High-pressure synthesis and superconductivity of the laves phase compound Ca(Al,Si)2 composed of truncated tetrahedral cages Ca at (Al,Si) 12

Article abstract of DOI:10.1021/ic400395n

The Zintl compound CaAl₂Si₂ peritectically decomposes to a new ternary cubic Laves phase Ca(Al,Si)₂ and an Al-Si eutectic at temperatures above 750 C under a pressure of 13 GPa. The ternary Laves phase compound can also be prepared as solid solutions Ca(Al_1-xSi _x)₂ (0.35 ≤ x ≤ 0.75) directly from the ternary mixtures under high-pressure and high-temperature conditions. The cubic Laves phase structure can be regarded as a type of clathrate compound composed of face-sharing truncated tetrahedral cages with Ca atoms at the center, Ca at Al,Si)₁₂. The compound with a stoichiometric composition CaAlSi exhibits superconductivity with a transition temperature of 2.6 K. This is the first superconducting Laves phase compound composed solely of commonly found elements.

Full text of DOI:10.1021/ic400395n

Article
pubs.acs.org/IC
High-Pressure Synthesis and Superconductivity of the Laves Phase
Compound Ca(Al,Si) Composed of Truncated Tetrahedral Cages
2
Ca@(Al,Si)12
Masashi Tanaka, Shuai Zhang, Kei Inumaru, and Shoji Yamanaka*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
*
S Supporting Information
ABSTRACT: The Zintl compound CaAl Si peritectically decomposes to a
2
2
new ternary cubic Laves phase Ca(Al,Si) and an Al−Si eutectic at temperatures
2
above 750 °C under a pressure of 13 GPa. The ternary Laves phase compound
can also be prepared as solid solutions Ca(Al1−xSi ) (0.35 ≤ x ≤ 0.75) directly
x 2
from the ternary mixtures under high-pressure and high-temperature
conditions. The cubic Laves phase structure can be regarded as a type of
clathrate compound composed of face-sharing truncated tetrahedral cages with
Ca atoms at the center, Ca@(Al,Si) . The compound with a stoichiometric
1
2
composition CaAlSi exhibits superconductivity with a transition temperature of
.6 K. This is the first superconducting Laves phase compound composed solely
2
of commonly found elements.
INTRODUCTION
anticotunnite structure (the PbCl -type, CN = 9), and then to
2
■
the InNi -type structure (CN = 11) under a much higher
2
Much attention has been paid to the synthesis of intermetallic
compounds under high-pressure and high-temperature
HPHT) conditions, in particular, in binary and ternary
pressure using a diamond anvil cell (DAC) at room
1
2,13
temperature.
A solid solution Mg2−xAl Si (0.3 < x < 0.8)
x
(
with the anticotunnite structure has been prepared under
HPHT conditions, which has also been found to become a
systems containing strongly electropositive elements (A) such
as alkali, alkaline-earth, and rare-earth metals, and covalent
bond network formers (E) such as Si and Ge. Various types of
new covalent subnetworks have been developed with unusual
E/A atomic ratios, which violate the electron counting rule,
according to the conventional Zintl concept. The compounds
have unusual coordination environments and/or excess
electrons in the networks. Note that most of them exhibit
superconductivity. We have shown that the barium-containing
type I silicon clathrate compound Ba Si can be obtained only
14
superconductor at T = 6.2 K when x > 0.5.
c
In ternary compounds, the HPHT effect is more
complicated. Ionic Zintl compounds melting congruently
under ambient pressure decompose peritectically under high
pressure. Recently, we have shown that a typical Zintl
compound CaAl Si , which melts at 938 °C under ambient
2
2
pressure, decomposes to a new ternary phase Ca Al Si
2
3
4
1
5
(
superconductor with T = 6.2 K) at 5 GPa and ∼600 °C.
c
8
46
1
The new ternary compound consists of two types of
interpenetrating subnetworks: the cagelike [Al Si ] and the
under HPHT conditions, which has been found to become a
superconductor with a superconducting transition temperature
3
4
layer structured [Ca ], which is isomorphous with black
2
of T = 8.0 K; this is the first superconductor that has a silicon
c
phosphorus. In this study, a much higher pressure is applied
clathrate network. In binary silicides under HPHT conditions,
silicon-rich compounds are generated, such as LaSi (T = 11.5
to CaAl Si . The compound undergoes a different type of
2
2
5
c
decomposition, and a new ternary compound Ca(Al,Si) with
2
K), LaSi (T = 6.7 K), Ba Si (T = 1.4 K), BaSi , SrSi , and
10
c
24 100
c
6
6
2
−6
the cubic Laves phase structure has been obtained. The
superconductivity of the ternary Laves phase compound is
examined. This is a part of the studies to develop new
superconductors composed of commonly found elements.
