BaRh2Si9-a new clathrate with a rhodium-silicon framework

Article abstract of DOI:10.1039/c3dt52775a

The semiconducting compound BaRh₂Si₉ is a new kind of intermetallic clathrate. It was obtained by reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a = 6.2221(5) A, b = 21.399(2) A, c = 6.2272(5) A, β = 90.306(7)°) displays a covalently bonded Rh-Si framework, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of 0.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba^+1.3) and negatively charged Rh atoms (Rh^-1). Si atoms with neighboring Rh atoms are positively charged, while those connected only with Si atoms are negatively charged.

Full text of DOI:10.1039/c3dt52775a

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BaRh Si – a new clathrate with a rhodium–silicon
2
9
framework
a,b
Cite this: Dalton Trans., 2014, 43,
2140
a
a
a
Walter Jung,* Alim Ormeci, Walter Schnelle, Hong Duong Nguyen,
a
a
Michael Baitinger and Yuri Grin*
The semiconducting compound BaRh
Si
2 9
is a new kind of intermetallic clathrate. It was obtained by
reacting BaSi, Rh and Si at 950 °C. The crystal structure (space group C2/c; Pearson symbol mC48, a =
.2221(5) Å, b = 21.399(2) Å, c = 6.2272(5) Å, β = 90.306(7)°) displays a covalently bonded Rh–Si frame-
6
work, in which four-connected Si atoms partly show unusually small bond angles. The Ba atoms are
encapsulated in large polyhedral cages formed by 18 Si and 4 Rh atoms. The compound is a diamagnetic
Received 4th October 2013,
Accepted 4th November 2013
p-type semiconductor, which is in agreement with band structure calculations resulting in a band gap of
+1.3
0
.12 eV. Quantum chemical calculations reveal positively charged Ba atoms (Ba ) and negatively
DOI: 10.1039/c3dt52775a
charged Rh atoms (Rh−1). Si atoms with neighboring Rh atoms are positively charged, while those con-
www.rsc.org/dalton
nected only with Si atoms are negatively charged.
transition metals, structural motifs appear which differ from
the clathrate networks. In this work, we introduce the semi-
1
. Introduction
Semiconducting framework compounds based on silicon net- conducting compound BaRh Si representing a new type of
2
9
works always attract interest in physics and materials science crystal structure based on four-bonded silicon atoms with
for their transport properties. By modifying both the crystal unexpected bond angles. This compound was originally found
structure and the composition, it is investigated whether the in the course of investigations on the phase relations in the
semiconducting properties can be designed to attain, e.g., system Ba/Rh/Si at 950 °C (τ -Ba8.5Rh17.5Si74 in ref. 5) and as a
1
promising thermoelectric materials. In the structures of many decomposition product of the clathrate-I phase Ba
silicon-rich binary compounds with alkali- or alkaline earth at low temperatures.
8
Rh2.7Si42.6
4
metals, the electropositive metal atoms are encapsulated in
large polyhedral cages formed by a typical four-bonded silicon
framework. However, while an empty allotrope Si46 would be a
2
. Methods
semiconductor, filled clathrates such as Ba8−xSi46 are metallic
1
2.1 Synthesis
conductors. Here the valence electrons of the electropositive
elements are not necessary for building up the four-bonded Starting materials for the preparation of BaRh Si were BaSi,
2
9
silicon framework and the excess electrons occupy conduction obtained by reaction of the elemental components in sealed
2
bands instead. To attain semiconducting behavior, the silicon tantalum containers under an Ar atmosphere, rhodium and
atoms can be substituted by group 13 atoms like in the clath- silicon (Ba, ChemPur, 99.9%; Rh, Degussa, powder 99.9%; Si,
3
rate-I phase Rb Ga Si
38
in which the four-bonded Ga atoms ChemPur, 99.999%, finely ground in a boron carbide mortar).
