High pressure synthesis and crystal structure of a ternary superconductor Ca2Al3Si4 containing layer structured calcium sub-network isomorphous with black phosphorus

Article abstract of DOI:10.1016/j.jssc.2012.11.011

The Zintl compound CaAl₂Si₂ is peritectically decomposed to a mixture of Ca₂Al₃Si₄ and aluminum metal at temperatures above 600 °C under a pressure of 5 GPa. The new ternary compound Ca₂Al₃Sl₄ crystalizes with the space group Cmc2₁ and the lattice parameters a=5.8846(8), b=14.973(1), and c=7.7966(5) A. The structure is composed of aluminum silicide framework [Al₃Si₄] and layer structured [Ca ₂] network interpenetrating with each other. The electron probe microanalysis (EPMA) shows the formation of solid solutions Ca ₂Al_3-xSi_4+x (x<0.6). The layer structured [Ca₂] sub-network is isomorphous with black phosphorus. The new ternary compound shows superconductivity with a transition temperature (T _c) of 6.4 K. The band structure calculation suggests that the superconductivity should occur through the conduction bands mainly composed of 3p orbitals of the aluminum silicide framework.

Full text of DOI:10.1016/j.jssc.2012.11.011

Journal of Solid State Chemistry 198 (2013) 445–451
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
High pressure synthesis and crystal structure of a ternary superconductor
Ca Al Si containing layer structured calcium sub-network isomorphous
2
3
4
with black phosphorus
n
Masashi Tanaka, Shuai Zhang, Yuki Tanaka, Kei Inumaru, Shoji Yamanaka
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
2 2 2 3 4
The Zintl compound CaAl Si is peritectically decomposed to a mixture of Ca Al Si and aluminum
Received 3 July 2012
Received in revised form
2 3 4
metal at temperatures above 600 1C under a pressure of 5 GPa. The new ternary compound Ca Al Sl
crystalizes with the space group Cmc2
1
and the lattice parameters a¼5.8846(8), b¼14.973(1), and
Si ] and layer structured
9
November 2012
˚
c¼7.7966(5) A. The structure is composed of aluminum silicide framework [Al
3
4
Accepted 10 November 2012
Available online 21 November 2012
[
Ca
formation of solid solutions Ca
isomorphous with black phosphorus. The new ternary compound shows superconductivity with a
transition temperature (T ) of 6.4 K. The band structure calculation suggests that the superconductivity
2
] network interpenetrating with each other. The electron probe microanalysis (EPMA) shows the
2
Al3ꢀxSi4þx (xo0.6). The layer structured [Ca ] sub-network is
2
Keywords:
Ca–Al–Si ternary compound
c
2 3 4
Ca Al Si
should occur through the conduction bands mainly composed of 3p orbitals of the aluminum silicide
High pressure synthesis
Superconductor
framework.
&
2012 Elsevier Inc. All rights reserved.
Black phosphorus
1
. Introduction
elements to show metallic conductivity. Ba
ducting compound with a transition temperature T
8
Si46 is a supercon-
¼8.0 K [6].
c
Zintl phases are ionic intermetallic compounds, consisting of
The binary clathrate compounds with excess electrons should be
classified into intercalation compounds rather than Zintl com-
pounds which fulfill the octet rule [7,8]. Clathrate compounds of
the intercalation type cannot be prepared by melting constituent
elements, but derived from thermal decomposition of Zintl
an electropositive, cationic component (alkali metal, alkaline
earth metal or lanthanoid) and an anionic component of less
electropositive elements of main groups. The electropositive
atoms lose all their valence electrons to the less electropositive
elements which form Zintl polyanions according to the octet rule.
Therefore, Zintl compounds are basically semiconductors or
moderate metallic conductors [1,2]. Ternary clathrate compounds
compounds such as NaSi. Ba
using high pressure and high temperature (HPHT) conditions on a
stoichiometric mixture of the Zintl compound BaSi and Si [6].
