Superconductivity in Alkaline Earth Metal-Filled Skutterudites BaxIr4X12 (X = As, P)

Article abstract of DOI:10.1021/jacs.7b04274

We report superconductive iridium pnictides Ba_xIr₄X₁₂ (X = As and P) with a filled skutterudite structure, demonstrating that Ba filling dramatically alters their electronic properties and induces a nonmetal-to-metal transition with increasing the Ba content x. The highest superconducting transition temperatures are 4.8 and 5.6 K observed for Ba_xIr₄As₁₂ and Ba_xIr₄P₁₂, respectively. The superconductivity in Ba_xIr₄X₁₂ can be classified into the Bardeen-Cooper-Schrieffer type with intermediate coupling.

Full text of DOI:10.1021/jacs.7b04274

Communication
pubs.acs.org/JACS
Superconductivity in Alkaline Earth Metal-Filled Skutterudites
BaxIr4X12 (X = As, P)
Yanpeng Qi,*,† Hechang Lei,† Jiangang Guo,† Wujun Shi,§,∥ Binghai Yan,# Claudia Felser,§
and Hideo Hosono*,†,‡
†Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Yokohama, Japan
‡Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Yokohama, Japan
§Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
∥School of Physical Science and Technology, ShanghaiTech University, 200031 Shanghai, China
#Department of Condensed Matter Physics, Weizmann Institute of Science, 7610001 Rehovot, Israel
S
* Supporting Information
knowledge, this is the first report of superconducting iridium
ABSTRACT: We report superconductive iridium pnic-
tides BaxIr4X12 (X = As and P) with a filled skutterudite
structure, demonstrating that Ba filling dramatically alters
their electronic properties and induces a nonmetal-to-
metal transition with increasing the Ba content x. The
highest superconducting transition temperatures are 4.8
and 5.6 K observed for BaxIr4As12 and BaxIr4P12,
respectively. The superconductivity in BaxIr4X12 can be
classified into the Bardeen−Cooper−Schrieffer type with
intermediate coupling.
pnictides with a filled skutterudite structure. The basic
structural features of BaxIr4X12 were examined by powder X-
ray diffraction (XRD) and electron probe microanalysis
(EPMA), and parameters of normal and superconducting
states were also established.
Samples with a nominal composition of BaxIr4As12 and
BaxIr4P12 were synthesized by high-pressure solid-state
reactions starting from BaAs/BaP, Ir, and As/P. BaAs and
BaP were synthesized by heating a stoichiometric mixture of Ba
and As or P at 973 K for 10 h. The starting material mixture of
the desired composition was placed in an h-BN capsule and
heated at 1473−1573 K and 5 GPa for 4 h using a belt-type
high-pressure apparatus. All starting materials and precursors
were prepared in a glovebox filled with purified Ar gas (H2O,
O2 < 1 ppm).
The crystalline phases of the thus prepared samples were
identified by powder XRD (D8 ADVANCE (Cu rotating
anode), Bruker). Rietveld refinement of XRD patterns was
performed using TOPAS3 software. Elemental compositions
were determined using EPMA (JXA-8530F, JEOL). The
dependence of direct-current electrical resistivity (ρ) on
temperature was measured at 2−300 K by a conventional
four-probe method. Magnetization (M) measurements were
performed using a vibrating sample magnetometer (Quantum
Design MPMS). Specific heat data were obtained by a
conventional thermal relaxation method (Quantum Design
PPMS).