MSi (M = Ca, Y, Lu; T = 4−7 K).
LaSi contains one-
5
2
3
c
dimensional sila-polyacene ribbons, i.e., Si ladder polymer. For
rare-earth germanides, LnGe (Ln = La (T = 7.0 K), Ce, Pr,
Nd, Sm, Gd, Tb) with a coordination number of CN = 8; more
5
c
recently, BaGe and CaGe (T = 6.8 K) with CN = 6 have
3
3
c
7
−10
EXPERIMENTAL SECTION
been reported.
These HPHT syntheses obey Le Chatelier’s
■
principle; the molar volume of the reactant of the system
decreases in the product by forming developed covalent
networks and high-coordination environments. Phase transi-
tions under HPHT conditions also obey the same rule. For
example, SrSi is transformed to the layered CaSi and the α-
High-Pressure and High-Temperature (HPHT) Synthesis.
Calcium metal was purified prior to use by sublimation at 800 °C
and deposition onto a water-cooled molybdenum plate under
1
6
vacuum. Calcium metal is very reactive and difficult to handle.
2
2
11
ThSi -type structure with reduced molecular volumes. Mg Si
Received: February 15, 2013
Published: May 8, 2013
2
2
with the antifluorite structure (CN = 8) is transformed to the
©
2013 American Chemical Society
6039
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Inorganic Chemistry
Article
CaSi was first prepared from a stoichiometric mixture of calcium
2
metal and silicon (99.999%) by arc melting under an argon
atmosphere. CaSi was used as a source of calcium as well as silicon.
2
CaAl Si was prepared from a stoichiometric mixture of CaSi and
2
2
2
aluminum, using arc melting. The ternary mixtures with nominal
compositions of Ca(Al1−xSi ) (0.15 ≤ x ≤ 0.75) were premelted using
x
2
an arc furnace, followed by remelting for 3 h at 1000−1200 °C using
radio frequency (RF) induction heating under an argon atomosphere
in an h-BN crucible with a cover. The ternary mixture with x = 0.5, was
identified to be the layered compound CaAlSi isomorphous with
17
AlB2. The cooled ternary mixtures were supplied for the HPHT
1
8
treatment, which was carried out using a Kawai-type multianvil press.
The ground powders of CaSi , CaAl Si , and the ternary mixtures were
2
2
2
separately loaded into a cylindrical h-BN cell (with an inner diameter
of 1.5−3.0 mm and a depth of 3.5−4.6 mm). The h-BN cell was
surrounded by a carbon tube heater and placed in a calcined
pyrophllite tube for thermal insulation. The cell, in turn, was housed in
a pierced Co-doped MgO octahedron. The assemblage was com-
pressed in the multianvil apparatus with a truncation edge length of 4−
6
mm up to 13−15 GPa and heated at 750−1000 °C for 1 h, followed
by a rapid quenching to room temperature. The sample temperature
was monitored and controlled using a chromel−alumel thermocouple.
Characterization. Single-crystal X-ray structural analysis was
performed on a Rigaku Model AFC7R Mercury CCD diffractometer
with Mo Kα radiation (λ = 0.71073 Å) coupled with CrystalClear
interface (Rigaku) for data collection. The single-crystal structure was
refined with the program SHELX97 and the WinGX software
1
9,20
package.
Powder X-ray diffraction (XRD) patterns were measured
by an imaging plate (IP) Guinier diffractometer (Huber, Model 670G)
using Cu Kα1 (λ = 1.540596 Å) radiation and a glass capillary
goniometer. TOPAS-Academic software package was used for the
Figure 1. Backscattered electron (BSE) images of the CaAl Si sample
2
2
peritectically decomposed under high-pressure and high-temperature
(HPHT) conditions of 13 GPa at (a) 750 °C and (b) 1000 °C; EPMA
analysis data are shown for some grains with different darknesses.
