Synthesis, crystal structure, and physical properties of the type-I clathrate Ba8-δNix□y Si 46- x - y

Article abstract of DOI:10.1021/ic2027626

Type-I clathrate phase Ba₈Ni_x□_ySi _46-x-y (□= vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba _8-δNi_x□_ySi_46-x-y crystallizes in the space group Pm3n (No. 223) with lattice parameter ranging from a = 10.3088(1) A for Ba_7.9(1)Ni _1.4(1)Si_44.6(1) to a = 10.2896(1) A for Ba _8.00(3)Ni_3.82(4)Si_41.33(6). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba_7.9(1)Ni_1.4(1)Si_44.6(1) and Ba _7.9(1)Ni_1.6(1)Si_44.4(1) are superconducting with T_c values of 6.0 and 5.5 K, respectively.

Full text of DOI:10.1021/ic2027626

Article
pubs.acs.org/IC
Synthesis, Crystal Structure, and Physical Properties of the Type-I
Clathrate Ba8−δNix□ySi46−x−y
U. Aydemir,* C. Candolfi, A. Ormeci, H. Borrmann, U. Burkhardt, Y. Oztan, N. Oeschler, M. Baitinger,
F. Steglich, and Yu. Grin*
Max-Planck-Institut fur Chemische Physik fester Stoffe, Nothnitzer Straße 40, 01187 Dresden, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Type-I clathrate phase Ba8Nix□ySi46−x−y (□ = vacancy) was obtained
from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤
0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba
deficiency were prepared from the melt by steel-quenching. Compositions were
established by microprobe analysis and crystal structure determination.
Ba8−δNix□ySi46−x−y crystallizes in the space group Pm3n (No. 223) with lattice
̅
parameter ranging from a = 10.3088(1) Å for Ba7.9(1)Ni1.4(1)Si44.6(1) to a = 10.2896(1) Å
for Ba8.00(3)Ni3.82(4)Si41.33(6). Single-crystal X-ray diffraction data together with
microprobe analysis indicate an increasing number of framework vacancies toward
compositions with higher Ni content. For all compositions investigated, Ni K-edge X-
ray absorption spectroscopy measurements showed an electronic state close to that of
elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower
and low thermal conductivity in the temperature range 300−773 K. Samples with
compositions Ba7.9(1)Ni1.4(1)Si44.6(1) and Ba7.9(1)Ni1.6(1)Si44.4(1) are superconducting with Tc values of 6.0 and 5.5 K, respectively.
by the dimensionless figure-of-merit, ZT = (α2T)/(ρλ).
Intermetallic clathrates usually show favorably low lattice
1. INTRODUCTION
The first type-I silicon clathrate, Na8Si46, was structurally
characterized in 1965.1 In the crystal structure, Si atoms form
thermal conductivity regardless of composition. Hence,
clathrates with high thermoelectric efficiency might be achieved
four-connected, covalently bonded framework with 20- and 24-
atom polyhedral cages filled by Na atoms. Following the Zintl−
Klemm concept, the valence electrons of Na are not needed to
by optimizing thermopower and electrical resistivity.16 In this
respect, clathrate systems with an extended homogeneity range
are promising candidates, because they allow for tuning the
form the four-bonded silicon framework. Na8Si46 shows
metallic behavior,2 which implies that the excess electrons
charge carrier concentration through chemical composition.
For example, clathrates in the systems Ba−Ni−Ge17,18 and Ba−
occupy antibonding (conduction) bands according to
Au−Si19,20 show either n- or p-type conduction behavior
[Na1+]8[(4b)Si0]46·8e−. Later, it was shown that the polyhedral
cages can also be filled by Ba atoms and clathrates can bear
even larger amount of excess electrons as Na2Ba6Si46 with
[Na1+]2[Ba2+]6[(4b)Si0]46·14e−.3 This phase attracted consi-
derable attention since it was found to be the first
superconductor based on an sp3-framework with Tc ≈ 4 K.4
A higher transition temperature of Tc ≈ 8 K was found for
Ba8Si46.5 Ternary superconducting clathrates have been
reported as well, in which silicon atoms are partially replaced
with other elements, e.g., Ge,6 Ga,7 Cu,8 Ag.9 However, in all
these cases, Tc was found to decrease with increasing content of
substitution atoms, and superconductivity vanishes after a
certain substitution level is reached. Generally, the number of
excess electrons in clathrate-I silicides is reduced by the
substitution atoms in the framework. In the Zintl phase
Ba8Ga16Si30, e.g., Si atoms are substituted by Ga resulting in the
charge balance [Ba2+]8[(4b)Ga1−]16[(4b)Si0]30.10−12 When full
charge balance is achieved, semiconducting clathrates can be
obtained. Such phases are of high interest for thermoelectric
properties,13−15 which depend on thermopower (α), electrical
resistivity (ρ), and thermal conductivity (λ), and characterized
depending on the content of transition metal.
For the system Ba−Ni−Si, a clathrate-I phase was originally
21
reported for only one composition Ba8Ni1.8Si44.2
.
Crystals of
this composition were directly obtained from the melt, whereas
the binary Ba8−xSi46 has been prepared only under high
pressure5,6,22,23 or, recently, by oxidation of Li2Ba4Si6 with
gaseous HCl.24 Remarkably, the reported lattice parameter of
Ba8Ni1.8Si44.2 (a = 10.285(5) Å) is significantly smaller than that
of Ba8Si46 (a = 10.328(2) Å).5
Herein, more systematic studies were undertaken to
determine the phase formation and the variations in the crystal
structure with Ni content. We report on the phase stability, the
homogeneity range and the crystal structure of the clathrate-I
phase Ba8−δNix□ySi46−x−y. X-ray absorption spectroscopy
(XAS) measurements were carried out to elucidate the
electronic state of Ni. We further demonstrate the possibility
of achieving superconducting single-phase samples with lower
Received: December 24, 2011
Published: March 28, 2012
© 2012 American Chemical Society
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Table 2. Atomic Coordinates, Displacement Parameters (in Å2), and Site Occupancies for Ba8Ni2.6Si43.3
(top),
□
0.1
□
□
Ba8Ni3.2Si42.4
(middle), and Ba8Ni3.7Si41.4
(bottom)
0.4
0.9
a
atom
site
x
y
z
Ueq
Occ.
