Physica C 338 Ž2000. 52–59
www.elsevier.nlrlocaterphysc
Effect of fluorination and high pressure on the structure and
properties of the Hg-bearing superconducting Cu mixed oxides
S.N. Putilin a,) , E.V. Antipov a , A.M. Abakumov a , M.G. Rozova a , K.A. Lokshin a ,
D.A. Pavlov a , A.M. Balagurov b, D.V. Sheptyakov b, M. Marezio c
a
Department of Chemistry, Moscow State UniÕersity, Moscow 119899, Russia
b
Frank Laboratory of Neutron Physics, JINR, Dubna 141980, Russia
c
MASPEC-CNR, 43010 Parma, Italy
Abstract
The Tc variation of HgBa 2 CuO4q d ŽHg-1201. and HgBa 2 CuO4 Fd can be achieved by a change in the carrier
concentration and by a compression of the structure under high pressure. Oxygenated and fluorinated series exhibit a
cupola-shaped behavior for the Tc vs. d dependence, but the curves are shifted away from each other along the d axis. NPD
showed double amount of extra fluorine in comparison with extra oxygen for the oxygenated Hg-1201 phases with close
Tc’s. An exchange of the extra oxygen by a double amount of fluorine causes a significant compression of the apical Cu`O
bond distances, while the in-plane ones, as well as Tc , do not vary. Fluorination of Hg-1223 resulted in a slight increase in Tc
in comparison with oxygenated material. The influence of the external pressure on the structure and Tc of Hg-1201 strongly
depends on the doping level. An increase in the extra oxygen content from underdoped to overdoped state results in the
larger compression of the apical Cu`O and Ba`O Hg distances while the HgO 2 dumbbell as well as a distance between Ba
and O from the ŽCuO 2 . layers become practically pressure independent. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Hg-bearing superconductors; High pressure; Fluorine doping
1. Introduction
HgBa 2 CuO4q d ŽHg-1201, Fig. 1. is one of the
most attractive compounds for investigating the relationship between structure and superconducting
properties owing to its simple structure and wide
)
Corresponding author. Fax: q7-95-939-4788.
E-mail address: [email protected] ŽS.N. Putilin..
range of superconducting compositions Žfrom underdoped to highly overdoped states..
Several structural investigations of Hg-1201, based
on powder neutron diffraction experiments, were
done. In general, there is good agreement between
the structural parameters refined from ambient pressure data. C.W. Chu et al. w1x were the first to detect
significant Tc growth for HgBa 2 Ca 2 Cu 3 O 8q d at extremely high pressure. The transition temperature for
this phase increased under external pressure and
reached 153 K at 15 GPa, while Nunez-Regueiro et
al. w2x reported 157 K for this phase at 23.5 GPa. The
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 0 1 - X
S.N. Putilin et al.r Physica C 338 (2000) 52–59
53
cies of Tc and structural parameters vs. the extra
oxygen and fluorine content.
2. Structure of Hg-1201 under high pressure
Fig. 1. Schematic representation of the Hg-1201 structure.
increase in transition temperature under pressure for
Cu-containing superconductors with hole type conductivity is a well-known phenomenon. The peculiarity of Hg-1223 and other members of the mercury
family is that their Tc is the highest ever detected for
any superconductor. These results show the possibility to reach superconductivity at 150–160 K in
Cu-based mixed oxides, if the phase with Cu`O
distance like in HgBa 2 Ca 2 Cu 3 O 8q d under high
pressure could be obtained.
The rate of variation of Tc under pressure depends on the doping level. Cao et al. w3x have
measured Tc for Hg-1201 in a wide d range and
shown that dTcrd P strongly depends on the doping
level. The underdoped and optimally doped Hg-1201
samples initially exhibit an increase in Tc with pressure of up to 6–8 GPa and with dTcrd P f 2 KrGPa,
while the rate is smaller and even becomes negative
for overdoped samples. The structural reason for
such behaviour of Hg-1201 samples with different
doping level has not been understood yet.