21
Rietveld analysis. The sample composition was determined by
electron probe microanalysis (EPMA) (JEOL, Model JCMA-733II),
using standards of CaSiO , Al O , and silicon for Ca, Al, and Si,
3
2
3
respectively. Backscattered electron (BSE) images were observed by
scanning electron microscopy (SEM) for microstructural analysis.
Magnetic susceptibility was measured using a superconducting
quantum interference device (SQUID) magnetometer (Quantum
Design, SQUID-VSM). The ab initio calculations of self-consistent
geometry optimization of the lattice parameter, the band structure, and
the electronic density of states were performed within the density
functional theory (DFT) framework, using the program CASTEP in
Table 1. Chemical Compositions Determined by EPMA on
the Samples Shown in Figure 1
Conditions
temperature pressure
Composition (at. %)
(
°C)
(GPa)
phase
Ca
Al
Si
7
50
13
Al−Si eutectic
0.01
22.29
33.33
80.98
32.13
24.77
19.00
45.58
41.89
2
2,23
Ca Al Si
2
3
4
Accelrys software suite.
The calculations were carried out using the
Ca(Al0.37Si0.63)2
general gradient approximation, Perdew−Burke−Ernzehof (GGA-
PBE) functional. Ultrasoft pseudo-potentials were used within the
default ultrafine setting.
1000
13
Al−Si eutectic
0.03
21.62
33.18
82.05
32.46
24.49
17.92
45.92
42.33
Ca Al Si
2
3
4
Ca(Al0.37Si0.63)2
RESULTS AND DISCUSSION
■1
. Decomposition of CaAl Si . The ternary Zintl
2
2
compound CaAl Si melts congruently at 938 °C under
CaAl0.74Si1.26 + Al−Si eutectic at 1000 °C. Some single crystals
of the unidentified compound suitable for X-ray structural
analysis were found from the sample treated at 1000 °C, and
the structure was determined to be isomorphous with the cubic
Laves phase, as shown below.
2. Single-Crystal Analysis. The crystallographic data of
CaAl0.74Si1.26 is shown in Tables 2 and 3. Some selected bond
distances are given in Table 4. The compound is isomorphous
2
2
ambient pressure. Under a pressure of 5 GPa, it decomposes
to Ca Al Si and aluminum at ∼600 °C, as previously
2
3
4
1
5
reported. In this study, a much higher pressure of 13 GPa
has been applied at 750 and 1000 °C for 1 h, and quenched to
room temperature within a few minutes. The BSE images of the
samples treated at 750 and 1000 °C are shown in Figures 1a
and 1b, respectively. At 750 °C, CaAl Si is decomposed to a
2
2
mixture of Ca Al Si , Al−Si eutectic, and an unidentified phase.
with the cubic Laves phase CaAl and crystallizes with the space
2
3
4
2
At 1000 °C, the Ca Al Si phase almost disappears, and the
̅
group Fd3m (No. 227); the lattice parameter a = 7.754(5) Å
2
3
4
product is composed of two phases: the unidentified phase and
an Al−Si eutectic. The compositions determined by EPMA are
shown in Table 1. The compositions of the unidentified phases
found in the products from the two different temperatures are
very similar: Ca 41.92, Al 20.83, Si 37.49 (total 100.24) in wt %,
corresponding to an atomic composition of CaAl0.74Si1.26 with
and Z = 8. The chemical composition determined by EPMA
was used as the occupancy of the 16c site in the structural
analysis. All of the constituent atoms occupy the special
positions, and thus only the atomic displacement parameters
and a scale factor were refined. The schematic structural model
for CaAl0.74Si1.26 is shown in Figure 2.
Ca:(Al + Si) = 1:2, i.e., Ca(Al0.37Si ) . This may suggest that
3. Synthesis from the Ternary Mixtures. The ternary
0.63 2
Ca Al Si formed at 750 °C is further decomposed to
solid solutions Ca(Al1−xSi ) with the cubic Laves phase
2
3
4
x 2
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Inorganic Chemistry
Article
Table 2. Details of the Crystal Structure Investigations for
CaAl0.74Si1.26
structure can be directly prepared from the corresponding
ternary mixtures using HPHT conditions of 13 GPa and 1000
°
C. Figure 3 shows the XRD pattern of one of the solid
parameter
value
CaAl0.74Si1.26
formula
formula weight
crystal system
space group
Rint
95.44
cubic
̅
Fd3m (No. 227)
0.0664
a (Å)
7.754(5)
3
V (Å )
466.2(5)
Z
8
dcalc
2.720
temperature (K)
λ (Mo Kα) (Å)
μ (mm−1)
293
Figure 3. Rietveld analysis of the Laves phase compound Ca-
(Al0.50Si0.50)2 (a = 7.8112(2) Å) based on the powder X-ray diffraction
0.71073
3.175
(XRD) pattern measured by an IP Guinier camera (Cu Kα ). Open
1
circles denote the observed data points, and the solid line represents
the calculated diffraction pattern.
