High-pressure synthesis and magnetic properties of the niobio-cuprate Nb0.9Sr2HoCu2.1O7.9

Article abstract of DOI:10.1016/j.physc.2008.05.033

Ho-based 1212 niobio-cuprate (Nb-1212Ho) was synthesized for the first time. An off-stoichiometric starting composition Nb_0.9Sr₂HoCu_2.1O_7.9 led a phase-pure polycrystalline sample under the synthesis condition at 1450 °C for 2 h at 6 GPa, approximating the true bulk composition. X-ray diffraction pattern clearly indicated an Nb-1212 type structure for the compound with lattice parameters a = b = 3.8505(1) A? and c = 11.574(3) A?, suggesting that the space group P4/mmm is likely for the structure. Unlike Nb-1212Y, the Nb-1212Ho compound showed neither superconductivity nor a magnetic order down to 1.8 K. A relatively large magneto-resistance approximately -7% was found at 30 K.

Full text of DOI:10.1016/j.physc.2008.05.033

Physica C 468 (2008) 1206–1209
Contents lists available at ScienceDirect
Physica C
journal homepage: www.elsevier.com/locate/physc
High-pressure synthesis and magnetic properties of the
niobio-cuprate Nb0.9Sr2HoCu2.1O7.9
*
S. Balamurugan , K. Yamaura, E. Takayama-Muromachi
National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 22 May 2008
Ho-based 1212 niobio-cuprate (Nb-1212Ho) was synthesized for the first time. An off-stoichiometric
starting composition Nb0.9Sr2HoCu2.1O7.9 led a phase-pure polycrystalline sample under the synthesis
condition at 1450 °C for 2 h at 6 GPa, approximating the true bulk composition. X-ray diffraction pattern
clearly indicated an Nb-1212 type structure for the compound with lattice parameters a = b = 3.8505(1) Å
and c = 11.574(3) Å, suggesting that the space group P4/mmm is likely for the structure. Unlike Nb-1212Y,
the Nb-1212Ho compound showed neither superconductivity nor a magnetic order down to 1.8 K. A rel-
atively large magneto-resistance approximately À7% was found at 30 K.
PACS:
74.72.Jt
74.25.Ha
74.25.Fy
Keywords:
Ó 2008 Elsevier B.V. All rights reserved.
High-pressure synthesis
New materials
Nb-1212
NbSr2HoCu2O8
1. Introduction
sized. In this study, we synthesized and characterized the Ho-
based niobio-cuprate (Nb-1212Ho). The data clearly showed that
In the presence of magnetic interactions superconductivity is
generally unstable against the formation of a long-range magnetic
order. Moreover, a small amount of magnetic impurities can kill
superconductivity completely, transforming the superconducting
state to another magnetic state. However, few superconductors
including RuSr2LnCu2O8Àd (Ru-1212) [1–3] and RuSr2(Ln,Ce)2-
Cu2O10Àd (Ru-1222), where Ln = Eu, Gd, Sm [4–6], were found to
show superconductivity in the presence of a magnetic order. Those
reports were surprising for the condensed matter physics commu-
nity, and much attention was concentrated on microscopic magne-
tism of the Ru-1212 and Ru-1222 compounds.
Since then, many materials were developed in order to shed
more light on the issue: for example, Nb [7,8] and Ta [9] replaced
1212 and 1222 compounds were synthesized and characterized. It
is noteworthy that a high pressure/temperature technique was
successful to expand varieties of the Ru-1212 series: the Ln ion
was replaced by Y, Ho, or Tb under synthesis conditions at few
GPa [10–14]. Subsequently, MSr2(Y1.5Ce0.5)Cu2O10 (M-1222Y;
M = Cr, Mn, Ru) compounds were synthesized in the way [15]. Very
recently, the Y-based niobio-cuprate Nb0.9Sr2YCu2.1O7.9 (Nb-
1212Y) was found to form under high-pressure [16,17]. In the
mean time, a Ho-based niobio-cuprate compound was also synthe-
Nb-1212Ho is neither superconducting nor magnetic down to
1.8 K, unlike Nb-1212Y.
2. Experimental details
A polycrystalline dense pellet of Nb-1212Ho (the nominal com-
position: Nb0.9Sr2HoCu2.1O7.9) was prepared by a solid-state reac-
tion technique under high pressure at 6 GPa. Fine powders of
Nb2O5 (99.9%, High purity chemicals), CuO (99.99%, Fluka), Cu
(99.9%, Fluka), Ho2O3 (99.9%, High purity chemicals), SrO2 (lab-
made), and SrCuO2 (lab-made) were used for the synthesis. SrCuO2
was prepared from CuO and SrCO3 (99.99%, Fluka) by a heating at
1000 °C for 4–5 days in air with three intermediate grinding steps,
and SrO2 was prepared from SrCl2 Á 6H2O in water using ammonia
(25%) and hydrogen peroxide (30%) solutions. Both lab-made pow-
ders were confirmed high quality by an X-ray diffraction technique
(see below).
The starting powders were mixed using an agate mortar and
pestle, followed by setting the mixture into a gold capsule. The
steps were completed in an argon filled glove box. It should be no-
ticed that the oxygen content 7.9 per the formula unit was crucial
to prepare a high-quality sample. The sealed capsule was then put
into a flat-belt type apparatus with a graphite heater, and those
were compressed and held at 6 GPa. The capsule was, then, heated
at 1450 °C for 2 h. After the thermal treatment, the capsule was
quenched in the press before releasing the pressure. A black and
well-sintered pellet was obtained.
* Corresponding author. Present address: Institut für Anorganische und Analyt-
ische Chemie University of Münster, Corrensstrasse 30 D-48149 Münster, Germany.
Tel.: +49 251 83 39983; fax: +49 251 83 36002.
E-mail addresses: asbalamurugan@yahoo.co.in, sarkarai@uni-muenster.de (S.
Balamurugan).
0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2008.05.033