Ba1
2a
0
0
0
0.00949(5)
0.0159(5)
0.0086(2)
0.00814(8)
0.0089(3)
0.0097(4)
1
Ba2
24k
6c
0.2474(8)
0.5163(3)
0
0.25
Ni1/Si1
Si2
1/4
0
1/2
x
0.433/0.55(1)
16i
24k
24k
0.18461(6)
x
1
Si31
Si32
atom
0
0
0.3087(2)
0.3148(2)
0.1164(2)
0.1279(2)
0.567(4)
0.433
U23
U11
U22
U11
U33
U12
U13
Ba1
0.00949(8)
0.0122(5)
0.0097(4)
0.0081(1)
0.0095(5)
0.0066(6)
site
U11
0
0
0
Ba2
0.0145(6)
0.0080(3)
U11
0.0211(10)
U22
0.0040(6)
0
0
Ni1/Si1
Si2
0
0
0
U11
−0.0008(1)
U12
0
U12
Si31
Si32
atom
0.0068(5)
0.0128(7)
0.0106(5)
0.0098(7)
0
0.0012(4)
0.0020(6)
Occ.
0
0
a
x
y
z
Ueq
Ba1
2a
0
0
0
0.00899(5)
0.0151(5)
0.0073(2)
0.00796(8)
0.0063(4)
0.0074(3)
1
Ba2
24k
6c
0.24105(7)
0.51441(7)
0
0
0.25
Ni1/Si1
Si2
1/4
1/2
x
0.533/0.404(7)
1
16i
24k
24k
24k
U11
0.18434(5)
x
Si31
Si32
Si33
atom
0
0
0
0.3067(2)
0.3151(2)
0.3278(13)
0.1122(2)
0.1281(2)
0.405(3)
0.533
b
0.1325(13)
0.009(2)
0.063(3)
U23
U22
U33
U12
U13
Ba1
Ba2
0.00899(9)
U11
U11
0
0
0
0.0019(2)
0.0079(5)
0.0080(1)
0.0082(7)
0.0073(5)
0.0188(10)
0.0069(3)
U11
0.0315(10)
U22
0.0027(5)
0
0
Ni1/Si1
Si2
0
0
0
U11
−0.0006(2)
0
U12
0
U12
Si31
Si32
atom
0.0069(7)
0.0066(5)
0.0037(7)
0.0082(5)
−0.0025(6)
−0.0001(4)
Occ.
0
0
a
site
x
y
z
Ueq
Ba1
2a
0
0
0
0.01052(5)
0.0214(8)
0.0088(2)
0.00981(7)
0.0076(6)
0.0099(3)
0.0080(9)
1
Ba2
24k
6c
0.24311(9)
0.51155(9)
0
0
0.25
Ni1/Si1
Si2
1/4
1/2
x
0.617/0.231(8)
1
16i
24k
24k
24k
0.18434(5)
x
Si31
Si32
Si33
atom
0
0
0
0.3072(3)
0.3122(1)
0.3216(5)
0.1094(3)
0.1231(1)
0.1343(5)
0.232(3)
0.617
0.152(4)
U23
U11
U22
U33
U12
U13
Ba1
0.01052(8)
0.0085(2)
0.0090(3)
0.0098(1)
0.0048(9)
0.0119(4)
0.0040(2)
U11
U11
0
0
0
Ba2
0.019(2)
0.0082(2)
U11
0.037(2)
U22
−0.0008(7)
0
0
0
Ni1/Si1
Si2
0
0
U11
−0.0006(1)
U12
0
U12
Si31
Si32
0.0095(11)
0.0089(4)
0.008(2)
0.0086(10)
0.0089(4)
0.012(2)
0
0
0
−0.0006(8)
0.0016(3)
−0.0027(14)
0
Si33
0
a
b
Ueq is defined as one-third of the trace of the orthogonalized Uij tensor, which is exp (−2π2 [h2a*2U11 + ... + 2 h k a*b*U12]). Refined in isotropic
approximation (Uiso)
open glassy carbon crucible (⌀ = 12 mm, l = 12 mm, Sigradur G,
HTW) and slowly heated to the melt using an induction furnace (5
Ni content. Finally, we investigated the thermoelectric
efficiency of the clathrate phase at high temperatures. A
thorough theoretical and experimental investigation of the
electronic band structure and the low-temperature transport
properties was already reported elsewhere.25
kW, coil ⌀ = 40 mm, l = 35 mm, Huttinger; IR pyrometer, Maurer).
̈
The melts were then cooled down to room temperature by pouring on
a steel plate. The resulting specimens were characterized by
metallography and powder X-ray diffraction (PXRD). For the
annealing, specimens were placed in glassy carbon crucibles and
welded in tantalum ampules which were in turn sealed in an argon-
filled quartz tube. To avoid any influence from the cooling process, we
quenched the ampules in water. Clathrate phase was obtained after 1
week of annealing at 1000 °C. Superconducting samples
Ba8−δNixSi46−x (δ ≈ 0.1; x = 1.4 and 1.6) with low Ni content were
obtained by fast cooling of the melt between two steel plates (steel-
2. EXPERIMENTAL SECTION
2.1. Synthesis. Polycrystalline samples were synthesized by
reaction of crystalline Ba (ChemPur, 99.9% metals basis), Ni powder
(ChemPur, 99.9% metals basis) and ground Si pieces (ChemPur,
99.9999% metals basis) in an argon-filled glovebox. Stoichiometric
amounts of elements with total weight of around 2 g were placed in an
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quenching).26 Besides the clathrate phase, the reaction products
mainly contained BaSi2 and α-Si, which were removed by treatment
with 3 M HCl for 8 h and afterward by 1 M NaOH for 8 h (see Figure
S1 in the Supporting Information).
2.2. Powder X-ray Diffraction. PXRD data were obtained using
an X-ray Guinier diffraction technique (Huber G670 camera, Cu-Kα1
radiation, λ = 1.54056 Å, graphite monochromator, 5° ≤ 2θ ≤ 100°,
Δ2θ = 0.005°). The reflection positions were determined by profile
deconvolution and corrected by LaB6 (a = 4.15683(9) Å at 295(2) K)
as internal standard. The unit cell parameters were calculated from
least-squares refinement and the Rietveld refinements were performed
by using WinCSD program package.27
2.3. Single-Crystal X-ray Diffraction. Room-temperature single-
crystal X-ray data were collected with a rotating-anode diffractometer
(RIGAKU Spider, Varimax optical system, Ag-Kα radiation, λ =
0.56087 Å). A multiscan absorption correction was applied during the
data collection. Crystal structure determination and refinement were
carried out with WinCSD software. Details regarding the data
collection and structure refinement are given in Table 1, atomic
coordinates and displacement parameters are listed in Table 2.
2.4. Microstructure Analysis. Samples stable to air and moisture
were embedded in epoxy resin substrate containing carbon fibers.