It was considered interesting to exchange the
extra oxygen in the Hg-bearing superconductors with
extra fluorine atoms. Oxygen and fluorine anions
have close crystallochemical behaviors, while their
formal charges differ significantly: y2 and y1,
respectively. Therefore, assuming a simple charge
transfer model, one can expect the extra fluorine
concentration to be twice as high for the fluorinated
phase for the same doping level. It would be also
interesting to determine and compare the dependen-
Three samples of Hg-1201 were prepared according to the procedure described in Ref. w4x. The
syntheses were performed in sealed silica tubes in a
furnace with controlled temperature gradient. The
sample in the underdoped state Žsample A, Tc f 75
K. in the final stage of the preparation process was
annealed in argon flow at 3508C. Sample ŽB., which
is optimally doped ŽTc f 97 K., was annealed at
2508C in 1 bar oxygen atmosphere. The third sample
ŽC, overdoped, Tc f 70 K. was annealed at 2708C
under oxygen pressure of 90 bar.
Neutron diffraction patterns of A and C samples
were measured at the D2B diffractometer at the ILL
ŽGrenoble. at several values of the external pressure,
while sample B was measured only at ambient pressure. The diffraction patterns in the range 208 - 2Q
- 1508 were obtained at 0, 0.5 and 0.85 GPa for
sample A and at 0, 0.5 and 0.7 GPa for sample C.
No peaks from impurity phases were found in any of
the diffraction patterns. A diffraction pattern of sample A was also obtained with the HRFD diffractome˚
ter at Dubna in the d-spacing range of 0.77 y 2.12 A
in order to make a comparison with the previously
obtained data, which would reveal possible systematic errors. The Rietveld refinements were carried
out with the use of the GSAS and MRIA programs.
Several hypotheses for the structural model were
checked during the refinements for all compositions.
They were: a deficiency in the occupation of the Hg
site; the possibility of splitting of the Ba site along
the c-axis; the presence of static displacements of Hg
in the basal plane and of O2 in a plane parallel to the
basal plane, the disordering of O3 in the basal plane
along the diagonal of the unit cell, and the possibility
of the existence of additional oxygen in the Ž0,0.5,0.
or Ž x,0.5,0. positions. None of these hypotheses was
found to be statistically significant. Therefore, the
refinements were carried out using the standard
structural model Žsee, for example, Ref. w5x.. The full
results of the refinement can be found in Ref. w6x
and, here, we only conclude that our three samples
were really under-, optimally and overdoped, with
mean contents of additional oxygen close to 0.06 "
S.N. Putilin et al.r Physica C 338 (2000) 52–59
54
Fig. 2. The unit cell parameters of Hg-1201 as a function of
external pressure for three samples. For the optimally doped state,
the point measured at P s 0 is shown together with the line slope
determined in Ref. w9x.
The variations of the a and c unit cell parameters
in Hg-1201 as the function of pressure are shown in
Fig. 2. For the optimally doped sample, the points
corresponding to ambient pressure Žmeasured at D2B.
are shown and the line slopes correspond to the
compressibilities derived from Ref. w9x. The compressibilities of the unit cell parameters and the main
interatomic distances in Hg-1201, defined as k q s
yŽ1rq . D qrD P Ž10y3 rGPa., where q is the specific parameter, are presented in the Fig. 3. In order
to calculate D qrD P, the linear least-squares fits of
the experimental points were used.