S. Balamurugan et al. / Physica C 468 (2008) 1206–1209
1207
The pellet was investigated by a powder X-ray diffraction meth-
od in a commercial apparatus (Rigaku, RINT 2200HF) using Cu-K
a
radiation (k = 1.54056 Å). The X-ray diffraction pattern was ob-
tained at room temperature between 5° and 80° in 2h. Static mag-
netic susceptibility (v) at 10 Oe and 10 kOe was measured in a
commercial magnetometer (Quantum Design, MPMS-XL) between
1.8 K and 300 K. Isothermal magnetization up to 70 kOe was mea-
sured at 1.8 K, 5 K, 10 K, 15 K, 20 K, 25 K, and 50 K in the same
apparatus.
Electric resistivity (q) and magneto-resistance (MR) were mea-
sured by a 4-probe method in a commercial apparatus (Quantum
Design, PPMS) between 2 K to 300 K. Ac-gage current was 0.1 mA
at a frequency of 30 Hz. Specific heat (Cp) measurements in a mag-
netic field of 0 kOe, 10 kOe, 30 kOe, 60 kOe, 90 kOe were conducted
on a piece of the polycrystalline pellet by a relaxation method in
the apparatus between 3.5 K and 37 K.
3. Results and discussion
Fig. 1 shows the powder X-ray diffraction pattern of the pellet.
All observed peaks were characterized by assuming the space
group P4/mmm for the unit cell with lattice parameters
a = b = 3.8505(1) Å and c = 11.574(3) Å, as were done for the Nb-
1212Y compound [16,17]. No impurities were detectable beyond
the X-ray pattern background, suggesting that the starting compo-
sition Nb0.9Sr2HoCu2.1O7.9 is likely the true bulk composition. Lets
us see the lattice parameters. Those are identical with those of
the Nb-1212Y compound within a precision of 0.23% [16,17]. Since
the ionic size of Ho3+ and Y3+ are nearly identical, the quantitative
similarity can be readily understood. Thus, we conclude that the
structure properties of Nb-1212Y and Nb-1212Ho are nearly
identical.
Fig. 2. Magnetic susceptibility of the Nb0.9Sr2HoCu2.1O7.9 compound measured at
10 Oe. Inset is a comparison with the data for the isostructural Nb0.9Sr2YCu2.1O7.9
compound (taken from Ref. [16]).
perature, by a least-squares method (all data points were used).
The analysis yielded hW = À13.3 K and peff (the effective Bohr mag-
neton) = 10.3 l
B/Ho. The peff was calculated from C = NAp2eff/3kB,
where NA and kB are Avogadro’s constant and Boltzmann’s constant,
respectively. The results suggest that 4f spins of Ho3+ (4f10, the cal-
culated effective Bohr magneton is 10.6 lB) are paramagnetic down
to the lowest limit. Although an anti-ferromagnetic interaction (see
the negative hW) governs the 4f spins, a magnetic order of the 4f
spins is suppressed probably in a similar way that was found for
HoBa2Cu3O7Àd [18].
We measured isothermal magnetization of the Nb-1212Ho
compound at T = 1.8 K to 50 K. The M vs. H curves to 70 kOe are
shown in Fig. 4. All the curves are monotonic and a magnetic sat-
uration is not seen, suggesting no magnetic orders within the
range. While the 4f spin contributions mask the 3d features, any
anomaly suggestive for superconductivity is not seen. Thus, overall
M vs. H features likely support the susceptibility results.
Fig. 5 shows temperature dependence of the electrical resistiv-
ity of the polycrystalline Nb-1212Ho compound with and without
a magnetic field of 70 kOe. It is semiconducting-like with room
The
v was measured on a part of the Nb-1212Ho pellet in a
zero-field-cool (ZFC) and a field-cool (FC) conditions at 10 Oe and
10 kOe between 1.8 K and 350 K. Fig. 2 shows a comparison be-
tween the 10 Oe data of the Nb-1212Ho and Nb-1212Y com-
pounds. The data for Nb-1212Y were taken from Ref. [16]. It
appears that Nb-1212Ho is paramagnetic-in-nature down to
1.8 K in marked contrast to what was found for Nb-1212Y.
Although 4f spin contributions are dominant over the data, mag-
netic anomalies are supposed to be visible if it is superconducting.
Fig. 3 shows the 10 kOe data of the Nb-1212Ho compound, plotted
in v
-T and vÀ1-T forms. The data clearly show that there are no sug-
gestive anomalies in regard to superconductivity, as well.
The vÀ1-T curve was fit to the Curie–Weiss law vÀ1(T) =
(T À hW)/C, where C is the Curie constant and hW is the Weiss tem-
temperature resistivity of 5.08
X-cm. Because the data mainly
Fig. 1. Powder X-ray diffraction pattern at room temperature of the polycrystalline
Fig. 3. Magnetic susceptibility of the Nb0.9Sr2HoCu2.1O7.9 compound measured at
Nb0.9Sr2HoCu2.1O7.9 compound.
10 kOe. The solid line is a fit to the Curie–Weiss law.