Grinding was performed with silicon carbide abrasive discs together
with alcohol including lubricants. Polishing was achieved in steps by
using different polishing discs of micrometer-sized diamond powders
(6, 3, 0.25 μm) in paraffin. Air or moisture sensitive samples were
polished in an Ar-filled glovebox using hexane as lubricant. Optical
microscopy images were obtained by a polarization microscope (Zeiss
Axioplan). Phase identification was carried out by using a Philips XL
30 scanning electron microscope (SEM) with integrated energy
dispersive X-ray (EDX) spectrometer. The composition of the
clathrate phase was determined with a Cameca SX 100 wavelength
dispersive X-ray (WDX) spectrometer using Ba6Ge25 and NiSi as
standards for the determination of Ba, Ni, and Si concentrations.
2.5. X-ray Absorption Spectroscopy. X-ray absorption near-
edge structure (XANES) and the extended X-ray absorption fine
structure (EXAFS) were studied in the spectral region of the Ni K-
edge (8333 eV). The spectra were recorded in transmission mode at
the EXAFS beamline C of HASYLAB at German Electron
Synchrotron (DESY). Further experimental details are given in the
Supporting Information (section S.1).
2.6. Thermal Analysis. Thermal behavior of the samples was
investigated with a Netzsch DSC 404C instrument. Bulk pieces of
about 30 mg were transferred to a glassy carbon crucible (⌀ = 4 mm, l
= 6 mm, Sigradur G, HTW) which was then sealed in an Nb crucible
(⌀ = 5 mm, 600 mg). The system was heated under Ar atmosphere
from room temperature up to 1300 °C applying different heating rates
of 2−10 °C/min.
2.7. Calculation Procedures. First-principles electronic structure
calculations within the local density approximation to the density
functional theory were performed using the all-electron, full-potential
local orbital method (FPLO).28 Perdew−Wang parametrization of the
exchange-correlation potential was employed.29 The calculations for
Ba8Nix□ySi46−x−y were performed on models with the compositions (x
= 0−4, 6; y = 0) and (x = 3, 4; y = 1). To compute heats of formation
and assess the stability of the clathrate phase, total energy calculations
for the elemental solids (bcc Ba,30 ferromagnetic fcc Ni,30 and α-Si30),
BaSi2,31 NiSi,32 and NiSi233 were carried out on fully optimized crystal
structures. In regard to the clathrates, however, only the end-members
(x = 0 and 6; y = 0) were fully optimized, whereas for the other
compositions experimentally determined values of the structural
simultaneously in the temperature range 300−773 K using a
commercial setup (ZEM-3, Ulvac-Riko) on prism-shaped specimens
of 2 × 2 × 6 mm3. Thermal diffusivity a was measured from 300 to
773 K using a Netzsch LFA 427 apparatus on specimens with 6 × 6 ×
1 mm3. The thermal conductivity was then calculated via λ = aCpρv
where Cp is the specific heat and ρv is the density. The specific heat
was considered to be constant and equal to the Dulong-Petit value
throughout the temperature range investigated. No correction for
thermal dilatation was applied.
The superconductivity of the clathrate phase with low Ni
concentrations was characterized by electrical resistivity measurements
in the temperature range 2−10 K, using the AC option of a physical
property measurements system (PPMS, Quantum Design). Copper
wires were used as contacts and attached to the samples using a tiny
amount of silver paint. The measurements were performed on prism-
shaped specimens with the size of 1.5 × 1 × 6.7 mm3 which were hot-
pressed at 300 °C under 0.5 GPa pressure.
3. RESULTS AND DISCUSSION
3.1. Isothermal Section of the System BaSi2−NiSi−α-Si
at 1000 °C. Homogeneous samples of the clathrate-I phase
Ba8Nix□ySi46−x−y were obtained for 2.4 ≤ x ≤ 3.8 and y ≤ 0.9
after annealing at 1000 °C for 1 week. The phase equilibria of
the clathrate phase at this temperature are shown in Figure 1.
The three-phase region between clathrate-I + BaSi2 + α-Si is
observed in the samples with the nominal compositions
Ba8Ni0.5Si45.5 (Figure 1a, sample 1), Ba8Ni2Si44 (Figure 1a,
sample 2), Ba22.5Ni2.5Si75 (Figure 1a, sample 3), and Ba10Ni2Si88
(Figure 1a, sample 4). The microstructure of these samples
always showed well separated grains of BaSi2, α-Si and
clathrate-I phase (C-I) in the respective amounts (Figure 2a
and Figure S2a-c in the Supporting Information). The lower
limit of Ni content in the homogeneity range Ba8Nix□ySi46−x−y
at 1000 °C was determined to be Ba8.00(5)Ni2.36(4)Si43.94(6) from
the sample 2 (Table 3). Two-phase regions were found at the
compositions Ba18Ni5Si77 (Figure 1a, sample 5), containing
BaSi2 and clathrate (Figure 2b), and Ba13Ni5Si82 (Figure 1a,
sample 6) showing a mixture of α-Si and clathrate (Figure 2c).
Samples with the composition Ba25Ni5Si70 (Figure 1a, sample
7) and Ba2.5Ni2.5Si95 (Figure 1a, sample 8) belong to three-
phase regions BaSi2 + clathrate-I + melt and clathrate-I + α-Si +
melt, respectively (see Figure S2d, e in the Supporting
Information). For the composition Ba8Ni4Si42 (Figure 1a,
sample 9), the clathrate phase is in equilibrium with the melt.
The microstructure of this sample shows large grains of the
clathrate phase surrounded by a finely grained mixture formed
from the melt after the sample was quenched from 1000 °C to
room temperature (Figure 2d). The composition
Ba8.00(3)Ni3.82(4)Si41.33(6) determined for the clathrate phase in
this sample represents the upper limit of Ni content at 1000 °C,
which was also confirmed from samples with higher nominal Ni
content (Table 3). Analysis of the clathrate-I phase by
wavelength dispersive X-ray spectroscopy (WDXS) shows a
distinct deviation from the nominal composition Ba8NixSi46−x
(Figure 1b, Table 3). In particular for higher Ni contents, the
ratio of concentrations (cNi + cSi)/cBa is influenced by the
presence of vacancies in the Ni−Si framework. Full occupancy
of the Ba sites as well as the existence of framework vacancies
were shown by structure analyses based on single crystal data
(see section 3.5).
parameters were used. In the crystal structure (space group Pm3n),
̅
Ni and Si atoms as well as vacancies share the 6c position as discussed
in crystal structure part (see section 3.5). For the calculations, partial
occupancies were modeled by using supercells which were constructed
according to the group−subgroup relations. The details regarding this
matter were explained in a recent publication.25
Clathrate products obtained at 1000 °C were investigated by
thermal analysis. For all compositions 2.4 < x < 3.8, the
peritectic decomposition of the clathrate-I phase was indicated
by a single, strong endothermic effect close to T = 1105 °C,
2.8. Physical Properties. For the measurements, specimens with
the required shape were cut from annealed ingots by using a diamond-
wire saw. Electrical resistivity and thermopower were measured
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Figure 2. Microstructure (SEM, BSE contrast) of samples in the
system BaSi2−NiSi−α-Si after annealing at 1000 °C: (a) Ba8Ni2Si44
(sample 2), (b) Ba18Ni5Si77 (sample 5), (c) Ba13Ni5Si82 (sample 6),
and (d) Ba8Ni4Si42 (sample 9). Sample numbers are given according to
Figure 1 and observed phases are marked.
estimated by microstructure analysis of samples 2 (Figure 4b)
and 3 (Figure 4c) to be 8 at % Ba, 34 at % Ni, 58 at % Si
(Figure 3, point τ).