Density functional theory (DFT) calculations were per-
formed using the Vienna Ab initio Simulation Package
(VASP)23 with a plane-wave basis, with interactions between
valence electrons and ion cores described using the projector-
augmented wave method.24,25 Exchange and correlation
energies were formulated using the generalized gradient
approximation with the Perdew−Burke−Ernzerhof function-
al.26 An experimentally determined lattice constant was used,
and atom positions were relaxed until the forces acting on the
atoms were less than 5 meV Å−1. Spin−orbit coupling was
included in the electronic structure calculations.
aged compounds exhibit crystal structures composed of
C
rigid covalently bonded cage-forming frameworks enclos-
ing differently bonded guest atoms, being not only of scientific
but also of significant technological interest. The ability of these
materials to accommodate guest filler species results in a wide
range of attractive features1−4 such as heavy-Fermion state
formation,5,6 multipole magnetic ordering,7,8 Fermi- and non-
Fermi liquid behavior,9 good thermoelectric properties,3,10 and
conventional or unconventional superconductivity.6,11−17
Among caged compounds, much attention is directed at
investigating filled skutterudites, which exhibit a general
formula of AT4X12, where A is an electropositive cation, T is
a transition metal, and X is a pnicogen (P, As, or Sb). The
skutterudite framework features eight tilted octahedra per unit
cell enclosing two icosahedral cages, with a transition metal
atom located at the center of each octahedron. This simple
structure acts as a prototype for a large class of compounds,
with its flexibility allowing the incorporation of different guest
atoms, from alkali and alkali earth to rare earth metals.
Although iridium skutterudites (IrAs3 and IrP3) have already
been known for a long time,18,19 no filled versions have been
reported so far.
The discovery of high-temperature superconductivity in iron
pnictides20 has rekindled the exploration of novel metal
pnictide- and chalcogenide-based superconductors.21,22 Herein,
we report the high-pressure synthesis of Ba-filled skutterudites
BaxIr4X12 (X = As, P), showing that these compounds exhibit
bulk superconductivity with transition temperatures of 4.8 K
(Ba0.85Ir4As12) and 5.6 K (Ba0.89Ir4P12). To the best of our
Received: April 26, 2017
Published: June 4, 2017
© 2017 American Chemical Society
8106
DOI: 10.1021/jacs.7b04274
J. Am. Chem. Soc. 2017, 139, 8106−8109

Journal of the American Chemical Society
Communication
Figure 1a shows the crystal structures of IrAs3 and Ba-filled
BaIr4As12, with the latter featuring Ba-filled icosahedra of As
Figure 1. Crystal structure and powder X-ray diffraction pattern of
BaxIr4As12. (a) Crystal structures of IrAs3 and BaIr4As12. (b) Observed
and fitted XRD patterns of BaxIr4As12. (c) Position of the (022) XRD
peak as a function of x.
atoms. The Ba content of BaxIr4As12 was determined by EPMA
as an average of eight measurements (Figure S1 and Table S1),
linearly increasing with increasing x, with saturation occurring
at x ≥ 0.85 for BaxIr4As12 and at x ≥ 0.89 for BaxIr4P12 (Figure
S2). Figure 1b shows the XRD pattern and Rietveld refinement
result for BaxIr4As12. Except for minor impurity peaks attributed
to IrAs3, all other peaks could be attributed to a filled
Figure 2. Electrical resistivity of BaxIr4As12 and BaxIr4P12 as a function
of Ba content. (a) Electrical resistivity of BaxIr4As12 as a function of
temperature. Electrical resistivity drop and low-temperature super-
conducting behavior of (b) BaxIr4As12 and (c) BaxIr4P12. Temperature
dependence of resistivity under different magnetic fields for (d)
Ba0.85Ir4As12 and (e) Ba0.89Ir4P12.
skutterudite structure (cubic, space group Im3 (No. 204), Z =
̅
2), shifting to smaller angles with increasing Ba content due to
successful filling (Figure 1c and Figure S3). The lattice
parameter (a) linearly increased from 8.4697 Å at x = 0 to
8.5605 Å at x = 0.85, with a similar phenomenon also observed
for BaxIr4P12 (Figure S4).