The pressure dependence of the Hg-1201 unit cell
parameters shows that the occupation of the O3
position does not strongly influence the lattice compressibility. It is remarkable that the compressibilities of the in-plane Cu`O1 and Hg`O3 bonds are
practically d independent, while the compressibility
of the apical bond distances, Cu`O2 and Hg`O2,
are strongly dependent upon the doping level. The
compressibility of the apical Hg`O2 bond is close to
k c for sample A, but with increasing doping level
Žsamples B and C., it becomes practically incompressible. The HgO 2 dumbbell is a very rigid element of the structure. The length of the Hg`O2
bond depends on the extra oxygen content in the
Hg-plane: an increase in the coordination number of
the Hg atoms results in an increase in the apical
0.01, 0.13 " 0.01 and 0.19 " 0.01, respectively. The
Tc vs. d dependence for all samples investigated can
be fitted by equation:
Tc s Tc ,max 1 y q Ž d y dopt .
2
Ž 1.
introduced by Presland et al. w7x with parameters
Tc,max s 97.8 " 0.8 K, q s 52 " 9, dopt f 0.128 "
0.005.
The dopt value turned out to be significantly
lower than the dopt given in Ref. w8x, but, at the same
time, it is ; 1.6 times higher than the value predicted by the simple ion doping model, that is dopt s
0.08. This assumes that there is the formation of two
holes in the ŽCuO 2 . layer per oxygen atom inserted
into the Hg-layer, the standard valences of the atoms
are: V Ba s VHg s q2, VO s y2, and the optimal
number of holes in the ŽCuO 2 . layer is n opt s 0.16.
Fig. 3. Compressibilities of the main Hg-1201 structural parameters for three doping levels.
S.N. Putilin et al.r Physica C 338 (2000) 52–59
Hg`O2 bond distance. This bond becomes incompressible with the presence of the significantly large
amount of O3 in the Hg-layer.
The compressibility of the apical Cu`O2 bond
length increases when the O3 content increases despite of the shortening this bond for larger d-values.
The compressibilities of the Ba`O distances strongly
depend on the doping level. The compressibility of
the Ba`O3 distance becomes larger when the doping
level increases. The compressibility of the Ba`O2
bond length becomes also larger for increases of the
extra oxygen content. The Ba`O1 distance does not
change under pressure, and the compressibility has
even a small negative value, while in the underdoped
and optimally doped samples, this bond decreases for
increasing pressure.
One of the most discussed questions in the literature is why Tc Ž P . for the Hg-1201 phases varies
with the doping level. This subject is discussed in a
few theoretical articles w10,11x. Even though each is
based on different models, all conclude that the Tc
increases are mainly due to the increase in Tc,max in
Eq. Ž1. and the variation of the charge carrier density
in the ŽCuO 2 . layer. From the experimental data on
the Tc vs. P and d dependence presented in Ref. w3x
it follows that dTcrd P f const.f 2 KrGPa. for d F
dopt at least up to 1.5 GPa pressures. The analysis of
Eq. Ž1. shows that in this case, the pressure-induced
charge transfer, d nrd P, would have to decrease for
increasing d , vanishing at d s dopt . From the homogeneous character of the structure compression, one
can conclude that the effect of the charge transfer
from the reservoir to the ŽCuO 2 . layers does not play
the dominant role in the Hg-1201 structure under
pressure at d F dopt . At the same time, the strong
shift of the Ba atoms in the overdoped state toward
the ŽHgOd . layer under applied pressure may be
considered as the signature of a significant charge
transfer into the ŽCuO 2 . layer.
We may conclude that at low Od concentration
the Hg-1201 structure compresses isotropically, i.e.
the compressibilities of the main interatomic distances correspond to the unit cell compressibility.
However, at higher Od concentrations, the Hg`O2
and Ba`O1 bonds become practically incompressible, while the Cu`O2 and Ba`O3 undergo a strong
compression. The analysis of the interatomic distances variations allows us to conclude that in the
55
under- and optimally doped states, the charge transfer from the reservoir to the ŽCuO 2 . layers plays a
minor role. While the decrease in Tc under applied
pressure for the overdoped Hg-1201 superconductor
may be explained by charge transfer enhancing overdoping.