1208
S. Balamurugan et al. / Physica C 468 (2008) 1206–1209
Fig. 4. Isothermal magnetization to 70 kOe of the Nb0.9Sr2HoCu2.1O7.9 compound at
T = 1.8 K, 5 K, 10 K, 15 K, 20 K, 25 K and 50 K.
represent nature of the 3d electrons and the ligand oxygen 2p
holes (4f electrons are highly localized), it is most likely that the
CuO2 layers are not doped with charge carries enough to establish
high-Tc superconductivity. Inset of Fig. 5 shows temperature
dependence of MR, calculated by the formula MR = 100 Â [(qH
–
q
0)/ 0], where qH and q0 stand for the electrical resistivity at
q
70 kOe and 0 kOe, respectively. A relatively large negative MR,
approximately À7%, was observed at ꢀ30 K, suggesting that the
applied magnetic field improved a charge-carrier hopping environ-
ment somewhat. Probably, the localized 4f spins play a key role in
the MR features.
In order to investigate the magnetic properties of Nb-1212Ho
further, Cp was measured. The data are shown in two forms Cp
vs. T (Fig. 6a) and Cp/T vs. T (Fig. 6b). The data clearly show an
anomaly around 5 K. To shed more light on the 5 K feature, we ap-
plied a magnetic field step by step to 90 kOe. We found that the
peak-like anomaly was dramatically suppressed by applying the
field. Obviously, the peak is magnetic in nature: it is most likely
due to a Schottky contribution from the localized and paramag-
Fig. 6. Specific heat of the Nb0.9Sr2HoCu2.1O7.9 compound in a magnetic field. The
data are plotted in (a) Cp vs. T and (b) Cp/T vs. T forms.
In summary, the Nb-1212Ho compound (Nb0.9Sr2HoCu2.1O7.9
)
shows no superconducting transition down to 1.8 K. The formal va-
lence of Cu is +2.05, if Nb is in the +5 state, suggesting that charge-
carrier density of the CuO2 layers is too low to turn on high-Tc
superconductivity. All data in this study support the picture. Since
the high-pressure condition was essential in the synthesis of the
Nb-1212 compounds, further efforts might be able to increase
the carrier density and then to turn on high-Tc superconductivity
through massive chemical tuning under the condition. Additional
studies are in progress.
netic 4f spins of Ho3+
.
Acknowledgements
The research was supported in part by the Grants-in-Aid for Sci-
entific Research from JSPS (18655080), and the Murata Science
Foundation, Kyoto, Japan. The author (S.B.) thanks support from
NIMS.
References
[1] A.C. McLaughlin, J.P. Attfield, Phys. Rev. B 60 (1999) 14605.
[2] C. Bernard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Golnik, E. Brücher, R.K.
Kremer, D.R. Noakes, C.E. StronachE.J. Ansaldo, Phys. Rev. B 59 (1999) 14099.
[3] A.C. McLaughlin, J.P. Attfield, J.L. Tallon, Int. J. Inorg. Mater. 2 (2000) 95.
[4] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151.
[5] I. Felner, U. Asaf, Y. Levi, O. Millo, Phys. Rev. B55 (1997) R3374.
[6] I. Zivkovic, Y. Hirai, B.H. Frazer, M. Prester, D. Drobac, D. Ariosa, H. Berger, D.
Pavuna, G. Margaritondo, I. Felner, M. Onellion, Phys. Rev. B 65 (2002) 144420.
[7] L.F. Matteiss, Phys. Rev. B 45 (1992) 2442.
Fig. 5. Temperature and magnetic field dependence of the electrical resistivity of
the Nb0.9Sr2HoCu2.1O7.9 compound. A polycrystalline sintered pellet was used. Inset
shows the magneto-resistance at 70 kOe.