8
8
act as an electron sink. Another possibility is the substitution Mixtures of the components (up to 1 g) were pressed to pellets
of silicon by transition metal atoms. In phases such as and arc melted several times under an Ar atmosphere (weight
4
,5
6–8
9–11
Ba Rh2.7Si46−x−y
,
Ba Ni Si
or Ba Au Si
the loss <0.5%). All operations were performed in an Ar filled glove
8
8
x
46−x−y
8
x
46−x
transition metal atoms are incorporated into the silicon frame- box. The as cast alloys consisted of BaRh Si as the main
2
9
work. However, in this case, the understanding of the influ- phase with varying amounts of Si, RhSi, BaSi , and the clath-
2
ence of the substituting elements on the physical properties is rate phase Ba Rh2.7Si42.6. Nearly single phase BaRh Si was
8
2
9
less straightforward. In compounds with higher content of obtained by annealing the as cast buttons in glassy carbon cru-
cibles enclosed in quartz glass ampoules under an Ar atmos-
phere at 950 °C for eight weeks. Alternatively, BaRh Si can be
a
2
9
Max Planck Institut für Chemische Physik fester Stoffe, Nöthnitzer Straße 40, 01187
synthesized in a solid state reaction from a compacted powder
mixture of BaSi, Rh and Si by repeated annealing at 950 °C
and regrinding.
Dresden, Germany
b
Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln,
Greinstraße 6, 50939 Köln, Germany. E-mail: grin@cpfs.mpg.de, jung@cpfs.mpg.de
2140 | Dalton Trans., 2014, 43, 2140–2146
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2
2
.2 X-Ray powder diffraction (XRPD)
Table 2 Atomic coordinates and displacement parameters (in pm ) for
a
2 9
BaRh Si
Phase analysis on powder samples was performed with the
X-ray Guinier diffraction technique (Huber G670 camera, Atom Site
x
y
z
Occ. Ueq
Cu Kα radiation, λ = 1.54056 Å, germanium monochromator,
1
Ba1
Rh1
Rh2
Si1
Si2
Si3
4e
4e
4e
8f
8f
8f
8f
4e
0
0
0
0.68717(6) 1/4
0.06959(7) 1/4
0.43173(7) 1/4
1
1
1
1
1
1
1
1
124(4)
60(4)
69(4)
93(10)
91(10)
92(10)
101(10)
92(13)
5
° ≤ 2θ ≤ 100°, Δ2θ = 0.005°). The reflection positions were
determined by profile deconvolution and the unit cell para-
1
2
meters were calculated from least-square refinement.
−0.0004(6) 0.1603(2)
0.2266(6) 0.3410(2)
0.3714(6) 0.4556(2)
0.3721(6) 0.0445(2)
0.0241(6)
0.2534(6)
0.1100(6)
0.3889(6)
1/4
2
.3 X-Ray single crystal diffraction
Si4
Si5
0
0.2505(3)
Single crystals of BaRh Si were selected under paraffin oil and
2
9
fixed in a glass capillary (borosilicate glass, ∅ = 0.2 mm, wall Atom
U
11
U
22
U
33
U
12
U
13
U
23
thickness 0.01 mm, Hilgenberg). The measurements were per-
formed using a CAD4 diffractometer (CAD4, Enraf-Nonius;
graphite monochromator, MoKα radiation). Absorption correc- Rh2
Ba1
Rh1
118(6)
69(6)
68(6)
100(20)
70(20)
100(20)
100(20)
90(20)
146(6)
62(7)
69(6)
80(20)
110(20)
100(20)
100(20)
80(20)
107(6)
49(6)
70(6)
100(20)
100(20)
70(20)
100(20)
100(20)
0
0
0
9(4)
6(5)
6(5)
21(14)
2(13)
13(13)
0(13)
0(20)
0
0
0
Si1
Si2
Si3
10(14)
−6(14)
18(14)
−11(14)
0
−7(14)
5(14)
4(13)
8(14)
0
tions were performed using a ψ-scan procedure. The crystal
1
3
structure was solved by direct methods with SHELXS and
1
2
refined with WINCSD. Details of the data collection and Si4
Si5
structure refinement are given in Table 1. Atomic parameters
are listed in Table 2, and selected interatomic distances and
angles in Tables 3 and 4, respectively.
a
U
eq is defined as one third of the trace of the orthogonalized Uij
2
tensor: Ueq = 1/3[U22 + 1/sin β(U11 + U33 + 2U13cosβ). The anisotropic
displacement factor T is defined as T = exp[−2π (U11a* h +⋯+ 2U23 b*
c* kl)].
