Neutron structural studies on the superconducting
„Nd1−xCax…„Ba1.6La0.4…Cu3Oz system

Amish G. Joshi,1,a� R. G. Kulkarni,2 W. B. Yelon,3 Ram Prasad,4 and M. R. Gonal4
1National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India
2Department of Physics, Saurashtra University, University Road, Rajkot 360 005, India
3Graduate Center for Material Research, University of Missouri, Rolla, Missouri 65409, USA
4Metallurgy Division, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India

�Received 9 October 2008; accepted 9 March 2009; published online 24 April 2009�

We have investigated the influence of Ca ion substitution on the structural and superconducting
properties of �Nd1−xCax��Ba1.6La0.4�Cu3Oz system. Magnetization, x-ray diffraction, and neutron
diffraction studies have been carried out on a series of compounds with x = 0.0 – 0.6. The
superconducting transition temperature Tc, determined from magnetization measurements, increases
with increasing Ca2+ substitution. Neutron diffraction studies reveal that these compounds
crystallize in a tetragonal structure �space group P4 / mmm�. A detailed analysis of the neutron
diffraction data reveals that Ca and La ions are intermixed at the nominal Ba and Nd sites. While
a major fraction of Ca ions occupy the usual Nd site, a small fraction occupies the Ba site.
Consequently, the corresponding amount of La substitutes at the nominal Nd site. The intermixing
of Ca and La sites randomizes the chain site oxygens leading to a tetragonal structure despite an
oxygen content close to 7.0 for all the Ca doped samples. Further increase in Ca content leads to
change in its coordination from sixfold to eightfold at x � 0.4. © 2009 American Institute of
Physics. �DOI: 10.1063/1.3116532�

I. INTRODUCTION

The normal state transport properties of oxide supercon-
ductors are unusual and systematic studies of them are there-
fore important for understanding high Tc superconductivity.
YBa2Cu3O7−� �Y123� superconductor has been extensively
studied; replacing Y by R �R = rare earth� does not affect
superconductivity except when R is Ce, Tb, or Pr. The hole
concentration p is an important parameter in the p-type ce-
ramic high Tc materials. The hole concentration can be var-
ied by changing the concentration of substituents at different
crystallographic sites �i.e., Y, Ba, or Cu� or by the ordering
of oxygens in basal plane. It is well known that in
R1+xBa2−xCu3O7−�, the light rare earth element �LR = La to
Nd� can occupy the Ba site as well as the R site. For LR
elements the ionic radii approach that of Ba,1,2 allowing for a
large degree of substitution by R for Ba without forming
second phase, which results in oxygen defects on the anti-
chain site. Nd has an ionic radius closest to that of Ba and,
therefore, exhibits the greatest amount of solubility with Ba
�x � 0.8�.3 When trivalent LR occupies the divalent Ba site,
charge balance requires 12 oxygen for each R is incorporated
into the structure. In the fully oxygenated orthorhombic
structure the extra oxygen occupies the antichain site
�O�5��.4–6 Since the basal plane acts as a charge reservoir, the
extra oxygen results in modification of the charge distribu-
tion in the structure. Superconductivity is suppressed due to
the localization of holes.6–8 Accompanying the LR3+ substi-
tution on the Ba2+ site is a change in the crystal structure
from orthorhombic to tetragonal �O-T� and an increase in the

flatness of the CuO2 plane.
9 A comparison of substituting

magnetic ions �such as Eu3+, Pr3+, Nd3+� �Refs. 7, 9, and 10�
and nonmagnetic ions �such as La3+� �Refs. 10–13� at Ba site
leads to the conclusion that besides magnetic pair breaking,
there is a considerable contribution to the Tc originating from
other effects such as defect scattering, carrier localization, or
hole filling.