the onset of a superconducting transition. The above
temperature increased with increasing x to Tc = 4.8 K (Figure
2b). Figure 2d shows the resistivity of BaxIr4As12 under different
magnetic fields, revealing that the superconducting transition
was suppressed. The upper critical field (μ0Hc2(0)) was
estimated as 6.9 T (Figure S7), yielding a Ginzburg−Landau
coherence length ξGL(0) of 6.9 nm. A strong diamagnetic signal
was clearly observed below Tc (Figure S8), indicating the bulk
superconductivity of BaxIr4As12, which was further confirmed
by the large specific heat jump at Tc (Figure 3a,b). Therefore,
the temperature dependence of Cp was fitted as Cp(T)/T = γ +
βT2 + δT4, which yielded an electronic specific heat coefficient
(γN) of 35.0 mJ mol−1 K−2 and β = 1.1 mJ mol−1 K−4. The
Debye temperature ΘD equaled 311 K, and the normalized
specific heat jump value, ΔC/γNTc, was estimated as 0.82, being
substantially smaller than the Bardeen−Cooper−Schrieffer
(BCS) weak coupling limit value of 1.43.27 Using the McMillan
formula for electron−phonon-mediated superconductivity,28
we obtained the electron−phonon coupling constant of
Ba0.85Ir4As12 as λe‑ph = 0.63, corresponding to a superconductor
with intermediate coupling. The superconducting transitions of
Figure 2a displays the temperature dependence of the
electrical resistivity of BaxIr4As12 between 2 and 300 K. The
corresponding data for IrAs3 indicate semiconducting behavior,
in agreement with the calculated band structure. However, Ba
filling results in dramatically altered electronic properties:
whereas a moderate change is observed at low x (x < 0.15), a
further increase of Ba content results in a sudden increase of
resistivity, which reaches a maximum at x = 0.52, with a rapid
resistivity decrease observed as x is increased further. The
resistivity of BaxIr4As12 exhibits a nonmonotonic evolution with
increasing x, which is sharply different from other filled
skutterudites.1,3 This anomalous behavior is attributed to the
change of charge carrier type, as revealed by the Hall resistivity
(ρyx) sign change from positive to negative (indicating a change
from p- to n-type conduction) with increasing x. In contrast to
BaxIr4As12, no such change was observed for BaxIr4P12, which
showed a gradually decreasing resistivity with increasing x
(Figures S5 and S6).
As x was increased further, a sharp drop of ρ(T) at a critical
temperature Tc = 3.2 K was observed for BaxIr4As12, marking
8107
DOI: 10.1021/jacs.7b04274
J. Am. Chem. Soc. 2017, 139, 8106−8109

Journal of the American Chemical Society
Communication
rA/dcage for BaxIr4As12 and other superconducting filled
skutterudites, where rA is the effective ionic radius for a 12-
coordinated site, and dcage is the distance from the central atom
A to the nearest cage-forming atoms (Table S2). The linear
relationship between ΘE and rA/dcage strongly demonstrates that
gfs is negatively correlated with rattling energy. Although low-
energy phonons originating from the presence of guest Ba
atoms may be crucial for the superconductivity of BaxIr4X12 (X
= As, P), no simple relationship between Tc and ΘE analogous
to that of β-pyrochlore oxides exists for filled skutterudites,
implying a different superconductivity mechanism or the
prevalence of other factors.38,39
Figure 4a shows the electronic phase diagram of BaxIr4As12,
demonstrating that Ba filling dramatically alters the electronic
Figure 3. Specific heat and Einstein temperature of Ba0.85Ir4As12. (a, b)
Specific heat data for Ba0.85Ir4As12 in the vicinity of the super-
conducting transition. (c) Normal-state specific heat data for
Ba0.85Ir4As12. Solid lines denote fitting curves discussed in the text,
with blue and red lines representing the individual contributions of
Debye and Einstein parts. (d) Plot of Einstein temperature vs rA/dcage
for superconducting filled skutterudites.
BaxIr4P12 are also illustrated in Figure 2c,e and Figure S9, with
relevant parameters summarized in Table 1.