CaAl Si is a typical ternary Zintl compound [9]. The structure
consists of two dimensional polyanion
honeycomb layers sandwiched by calcium layers. It is interesting to
note the
-form layer structured nitride chloride MNCl (M¼Zr, Hf)
have similarly corrugated polycation
chlorine layers [10]. This is a positive version of Zintl compound
[11]. The alkali metal (A) intercalated compounds A MNCl are
superconductors with relatively high T s. The highest T was
observed for Li and THF (tetrahydrofuran) cointercalated compound
Li0.48(THF) HfNCl at 25.5 K [12]. CaAl Si is semimetallic [13,14].
It would be interesting to dope holes to CaAl Si by a partial
substitution of Ca with Na, Na Ca1ꢀx[Al Si ]. Although the target
8
Si46 can be synthesized only by
2
such as Sr
8
Ga16Ge30 and Ba
8
Ga16Ge30 can be classified into Zintl
2
2
2
N
2ꢀ
compounds, where 14 group elements Si, Ge, and Sn forming the
framework with polyhedral cages are partially substituted with
lower valent elements such as Al, Ga, and In [3]. The resulting
deficiency in the charge to form octets for the covalent bonded
clathrate framework is balanced by the electrons from the
encapsulated metal atoms (alkali and alkaline earth atoms) in
the cages. Zintl clathrate compounds can be usually prepared by
simple melting of the elemental mixtures at ambient pressure.
2
[Al Si
2
]
with corrugated
b
2
N
2þ
[M N
2 2
]
sandwiched by
x
c
c
Binary clathrate compounds such as Na
8
x
Si46, Na Si136 [4,5],
y
2
2
and Ba Si46 [6] have excess electrons in the covalent framework;
the electrons from the trapped metal atoms are doped into the
conduction bands of the semiconducting framework of 14 group
8
2
2
x
2
2
compound has not yet been obtained, in an attempt to prepare the
hole doped compound using HPHT conditions, we have found that
the Zintl compound CaAl
to Ca Al Si under pressure, which shows superconductivity.
This paper reports the crystal structure and the superconducting
2 2
Si undergoes peritectic decomposition
n
Corresponding author. Fax: þ81 824 24 7740.
2
3
4
E-mail address: syamana@hiroshima-u.ac.jp (S. Yamanaka).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2012.11.011

4
46
M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
properties of the new ternary compound Ca
HPHT conditions.
2
Al
3
Si
4
prepared under
performed by a Shimazu Thermal Analyzer model TA-50WS using
h-BN crucibles under argon for a heating rate of 10 1C/min.
Magnetic susceptibilities were measured using a SQUID (super-
conducting quantum interference device) magnetometer (Quan-
tum Design MPMS-5) under a field of 20 Oe. The ab initio
calculations of geometry optimization, the density of states, and
band structure were performed within the density functional
theory (DFT) framework, using the program CASTEP [21,22] in
Accelrys software suit. The calculations were carried out using the
GGA-PBE (General gradient approximation, Perdew-Burke-Ernze-
hof) functional. Ultrasoft pseudopotentials were used within a
plane wave basis with cut off energy of 310 eV.
2
. Experimental
.1. Materials
CaSi was prepared by melting a stoichiometric mixture of Ca
2
2
metal and Si (99.98%) under an argon atmosphere using a
tantalum crucible and RF (radio frequency) induction heating, or
using an electric arc melting on a water-cooled copper holder. The
Ca metal was purified prior to the use by sublimation at 800 1C,
and deposition onto a water-cooled Mo plate under vacuum [15].
NaSi was prepared from a stoichiometric mixture of Na metal
3. Results and discussion
(
499.9%) and Si. A slight excess of Na metal was mixed with
silicon powder, and loaded in a tantalum crucible, which was in
turn sealed in a stainless steel tube using Swagelock fittings in an
Ar filled glove box (MBRAUN). The sealed tube was heated at
3
.1. An attempt to prepare a solid solution of the Zintl compound
CaAl Si
2
2
6
50 1C for three days [5]. The ternary compound CaAl
prepared by melting a stoichiometric mixture of Ca, Si and Al
99.9%) using RF induction heating under an argon gas atmo-
sphere in an h-BN crucible with a cover.