By studying La substituted Nd123 compounds, we
have shown that increasing La concentration in
Nd�Ba2−yLay�Cu3Oz �0 � y � 0.5� causes a reduction in Tc
due to hole filling, and a structural phase transition �O-T� is
observed at y = 0.3.11 A similar structural transformation is
observed with nonmagnetic La3+ at Ba2+, with magnetic rare
earth R �R = Eu, Ho, Dy� substitutions.10,12,13 This Tc
is expected to revive when divalent Ca is substituted for
trivalent rare earth R. When Ca is incorporated in
�Nd0.94Ca0.06�Ba2Cu3O7−�, the sample is observed to be more
metallic at 8 GPa and 523 K.14 On the other hand, thermo-
electric power measurements have shown that La doping in
Nd1−xLaxBa2Cu3Oy causes a decrease in hole concentration
smaller than that of Ca doping in Nd1−xCaxBa2Cu3Oy, which
suggests that some La ��26 % � would enter in to the Ba site
with the generation of additional BaCuO2+v phase.

15 Our
present efforts are to understand the effect of simultaneous
substitution of Ca and La at the Nd and Ba sites, respec-
tively, on structure and superconductivity of �Nd1−xCax�
�Ba1.6La0.4�Cu3Oz system. If all the La atoms occupy the Ba
site, the oxygen content would have to increase to maintain
charge balance. As Ca �1.12 Å, in eightfold coordination� is
slightly larger than Nd �0.995 �, there should be a prefer-
ence for Ca to occupy the Ba-site rather than the Nd site,
which is solely based on lattice strain arguments.

a�Author to whom correspondence should be addressed. Electronic mail:
amish@mail.nplindia.ernet.in.

JOURNAL OF APPLIED PHYSICS 105, 083919 �2009�

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http://dx.doi.org/10.1063/1.3116532
http://dx.doi.org/10.1063/1.3116532
http://dx.doi.org/10.1063/1.3116532


However, this preference is not observed and is expected to
be due to some intersite mixing of Ca and La at the Ba and
Nd sites, respectively. For this reason neutron diffraction
studies were performed to determine the relative site occu-
pations of the Ca and La ions. Systematic investigations
of neutron diffraction and magnetization measurements of
compounds having the stoichiometric compositions
�Nd1−xCax��Ba1.6La0.4�Cu3Oz with x = 0.0 to 0.6 have been
studied in detail. The interrelationship between the supercon-
ducting transition temperature �Tc� and structural changes
has also been discussed in the context of the carrier concen-
tration.

II. EXPERIMENTAL DETAILS

The �Nd1−xCax��Ba1.6La0.4�Cu3Oz, series of compounds
with 0 � x � 0.6, were synthesized via a solid state reaction,
the details of which are given in Ref. 11. Powder x-ray dif-
fraction patterns of the compounds were obtained in the 2�
range 20 ° � � � 80° using Cu K� radiation.

Room temperature neutron diffraction patterns were ob-
tained at the University of Missouri Research Reactor Facil-
ity using a position sensitive detector �PSD� diffractometer.
This diffractometer is equipped with a Si �511� monochro-
matic, horizontally curved with about 11 – 12 m radius, and
vertically segmented with 1.5 m radius and with nine asym-
metrically cut blades. For collecting the diffraction data, the
sample was contained in a thin-walled vanadium can of
about 3 – 4 mm diameter and about 5 cm long and rotated
about the long axis to average out any preferred orientations.
The data were obtained in a 2� range of 5 ° � � � 105° �at
0.05° intervals�, using a PSD �with five detector elements�
that covered a range of 20° at a time. Neutrons of wave-
length of 1.4875 Å were used for the diffraction experi-
ments. Magnetization measurements were performed using a
superconducting quantum interference device magnetometer
�Quantum Design� in the temperature range of 1.7 – 300 K
and an applied field of 50 Oe.

III. RESULTS AND DISCUSSION

X-ray powder diffraction studies were performed on se-
ries of compounds �Nd1−xCax��Ba1.6La0.4�Cu3Oz with x
= 0.0 – 0.6, showed them to be single phase. A fit of a typical
neutron diffraction pattern is shown in Fig. 1. Figure 2 shows
the typical zero-field-cooled �ZFC� and field-cooled �FC�
magnetization data of �Nd1−xCax��Ba1.6La0.4�Cu3Oz with x
= 0.0, 0.1, 0.2, and 0.4 with an applied filed of 50 Oe. In both
ZFC and FC states data were acquired during warming of the
sample. The superconducting transition temperature �Tc� is
defined as the onset of diamagnetic response. It is seen from
Figs. 2 and 5 that Tc increases with increasing Ca concentra-
tion, which is in agreement with an earlier report.11 For the
sample with x = 0.1, the diamagnetic signal is small, suggest-
ing that superconductivity is not a bulk property of the
sample. It is evident from the figure that the Meissner frac-
tion increases with increasing Ca content, which may indi-
cate that a superconducting phase is forming while more

Nd3+ is replaced by Ca2+. However, neither x-ray diffraction
nor neutron diffraction shows any evidence of another phase
at room temperature.