absorption correction
θmax
numerical
27.130
index ranges
−9 < h < 6, −9 < k, l < 9
829
solutions Ca(Al0.50Si ) and the Rietveld analysis result. The
number of total reflections
number of unique reflections
observed [I ≥ 2σ(I)]
number of variables
0.50 2
pattern fits well with the calculated powder pattern based on
the single-crystal data given in Table 3. The nominal
compositions of the ternary mixtures were almost maintained
in the solid solutions obtained by the HPHT treatment. The
39
27
4
GOF on F 2
1.150
0
Laves phase Ca(Al1−xSi ) (x ≤ 0.05) can be prepared under
R1/wR2 [I ≥ 2σ(I)]
R1/wR2 (all data)
residual density (e− Å−3
0.0202/0.0528
0.0287/0.0546
0.222/−0.297
x 2
ambient pressure. The lattice parameters of the solid solutions
are plotted against the EPMA compositions x in Ca(Al1−xSix)2
in Figure 4, which obey the Vegard’s law over the entire range
)
Table 3. Atomic Coordinates and Equivalent Displacement
Parameters for CaAl0.74Si1.26
atom
Ca
Al0.74Si1.26
site
x
y
z
Ueq (Å2)
8b
5/8
1/2
1/8
1/2
1/8
0
0.0182(6)
0.0178(6)
16c
Table 4. Lattice Parameter a, and Bond Distances for the
Cubic Laves Phase Compounds CaAl0.74Si1.26, CaAl , and
2
CaSi2
Bond Distance (Å)
CaM2
a (Å)
Ca−Ca
Ca−M
M−M
ref
Figure 4. Lattice parameter of the Laves phase solid solutions
Ca(Al1−xSi ) versus silicon content (x). The lattice parameters of the
CaAl0.74Si1.26
CaAl2
7.754(5)
8.04
3.358
3.48
3.29
3.215
3.33
3.15
2.742
2.84
2.68
this study
25
x
2
end members CaAl and CaSi are given in Table 4.
a
2
2
CaSi2
7.59
this study
a
Calculated by geometrical optimization (see text).
between CaAl and CaSi . The preparation of the binary Laves
2
2
phase CaSi2 has been attempted using a similar HPHT
condition of 13 GPa and 1000 °C. However, the starting
CaSi was transformed to a high-pressure polymorph, the
2
tetragonal CaSi that is isomorphous with the α-ThSi2
2
24
structure. When a higher pressure of 15 GPa at 1000 °C
6
was used, a different binary compound CaSi was obtained.
3
The lattice parameter of the hypothetical binary Laves phase
compound CaSi was estimated to be a = 7.590 Å by geometry
2
optimization using CASTEP. This lattice parameter is
employed in Figure 4 as an end member. Note that the
Vegard’s law for the solid solution Ca(Al1−xSi ) can be
x 2
extended over the entire compositional range between the
coupled end members, CaAl and CaSi . Although we have not
2
2
yet succeeded in the preparation of the Laves phase compound
CaSi , the expected lattice parameter used in Figure 4 should be
reasonable.
Figure 2. Schematic representation of the structure of the new Laves
2
phase compound, Ca(Al1−xSi ) .
x
2
The lattice parameters and bond distances of calcium-
containing Laves phase compounds with various metals, CaM2
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Inorganic Chemistry
M = Al, Ni, Rh, Pd, Ir, Pt, Li) found in a crystallographic
Article
(
2
5
database are compared with those of CaSi in Figure 5. Since
2
Figure 5. Bond distances dM−M (M−M) (solid circles, ●) and d
Ca−Ca
Figure 6. Temperature dependence of the magnetic susceptibility of
Ca(Al0.50Si0.50)2 measured in zero field cooling (ZFC) and field cooling
(
Ca−Ca) (open circles, ○) of some calcium-containing Laves phase
compounds CaM2 against the shortest M−M distance in the
(FC) processes under a magnetic field strength of 2.5 Oe. The inset
corresponding elemental metals; the values for CaSi are taken from
2
shows dc magnetization of the same sample at 1.9 K, versus magnetic
field strength.