2
2 2
2
.4 Thermal analysis
For the measurement, 23 mg of substance were filled in a
glassy carbon crucible (∅ 4 mm, Sigradur G, HTW) and sealed
in a Nb ampoule (∅ 5 mm). A Netzsch heat–flux calorimeter
DSC 404 C was used with a heating rate of 5 K min− and
Table 3 Selected interatomic distances (Å)
Ba1
–2 Si4
–2 Si5
3.272(4)
3.388(2)
3.394(3)
3.458(4)
3.460(4)
3.467(4)
3.485(4)
3.684(4)
3.706(4)
4.002(1)
4.023(1)
Si1
Si2
Si3
Si4
Si5
–1 Si2
–1 Si5
–1 Rh1
–1 Si2
2.387(5)
2.390(6)
2.398(4)
2.434(5)
1
–2 Si5
–2 Si1
–2 Si2
T
max = 1473 K.
2
.5 SEM
–2 Si1
2 Si2
–2 Si1
–1 Si1
–1 Si5
–1 Rh2
–1 Si1
2.387(5)
2.396(6)
2.399(4)
2.434(5)
–
Samples were polished on air with discs of micrometer-sized
diamond powders in paraffin and investigated using a Philips
XL 30 scanning electron microscope (LaB cathode). Energy
6
dispersive X-ray spectroscopy (EDXS) was performed with an
attached EDAX Si (Li) detector.
–2 Si2
–2Rh1
–2 Rh2
–1 Si3
–1 Si4
–1 Rh1
–1 Rh2
2.361(5)
2.433(5)
2.446(4)
2.527(4)
Rh1
Rh2
–2 Si1
2.398(4)
2.446(4)
2.525(4)
2.708(4)
–2 Si3
–2 Si4
–2 Si3
–1 Si4
2.358(5)
2.433(5)
2.435(4)
2.525(4)
2 9
Table 1 Crystallographic data of BaRh Si
–
1 Si3
–2 Si2
2.399(4)
2.435(4)
2.527(4)
2.688(4)
–1 Rh2
–1 Rh1
Refined composition
Crystal system
Space group
2 9
BaRh Si
Monoclinic
C2/c
–2 Si4
–2 Si3
–2 Si4
–1 Si1
2.390(6)
2.390(6)
2.396(6)
2.396(6)
Lattice parameters (powder data)
a = 6.2221(5) Å
b = 21.399(2) Å
c = 6.2272(5) Å
β = 90.306(7)°
–
–
–
1 Si1
1 Si2
1 Si2
3
V = 829.1(2) Å
Formula units/cell, Z
Calculated density
Absorption coefficient
Crystal size
Diffractometer
Radiation
4
2
.6 Magnetic susceptibility
−
3
4.774(1) g cm
9.8 mm−
1
A polycrystalline sample of composition BaRh Si (m = 80 mg)
2
9
3
0.1 × 0.05 × 0.01 mm
was mounted in a polyethylene holder and measured in a
SQUID magnetometer (MPMS-XL7, Quantum Design) in fields
μ0H from 2 mT to 7 T and in the temperature range 2–400 K.
The diamagnetic contribution of the sample holder was
subtracted.