The enhancement of Tc under pressure is due to
an increase in the Tc,max value, caused by appropriate
variation of interatomic distances. To simulate such
effect, we have investigated an influence of the
exchange of extra oxygen by extra fluorine on the
structure and properties of Hg-1201 and Hg-1223
superconductors.
3. Fluorination of Hg-1201 and Hg-1223 phases
Crucial point for the synthesis of fluorine substituted Hg-bearing superconductors is a high thermodynamic stability of BaF2 , which can be formed due
to decomposition reactions. To avoid this phase formation, we have used a soft fluorinating agent XeF2
w12x. A two-step procedure, first deep reduction and
then fluorination, was used to obtain fluorinated
Hg-1201 and Hg-1223 phases.
Single-phase samples of Hg-1201 were firstly reduced in a dynamic vacuum to the composition
HgBa 2 CuO4.01 determined by iodometric titration.
Obtained samples were monophase and exhibited
superconductivity with Tc s 61 K. The sample of the
Hg-1223 phase with cation stoichiometry
Hg 0.8 Ba 2 Ca 2 Cu 3.2 O 8q d was obtained from metal
oxides by two temperature synthesis, with CoOr
Co 3 O4 mixture for regulation of oxygen pressure
inside quartz tube. It contained less than 5% of the
impurities ŽCaO, BaCuO 2 .. This sample was then
reduced at 5008C with Ti-getter in sealed quartz tube
and exhibited Tc f 100 K. Due to impurity presence,
it was difficult to estimate an amount of interstitial
oxygen in the sample by iodometric titration. The
reduced Hg-1201 and Hg-1223 samples were subsequently fluorinated by XeF2 at 150–2008C for 10–15
h. All operations were made in a glove box in a dried
N2 atmosphere excluding the presence of O 2 . Syntheses were carried out in Ni-crucibles placed in
N2-filled and sealed copper tubes. Two fluorinated
samples HgBa 2 CuO4 Fd ŽD and E. with Tc s 97 K
and ; 80 K, respectively, and fluorinated Hg-1223
were obtained. X-ray diffraction pattern of the fluori-
56
S.N. Putilin et al.r Physica C 338 (2000) 52–59
Fig. 4. X-ray diffraction pattern of the fluorinated Hg-1223 phase
with theoretical peak positions.
˚ cs
nated Hg-1223 phase Ž a s 3.8501Ž2. A,
˚ . with theoretical peak positions is shown
15.773Ž3. A
in Fig. 4. Only traces of the impurities with the
intensity of lines less than 2% are visible. Additionally, the reduced Hg-1223 sample was treated in
˚ cs
oxygen flow at 3008C Ž a s 3.8524Ž4. A,
˚ .. Results of the AC magnetic suscepti15.819Ž4. A
bility measurements for oxygenated and fluorinated
samples of Hg-1223 phase are shown in the Fig. 5.
Slight increase in the transition temperature to superconducting state was detected for fluorinated sample
in comparison with the oxygenated one Ž138 and 135
K, respectively..
The samples were studied by NPD. Neutron
diffraction experiments were performed with the
Fig. 5. AC magnetic susceptibility curves for reduced Ž^., oxygenated Ž`. and fluorinated ŽI. Hg-1223 samples.
high-resolution Fourier diffractometer ŽHRFD. at the
IBR-2 pulsed reactor in Dubna. Diffraction patterns
were measured for all samples at room temperature.
Data processing was carried out by the Rietveld
method.
The initial parameters for the neutron data refinement of the samples D and E were chosen as those
obtained for the oxygenated Hg-1201 sample w5x. To
avoid a correlation between occupation and thermal
factors, the refinements were carried out with fixed
thermal parameters for the Ba, Cu, O, and F atoms.
The final refinement results can be found in Ref.
w13x, where we conclude that the fluorinated Hg-1201
samples as oxygenated ones have a stoichiometric
cation composition with only one site for the oxidizing extra anion.