The primary crystallization field of α-Si is subdivided into
three parts. At low Ni content, it is in contact with the primary
crystallization field of BaSi2. Microstructure analysis of sample 4
(Figure 4d) showed large and well-shaped crystallites of the
primary phase α-Si. On further cooling, smaller but distinctly
shaped crystallites of BaSi2 and α-Si appear. With increasing Ni
content, the peritectic reaction L + α-Si → C−I is observed at
1105 °C. Hence, the microstructure of samples 5−7 contains
grains of α-Si, enveloped by the crystallites of the clathrate-I,
and remaining BaSi2 (sample 7, Figure 4e). Further increase of
Ni content leads to formation of the clathrate-I phase besides
α-Si. The microstructure of sample 8 (Figure 4f) and sample 9
(Figure 4g) contains primary grains of α-Si that are almost
entirely enveloped by NiSi2 crystallites. Upon crystallization of
NiSi2, the crystallization field of the clathrate phase is reached
as both phases are formed with well-shaped crystals. At lower
temperatures, the eutectic channel between clathrate and NiSi
is reached, represented in the microstructure by areas with a
finely grained mixture of NiSi and the clathrate phase. The
primary crystallization field of BaSi2 is observed in the samples
10 and 2 in which primary grains of BaSi2 are in contact with
clathrate-I (Figure 4h) or NiSi crystallites (Figure 4b),
respectively. The primary crystallization of NiSi is observed in
sample 3 (primary grains of NiSi in contact with BaSi2
crystallites, Figure 4c). The small field of crystallization of
NiSi2 may be expected close to the binary system α-Si−NiSi
and was not further characterized. The primary crystallization
of the clathrate-I phase is observed for the samples 11−14
(region C-I, Figure 3) showing large areas of clathrate-I phase
in contact with BaSi2 and NiSi (samples 11−13, Figure 4i) or
NiSi and NiSi2 (sample 14), respectively. Almost equal
amounts of clathrate-I and BaSi2 phases are observed in sample
13 (Figure 4j) suggesting that the eutectic channel is running
close to this concentration point.
Figure 1. (a) Isothermal section of the system BaSi2−NiSi−α-Si at
1000 °C based on the investigated samples Ba8Ni0.5Si45.5 (1),
Ba8Ni2Si44 (2), Ba22.5Ni2.5Si75 (3), Ba10Ni2Si88 (4), Ba18Ni5Si77 (5),
Ba13Ni5Si82 (6), Ba25Ni5Si70 (7), Ba2.5Ni2.5Si95 (8), and Ba8Ni4Si42 (9).
The part of the phase diagram, which should be liquid at 1000 °C is
only estimated and shown as hatched gray area. (b) The homogeneity
range of the clathrate phase based on the WDXS data is shown as thick
line. The refined single crystal compositions (I, II, III) are shown with
squares. The dashed-line represents the isoconcentrate of 14.81 at %
Ba (Ba8NixSi46−x).
followed by a broad endothermic peak indicating the liquidus
(see Figure S3 in the Supporting Information). On cooling,
several exothermic effects appeared in agreement with the
incongruent (peritectic) formation of the clathrate phase.
3.2. Estimation of Primary Crystallization Field of the
Clathrate-I Phase. The type-I clathrate Ba8Nix□ySi46−x−y is
the only known ternary phase in the composition triangle
BaSi2−NiSi−α-Si.21,25 For the binary systems BaSi2−Si and
NiSi−Si, a eutectic composition was reported at 77 at % Si
(Figure 3, point ε1),34 and at 56 at % Si (point ε2),35
respectively. The eutectic point ε3 of the quasi-binary system
BaSi2−NiSi was assigned from sample 1 to be 20 at % Ba, 20 at
% Ni, 60 at % Si with a melting temperature at around 920 °C
(see Figure S4 in the Supporting Information). At this
composition, the microstructure contains finely distributed
crystallites of BaSi2 and NiSi (Figure 4a). The ternary eutectic
point of BaSi2, NiSi and the clathrate phase was roughly
3.3. Metastable Compositions. The clathrate phase with
distinctly lower Ni content was obtained by steel-quenching of
molten samples. The compositions were determined from
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Table 3. Lattice Parameter and Composition of the Clathrate Phase (C−I) in the Samples Annealed at 1000 °C for 1 Week
nominal composition
products (PXRD & EDXS)
composition (WDXS)
a (Å)
Ba8Ni0.2Si45.8
Ba8Ni0.5Si45.5
Ba8Ni1Si45
C-I (<10%), BaSi2 (>40%), α-Si (>50%)
C-I (<20%), BaSi2 (>40%), α-Si (>40%)
C-I (>40%), BaSi2 (<30%), α-Si (<30%)
C-I (>80%), BaSi2 (<10%), α-Si (<10%)
C-I (>80%), BaSi2 (<10%), α-Si (<10%)
C−I (>95%), BaSi2 (<3%), α-Si (<2%)
C-I (>95%), BaSi2 (<3%), α-Si (<2%)
C-I (>96%), BaSi2 (<2%), α-Si (<2%)
C-I (>98%), BaSi2 (<1%), α-Si (<1%)
C-I (>98%), BaSi2 (<1%), α-Si (<1%)
C-I (>98%), BaSi2 (<1%), α-Si (<1%)
C-I (>98%), BaSi2 (<1%), α-Si (<1%)
C-I (>98%), BaSi2 (<1%), α-Si (<1%)
C-I (>99%), α-Si (<1%)
Ba8.0(3)Ni2.5(1)Si42.0(2)
10.2994(1)