The neutron diffraction data were analyzed by Rietveld
refinement procedure using the generalized structural analy-
sis system �GSAS� program. The structure refinement carried
out with orthorhombic structure, space group Pmmm. It was
realized that lattice parameters a and b are nearly the same,
exhibit possible formation of tetragonal phase. Therefore,
neutron diffraction data on all samples were refined on the
basis of a tetragonal structure �space group P4 / mmm�. Neu-
tron scattering factors used were �in units of femtometers�
8.27 for La, 4.90 for Ca, 5.25 for Ba, 7.69 for Nd, 7.718 for
Cu, and 5.805 for O. Structural parameters such as atomic
coordinates, fractional occupancies, and the thermal param-
eter �Ui� for different atoms �including various oxygen sites

FIG. 1. Typical observed �dots� and the calculated �solid curve� neutron
diffraction patterns for �Nd1−xCax��Ba1.6La0.4�Cu3Oz samples with x = 0.0,
0.1, 0.2, and 0.4 at room temperature. The difference pattern for each sample
is shown at the bottom.

083919-2 Joshi et al. J. Appl. Phys. 105, 083919 �2009�

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assigned to the standard R-123 structure�, obtained from Ri-
etveld analysis, are listed in Table I.

In R-123-type compounds, the unit cell is considered as
an assembly of three ABO3-type pervoskite cells stacked one
on top of the other with the central atom being R, in the
middle cell, and Ba in other two. This would result in a
chemical formula RBa2Cu3O9, and would contain a three-
dimensional network of Cu–O bonds. The lower dimension-
ality and oxygen content O7−� arise from the missing oxygen
atoms surrounding the R-site, Cu–O chains in the basal
planes. The R and the two Ba atoms occupy crystallographi-
cally distinct sites with eightfold and tenfold oxygen coordi-
nations, respectively. There are two different types of Cu
atoms, labeled as Cu�1� �0,0,0� for the Cu in the basal plane

and Cu�2� �0 , 0 , z� for the Cu in the two Cu–O planes. The
Cu�1� atoms have strong covalent bonds to four O2 atoms in
a square planner configuration, while Cu�2� atoms are
bonded to five oxygens in pyramidal symmetry. However,
Cu – O2 planes extend indefinitely in two directions �in the
ab-plane�, while Cu–O chains extend indefinitely in only one
direction �the b-axis direction�. The oxygen sites in the
Cu – O2 planes are identified as O�2� �

1
2 , 0 , z� and O�3�

�0 , 12 , z�. The oxygen site in the Ba–O plane is designated as
O�4� �0 , 0 , z� and is often called the “apical oxygen.” The
oxygen sites in basal plane, often called as Cu–O chains, are
designated as O�1��0 , 12 , 0� �along b-axis� and O�5��

1
2 , 0 , 0�

�along a-axis�. In orthorhombic R-123, the O�1� sites are
fully occupied, while the O�5� sites are nearly vacant, giving
rise to b � a. The O-T transition occur either by removing
O�1� �oxygen deficient R-123 with oxygen stoichiometry
�7.0� or by filling the O�5� sites �e.g., trivalent La at Ba site,
resulting in oxygen stoichiometry �7.0�.12,13 Moreover,
an interesting situation can occur via redistribution of
O�1� and O�5� with oxygen stoichiometry close to 7.0,
which is the unique case for the present case of
�Nd1−xCax��Ba1.6La0.4�Cu3Oz. In the tetragonal system with
a = b �Cu�1� – O�1� = Cu�1� – O�5��, the O�1� and O�5� are in-
distinguishable, and the same is true for O�2� and O�3� sites
in the Cu – O2 planes.