The occupancy of the fluorine position was refined. The values of nŽ F . s 0.24Ž2. and nŽ F . s
0.32Ž2. were obtained for samples D and E, respectively. Thus, the nŽ F . values are significantly larger
than the values of 0.124Ž9. and 0.19Ž1. obtained for
the oxygenated Hg-1201 samples with close Tc values w5x. This comparison is quite correct since oxygenated and fluorinated materials were characterized
by the same NPD facility and initial Hg-1201 samples were prepared by the same synthesis technique.
Fig. 6 shows the dependencies of the Tc values
vs. extra oxygen or fluorine concentration. For the
fluorinated series, we also used the initial non-fluo-
Fig. 6. The behavior of Tc vs. extra oxygen or fluorine content for
Hg-1201.
S.N. Putilin et al.r Physica C 338 (2000) 52–59
rinated sample as the first point. It can obviously be
seen that in both cases, there are parabolic-shaped
curves that are shifted away from each other along
the d axis. One can conclude that fluorine indeed
oxidizes Žlike oxygen. the ŽCuO 2 . layers in the
Hg-1201 structure, followed by a change in Tc , but
the amount of inserted fluorine is approximately
twice that of oxygen to achieve the same Tc values
and, consequently, the same doping level. Taking
into account the different formal valences of these
anions Žy1 and y2, respectively., we conclude that
inserted extra oxygen creates the twice as many
holes as fluorine.
The amount of inserted fluorine Žas well as oxygen for the oxygenated samples., however, was found
to be significantly larger than could be expected for
the optimally doped phase, assuming an optimal hole
number popt s 0.16 Ž d F should be equal to 0.16
instead of 0.24, and d O should be equal to 0.08
instead of 0.12 if VBa s VHg s q2, VO s y2 and
V F s y1.. Therefore, we can conclude that the doping mechanism in the Hg-1201 superconductor is
more complex than a simple oxidation of the ŽCuO 2 .
layers by an inserted fluorine or oxygen. They can
oxidize not only the ŽCuO 2 . layers, but, also, possibly, the HgO 2 ‘‘dumbbell’’ as well. The carrier
concentration in the conducting band is a result of
the delicate charge balance among these fragments.
Another important conclusion can be made if we
plot the dependence of Tc vs. the a-parameter Žthe
doubled in-plane Cu`O1 bond length. for the oxy-
Fig. 7. The behavior of Tc vs. the a-parameter of the unit cell of
Hg-1201.
57
Fig. 8. Bond distances Hg ` O2 Žleft scale, open symbols. and
Cu ` O2 Žright scale, full symbols. as a function of extra oxygen
or fluorine content. Lines through points are guides to the eye.
genated and fluorinated samples ŽFig. 7.. For this
graph, the values of the a-parameter were taken only
from the X-ray data that was treated in a similar way
to exclude possible systematic errors. All data points
can be fitted by one parabolic-like function with a
˚ This fact
maximum around 97 K at a s 3.882 A.
supports the conclusion that this distance and the
carrier concentration Ž VCu ., are crucial parameters
determining the magnitude of Tc .
Variation of the apical Cu`O2 bond distance
caused by the extra anion exchange when d F f 2 d O ,
however, is well-pronounced in contrast to the inplane ones. Fig. 8 shows practically a linear dependence of the apical Cu`O2 distance vs. d Žoxygen
or fluorine.. An increase in the amount of the extra
anion in the Hg-layer results in a compression of the
apical Cu`O2 distance. These distances differ significantly between the fluorinated and oxygenated Hg1201 phases with close Tc and in-plane Cu`O1 bond
lengths. For instance, the difference between these
distances in the phases with Tc s 97 K is about 0.04
Å, which is much larger than the standard deviation
and the difference between the c-parameters.
The origin of this unusual phenomenon may be
explained if we also take into account the variation
of the apical Hg`O2 bond distance. This bond length
in the fluorinated Hg-1201 phases is significantly
larger in comparison with that in the oxygenated
compounds ŽFig. 8, right., where this distance varies
˚ Ž d s 0.057. up to 1.990 A˚ Ž d s 0.19.
from 1.963 A
due to the shift of the O2 atoms towards the more
oxidized Cu cations.
