of the
extensively by Beyers et al [61], Zeiske et al. [62], and
oxygen ions had occurred in the specimens. Diffraction Ourmazd and Spence [63]. De Fontaine et al. [64] sug-
patterns from the yttrium specimens (Y6.85, Y6.63, gested that this supercell is stabilized at low tempera-
and Y6.55, as defined in Table 1) contained elongated ture. It is now generally agreed that the degree of
superlattice reflections (streaks) lying along the [100] plateau behavior of BajYCujOy.^ depends on the degree
direction and centered on positions g + '/2OO in recipro- to which the ordered laxbxc supercell is stabilized
cal space (where ^ is a reciprocal lattice vector of the [65].
The gadolinium specimen was unique in that there
conventional orthorhombic structure).
A typical image of a crystallite in the yttrium speci- was very little twinning in the crystallites studied; all
men, Y6.55, together with a diffraction pattern from the
the other lanthanide specimens were twinned.
Diffraction patterns from the neodymium, samarium
crystallite, are shown in Fig. 12. The image, Fig. 12(a),
shows an approximately regular arrangement of twins. and gadolinium specimens (Nd6.90, Sm6.75 and
From the diffraction pattern. Fig. 12(b), we can deter-
Gd6.79) showed no evidence of superlattice formation
mine that the habit plane of the twins is (110), as of the type corresponding to the doubling of the unit
expected, and that the orthorhombicity {bid) of the cell dimension along the a axis. It is conceivable that
material is 1.01, which agrees with the value of 1.008 the 0(B) phase in compounds with smaller size R, has
a doubling of the unit cell dimension along the a axis,
obtained previously [37]. Diffuse scattering spots were
corresponding to the absence of oxygen in every other
also observed in the diffraction patterns, as indicated by
the arrows. Despite the twinning, by a careful examina- Cu-0 chain along the b axis. For the larger size R, the
tion of the diffraction pattern far from the transmitted compositions with lower T^ plateau that we investigat-
ed correspond to those deviating significantly from the
beam where the splitting of the orthorhombic matrix
spots was greatest, it was possible to determine that the oxygen content of 6.50. We postulate that these lower
superlattice streaks were at ^ + '/2OO and not g + 0/40. T^ plateau regions of 55 K, 52 K, and 38 K in the Nd,
Sm and Gd samples may result from another
This corresponds to a doubling of the unit cell dimen-
sion along the a axis, suggesting that the oxygen atoms
orthorhombic phase with a superlattice cell of the type
of every other CuO chain, running along the b axis, are corresponding to different oxygen vacancies. A super-
removed. These results agree with those reported by lattice cell other than the type with a' = 2a has also been
Alario-Franco et al. [42], who observed diffuse scatter- reported. For example, Alario-Franco et al. [42] report-
ing in a BajYCujOy.ft 5= 0.5 sample. For the yttrium ed a superlattice type in the Y compound corresponding
to an oxygen content of 6.85; this superlattice can be
specimen, Y6.85, the streaks at '/2OO were particularly
faint and long, as is expected as the oxygen content
indexed on a unit cell of 2^2a^x2^2a^x3a^, where
increases. In all cases where superlattice streaks were a^ is the basic cubic perovskite cell dimension.
50