The observed variations of lattice parameter and cell vol-
ume with Ca content are shown in Fig. 3. It is observed that
the lattice parameter a and the cell volume both decrease up
to x = 0.4. The decrease in lattice parameter reflects the de-
crease in Cu�2�–O�2� distance and indicates a decrease in
flatness of the Cu – O2 plane with the decrease in “buckling
angle” �Cu�2�–O�2�–Cu�2��, which helps to revive Tc from
37 K �x = 0.0� to 84 K �x = 0.4�.

Initially structure refinement model assumed with Nd/R
and Ba / La ratios fixed by initial stoichiometry, but site oc-
cupancies were allowed to vary. In the subsequent analysis of
neutron diffraction data, it was assumed that Nd and Ba fully
occupy their normal sites. After fixing Nd and Ba occupan-
cies, the Ca and La occupancies were allowed to vary freely
both at the actual sites �Ca at Nd site and La at Ba site� and
the nominal sites �La at Nd site and Ca at Ba site� with the
condition that combined fractional and nominal at R site �Ca
at Nd + La at Nd site� and Ba site �La at Ba + Ca at Ba� add up
to unity �i.e., full occupancy�. In order to maintain a mean-
ingful and reliable variation in Ca and La occupancies at the
two sites, this procedure was adopted uniformly for all the
samples. The variation in occupancies for La and Ca at dif-
ferent sites is shown in Figs. 4�a� and 4�b�, respectively. The
intermixing behavior of La and Ca occupancy variation with
R �R = Nd, Dy, and Y� through neutron diffraction has also
been observed in La-1113, La-2125, and R-123 systems.16–19

It is well known that Ca substitution at the R-site de-
creases the oxygen content of R-123 compounds; however,
La substitution at the Ba-site increases the oxygen
content.12,13 Analysis of the neutron diffraction data reveals
that some Ca atoms occupy the actual Nd-site while some
occupy the Ba site. Consequently, corresponding amount of
La substitutes at the Nd site. The oxygen content, obtained
from refinement of neutron data, is found to be close to 7.0

FIG. 2. �Color online� ZFC and FC magnetization of �Nd1−xCax�
�Ba1.6La0.4�Cu3Oz samples with x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 as a
function of temperature in an applied filed of 50 Oe showing superconduct-
ing transition temperature.

083919-3 Joshi et al. J. Appl. Phys. 105, 083919 �2009�

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for all Ca-doped �Nd1−xCax��Ba1.6La0.4�Cu3Oz compounds
with x = 0.0 – 0.6. The cross substitution of La at the Nd site
and Ca at the Ba site randomizes the chain site oxygen’s
bringing the O�1� and O�5� site occupancies of oxygen in the
basal planes to same level and forces the compound to crys-
tallize in a P4 / mmm tetragonal structure.

Here it is worth mentioning that the substitution of Ca2+

for Nd3+ can increase the number of mobile holes provided
the oxygen content remains nearly constant. An increased
number of holes oxidizes the Cu – O2 layer. The oxidation of
Cu – O2 layer removes electrons from the x

2 − y2 orbitals,
which have antibonding character in the in-plane Cu–O
bonds. Therefore, as number of holes �nH� increases, the in-
plane Cu�2�–O�2� bond length �rCu–O� is shortened.

20,21 The
effective Cu valence, estimated from the overall oxygen con-

TABLE I. Lattice parameters, cell volume, fractional occupancies, positional, and thermal parameter for �Nd1−xCax��Ba1.6La0.4�Cu3Oz system obtained from
Rietveld refinements of the neutron diffraction data. For the tetragonal refinements, O�1� and O�5� are equivalent, and O�2� and O�3� are equivalent. Numbers
in parentheses are statistical standard deviations of the last significant digits.