Table 1. Structural and Superconductivity-Related
Parameters of Ba0.85Ir4As12 and Ba0.89Ir4P12
Compound
Space group (Z)
Ba0.85Ir4As12
Ba0.89Ir4P12
Cubic Im3 (No. 204),
Z = 2
̅
Lattice parameter a, Å
8.5605
4.8
8.1071
5.6
Figure 4. Phase diagram and electronic structures of BaxIr4As12. (a)
Phase diagram of BaxIr4As12. (b) DOS for (IrAs3)4 and BaIr4As12
around the Fermi level, with the Fermi energy taken as zero.
Transition temperature Tc, K
Upper critical field Hc2, T
Coherence length ξGL(0), nm
Debye temperature ΘD, K
Einstein temperature ΘE, K
ΔC/γTc
6.9
2.9
6.9
10.7
368
311
92
properties of this compound. For x < 0.15, a weak
enhancement of the overall magnitude of ρ is observed.
Subsequently, the value of ρ(5 K), i.e., normal-state resistivity
just above the superconducting transition temperature, rapidly
increases with increasing x, reaching a maximum at x = 0.52.
For x ≥ 0.52, a dramatic decrease of resistivity by more than 6
orders of magnitude is observed, indicating a nonmetal-to-metal
transition. Superconductivity starts to appear at x ≥ 0.67, with
Tc monotonically increasing with x and reaching a maximum of
4.8 K at x = 0.85 (samples with x ≥ 0.85 could not be
synthesized under the present conditions). A phase diagram
was also obtained for BaxIr4P12, with a maximum Tc of 5.6 K
observed for x = 0.89 (Figure S10). The electronic band
structure of BaxIr4X12 (X = As and P) was investigated by DFT
calculations (Figures S11 and S12). Figure 4b shows the
calculated total and partial near-EF density of states (DOS)
curves for (IrAs3)4 and BaIr4As12, revealing that IrAs3 is a
normal semiconductor with a narrow band gap (ΔE = 0.16 eV),
in agreement with the above-mentioned experimental results.
Ba filling did not significantly alter states around the Fermi
150
0.82
0.63
1.04
0.62
Electron−phonon coupling constant λe‑ph
Similarly to other caged compounds such as Si−Ge
clathrates29 and β-pyrochlore oxides,30 an important structural
feature of Ba-filled skutterudites is Ba atom rattling. Figure 3c
shows the normal-state specific heat data for Ba0.85Ir4As12,
revealing broad peaks at ∼20 K that manifest the presence of a
low-energy Einstein vibration.31−33 This so-called boson peak
always involves local vibration modes34 or transverse acoustic
phonon anomalies.35 Data fitting using a combination of Debye
and Einstein oscillator models yielded Einstein temperatures
ΘE = 92 and 150 K for Ba0.85Ir4As12 and Ba0.89Ir4P12,
respectively.
We subsequently considered characteristic energies of
rattling, i.e., 92 K for Ba atoms in BaxIr4As12. The rattling
energy has been sorted out in terms of the guest free space
(gfs) in caged compounds.36,37 Figure 3d shows plots of ΘE vs
8108
DOI: 10.1021/jacs.7b04274
J. Am. Chem. Soc. 2017, 139, 8106−8109

Journal of the American Chemical Society
Communication
́
(8) Bauer, E. D.; Slebarski, A.; Frederick, N. A.; Yuhasz, W. M.;
Maple, M. B.; Cao, D.; Bridges, F.; Giester, G.; Rogl, P. J. Phys.:
Condens. Matter 2004, 16, 5095.
level, which still mainly originated from the interactions of
strongly hybridized As 4p and Ir 5d orbitals, whereas the Fermi
level was shifted toward the conduction band (CB), finally
crossing it. Ba 6s states were mainly above the Fermi level.
Overall, these results were consistent with Ba 6s to [Ir4As12]
polyanion charge transfer.16,17 For BaIr4As12, the total DOS at
EF, N(EF), equaled 9.11 states eV−1 f.u.−1, corresponding to a
(9) Bauer, E.; Berger, S.; Galatanu, A.; Paul, C.; Mea, M. D.; Michor,
H.; Hilscher, G.; Grytsiv, A.; Rogl, P.; Kaczorowski, D.; Keller, L.;
Hermannsdorfer, T.; Fischer, P. Phys. B 2002, 312−313, 840.