2 2
Si was
In an attempt to dope holes to CaAl
.7 CaSi
was heated under an ambient pressure condition at 800 1C for 18 h
2
Si
2
, a molar mixture of
Si
0
2
þ0.3 NaSiþ2 Alþ0.3 Si for a composition Na0.3Ca0.7Al
2
2
(
in a tantalum tube, which was sealed in a stainless steel tube with
Ar. The product was found to be
Si
þNaAlSiþAlþSi. The EDX study revealed that no Na atoms
were doped in the CaAl Si phase. It is likely that Na atoms cannot
replace Ca of CaAl Si to form solid solutions under ambient
pressure conditions. We have also treated the same reactant
mixture under an HPHT condition of 5 GPa at 950 1C for 1 h,
followed by cooling to room temperature during 1 h. The target
solid solution was not obtained again, but the product was a
mixture of an unidentified compound (I) and Al metal. The EDX
analysis showed that the unidentified compound (I) had
a mixture of CaAl2-
2
.2. High pressure synthesis
2
2
2
High pressure and high temperature (HPHT) syntheses were
2
2
carried out using a Kawai-type multianvil press [16]. The cell
assembly and synthesis procedures were very similar to those
2
described elsewhere [17]. A mixture of CaSi and Al, Si, and/or NaSi
powders was filled in a cylindrical h-BN cell with an inner diameter
of 4–6 mm and 4–6 mm in depth, which was surrounded by a
graphite tube heater. The heater was in turn surrounded by a
2
calcium stabilized ZrO tube for thermal insulation. The whole
2 3 5
an approximate composition of Ca Al Si without containing
sample assembly was centered in a pierced CoO doped MgO
octahedron with an edge length of 10–18 mm, which was placed
in the center of eight truncated tungsten carbide cubes as anvils
separated by pyrophyllite gaskets, and compressed in a multianvil
apparatus up to 5–13 GPa. The sample in the h-BN cell was heated
by resistive heating of the graphite tube. Typical HPHT conditions
were 5 GPa at 950–1000 1C, and the sample was cooled down to
room temperature using different thermal protocols. The sample can
be quenched to room temperature within a few minutes when the
electric power supply is turned off.
sodium.
In order to prepare the unidentified ternary compound (I), a
mixture of CaSi Al Si
þAlþSi with a nominal composition Ca
without Na was treated under an HPHT condition of 5 GPa,
000 1C for 1 h, followed by cooling slowly to room temperature
2
2
3
5
1
during 2 h. The BSE image of the product is shown in Fig. 1. The
product obtained after the slow cooling had a uniform composition
in all grains; the composition was determined to be Ca 29.54,
2.3. Characterization
Single crystal X-ray structural analysis was performed by a
Rigaku AFC7R Mercury CCD diffractometer with MoK
a radiation
˚
(
l
¼0.71073 A) coupled with CrystalClear interface (Rigaku) for
data collection. The single crystal structure was refined with the
program SHELX97 [18] and the WinGX software package [19].
Powder X-ray diffraction (XRD) patterns were measured by an
imaging plate Guinier diffractometer (Huber 670 G) using CuK
a
1
˚
(
l
¼1.540596 A) radiation and a glass capillary goniometer.
TOPAS-Academic software package was used for the Rietveld
analysis [20]. Secondary electron (SE) images were observed by
a scanning electron microscope (Hitachi-3400). Back-scattered
electron (BSE) image analysis was used to observe different
compositions in microstructures. Electron probe microanalysis
(
(
(
EPMA) was carried out by means of wavelength-dispersive X-ray
WDX) analysis (JEOL, JCMA-733II) and energy-dispersive X-ray
EDX) analysis (Hitach-3400, EDAX Genesis XM2), using stan-
3 2 3
dards of CaSiO , Al O , and Si for Ca, Al, and Si, respectively.
Fig. 1. BSE image of the sample obtained from a nominal composition mixture of
The samples were embedded in epoxy resin and polished to
avoid topographic effect. Differential thermal analysis (DTA) was
Ca
2 3 5
Al Si by heating at 1000 1C under a pressure of 5 GPa, followed by cooling to
room temperature during 1 h.

M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
447
Al 27.02, Si 44.19 (total 100.75) in wt% by WDX analysis, corre-
sponding to Ca2.0Al2.72Si4.27 in atomic ratio, which is close to the
nominal composition. Single crystals suitable for X-ray structural
analysis were obtained, and supplied for the analysis.