CAD4
MoKα
30°
−8/8, −5/30, −8/8
3043
θmax for data collection
Ranges for h, k, l
Reflections measured
Unique
1152
I > 4σ(I)
int
737
R
R
0.0526
0.0590
58
2.7 Thermal and electric transport measurements
σ
Refined parameters
Thermopower, thermal and electrical conductivity measure-
ments from 2 K to 350 K were simultaneously performed by a
R(F)
0.0457
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2
Table 4 Selected interatomic angles at the Si atoms in the Rh–Si XRPD were the clathrate-I phase, Si, RhSi, and BaSi , but no
framework
BaRh Si . In contrast, on rapid cooling of arc-melted samples
2
9
with the same nominal composition on a water-cooled copper
hearth, the formation of the clathrate-phase is suppressed to a
large extent. Instead, BaRh Si is obtained as the main phase
Rh1–Si1–Si5
Rh1–Si1–Si2
Rh1–Si1–Si2
Si2–Si1–Si5
Si2–Si1–Si5
Si2–Si1–Si5
108.0(2)
113.8(2)
114.1(2)
115.1(2)
115.4(2)
89.8(2)
Rh1–Si4–Si3
Rh1–Si4–Si4
Rh1–Si4–Rh2
Rh2–Si4–Si3
Rh2–Si4–Si4
Si3–Si4–Si4
66.20(14)
111.7(2)
124.5(2)
111.2(2)
117.4(2)
115.1(2)
2
9
with additional by-products Si, Rh
amount, clathrate-I.
3 4 2
Si , BaSi , and, in a small
The thermal stability of BaRh Si was investigated by DSC
2
9
Rh2–Si2–Si1
Rh2–Si2–Si1
Rh2–Si2–Si5
Si1–Si2–Si1
Si1–Si2–Si5
Si1–Si2–Si5
114.6(2)
115.6(2)
107.9(2)
90.2(2)
113.5(2)
114.4(2)
Si1–Si5–Si1
Si1–Si5–Si2
Si1–Si5–Si2
Si1–Si5–Si2
Si1–Si5–Si2
Si2–Si5–Si2
72.2(2)
130.5(2)
130.5(2)
131.1(2)
131.1(2)
72.1(2)
and annealing experiments. After annealing experiments at
70 °C, no decomposition was observed. After 12 h at 1000 °C
and cooling within a few minutes, the sample consisted of
BaRh Si , clathrate phase, Si, Rh Si , and BaSi indicating
9
2
9
3
4
2
decomposition of the compound. This is in agreement with a
DSC experiment which showed on heating one pronounced
endothermic effect at 980 °C.
Rh1–Si3–Si4
Rh1–Si3–Si3
Rh1–Si3–Rh2
Rh2–Si3–Si4
Rh2–Si3–Si3
Si3–Si3–Si4
112.5(2)
117.0(2)
125.2(2)
65.60(14)
113.3(2)
114.7(2)
3
.2 Crystal structure
The structure of BaRh Si was solved in the centrosymmetric
2
9
monoclinic space group C2/c. The lattice parameters (Table 1)
are close to an orthogonal unit cell and in fact the arrange-
ment of most of the atoms (Ba1, Rh1, Rh2, Si1, Si2, Si5) can
nearly be described in the orthorhombic space group Cmcm.
However, the silicon sites Si3 and Si4 clearly break the ortho-
rhombic symmetry. All crystallographic sites were found to be
fully occupied, which is in agreement with the EDXS analysis.
The displacement parameters Uij were widely isotropic. The
crystal structure of BaRh Si is characterized by a 3D Si–Rh
framework in which the Rh atoms are located within adjacent
layers extending perpendicular to the b axis shortly below and
above y = 0 and y = 1/2 (Fig. 1a). The rhodium atoms are co-
ordinated by 6 silicon atoms forming heavily distorted trigonal
prisms, which are corner-connected (Fig. 2). Between the two
relaxation method and ac resistance measurements, respecti-
vely, using a commercial measurement system (PPMS, Quantum
Design). The thermal and electrical contacts were glued to the
sample with a silver filled conductive epoxy adhesive.
2
.8 Quantum-chemical calculation procedures
Electronic structure calculations were performed by using the
1
4
2
9
all-electron, full-potential local orbital (FPLO) method within
the local density approximation to the density functional
theory. Perdew–Wang parameterization for the exchange–corre-
1
5
lation potential was employed. The electron localizability
1
6
indicator in the ELI-D representation was calculated with the
1
7
module implemented in the program package FPLO. Topo-
logical analysis of the electron density and the ELI-D was
double layers [RhSi ], large polyhedral cavities accommodate
6
1
8
the Ba atoms. The polyhedra around the Ba atoms, marked in
blue in Fig. 1b and shown separately in Fig. 3a, are also
arranged in double layers and consist of 18 Si atoms (Si1, Si2,
carried out by using the program DGRID. A combined analy-
sis of the electron density and ELI-D was performed using the
1
9
basin intersection technique. The atomic charges are calcu-
lated using the quantum theory of atoms in molecules
2
0
(QTAIM) approach.