(10) Sales, B. C.; Mandrus, D.; Williams, R. K. Science 1996, 272,
1325.
(11) Gunnarsson, O. Rev. Mod. Phys. 1997, 69, 575.
(12) Imai, M.; Akaishi, M.; Sadki, E. H.; Aoyagi, T.; Kimura, T.;
Shirotani, I. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75,
184535.
̈
π2
coupling constant λ = γN/γ − 1 = 1.1(γ = kB2N(EF)) and
0
0
3
providing further evidence of intermediate coupling in
BaIr4As12.
In conclusion, we identified BaxIr4As12 and BaxIr4P12 as new
superconductors with a filled skutterudite structure, observing
insulator-to-metal and metal-to-superconductor transitions with
increasing Ba content. Analysis of normal-state specific heat
data revealed the presence of a local low-energy Einstein mode
originating from the presence of guest Ba atoms, which may
favor superconductivity. Both experimental and DFT calcu-
lation results suggested that BaxIr4X12 (X = As, P) can be
categorized as Bardeen−Cooper−Schrieffer superconductors
with intermediate coupling.
(13) Kawaji, H.; Horie, H.-o.; Yamanaka, S.; Ishikawa, M. Phys. Rev.
Lett. 1995, 74, 1427.
(14) Tanigaki, K.; Shimizu, T.; Itoh, K. M.; Teraoka, J.; Moritomo,
Y.; Yamanaka, S. Nat. Mater. 2003, 2, 653.
(15) Yonezawa, S.; Muraoka, Y.; Matsushita, Y.; Hiroi, Z. J. Phys.:
Condens. Matter 2004, 16, L9.
(16) Bauer, E.; Grytsiv, A.; Chen, X.-Q.; Melnychenko-Koblyuk, N.;
Hilscher, G.; Kaldarar, H.; Michor, H.; Royanian, E.; Giester, G.;
Rotter, M.; Podloucky, R.; Rogl, P. Phys. Rev. Lett. 2007, 99, 217001.
(17) Gumeniuk, R.; Schnelle, W.; Rosner, H.; Nicklas, M.; Leithe-
Jasper, A.; Grin, Y. Phys. Rev. Lett. 2008, 100, 017002.
(18) Rundqvist, S. Nature 1960, 185, 31.
(19) Kjekshus, A. Acta Chem. Scand. 1961, 15, 678.
(20) Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. J. Am.
Chem. Soc. 2008, 130, 3296.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b04274.
(21) Qi, Y.; Matsuishi, S.; Guo, J.; Mizoguchi, H.; Hosono, H. Phys.
Rev. Lett. 2012, 109, 217002.
Chemical composition and structure analysis of all
samples, properties of other superconducting filled
skutterudites, Hall Effect and superconducting proper-
ties, and DFT calculation results (PDF)
(22) Qi, Y.; Guo, J.; Lei, H.; Xiao, Z.; Kamiya, T.; Hosono, H. Phys.
Rev. B: Condens. Matter Mater. Phys. 2014, 89, 024517.
(23) Kresse, G.; Furthmuller, J. Phys. Rev. B: Condens. Matter Mater.
̈
Phys. 1996, 54, 11169.
(24) Blochl, P. E. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50,
̈
17953.
AUTHOR INFORMATION
(25) Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys.
1999, 59, 1758.
(26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
3865.
(27) Bardeen, J.; Cooper, L. N.; Schrieffer, J. R. Phys. Rev. 1957, 108,
1175.
(28) McMillan, W. L. Phys. Rev. 1968, 167, 331.
(29) Nolas, G. S.; Cohn, J. L.; Slack, G. A.; Schujman, S. B. Appl.
Phys. Lett. 1998, 73, 178.