The structure can be viewed as being composed of aluminum
silicide framework with an approximate composition [Al Si ] and
two dimensional Ca sub-network [Ca ] interpenetrating with
each other. It is interesting to note that the Ca sub-network is
isomorphous with black phosphorus containing six membered
hexagonal chair network [23]. The bond distances between Ca
3
4
2
3.2. X-ray single crystal analysis
˚
atoms in the layer are 3.598–3.914 A (Fig. 2(c)), close to the
˚
distance 3.960(1) A in Ca metal of the fcc structure [24]. The
The single crystals with a composition Ca2.0Al2.72Si4.27 were
resemblance of the structure between the Ca layer of this
compound and black phosphorus need not imply that there is a
similar type chemical bonding in the two different compounds. In
black phosphorus, phosphorus atoms form a covalent bonding
network with octets. On the contrary, in the present compound, it
appears that Ca atoms only fill the space formed by Si and Al
successfully performed. The crystallographic data are listed in
Tables 1 and 2. Some selected bond distances are given in Table 3.
The structure is schematically shown in Fig. 2. The compound
crystallizes with the space group Cmc2
1
, and the lattice parameters
˚
a¼5.8846(8), b¼14.973(1), c¼7.7966(5) A, and Z¼4. Although the
X-ray analysis cannot distinguish the Al and Si sites, it is reasonable
to assume that the atoms concerning the shortest bond distance
network. It should be noted that the [Ca
same space group Cmca as black phosphorus; the aluminosilicide
framework [Al Si ] has the acentric space group Cmc2 . The
aluminosilicide framework is composed of two kinds of two
dimensional layers, A with a composition [AlSi ] and B with a
composition [Al Si ] as shown in Fig. 2(b), which are linked by Si–
Si bonds along the b direction. The two dimensional Ca network
and the B [Al Si ] layer are interpenetrating each other. Silicon
atoms form rectangular bend Si units with horseshoe-like shape
2
] sub-network has the
˚
2
.476(2) A should be attributed to Si atoms (Si1 and Si2). All the
other sites in the [Al, Si] framework are attributed to Al atoms,
resulting in the composition of Ca Al Si . As mentioned above, the
3
4
1
2
3
4
2
WDX analysis of the chemical composition revealed that the crystal
had a composition Ca:Al:Si¼2: 2.72: 4.27 in atomic ratio, suggesting
that part of the Al sites are substituted by Si atoms, resulting in the
2
2
2
2
composition Ca
substitution was not considered, but the stoichiometric composition
of Ca Al Si was used. The compound is hereafter called the
Ca Al Si phase including solid solutions Ca
2
Al3ꢀxSi4þx (x¼0.27). In the structural analysis, the
4
4
as shown in Fig. 2(d). The horse shoes [Si ] are coupled through Al
2
3
4
to form the 3D framework.
2
3
4
2
Al3ꢀxSi4þx.
3.3. Thermal decomposition of CaAl
2
Si
2
under pressure
Table 1
Details of the crystal structure investigations for Ca
Al
2 3
Si .
4
Fig. 3 shows a DTA result of CaAl
Si
2 2
under ambient pressure in
argon atmosphere. A large endothermic peak starting from 938 1C is
due to a melting of CaAl Si . A weak endothermic peak with the top
at 946 1C is tentatively attributed to the melting of CaAl Si
contacting with the inner surface of an h-BN measurement cell.
The intensity of this peak increased slightly, when the measurement
was repeated using the same cell keeping the same sample, which
would have a more intimate contact with the h-BN cell after the
melting in the first run. The two endothermic peaks start from the
same temperature of 938 1C. In the cooling process, the correspond-
ing exothermic peak due to recrystallization was observed. It is
Formula
Ca
546.89
0.15 0.10 0.10
Cmc2 (No. 36)
0.0357
2 3 4
Al Si
2
2
Formula weight
Crystal size (mm)
Space group
2
2
1
R
int
a ( A˚ )
b ( A˚ )
5
1
7
.8846(8)
4.973(1)
.7966(5)
c ( A˚ )
V ( A˚ 3), Z
687.0(1), 4
3
d
calc (g/cm )
Temperature (K)
MoK
( A˚ )
2.644
293
2 2
concluded that CaAl Si melts congruently at 938 1C like the isotypic
0.71073
l
m
a
1
ꢀ
(mm
)
2.624
Absorption correction
Numerical
Table 3
o
%
)
27.45
˚
y
max
(
Selected bond distances (A).