Table 4.
the Ca and the metal (M) atoms occupy the special positions,
the atomic distances Ca−Ca and M−M can be calculated from
the lattice parameter a: dM−M
= =
( 2 /4)a, and dCa−Ca
(
3 /4)a, respectively. These atomic distances are compared
with those of the corresponding metal elements in the figure.
Note that, in the Laves phase CaM , the atomic distance M−M
2
is very close to those of the elemental metals. The Ca−Ca
atomic distances of the Laves phase CaM are shorter than that
2
of calcium metal. This can be interpreted in terms of the
electronegativity difference between Ca and M. The electro-
negativity of M atoms is larger than that of the Ca atom (except
Li). The Ca atom is partially ionized. Note that the Si−Si
Figure 7. T and Meissner volume fraction of the Laves phase solid
c
solutions Ca(Al1−xSi ) as a function of Si content x. The open circles
x
2
distance in CaSi (2.68 Å) is longer than that of the covalent
show the values of the CaAl
Si decomposed under HPHT and left for
2 2
2
3
months under ambient pressure.
distance Si−Si (2.35 Å) in the diamond structure. According to
the Mulliken charge calculation by CASTEP, the charge
+0.79
−0.40
temperature dependence of the magnetization under varying
magnetic field strengths up to 7 T; this is shown in Figure 8.
distribution in the Laves phase CaSi is Ca (Si
)2. The
2
large electronegativity difference between Ca and Si appears to
be the reason of the long Si−Si distance in the Laves phase
structure. The Si atoms in the Laves phase CaSi are 6-
2
coordinated, similar to the high-pressure polymorph of silicon
with the β-tin structure.
4. Superconductivity. The temperature dependence of the
magnetic susceptibility of the sample Ca(Al0.50Si ) measured
0.50 2
under a magnetic field of 2.5 Oe is shown in Figure 6. A clear
superconducting transition was observed at T = 2.6 K. The
c
inset of the figure shows the magnetization curve of the same
sample. This is a Type II superconductor; the lower critical field
(
H ) is as low as 5 Oe. The magnetic susceptibility of the solid
c1
solutions were measured using zero field cooling (ZFC) and
field cooling (FC) modes under a magnetic field of 2.5 Oe,
which is lower than the Hc1 = 5 Oe magnetic field. Figure 7
Figure 8. Temperature dependence of H (T) of the Laves phase
c2
compound Ca(AlSi). The dotted line shows the WHH relationship to
shows the T and the superconducting volume fraction of the
c
estimate H (0).
c2
solid solutions Ca(Al1−xSi ) estimated from the FC mode at
x 2
1
.9 K against the silicon content (x). Although the T is almost
c
constant at 2.6 K over the solid solutions, the Meissner
exclusion volume abruptly decreases with a deviation from the
stoichiometric composition CaAlSi. This finding indicates that
only the solid solution having a composition of Ca(Al1−xSix)2
with x very close to 0.5 can become a superconductor. As will
be discussed later, it is an interesting issue why only the Laves
phase compound with the stoichiometric composition shows
superconductivity specifically. The temperature dependence of
The initial slope (dH /dT) at T is calculated to be 1500 Oe/
c2
c
K. We assume the Werthamer−Helfand−Hohenberg (WHH)
relation
⎛
dH ⎞
c2
H (0) = 0.693⎜ −
⎟ T
T
c
c2
c
⎝
dT ⎠
the upper critical field (H (T)) was determined from the
c2
6
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Inorganic Chemistry
Article
2
6
for the Type II superconductor in a dirty limit; the H (0)
mixture Ca(Al1−xSi ) as single phase without the Al−Si
c2
x 2
value is estimated to be ∼2700 Oe. The coherence length (ξ) is
eutectic phase are unchanged, even after 4 months.
calculated to be ∼35 nm, using a relation of
5. Structural Consideration of the New Laves Phase
Compound with a Stoichiometric Composition CaAlSi.