10.2997(1)
10.3007(1)
10.3011(2)
10.3007(1)
10.2982(1)
10.2981(1)
10.2958(1)
10.2951(1)
10.2947(1)
10.2929(1)
10.2923(1)
10.2922(1)
10.2904(1)
10.2897(1)
10.2906(1)
10.2896(1)
10.2907(1)
Ba8.0(3)Ni2.5(1)Si42.3(3)
Ba8.00(3)Ni2.43(2)Si43.30(4)
Ba8.00(5)Ni2.36(4)Si43.94(6)
Ba8.00(3)Ni2.44(2)Si42.81(3)
Ba8.00(3)Ni2.62(1)Si42.57(3)
Ba8.00(2)Ni2.64(2)Si43.23(4)
Ba8.00(5)Ni2.73(1)Si42.89(5)
Ba8.00(2)Ni2.81(2)Si42.10(3)
Ba8.00(2)Ni2.83(1)Si42.61(1)
Ba8.00(1)Ni2.93(1)Si42.50(1)
Ba8.00(2)Ni3.11(1)Si42.26(2)
Ba8.00(2)Ni3.17(1)Si43.03(2)
Ba8.00(7)Ni3.42(3)Si41.99(4)
Ba8.00(3)Ni3.46(4)Si41.56(7)
Ba8.00(4)Ni3.72(9)Si41.83(5)
Ba8.00(3)Ni3.82(4)Si41.33(6)
Ba8.00(1)Ni3.62(3)Si41.16(3)
Ba8Ni2Si44
Ba8Ni2Si44
Ba8Ni2.5Si43.5
Ba8Ni2.5Si43.5
Ba8Ni2.65Si43.35
Ba8Ni2.75Si43.25
Ba8Ni2.75Si43.25
Ba8Ni2.85Si43.15
Ba8Ni3Si43
Ba8Ni3Si43
Ba8Ni3.5Si42.5
Ba8Ni3.5Si42.5
Ba8Ni4Si42
C-I (>99%), NiSi (<1%)
C-I (>98%), BaSi2, NiSi, NiSi2 (<2%)
C-I (>98%), BaSi2, NiSi, NiSi2 (<2%)
C-I (>90%), BaSi2, NiSi, NiSi2 (<5%)
Ba8Ni4Si42
Ba8Ni6Si40
3.4. Heat of Formation and Phase Stability. The phase
stability of clathrates can be estimated by comparison of the
ground-state energies.36,37 The heat of formation of
Ba8Nix□ySi46−x−y was computed on the basis of the total
energies obtained from first principles electronic structure
calculations by considering the following balance
8Ba + xNi
bcc
+ (46 − x − y)α‐Si
fcc;FM
= Ba Ni □ Si
+ ΔH (x, y)
8
x
y
46−x−y
f
(1)
The stability of the clathrate phase with respect to the
competing phases BaSi2, NiSi, NiSi2, and α-Si was also explored
(see Table 5). Since both NiSi and NiSi2 can form by
decomposition of the clathrate phase, we took into account the
two limiting balances:
Ba Ni □ Si
8
x
y
46−x−y
□
= 8BaSi + xNiSi + (30 − 2x − y)α‐Si + ΔE (x, y)
2
1
Figure 3. Estimated primary crystallization fields in the system BaSi2−
NiSi−α-Si. The investigated compositions are numbered as
Ba20Ni20Si60 (1), Ba12Ni30Si58 (2), Ba5Ni40Si55 (3), Ba8Ni0.5Si45.5 (4),
Ba10Ni2Si88 (5), Ba8Ni3Si43 (6), Ba8Ni4Si42 (7), Ba5Ni20Si75 (8),
Ba5Ni30Si65 (9), Ba25Ni5Si70 (10), Ba8Ni6Si40 (11), Ba8Ni10Si36 (12),
Ba20Ni10Si70 (13), and Ba10Ni20Si70 (14). Binary and ternary eutectics
are marked with ε and τ, respectively.
(2a)
Ba Ni □ Si
8
x
y
46−x−y
= 8BaSi + xNiSi + (30 − 3x − y)α‐Si + ΔE (x, y)
2
2
2
(2b)
In agreement with the phase diagram, eq 2b is more realistic
because NiSi2 was calculated to be more stable than NiSi + α-Si
by 46.6 meV/atom.
Rietveld refinement on PXRD data and qualitatively confirmed
by EDXS on pressed-powder after removing BaSi2 and α-Si
with dilute HCl and NaOH (Table 4 and Table S4 in the
Supporting Information). From the starting composition
Ba8Ni0.2Si45.8, a clathrate phase with the composition
Ba7.9(1)Ni1.4(1)Si44.6(1) (a = 10.3088(1) Å) was formed as a
minority product together with BaSi2 and α-Si. Similarly,
Ba7.9(1)Ni1.6(1)Si44.4(1) (a = 10.3078(1) Å) was obtained starting
from Ba8Ni0.5Si45.5. In the DSC experiment, single phase
Ba7.9(1)Ni1.6(1)Si44.4(1) showed an exothermic effect at ∼715 °C
on heating with 10 K/min indicating the decomposition of a
metastable phase (see Figure S5 in the Supporting
Information). Therefore, it is possible that the steel-quenched
clathrate products with low Ni content belong to a Ni-stabilized
The heat of formation ΔHf of Ba8Nix□ySi46−x−y (for y = 0)
varies linearly with Ni content with two different slopes in the
ranges 0 ≤ x ≤ 3 and 3 ≤ x ≤ 6 (Figure 5 inset). The additional
energy saved upon forming a clathrate with one more Ni atom
per formula unit for x up to 3 is almost twice that for x > 3. In
this context, it is remarkable that the composition with x = 3
was also found to be special as it delineates two regions of
different transport behavior.25 Examination of Table 5 and
Figure 5 reveals that: (i) all clathrates with compositions x ≤ 4
are stable according to eq 2a; (ii) the binary clathrate Ba8Si46
and the compositions with 1 < x < 4 are stable according to eq
2b; (iii) the composition with x = 3 is the most stable one; and
(iv) compositions with vacancies (y = 0) are unstable. ΔE1 and
high-pressure phase Ba8−δNixSi46−x
.
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Table 5. ΔHf (eq 1), ΔE1 (eq 2a), and ΔE2 (eq 2b) in meV
a
atom−1 for Ba8Nix ySi46−x−y
□
(x, y)
ΔHf
ΔE1
ΔE2
(0, 0)
(1, 0)
(2, 0)
(3, 0)
(3, 1)
(4, 0)
(4, 1)
(6, 0)
171.3
193.3
218.2
243.9
222.0
257.0
231.3
285.8
−1.8
−2.2
−5.6
−9.7
16.6
−1.2
29.4
13.1
−1.8
0.4
−0.4
−1.9
24.6
9.2
39.9
28.2
a
Negative values of ΔE indicate that the clathrate phase is more stable
than the competing phases.
●
▲
Figure 5. Enthalpies ΔE1 ( ), ΔE2 ( ), and heat of formation (inset)
as a function of nominal Ni content for vacancy-free compositions.