It is known that the apical Hg`O2 bond is a very
strong covalent bond and even under high pressure,
58
S.N. Putilin et al.r Physica C 338 (2000) 52–59
this distance does not decrease significantly: 1.980Ž4.
˚ at ambient conditions and under
and 1.973Ž19. A
5.07 GPa, respectively w9x. Therefore, the more probable origin of the elongation of the Hg`O2 distance
is not a variation of the formal Cu valence, but an
interaction between Hg and the extra anions Ževen
located far from the Hg atom.. An increase in the
coordination number of Hg by the inserted extra
anions is accompanied by a shift of the O2 atoms
away from the Hg cations towards the Cu atoms,
thus, elongating the Hg`O2 distance and shortening
the Cu`O2 one. We can conclude that the exchange
of extra oxygen for double the amount of fluorine
causes a variation of the apical Cu`O2 distances,
predominantly, while the in-plane distance and Tc
remain the same. This transformation can be formally considered as anisotropic compression Žalong
the c axis. of the CuO6 octahedron.
Our observation that the apical distances in Hg1201 depend smoothly on the number of anions in
the Hg-plane rather than on their charge can provide
a new explanation of the well-known fact that the Tc
of Hg-bearing superconductors increases under high
pressure. Compression of the Cu`O2 bonds in the
fluorinated Hg-1201 phases can be considered to be
equivalent to an application of approximately 2 GPa
of uniaxial pressure along the c axis. However, our
study showed no difference in Tc for the fluorinated
or oxygenated optimally doped Hg-1201 compounds.
The uniaxial pressure experiments performed on a
crystal of YBa 2 Cu 3 O 7 also showed no variation of
Tc when pressure was applied along the c axis to
provide for compression of the apical Cu`O2 bond
distances w14x. These results agree with our observations and support the conclusion that compression of
the in-plane Cu`O1 distances is the most probable
origin of an enhancement of Tc under pressure in the
Hg-based superconductors. Decrease of the a-parameter for fluorinated Hg-1223 in comparison with
˚ respectively.
oxygenated one Ž3.8501 and 3.8524 A,
followed by slight increase in transition temperature
confirm this hypothesis.
4. Conclusions
The compression of the Hg-1201 structure under
external pressure strongly depends on the doping
level. An increase in the extra oxygen content from
underdoped to overdoped state results in the larger
compression of the apical Cu`O and Ba`O Hg distances while the HgO 2 dumbbell as well as a distance between Ba and O from the ŽCuO 2 . layers
become practically pressure independent. We conclude that in the under- and optimally doped states,
the charge transfer from the reservoir to the ŽCuO 2 .
layers plays a minor role. While the decrease of Tc
under applied pressure for the overdoped Hg-1201
superconductor may be explained by charge transfer
enhancing overdoping.
Oxygenated and fluorinated series exhibit a
cupola-shaped behavior for the Tc vs. d dependence,
but the curves are shifted away from each other
along the d axis. NPD showed double amount of
extra fluorine in comparison with extra oxygen for
the oxygenated Hg-1201 phases with close Tc ’s. An
exchange of the extra oxygen by a double amount of
fluorine causes a significant compression of the apical Cu`O bond distances, while the in-plane ones,
as well as Tc , do not vary. Fluorination of Hg-1223
resulted in a slight in-plane compression and Tc
increase in comparison with the oxygenated material.
Acknowledgements
The authors would like to thank Dr. V.A. Alyoshin
and Mrs. D.A. Mikhailova for the sample preparation, and Dr. P.E. Kazin for the magnetic measurements. The work has been carried out with the
support of the Russian Scientific Council on Superconductivity ŽPoisk. and the Russian Foundation for
Basic Research Ž97-02-17103..
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