■
Corresponding Authors
*hosono@msl.titech.ac.jp
*qi-yanpeng@hotmail.com
ORCID
Yanpeng Qi: 0000-0003-2722-1375
Notes
The authors declare no competing financial interest.
(30) Bruhwiler, M.; Kazakov, S. M.; Karpinski, J.; Batlogg, B. Phys.
̈
Rev. B: Condens. Matter Mater. Phys. 2006, 73, 094518.
(31) Keppens, V.; Mandrus, D.; Sales, B. C.; Chakoumakos, B. C.;
Dai, P.; Coldea, R.; Maple, M. B.; Gajewski, D. A.; Freeman, E. J.;
Bennington, S. Nature 1998, 395, 876.
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for World-
Leading Innovative R&D on Science and Technology (FIRST)
and the Element Strategy Initiative to form a Research Core by
MEXT, Japan.
(32) Johari, G. P. Chem. Phys. 2003, 287, 273.
(33) Guo, J.; Yamaura, J.-i.; Lei, H.; Matsuishi, S.; Qi, Y.; Hosono, H.
Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 140507.
(34) Duval, E.; Boukenter, A.; Achibat, T. J. Phys.: Condens. Matter
1990, 2, 10227.
REFERENCES
■
(1) Nolas, G. S.; Morelli, D. T.; Tritt, T. M. Annu. Rev. Mater. Sci.
1999, 29, 89.
(2) Sato, H.; Sugawara, H.; Aoki, Y.; Harima, H. Handbook of
Magnetic Materials; Elsevier: Amsterdam, 2009.
(3) Uher, C. Semicond. Semimetals 2001, 69, 139.
(4) Sales, B. C. Handbook on the Physics and Chemistry of Rare Earths;
Elsevier: Amsterdam, 2003; Vol. 33.
(5) Ho, P. C.; Yuhasz, W. M.; Butch, N. P.; Frederick, N. A.; Sayles,
T. A.; Jeffries, J. R.; Maple, M. B.; Betts, J. B.; Lacerda, A. H.; Rogl, P.;
Giester, G. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72,
094410.
(6) Bauer, E. D.; Frederick, N. A.; Ho, P. C.; Zapf, V. S.; Maple, M. B.
Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 100506.
(7) Leithe-Jasper, A.; Schnelle, W.; Rosner, H.; Senthilkumaran, N.;
Rabis, A.; Baenitz, M.; Gippius, A.; Morozova, E.; Mydosh, J. A.; Grin,
Y. Phys. Rev. Lett. 2003, 91, 037208.
(35) Chumakov, A. I.; Monaco, G.; Monaco, A.; Crichton, W. A.;
Bosak, A.; Ruffer, R.; Meyer, A.; Kargl, F.; Comez, L.; Fioretto, D.;
̈
Giefers, H.; Roitsch, S.; Wortmann, G.; Manghnani, M. H.; Hushur,
́
A.; Williams, Q.; Balogh, J.; Parlinski, K.; Jochym, P.; Piekarz, P. Phys.
Rev. Lett. 2011, 106, 225501.
(36) Matsuhira, K.; Sekine, C.; Wakeshima, M.; Hinatsu, Y.; Namiki,
T.; Takeda, K.; Shirotani, I.; Sugawara, H.; Kikuchi, D.; Sato, H. J.
Phys. Soc. Jpn. 2009, 78, 124601.
(37) Hiroi, Z.; Onosaka, A.; Okamoto, Y.; Yamaura, J.-i.; Harima, H.
J. Phys. Soc. Jpn. 2012, 81, 124707.
(38) Hiroi, Z.; Yamaura, J.-i.; Hattori, K. J. Phys. Soc. Jpn. 2012, 81,
011012.
(39) Nagao, Y.; Yamaura, J.-i.; Ogusu, H.; Okamoto, Y.; Hiroi, Z. J.
Phys. Soc. Jpn. 2009, 78, 064702.
8109
DOI: 10.1021/jacs.7b04274
J. Am. Chem. Soc. 2017, 139, 8106−8109