ΔE2 having different signs for cases x = 1 and x = 4 should be
considered taking two aspects into account: the ratio of the
balances in eqs 2a and 2b is indeterminant and none of the
clathrate models was optimized in a supercell. Although both
aspects may be important for the x = 1 case, only the former is
crucial for the x = 4 case. Furthermore, the presence of
vacancies at the 6c site and the associated entropy contributions
of phonons to the free energy introduce more uncertainty to
the case of x = 4. Since after three Ni atoms, addition of an
extra Ni atom saves less energy, the competing phases quickly
become more favorable for x > 3. This result reflects a tendency
to avoid two Ni atoms to be situated at consecutive 6c positions
of the same six-ring, which leads to increased bond distances
between the atoms at the 24k sites. Finally, Ba8Ni6Si40 (x = 6) is
found to be unstable in agreement with the fact that clathrate-I
at this composition has not been observed yet.
Figure 4. SEM images (BSE contrast) of the samples obtained by fast
cooling from the melt in the system BaSi2−NiSi−α-Si. The nominal
compositions of the samples are: (a) Ba20Ni20Si60 (sample 1), (b)
Ba12Ni30Si58 (sample 2), (c) Ba5Ni40Si55 (sample 3), (d) Ba8Ni0.5Si45.5
(sample 4), (e) Ba8Ni4Si42 (sample 7), (f) Ba5Ni20Si75 (sample 8), (g)
Ba5Ni30Si65 (sample 9), (h) Ba25Ni5Si70 (sample 10), (i) Ba8Ni6Si40
(sample 11), and (j) Ba20Ni10Si70 (sample 13). Sample numbers are
given according to Figure 3 and observed phases are marked.
Table 4. Lattice Parameter and Composition of the Clathrate
The effect of vacancy formation on phase stability of
clathrates was investigated for compositions with x = 3 and 4
using the following balances:
Phase (C−I) in the Samples Obtained by Steel-Quenching
nominal
products
(PXRD & EDXS)
composition
(EDXS)
a
(Å)
composition
Ba8Ni0.2Si45.8 C-I (<5%), BaSi2 (>40%),
Ba7.5Ni1.6Si44.4
10.3088(1)
10.3078(1)
10.2932(1)
10.2927(1)
10.2935(1)
Ba Ni Si = Ba Ni Si □ +α‐Si + ΔE
v
(3a)
(3b)
8
3
43
8
3
42
1
α-Si (>50%)
Ba8Ni0.5Si45.5 C-I (<10%), BaSi2 (>40%), Ba7.5Ni1.8Si44.2
α-Si (>40%)
Ba Ni Si = Ba Ni Si □ +α‐Si + ΔE
42 41 v
8
4
8
4
1
Ba8Ni3Si43
C-I (>60%), BaSi2 (<20%), Ba8Ni3.0Si45.6
α-Si (<20%)
The computed vacancy formation energies are −1.4 and −1.6
eV for one vacancy per formula unit for x = 3 and 4,
respectively, leading to the conclusion that internal energy
contributions alone would favor vacancy-free compositions
Ba8Ni3Si43 and Ba8Ni4Si42. However, the formation of
Ba8Ni3.5Si42.5 C-I (>70%), BaSi2 (<15%), Ba8Ni3.0Si42.3
α-Si (<15%)
Ba8Ni6Si40
C-I (>80%), BaSi2 (<15%), Ba8Ni3.2Si45.9
NiSi(<5%)
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vacancies, which are observed in the crystal structure (see
following section) may be favored by entropy contributions.
3.5. Crystal Structure. The clathrate-I structure (space
by vacancies, Ni, and Si, the atoms in the neighboring 24k site
may be located at three different positions. In particular, the
presence of vacancies at the 6c site leads to a shift of the
neighboring atoms toward the vacancy position. Because of the
small amount of vacancies in crystal I, position Si33 could not
be resolved. The interatomic distances d(Si1−Si31) varying in
the range 2.40−2.46 Å are slightly longer than the Si−Si
distance in α-Si (∼2.35 Å), but agree well with those observed
for other silicon clathrates, e.g., Na2Ba6Si46 (d(Si1−Si3) ≈
2.39,47 2.40 Å48), Ba8Si46 (d(Si1−Si3) ≈ 2.42 Å),5 or Cs7.8Si46
(d(Si1−Si3) ≈ 2.44 Å).49 The distances d(Ni−Si32) ≈ 2.28−
2.33 Å are in the range with those reported in NiSi as d(Ni−Si)
≈ 2.29−2.40 Å,50 or in NiSi2 as d(Ni−Si) ≈ 2.34 Å.33 The
shorter distance d(Ni−Si) compared to d(Si−Si) explains the
contraction of the framework with increasing Ni content as
revealed by PXRD data (Figure 7). Distances d(□−Si33) are
group Pm3n, No. 223) is represented by two Wyckoff sites for
̅
guest atoms (2a, 6d) and three sites for the atoms in the host
framework (6c, 16i, 24k; Figure 6). The framework, which
Figure 6. Crystal structure of the type-I clathrate. 20- and 24-atom
polyhedra are shown in light and dark blue, respectively. Guest atoms
at 2a (gray) and 6d (brown) as well as atoms (or vacancies) in the
host framework at 6c (white), 16i (green), and 24k (red) sites are
shown.
consists of four-bonded atoms forming twisted pentagons and
planar hexagons, encloses 20-vertex [512] polyhedra centered
by a 2a site and 24-vertex [62512] polyhedra centered by a 6d
site. The hexagons formed by atoms at 6c and 24k sites are
connected via common 6c atoms to infinite chains along ⟨100⟩.
Atoms at the 6c site join two hexagons with bond angles around
120° making this coordination environment unfavorable
especially for four-bonded Si atoms. Hence, vacancies or
substitution atoms are preferentially found at this site.
Crystal structure determinations were performed on three
single crystals selected from samples annealed at 1000 °C with
WDXS compositions Ba8.00(3)Ni2.62(1)Si42.57(3) (I),
Ba8.00(2)Ni3.17(1)Si43.03(2) (II), and Ba8.00(4)Ni3.72(9)Si41.83(5) (III)
(Table 3, Figure 1b). Structure refinement revealed in all cases
full occupancy of Ba1 at 2a site and Ba2 at 6d site. Ba2 in
oversized 24-atom cages shows relatively large anisotropic
displacement which is usually interpreted as off-centered
positioning of the guest atoms.38,39 When the Ba2 atoms
were refined at the 24k site with 0.25 occupancy, the residual
values were considerably lowered (e.g., RF for I drops from 3.9
to 3.0%). In the next step, the Ni to Si ratio of the framework
site 6c was refined assuming full site occupancy, while sites 16i
and 24k were refined with full Si occupancy. With such
distribution of Ni and Si, the number of Ni atoms per formula
unit was obtained to be 2.49(6) for I, 2.90(5) for II, and
3.05(4) for III which is distinctly lower than the respective Ni
content obtained from WDXS analyses. The discrepancy can be
explained by the presence of vacancies at the 6c site, which was
also indicated by the slightly increasing displacement
anisotropy of the Si atoms at the 24k site. This sizable
anisotropy has been correlated with the amount of vacancies as
shown for Cs8Sn44□2,40 K8Ge44□2,41 and for ternary variants
Ba8TMx□yGe46−x−y (TM = Ni,42 Pd,43 Pt,44 Zn,45,46 etc.).