Φ0
H (0) =
The ternary silicide CaAlSi isomorphous with the AlB type
c2
2
2
2πξ
layered structure is known as a superconductor with T = 7.7
c
17
where Φ is the flux quantum. These values are comparable to
those of the layer-structured CaAlSi, which is considered to be
K. The layer structured CaAlSi is also isomorphous with the
high-T superconductor MgB (T = 39 K). The Laves phase
0
33
c 2 c
27
a weak-coupling BCS superconductor.
compound CaAlSi of the present study is a new polymorph of
the high-pressure phase. In the preparation of the high-pressure
phase with the stoichiometric composition, we have prepared
the layered compound CaAlSi first, and treated it under HPHT
conditions. In the layered polymorph, Si and Al atoms are
completely ordered in the ab plane, such as B and N in h-BN,
and stacked with Ca atoms between the layers with or without a
Superconductivity of transition-metal Laves phases has been
extensively studied such as AB (A = Ca, Sr, Sc, Y, La, Zr, Hf,
2
2
8−30
Th, Lu; B = V, Mo, Re, Ru, Os, Rh, Ir, Pt, Te).
Most of the
compounds are composed of transition metals. Note that the
compound of the present study does not include transition
metals, but contains a (Si,Al) covalent network.
As described above, the Zintl compound CaAl Si2 is
decomposed to a mixture of the Laves phase compound
Ca(Al0.37Si ) and an Al−Si eutectic at 13 GPa and 1000 °C.
Figure 9 shows the time dependence of the magnetic
2
7,34
2
superlattice along the c-axis.
In the Laves phase CaAlSi, the
structure can be regarded as being composed of Al Si
2
2
0.63 2
tetrahedra by corner sharing, as shown in Figure 2, the Ca
atoms being placed in the space formed by the Al Si
2
2
tetrahedral network; Ca atoms have the same arrangement as
in the diamond. In the layered CaAlSi, the network consists of
2
only Al−Si sp bonds, while in the Laves phase compound, the
tetrahedral unit [Al Si ] contains Si−Si, Al−Al, and Al−Si
2
2
bonds. There are many possible ways to connect the [Al Si ]
2
2
tetrahedra in a three-dimensional (3D) network. The [Al Si ]
2
2
tetrahedral units should have 3-fold symmetries in the unit cell
with space group Fd3m, and, at the same time, the tetrahedra
̅
should be linked by sharing corners, maintaining the
stoichiometric composition CaAlSi. It is difficult to make an
ordered structure, even if a very large super cell is used. We are
tentatively assuming an averaged structure. The hybridization of
the s/p orbitals between Si−Si Al-Al, and Al−Si must be taken
into account. The s/p orbitals of the covalent network should
also be hybridized with Ca orbitals. It is an interesting but
difficult issue why only the Laves phase compound with the
stoichiometric composition CaAlSi shows the superconductiv-
Figure 9. Temperature dependence of the magnetic susceptibility in
ZFC mode of the CaAl Si decomposed under a pressure of 13 GPa at
2
2
1000 °C.
ity among the solid solutions Ca(Al1−xSi ) with widely varying
x 2
susceptibility in ZFC mode of the decomposed product. The
as-prepared sample obtained after the HPHT treatment shows
silicon and aluminum contents.
If we focus on the environment of the Ca atoms, the
structure can be regarded as being composed of large Al and Si
tetrahedral cages @Al Si with all of the four corners truncated,
a superconducting transition at T = 6 K, which is higher than
c
that (T = 2.6 K) of the ternary Laves phase compounds. As can
c
6
6
be seen from the figure, the superconducting volume fraction
and the T value decrease with time, and after 3 months, the T
as shown in Figure 10. These are linked by face sharing with
each other, with Ca atoms being placed at the center of the
large truncated tetrahedral cages. The structure can be regarded
c
c
value has decreased to ∼2.5 K, corresponding to that of the
Laves phase compound. The T and the superconducting
c
Meissner volume fraction measured on the decomposed sample
after 3 months are shown in Figure 7 by open circles, together
with the values of the solid solutions.
It is well-known that the superconductivity of Al metal is
enhanced by supersaturation of Si in the Al−Si binary system
3
1,32
by means of rapid solidification under high pressure.