Index ranges
ꢀ7oho4, ꢀ19oko18, ꢀ8olo10
Total reflections
Unique reflections
2639
Si1–
Si2–
Si2
Si1
Al1
Al1
Al2
Al2
Al3
Al3
2.476(2)
2.510(3)
2.568(3)
2.596(3)
2.544(3)
2.819(3)
2.557(3)
2.808(3)
Ca1–
Al1–
Ca2
3.914(3)
709
Ca2
3.598(2)
Observed [IZ2
s
(I)]
667
Al2
2.816(4)
No. of variables
49
Al3
2.909 (4)
3.031–3.511
3.081–3.405
2
o
GOF on F
R1/wR2 [IZ2
R1/wR2 (all data)
Largest diff. peak and hole, e /A
1.129
Ca1–
Ca2–
(4 Si, 5 Al)
(4 Si, 5 Al)
s(I)]
0.0326/0.0907
0.034/0.0913
0.602, ꢀ0.574
ꢀ
˚
3
Table 2
Atomic coordinates, anisotropic and equivalent displacement parameters ( A˚ 2) for Ca
2 3 4
Al Si .
Atom
Position
x
y
z
U
11
U
22
U
33
U
12
U
13
U
23
U
eq
Ca1
Ca2
Al1
Al2
Al3
Si1
4a
4a
4a
4a
4a
8b
8b
0
0.1400(1)
0.3635(1)
0.2477(2)
0.0601(1)
0.4375(2)
0.1970(1)
0.0324(1)
0.6428(2)
0.4736(2)
0.0252(3)
0.0000(3)
0.1049(3)
0.2957(2)
0.3029(3)
0.0160(6)
0.0163(6)
0.0157(9)
0.0125(8)
0.0111(8)
0.0156(5)
0.0309(6)
0.0147(8)
0.0138(8)
0.0135(8)
0.0135(8)
0.0155(13)
0.0088(10)
0.0110(11)
0.0115(6)
0.0397(10)
0
0
0
0
0
0
0
0
0
0
0.0017(6)
-0.0005(7)
0.0099(12)
0.0023(8)
0.0017(9)
0.0000(6)
0.0014(12)
0.0147(4)
0.0146(4)
0.0229(6)
0.0110(4)
0.0111(5)
0.0138(3)
0.0292(4)
0
0
0.0374(13)
0.0116(11)
0.0112(11)
0.0142(6)
0.0171(7)
0
0
0.2132(2)
0.2517(2)
ꢀ0.0002(4)
0.0001(6)
0.0150(7)
Si2
0.0012(7)
Note: Ueq is defined as one third of the trace of the orthogonalized U tensor.

4
48
M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
Fig. 2. Schematic illustration of the crystal structure of (a) Ca
2 3 4 2 2 2 2
Al Si composed of (b) A [AlSi ] and B [Al Si ] layers, and (c) [Ca ] layer isomorphous with black phosphorus;
silicon forms [Si ] rectangular horseshoe-shaped units, which are linked by Al atoms as shown in (d).
4
SrAl
measured after cooling confirmed the crystallization of CaAl
CaAl Si was also treated at HPHT condition of 950 1C and 5 GPa
Si
2 2
melting at 1020 1C [25]. The XRD pattern of the sample
2 2
Si .