In the next step of the structure refinement, the Ni content
was fixed according to WDXS analysis. The final structure
5
○
Figure 7. Lattice parameter of Ba8Si46 ( ), Ba8Si46 (extrapolated
24
22
■
▼
based on our data and Ba8−δSi46 ) ( ), Ba7.76Si46
( ), Ba7.6Si46
6
▲
(
), Ba8−δNixSi46−x obtained by steel-quenching (◓), and clathrate
●
samples annealed at 1000 °C ( ) with respect to determined Ni
content. For our samples, the errors of the lattice parameters are
smaller than the size of the symbols used.
found to be ∼2.15 Å for II and ∼2.19 Å for III. Consequently,
the clathrate Ba8Nix□ySi46−x−y constitutes a rare example of a
Si-based clathrate with framework vacancies. The only other
example was Ba8Al14Si31□1,51 although a very similar
composition Ba8Al15Si31 was reported later without framework
vacancies.52
The crystal structure of the phase Ba8−δNixSi46−x obtained by
steel-quenching (see section 3.3) was refined from PXRD data
for samples with nominal δ = 0, x = 0.2, and x = 0.5. In the
structure refinement, the site 2a was found to be partially
occupied by Ba atoms in both samples. However, Ba2 at 6d site,
the framework sites Ni1/Si1 at 6c, Si2 at 16i, and Si3 at 24k
were refined with full occupancy (see Figure S6 and S7 and
Tables S4−S6 in the Supporting Information). Accordingly, the
compositions were determined to be Ba7.9(1)Ni1.4(1)Si44.6(1) and
Ba7.9(1)Ni1.6(1) Si44.4(1) for the samples prepared with x = 0.2 and
x = 0.5, respectively.
3.6. X-ray Absorption Spectroscopy. It was reported by
several studies dealing with Ni compounds (e.g., KNiIO6,
LiNiO2, NiO; LaNiO3, LaNi2O5, LaNiO2) that the energy of
the X-ray absorption edge correlates well with Ni valency and
refinements yielded the compositions Ba8Ni2.6Si43.3
□
for I,
0.1
Ba8Ni3.2Si42.4 for II, and Ba8Ni3.7Si41.4 for III (see
□
□
0.9
0.4
Tables 1 and 2 and Tables S1−S3 in the Supporting
Information). Because of the mixed occupancy of the 6c site
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shifts to higher values as the valency increases.53,54 X-ray
absorption spectra of Ba8Nix□ySi46−x−y were determined close
to the Ni K-edge (8333 eV) on four samples annealed at 700
°C with determined Ni contents x = 2.6, 3.1, 3.4, and 3.8
(Figure 8a). For comparison, the spectra of Ni foil, NiSi, NiI2,
Figure 9. k2-weighted Ni K-edge EXAFS (inset) and associated
□
Fourier transform for the clathrate Ba8Nix ySi46−x−y with determined
compositions Ba8Ni2.6Si42.6, Ba8Ni3.1Si42.3, Ba8Ni3.4Si42, and
Ba8Ni3.8Si41.3
.
3.7. Thermoelectric Properties. In the previous report,25
the transport properties of Ba8Nix□ySi46−x−y were discussed in
detail for the temperature range 2−350 K. For all compositions
investigated, the clathrate phase shows metallic behavior in
which both holes and electrons are involved in the electrical
conduction. Herein, the previous work is complemented by the
determination of the thermoelectric properties in the temper-
ature range 300−773 K (Figure 10). The electrical resistivity of
the samples increases linearly with increasing temperature in
agreement with the metallic-like behavior observed in the low
temperature range, and reaches the values between 3.5 and 5.9
μΩm at 750 K (Figure 10a). The substitution of Si by Ni
results in a slight increase of the electrical resistivity. Regardless
of the composition, the thermopower values remain negative
above room temperature, indicating a dominant contribution
from electrons (Figure 10b). The metallic character of these
samples results in moderate values of thermopower that
increase in absolute values with temperature reaching the
values between −30 and −55 μV K−1 at 750 K. A clear trend of
the thermopower values as a function of the Ni content was not
observed. This behavior can be attributed to a balance between
the hole and electron contributions as previously shown in the
low-temperature investigation.25
For all compositions, the thermal conductivity values are
weakly temperature dependent ranging between 4.0 and 6.5 W
m−1 K−1 at 773 K (Figure 10c). The measured values decrease
with increasing Ni content up to x = 3.4. The observed
behavior against x mainly reflects the variation of the electrical
resistivity as evidenced by the temperature dependence of the
lattice and electronic thermal conductivity displayed in Figure
10d. To separate these two contributions, the electrical
resistivity values were used to estimate the electronic
contribution, λe, via the Wiedemann−Franz law λe = LT/ρ.
The Lorenz number L was assumed as a first approximation to
be equal to the value of a degenerate electron gas L = L0 = 2.44
× 10−8 V2 K−2. The lattice thermal conductivity values λL are in
the expected range for Si-based clathrates and decrease with
increasing temperature as a result of dominant phonon−
Figure 8. Normalized Ni K-edge XANES spectra: (a) The clathrate
□
Ba8Nix ySi46−x−y with determined compositions Ba8Ni2.6Si42.6
,
Ba8Ni3.1Si42.3, Ba8Ni3.4Si42, and Ba8Ni3.8Si41.3; (b) Ni foil, NiSi, NiO,
and NiI2. Spectra are shifted vertically for clarity.
and NiO were also recorded (Figure 8b). The Ni K-edge
XANES and EXAFS regions of all clathrate samples show
qualitatively identical features. Near the Ni K-edge, the spectra
exhibit distinct characteristics dominated by the dipole-allowed
transition of the Ni-1s electron to unoccupied 4p states (Figure
8a). The position of the Ni absorption edge was found to be
very close to those of Ni foil and NiSi and distinctly separated
from those of NiI2 and NiO. Accordingly, the electronic state of
Ni in clathrate Ba8Nix□ySi46−x−y is close to that in the element.
EXAFS data for all clathrate samples show almost identical
features pointing to very similar local environments (Figure 9).