The
T value of aluminum increases from 1.2 K to 6.2 K for Al−30
c
at. % Si. It is very likely that the superconductivity with T = 6 K
c
of the CaAl Si peritectically decomposed under pressure is due
2
2
to the presence of the Al−Si eutectic phase, in which aluminum
is supersaturated with silicon. While the sample is maintained
under ambient pressure, Si supersaturated in Al is released, and
the T value decreases, coupled with the decrease of the
c
superconducting volume fraction. The lattice parameter of the
Laves phase compound was found to be essentially unchanged
with time, and moreover, the superconducting volume fraction
of the Laves phase samples synthesized from the ternary
Figure 10. Schematic structural illustration of the Laves phase
compound Ca(AlSi) composed of truncated tetrahedral cages with Ca
atoms in the center, Ca@Al Si .
6
6
6
043
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Inorganic Chemistry
Article
as a type of clathrate structure composed of new clathrate cages
Ca@Al Si .
AUTHOR INFORMATION
■
*
6
6
Corresponding Author
E-mail: syamana@hiroshima-u.ac.jp.
6
. Electronic Band Structure. The electronic band
structures of the two types of Laves phase compounds CaAl2
and CaSi were calculated based on the crystallographic data
Notes
2
given in Tables 3 and 4, using CASTEP. The structure of CaSi2
was estimated by geometrical optimization, as described
previously. The electron densities of states (DOS) thus
obtained are compared in Figure 11. The band structure [Si2]
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by the Japan Society for the
Promotion of Science (JSPS) through its “Funding Program for
World-Leading Innovative R&D on Science and Technology
(
FIRST) Program”.
REFERENCES
■
(
2
1) Yamanaka, S.; Enishi, E.; Fukuoka, H.; Yasukawa, M. Inorg. Chem.
000, 39, 56−58.
2) Yamanaka, S.; Izumi, S.; Maekawa, S.; Umemoto, K. J. Solid State
(
Chem. 2009, 182, 1991−2003.
(
(
3) Rachi, T.; et al. Phys. Rev. B 2005, 72, 144504.
4) Yamanaka, S.; Maekawa, S. Z. Naturforsch. B 2006, 61, 1493−
1
499.
(5) Wosylus, A.; Prots, Y.; Burkhardt, U.; Schnelle, W.; Schwarz, U.;
Grin, Y. Z. Naturforsch. B 2006, 61, 1485−1492.
Figure 11. Electronic density of states (DOS) for CaAl , CaSi , and
(6) Schwarz, U.; Wosylus, A.; Rosner, H.; Schnelle, W.; Ormeci, A.;
2
2
[
Si ] with the cubic Laves phase structure calculated using CASTEP
2
̈
Meier, K.; Baranov, A.; Nicklas, M.; Leipe, S.; Muller, C. J.; Grin, Y. J.
software.
Am. Chem. Soc. 2012, 134, 13558−13561.
(
(
7) Fukuoka, H.; Yamanaka, S. Phys. Rev. B 2003, 67, 094501.
8) Fukuoka, H.; Baba, K.; Yoshikawa, M.; Ohtsu, F.; Yamanaka, S. J.
Solid State Chem. 2009, 182, 2024−2029.
9) Fukuoka, H.; Tomomitsu, Y.; Inumaru, K. Inorg. Chem. 2011, 50,
372−6377.
10) Schnelle, W.; Ormeci, A.; Wosylus, A.; Meier, K.; Grin, Y.;
(
6
(
with the Laves phase structure without containing Ca has been
calculated, and its DOS is compared with that of CaSi in the
2
figure. The [Si ] has a metallic band structure with nonzero
2
Schwarz, U. Inorg. Chem. 2012, 51, 5509−5511.
DOS at the Fermi level. The profile of the DOS of CaSi is
2
(11) Imai, M.; Kikegawa, T. Chem. Mater. 2003, 15, 2543−2551.
(12) Hao, J.; Zou, B.; Zhu, P.; Gao, C.; Li, Y.; Liu, D.; Wang, K.; Lei,
W.; Cui, Q.; Zou, G. Solid State Commun. 2009, 149, 689−692.
much different from that of [Si ], implying that the band
2
structure of CaSi cannot be discussed from a rigid band model,
2
(13) Yu, F.; Sun, J.-X.; Yang, W.; Tian, R.-G.; Ji, G.-F. Solid State
unlike clathrate compounds. This is due to the strong
hybridization of the Ca 3d orbitals and the Si 3p orbitals near
the Fermi level. In the calculation of the band structure of the
Laves phase CaAlSi, we need information on the distribution of
Al and Si atoms in the network. It should also be hybridized
with the Ca 3d orbitals, and would be more complicated. We
are tentatively estimating that the band structure of the Laves
phase CaAlSi can be an intermediate, or averaged one between
those of CaAl and CaSi . Neutron diffraction and theoretical
Commun. 2010, 150, 620−624.