2
2
for 1 h, followed by cooling to 500 1C during 3 h before quenching to
room temperature. The compound was found to be decomposed
into a mixture of a phase similar to the Ca
XRD measurement. CaAl Si was similarly compressed under a
pressure of 5 GPa, and then heated up to a lower temperature of
00 1C. After being kept for 1 h at the temperature, the sample was
2 3 4
Al Si phase and Al by
2
2
6
quenched to room temperature. The SE and BSE images of the
quenched sample are shown in Fig. 4. In the BSE image three kinds
of domains with different gray color levels are observed; middle
gray color level with a composition CaAl
with an approximate composition Ca Al
level for an Al rich composition. It appears that the sample is not
subjected to melting at this temperature, but part of CaAl Si starts
to decompose to the Ca Al Si
phaseþAl. The XRD pattern showed
that the product was a mixture of undecomposed CaAl Si
þthe
Ca Al Si
phaseþAl metal, in good agreement with the BSE image
analysis. A similar study has been done on the CaAl Si sample
2
Si
2
, light gray color level
2
3
Si
4
, and dark gray color
2 2
Fig. 3. DTA analysis of CaAl Si in argon atmosphere for a heating rate of 10 1C/
min using h-BN crucibles.
2
2
2
3
4
2
2
2 3 4
3.4. Thermal properties of the Ca Al Si phase
2
3
4
2
2
A mixture of CaSi
2
and Al with a nominal composition
heated at 750 1C for 1 h under 5 GPa, followed by quenching to
Ca
2
Al Si was treated under a HPHT condition of 5 GPa at
3
4
room temperature. As shown in Fig. 5, the quenched sample had a
950 1C for 1 h, followed by cooling to 550 1C in 2 h. The XRD
pattern of the product is shown in Fig. 6. The pattern can fit very
well with the calculated powder pattern based on the single
crystal data given in Table 2. The lattice parameters were
uniform BSE image, showing that CaAl
posed to the phase Ca Al Si
þAl. The EDX analysis of Al along the
dashed line shown in Fig. 5 evidenced the deposition of Al
metal between the grains of the Ca Al Si phase. Part of Al is
trapped within the grains as dark spots in the BSE image. The BSE
image analysis and the XRD data clearly support that CaAl Si peri-
tectically decomposes to the Ca Al Si phase and Al at about 600 1C.
2 2
Si is completely decom-
2
3
4
˚
2
3
4
determined to be a¼5.8605(5), b¼15.036(1), c¼7.7659(7) A, in
good agreement with those of Table 1. The powder pattern of the
unidentified compound obtained from a nominal composition of
2
2
2
3
4
2 2
Na0.3Ca1.7Al Si used in an attempt to obtain Na doping can also

M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
449
Fig. 5. BSE image of the CaAl
Si
2 2
heated at 750 1C for 1 h, followed by quenching
to room temperature. The composition of the grains is corresponding to the
Ca Al Si phase. The EDX analysis of Al along the dashed line is shown in yellow
color. The grain boundaries and the dark spots within the grains have high Al
contents, showing that Al is deposited by the peritectic decomposition of CaAl Si
For interpretation of the references to color in this figure caption, the reader is
2
3
4
2
2
.
(
referred to the web version of this article.)
2 2
Fig. 4. BSE (a) and SE (b) images of the CaAl Si sample heated up to 600 1C under
a pressure of 5 GPa for 1 h, followed by quenching to room temperature. The
yellow circles show the domains of light gray color with a composition corre-
sponding to the Ca
2
Al
3
Si
4
phase, which are formed by decomposition of the
Si . The red arrows
starting CaAl Si . The domain with dark gray color is CaAl
2
2
2
2
show Al metal phase (dark gray color) formed by the decomposition. (For
interpretation of the references to color in this figure caption, the reader is
referred to the web version of this article.)
Fig. 6. Powder X-ray diffraction pattern (measured Guinier camera, CuKa1) of the
product obtained from a nominal composition of Ca Al Si using HPHT condition
2 3 4
be identified as the Ca
2
Al
3
Si
4
phase, where Na may act as flux for
of 5 GPa at 1000 1C. The pattern was analyzed by the Rietveld software to obtain
crystal growth, and is not included in the crystals.
The ternary mixture with a nominal composition Ca
refined lattice parameters using the crystallographic data shown in Table 2.
2
3 4
Al Si
was also heated up to 1100 1C under a pressure of 5 GPa for 1 h,
and quenched to room temperature in a few minutes. Fig. 7 shows
the BSE image of the quenched sample. The light gray color zones
from the melt under pressure. However, the crystallization rate is
2 2
not so high that CaAl Si is deposited as a metastable phase. It is
2 3 4
reasonable to estimate that the Ca Al Si phase has a melting
growing dendritically has a composition similar to the Ca
phase, the dark gray color zone surrounding the dendritic core
was found to be CaAl Si and black part is an Al–Si eutectic phase.