The major peak in the Fourier-transformed curves reflects the
4-fold coordination of the Ni atoms by Si atoms with d(Ni−Si)
≈ 2.2 Å. The broad peak at around 3.7 Å is associated to Ni−
Ba2 contacts. A related EXAFS analysis was performed on the
clathrate-I Ba8Cu4Si42 revealing that Cu atoms are located at the
6c site,55 which is in agreement with the crystal structure
investigations.8,21,56
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□
Figure 10. Transport properties of the clathrate Ba8Nix ySi46−x−y with determined compositions Ba8Ni2.6Si42.6 (red circle), Ba8Ni2.8Si42.1 (blue
square), Ba8Ni3.1Si42.3 (green triangle), Ba8Ni3.4Si42 (orange triangle), and Ba8Ni3.8Si41.3 (aqua plus sign). Temperature dependence of (a) the
electrical resistivity, (b) the thermopower, (c) the total thermal conductivity, and the lattice and (d) electronic thermal conductivities (inset) are
shown.
phonon interactions. In addition, λL decreases slightly as the Ni
concentration increases. The combination of the transport
properties results in a maximum ZT value of around 0.1 at 700
K for the sample with x = 3.8. The metallic nature of these
samples is the main reason for the low ZT values.
3.8. Superconductivity. Low-temperature electrical resis-
tivity ρ(T) of Ba8−δNixSi46−x phase with low Ni content shows
superconducting behavior (Figure 11). The ρ values of the
samples with compositions Ba7.9(1)Ni1.4(1)Si44.6(1) and
Ba7.9(1)Ni1.6(1)Si44.4(1) remained quasi-constant down to the
superconducting transition with onsets at Tc = 6 and 5.5 K and
zero-resistance temperatures 3.5 and 3 K, respectively. The
broad superconducting transitions of ΔTc ∼ 2.5 K for both
compounds may be related to inhomogeneities in the Ni
content arising from rapid cooling. Since X-ray diffraction
measurements together with the EDXS analyses revealed no
detectable impurity phases, this indicates that the super-
conducting transition observed is an intrinsic property of the
clathrate phase at these compositions. The presence of Ni
results in a slight decrease of Tc compared to Ba8Si46 where a
Figure 11. Electrical resistivity of the steel-quenched samples as a
superconducting state sets in at 8 K. Higher Ni contents (x ≥
function of temperature: Ba7.9(1)Ni1.4(1)Si44.6(1) (red circle) and
2.4) resulted in a complete suppression of superconductivity
Ba7.9(1)Ni1.6(1)Si44.4(1) (blue square), showing the superconducting
down to 0.4 K as shown by low-temperature electrical resistivity
measurements.25 This sharp decrease in Tc with Ni content
reflects the evolution of the density of states at the Fermi level,
N(EF), which shows only slight variation for 0 ≤ x ≤ 2.0 before
exhibiting a drastic decrease between x = 2.0 and x = 3.0.25
transition with onsets at 6.0 and 5.5 K, respectively. The inset figure
illustrates the evolution of the critical temperature Tc as a function of
the Ni content. The Tc value of Ba8Si46 (x ≠ 0) is taken from
Yamanaka et al.5.
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(2) Stefanoski, S.; Martin, J.; Nolas, G. S. J. Phys.: Condens. Matter
2010, 22, 485404.
(3) Yamanaka, S.; Horie, H.; Nakano, H.; Ishikawa, M. Fullerene Sci.
Thus the decrease in the electron−phonon coupling strength in
Ba8−δNix□ySi46−x−y originates from the Ni-content-dependent
variations in the electronic band structure.25 A decrease in Tc is
a feature shared by all ternary type-I clathrates investigated so
far showing that alloying on either the Ba or Si site tends to
suppress superconductivity. However, the variation in Tc
strongly depends on the substituting element as found out in
several studies on Ba8MxSi46−x for M = Ge,6 Ga,7 or Cu.8
Although a superconducting state still develops in the Ge- and
Ga-substituted systems up to x = 25 and x = 10, respectively, a
lower Cu content of x = 4 is sufficient to drop Tc from 8 K
down to 2.9 K. In the present case, Ni has a stronger influence
since the superconductivity disappears for x ≥ 2.4.
Technol. 1995, 3, 21−28.
(4) Kawaji, H.; Horie, H.; Yamanaka, S.; Ishikawa, M. Phys. Rev. Lett.
1995, 74, 1427−1429.
(5) Yamanaka, S.; Enishi, E.; Fukuoka, H.; Yasukawa, M. Inorg. Chem.
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The homogeneity range of the clathrate-I phase
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≤ y ≤ 0.9 at 1000 °C. The experimental result is in agreement
with the heat of formation calculations. Reinvestigation of the
crystal structure showed that the clathrate-I phase contains
framework vacancies increasing in number with Ni content
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investigations indicate that the electronic state of the Ni atoms
is close to that of the element. The electronic transport
behavior of all samples exhibits metallic-like behavior with
moderate thermopower values at high temperatures. The
thermal conductivity values for all compositions are low as in
other clathrate phases. A maximum ZT value of around 0.1 was
obtained at 700 K for the sample with x = 3.8. The metastable
clathrate phase Ba8−δNixSi46−x was prepared by steel-quenching
and shows superconducting transition with onset temperatures
at 6.0 and 5.5 K for x = 1.4 and 1.6, respectively.
ASSOCIATED CONTENT
■
S
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* Supporting Information
X-ray crystallographic data in CIF format, X-ray powder
patterns, DSC results, SEM images, tables of crystallographic
data, atomic coordinates, displacement parameters, site
occupancies, and interatomic distances. This material is
available free of charge via the Internet at http://pubs.acs.org.
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J. L. Phys. Rev. B 2005, 71, 094504.
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AUTHOR INFORMATION
Baitinger, M.; Lichte, H.; Grin, Yu. Inorg. Chem. 2011, 50, 4523−4528.
(25) Candolfi, C.; Aydemir, U.; Ormeci, A.; Baitinger, M.; Oeschler,
N.; Steglich, F.; Grin, Yu. Phys. Rev. B 2011, 83, 205102.
(26) Aydemir, U.; Candolfi, C.; Borrmann, H.; Baitinger, M.; Ormeci,
A.; Carrillo-Cabrera, W.; Chubilleau, C.; Lenoir, B.; Dauscher, A.;
Oeschler, N.; Steglich, F.; Grin, Yu. Dalton Trans. 2010, 39, 1078−
1088.
■
Corresponding Author
*E-mail: aydemir@cpfs.mpg.de; grin@cpfs.mpg.de. Tel.: +49-
351-46464000.
Notes
The authors declare no competing financial interest.
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Baumgartner, B.; Wolfel, E. Mater. Sci. Forum 1993, 335, 133−136.
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ACKNOWLEDGMENTS
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The authors acknowledge Petra Scheppan, Monika Eckert,
Renate Hempel-Weber, and members of the Kompetenzgruppe
Struktur for providing experimental support. C.C. acknowl-
edges the financial support of the CNRS-MPG program. M.B.
and Y.G. gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (SPP 1415, “Kristalline
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