14) Ji, S.; Tanaka, M.; Zhang, S.; Yamanaka, S. Inorg. Chem. 2012,
1, 10300−10305.
15) Tanaka, M.; Zhang, S.; Tanaka, Y.; Inumaru, K.; Yamanaka, S. J.
(
5
(
Solid State Chem. 2013, 198, 445−451.
(16) Nakano, H.; Yamanaka, S. J. Solid State Chem. 1994, 108, 260−
266.
(17) Imai, M.; Nishida, K.; Kimura, T.; Abe, H. Appl. Phys. Lett. 2002,
8
(
(
0, 1019−1021.
2
2
18) Kawai, N.; Endo, S. Rev. Sci. Instrum. 1970, 41, 1178−1181.
studies are required.
19) Sheldrick, G. M. Program for Structure Refinement; University of
Go
(20) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838.
21) Coelho, A. TOPAS-Academic V4.1: General profile and structure
analysis software for powder diffraction data; Coelho Software: Brisbane,
Australia, 2007.
̈
ttingen, Germany, 1997.
CONCLUSIONS
■
(
A new ternary Laves phase compound Ca(Al Si ) (0.35 ≤ x
1−x x 2
≤
0.75) has been prepared using a high-pressure and high-
temperature (HPHT) condition of 13 GPa at 1000 °C. The
lattice parameters of the ternary solid solutions can fit on a
linear line tying the parameters of CaAl and CaSi . The
stoichiometric compound CaAlSi shows superconductivity at Tc
2.6 K. The Laves phase compound with a covalent network
(22) CASTEP is available from Accelrys, San Diego, CA (http://
www.accelrys.com).
(23) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.;
Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter. 2002,
2
2
1
(
1
(
4, 2717.
24) Silverman, M. S.; Soulen, J. R. J. Phys. Chem. 1963, 67, 1919−
920.
25) Inorganic Crystal Structure Database (ICSD), Fachinforma-
=
can be regarded as a type of clathrate compound composed of
truncated tetrahedra with Ca atoms in the center, Ca@Al Si .
6
6
tionzentrum (FIZ) Karlsruhe and the National Institute of Standards
and Technology (NIST).
ASSOCIATED CONTENT
Supporting Information
■
(26) Werthamer, N. R.; Helfand, E.; Hohenberg, P. C. Phys. Rev.
*
S
1
966, 147, 295.
CIF file for Ca(Al0.37Si ) . This material is available free of
(27) Kuroiwa, S.; Sagayama, H.; Kakiuchi, T.; Sawa, H.; Noda, Y.;
0.63 2
charge via the Internet at http://pubs.acs.org.
Akimitsu, J. Phys. Rev. B 2006, 74, 014517.
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044
dx.doi.org/10.1021/ic400395n | Inorg. Chem. 2013, 52, 6039−6045

Inorganic Chemistry
Article
(
1
(
28) Matthias, B. T.; Geballe, T. H.; Compton, V. B. Rev. Mod. Phys.
963, 35, 1−22.
29) Phillips, J. C. Physics of High-T Superconductors; Academic Press:
c
New York, 1989.
30) Vonsovsky, S. V.; Izyumov, Y. A.; Kurmaev, E. Z. Super-
conductivity in Transition Metals; Springer, New York, 1982.
31) Degtyareva, V. F.; Chipenko, G. V.; Ponyatovskii, E. G.;
Rashchupkin, V. I. Sov. Phys. Solid State 1984, 26, 733−734.
32) Chevrier, J.; Pavuna, D.; Cyrot-Lackmann, F. Phys. Rev. B 1987,
6, 9115−9121.
33) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.;
Akimitsu, J. Nature 2001, 410, 63−64.
34) Sagayama, H.; Wakabayashi, Y.; Sawa, H.; Kamiyama, T.;
(
(
(
3
(
(
Hoshikawa, A.; Harjo, S.; Uozato, K.; Ghosh, A. K.; Tokunaga, M.;
Tamegai, T. J. Phys. Soc. Jpn. 2006, 75, 043713.
6
045
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