The XRD pattern showed that the quenched product was com-
posed of CaAl Si as a major phase, the Ca Al Si phase and Al–Si.
It is strange that CaAl Si can exist in the quenched sample
treated under 5 GPa, which should be decomposed to the
Ca Al Si
phaseþAl under high pressure. As shown above, this
ternary mixture gave a uniform Ca Al Si phase by annealing. It is
likely that CaAl Si obtained by quenching the melt under
pressure is a metastable phase, and going to change to the
Ca Al Si phase by the reaction with Al–Si eutectic if the cooling
rate was not so rapid. The Ca Al Si phase is first recrystallized
2
Al
3
Si
4
temperature below 1100 1C.
2
2
2 3 4
3.5. Superconductivity of Ca Al Si
2
2
2
3
4
The temperature dependence of the magnetic susceptibility of
Al3ꢀxSi4þx (x¼0.27) was measured under a field of 20 Oe, and
shown in Fig. 8. The compound clearly shows a superconducting
transition at T
¼6.4 K with a superconducting shielding volume
fraction 22%, indicating bulk superconductivity. The sample with a
stoichiometric composition Ca Al Si shown in Fig. 6 also showed
superconductivity with T
¼6.2 K. Another sample with a different
composition Ca
Al3ꢀxSi4þx (x¼0.61) was obtained under a higher
pressure of 13 GPa at 1200 1C, which was not a single phase, but
2
2
Ca
2
2
3
4
c
2
3
4
2
2
2
3
4
c
2
3
4
2
2
3
4

4
50
M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
Fig. 7. BSE image of the sample with a nominal composition Ca
100 1C for 1 h, followed by a rapid quenching to room temperature. Three kinds
of different gray color levels are observed. The middle gray color domains are
CaAl Si , the light gray color domains have a composition corresponding to the
Ca Al Si phase, and the dark gray color domain is Al–Si eutectic.
2 3 4
Al Si heated at
1
2
2
2
3
4
2 3 4
Fig. 9. DOS profiles of Ca Al Si (a) and the contributions of Al (b) and Si (c); the
Ca 3d contribution is shown in (a) by hatching.
Fermi level. This would be related to the compositional depen-
Fig. 8. Temperature dependence of the magnetic susceptibility of Ca
x¼0.27) measured in zero-field cooled (ZFC) and field cooled (FC) modes. A
superconducting transition was observed at T
¼6.4 K.
2
Al3ꢀxSi4þx
c
dence of the T .
(
c
4. Conclusions
showed superconductivity with T
fraction480%. It is likely that the T
although the relation between the composition and T
c
¼5.4 K and a shielding volume
depends on the composition,
is not clear.
A typical Zintl compound CaAl
ent pressure, which decomposes peritectically to a mixture of the
Ca Al Si phase and Al under HPHT conditions. The new ternary
compound shows superconductivity with T
¼6.4 K. The X-ray
structural study revealed that the new ternary compound consists
of [Al Si ] framework and layer structured [Ca ] sub-network
2 2
Si melts congruently at ambi-
c
c
2
3
4
c
3.6. Ab intio calculation
3
4
2
The band structure of Ca
2
Al
3
Si
4
was calculated on the basis of
isomorphous with black phosphorus.
the crystal structure after geometrical optimization using CASTEP.
Fig. 9 shows the calculated partial density of states (PDOS). The
compound is metallic with the DOS at the Fermi level. The main
contribution to the conduction bands near the Fermi level is
coming from the p orbitals of Si and Al. A small contribution of the
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’’, and by a Grant-in-Aid for Scientific Research
(Nos. 19105006 and 19051011) of the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
3
d orbital of Ca can be seen. It is likely that the superconducting
current will pass through [Al Si ] network. As found for the
composition of the crystal, a part of the Al sites are replaced
with Si to form solid solutions Ca Al3ꢀxSi4þx. This will shift the
Fermi level toward a higher energy, and change the DOS at the
3
4
2

M. Tanaka et al. / Journal of Solid State Chemistry 198 (2013) 445–451
451
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