S. Balamurugan et al. / Physica C 468 (2008) 1206–1209
1209
[8] M.J. Rey, Ph. Dehaudt, J. Joubert, A.W. Hewat, Physica C 167 (1990) 162.
[9] N. Murayama, E. Sudo, K. Kani, A. Tsuzuki, S. Kawakami, M. Awano, Y. Torii, Jpn.
J. Appl. Phys. 27 (1988) L1623.
[10] V.P.S. Awana, T. Kawashima, E. Takayama-Muromachi, Phys. Rev. B 67 (2003)
172502.
[11] W.B. Yelon, Q. Cai, H.A. Blackstead, M. Kornecki, V.P.S. Awana, H. Kishan, S.
Balamurugan, E. Takayama-Muromachi, J. Appl. Phys. 101 (2007) 09G104.
[12] I. Zivkovic, V.P.S. Awana, H. Kishan, S. Balamurugan, E. Takayama-Muromachi,
I. Felner, J. Appl. Phys. 101 (2007) 09G112.
[14] H. Takagiwa, J. Akimitsu, H. Kawano-Furukawa, H. Yoshizawa, J. Phys. Soc. Jpn
70 (2001) 336.
[15] V.P.S. Awana, S. Balamurugan, H. Kishan, E. Takayama-Muromachi, Mater. Lett.
61 (2007) 774.
[16] S. Balamurugan, V.P.S. Awana, E. Takayama-Muromachi, J. Appl. Phys. 101
(2007) 09G110.
[17] H.A. Blackstead, W.B. Yelon, M. Kornecki, M.P. Smylie, Q. Cai, J. Lamsal, V.P.S.
Awana, S. Balamurugan, E. Takayama-Muromachi, Phys. Rev. B 75 (2007)
140514. R.
[13] H.A. Blackstead, W.B. Yelon, M. Kornecki, M.P. Smylie, Q. Cai, J. Lamsal, V.P.S.
Awana, S. Balamurugan, E. Takayama-Muromachi, Phys. Rev. B 76 (2007)
094507.
[18] A. Furrer, P. Brüesch, P. Unternährer, Phys. Rev. B 38 (1988) 4616.