Parameter x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5 x = 0.6

Space group P4 / mmm P4 / mmm P4 / mmm P4 / mmm P4 / mmm P4 / mmm P4 / mmm
a �� 3.895 05�24� 3.888 27�24� 3.882 39�22� 3.878 95�24� 3.876 81�26� 3.882 49�29� 3.882 3�4�
b �� 3.895 05�24� 3.888 27�24� 3.882 39�22� 3.878 95�24� 3.876 81�26� 3.882 49�29� 3.882 3�4�
c �� 11.689 7�8� 11.719 6�8� 11.721 2�7� 11.716 0�7� 11.715 7�8� 11.725 4�9� 11.720 5�14�

V �Å3� 177.349�19� 177.184�19� 176.673�17� 176.281�19� 176.083�21� 176.746�23� 176.66�4�
Nd � 12 ,

1
2 ,

1
2

�
Ui � 100 �Å2� 0.65�9� 0.67�13� 0.58�11� 1.14�13� 0.74�14� 1.03�17� 1.02�29�
NNd 1.000 0 0.900 0 0.800 0 0.700 0 0.600 0 0.500 0 0.400 0
NCa 0.083�23� 0.188�20� 0.243�21� 0.299�24� 0.308�28� 0.387�44�
NLa 0.000 0 0.017�23� 0.012�20� 0.057�21� 0.101�24� 0.192�28� 0.213�44�
Ba � 12 ,

1
2 , z�

Z 0.182 68�30� 0.185 61�28� 0.187 76�24� 0.187 09�25� 0.186 91�27� 0.186 77�33� 0.185 9�6�
Ui � 100 �Å2� 1.57�9� 1.49�11� 1.56�10� 1.57�10� 1.23�11� 0.73�13� 0.57�20�
NBa 1.6 1.6 1.6 1.6 1.6 1.6 1.6
NLa 0.4 0.384�24� 0.388�20� 0.342�22� 0.298�24� 0.208�28� 0.188�44�
NCa 0.016�24� 0.012�20� 0.058�22� 0.102�24� 0.192�28� 0.212�44�
Cu�1� �0,0,0�
Ui � 100 �Å2� 1.87�10� 2.04�10� 2.17�9� 2.46�10� 2.41�11� 2.35�13� 3.82�27�
N 1.000 0 1.000 0 1.000 0 1.000 0 1.000 0 1.000 0 1.000 0
Cu�2� �0,0,z�
Z 0.349 50�22� 0.350 93�22� 0.351 80�19� 0.352 14�21� 0.351 73�22� 0.351 93�27� 0.351 2�4�
Ui � 100 �Å2� 0.74�6� 0.82�5� 0.75�5� 0.90�5� 0.66�6� 0.59�7� 0.53�11�
N 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0

O�1� �0, 12 ,0�
Ui � 100 �Å2� 3.54�28� 4.08�32� 4.72�31� 5.23�31� 4.43�33� 4.67�38� 2.62�54�
N 0.533�10� 0.475�10� 0.456�9� 0.493�10� 0.474�11� 0.498�13� 0.466�18�
O�2� � 12 ,0,z�
Z 0.370 03�20� 0.369 93�19� 0.369 71�16� 0.368 51�17� 0.368 82�18� 0.368 97�21� 0.368 20�34�
Ui � 100 �Å2� 1.15�6� 1.24�6� 1.25�5� 1.44�5� 1.23�6� 1.02�7� 1.13�11�
N 4.000 0 4.000 0 4.000 0 4.000 0 4.000 0 4.000 0 4.000 0
O�4� �0,0,z�
Z 0.156 66�40� 0.155 22�43� 0.154 94�38� 0.154 27�41� 0.154 33�43� 0.153 95�56� 0.152 84�11�
Ui � 100 �Å2� 3.61�15� 3.66�14� 3.80�12� 3.98�12� 3.51�13� 3.93�16� 4.91�29�
N 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0 2.000 0

O�5� � 12 ,0,0�
Ui � 100 �Å2� 3.54�28� 4.08�32� 4.72�31� 5.23�31� 4.43�33� 4.67�38� 2.62�54�
N 0.533�10� 0.475�10� 0.456�9� 0.493�10� 0.474�11� 0.498�13� 0.466�18�
Total oxygen
content/formula
unit

7.066�20� 6.950�20� 6.912�18� 6.986�20� 6.948�22� 6.996�26� 6.932�36�

Cu valence 2.244 2.2 2.208 2.290 6 2.298 6 2.364 2.354 67

FIG. 3. �Color online� Variation in lattice parameter and cell volume with
Ca concentration.

083919-4 Joshi et al. J. Appl. Phys. 105, 083919 �2009�

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tent �z�, obtained from neutron diffraction data is listed in
Table I, which increases with increasing Ca concentration.
Simultaneous increase in Tc with Cu valence is observed and
showed maximum Tc of 84 K at x = 0.4. The introduction of
Ca into �Nd1−xCax��Ba1.6La0.4�Cu3Oz alters the normal and
superconducting properties substantially.11 The ionic radii of
Ca2+ in sixfold and eightfold coordination are significantly
different. The ionic size of Nd3+ in eightfold coordination is
0.995 Å, which matches more closely with that of Ca2+ in
sixfold �0.99 � than that of eightfold coordination �1.12 �.
The decrease in lattice parameter a and cell volume up to
x = 0.4 indicates that Ca at the Nd site would have preferred a
sixfold coordination. The lattice parameter c remains nearly
unchanged which is in good agreement with earlier reports of
the possibility of sixfold coordination in those samples22,23

with Ca2+ substitution at R site in fully oxygen annealed
Y-123 systems. As Ca content increases in the regime x
� 0.4, some Ca2+ might change its coordination from sixfold
to eightfold, which occupy Ba site and corresponding
amount of La occupy at Nd site �Figs. 4�a� and 4�b�; Table I�.
Therefore, lattice parameter a and cell volume increase due
to the fact that La3+ and Ca2+ in eightfold coordination are
bigger than Nd, which indicate an increase in the interplane
Cu�2�–O�2� bond distances and suggest a decrease in the
number of holes in Cu – O2 planes, which also results in an
increase in the buckling angle. As the buckling angle in-
creases, the Cu – O2 planes become flatter which results in
suppression of superconductivity.

Magnetization and neutron diffraction results may be
summarized as follows.

�1� The superconducting transition temperature Tc increases
with increasing Ca content up to x = 0.4.

�2� With progressive Ca doping at the Nd site, the lattice
parameter a decreases, but c remains nearly unchanged,
suggesting that Ca2+ may adopt sixfold coordination.

�3� Ca2+ may change its coordination from sixfold to eight-
fold with simultaneous increases in both lattice param-
eter and cell volume for further increase in Ca content
x � 0.4.

It is evident from Fig. 5 that Tc increases with increasing
x up to x = 0.4 and thereafter, decreases slightly for x � 0.4.
The most spectacular proof that Ca doping enhances super-
conductivity is the phase �Nd0.6Ca0.4��Ba1.6La0.4�Cu3Oz,
which exhibits a Tc of 84 K, whereas the Ca-free cuprate
Nd�Ba1.6La0.4�Cu3Oz does not superconduct down to 10 K
and shows Tc

on = 37 K.11 The substitution of Ca for Nd intro-
duces oxygen vacancies and nonstoichiometry, which lead to
hole doping. Therefore, this implies that the reduction in
Tc from 91 K for pristine Nd-123 to 37 K for
Nd�Ba1.6La0.4�Cu3Oz is compensated by appropriate hole
doping with Ca. Hence, the oxide displaying the optimum
Tc 	 84 K at x = 0.4 is identified as a compensated oxide
whose Tc lies close to that of pristine Nd-123 �Tc = 91 K�.
Further increase in x from 0.4 to 0.6 causes Tc to decrease
from 84 K �x = 0.4� to 78.5 K �x = 0.6� due to excess hole
doping from Ca.

IV. CONCLUSIONS

We conclude that Ca doping enhances superconductivity
for doping level up to x = 0.4; thereafter it shows a decrease
in Tc with increasing x due to excess hole doping. Neutron
diffraction data suggest that there is intermixing of Ca and
La occupancies. At higher doping level x � 0.4 indicates pos-
sibility of eightfold coordination of Ca.

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FIG. 4. �Color online� Intermixing of occupancies with Ca concentration.
�a� Variation in the La occupancy on Nd and Ba site with Ca concentration.
�b� Variation in the Ca occupancy at Nd and Ba site with Ca concentration.

FIG. 5. �Color online� Variation in transition temperature Tc with Ca
concentration.

083919-5 Joshi et al. J. Appl. Phys. 105, 083919 �2009�

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