3
. Results and discussion
3
.1 Phase relations
Three ternary compounds have been known in the system Ba–
2
1
22
Rh–Si, namely BaRh
2
Si
phase Ba Rh Si (2.4 < x < 2.7; y = 0.7 for x = 2.7). The
46−x−y
2
,
BaRh
3
Si
2
,
and the clathrate-I
4
8
x
new compound BaRh
the high-temperature phase Ba
the clathrate phase decomposed into BaRh Si , BaSi and Si.
2
Si
9
was first identified by annealing of
8
Rh Si46−x−y at 840 °C, where
x
2
9
2
2 9
At the nominal composition BaRh Si , the relative amounts
of phases formed on cooling from the melt depend strongly on
the cooling rate. In a sample prepared in a glassy carbon cruci-
ble in a high frequency furnace and cooled from the melt to
about 800 °C within three minutes and then to room tempera-
Fig. 1 Crystal structure of BaRh
are drawn in blue, Rh atoms in green and silicon atoms in red. (a) The
distorted [RhSi ]-prisms are highlighted in green, (b) the double-layers
polyhedra are highlighted in blue.
2 9
Si in projection along [001]. Ba atoms
6
ture outside the furnace, the components found by means of of BaSi18Rh
4
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coordination sphere. The upper part of the BaSi18Rh
4
poly-
hedron with a slightly distorted square (Si ) at the top, four
4
6
adjacent hexagonal faces (Si ), and four lateral rhombic faces
(
3
Si Rh) is closely related to a truncated octahedron, known f.e.
from the structure of sodalite. Deviating from a truncated octa-
hedron, the bottom of the polyhedron is formed by each of the
two bent hexagonal (Si Rh ) and pentagonal faces (Si Rh ).
4 2 3 2
Every polyhedron is face-connected to 8 others. Four of them,
connected by rhombic faces, are in the same orientation, while
those sharing hexagonal faces are in reverse orientation
(
Fig. 1b). All the Si atoms are connected to 4 nearest neigh-
bors. These are four silicon atoms in the case of Si5, three
silicon atoms and one rhodium atom for Si1 and Si2, and two
silicon and two rhodium atoms for Si3 and Si4. The distances
roughly correspond to the sum of the covalent radii rSi = 1.17 Å
6
Fig. 2 Layer of distorted [RhSi ]-polyhedra perpendicular to [010].
2
3
and rRh = 1.25 Å: d(Si–Si) = 2.358(5)–2.434(5) Å, d(Si–Rh) =
.398(4)–2.527(4) Å. While bonding angles in four-connected
silicon frameworks are usually close to the tetrahedral angle of
09.5°, the respective angles in BaRh Si are most remarkable.
For Si1 and Si2, which constitute the square at the top of the
BaRh Si polyhedron (Fig. 3a) the angles Si2–Si1–Si2 and Si1–
2
1
2
9
2
9
Si2–Si1 are close to 90°. Even smaller angles are observed for
Fig. 3 (a) Local atomic arrangement around a polyhedron BaSi18Rh
b) Building elements with unusually small bond angles highlighted.
4
.
(
Si4, Si5; d(Ba–Si) = 3.272(4) Å–3.706(4) Å) and four Rh atoms
d(Ba–Rh) = 4.002(1)–4.023(1) Å). The Si3 atoms with a distance
(
Fig. 4 (a) Total electronic density of states together with Rh and Ba d-state
of 4.435(4) Å to the Ba atoms are not considered as part of the contributions. (b) Partial density of states due to Si 3p-state contributions.
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the Si5 atoms, which, together with Si1, Si2 and a rhodium one Rh neighbor are positively charged by 0.1 electron, and Si3
atom constitute the lateral rhombic faces of the polyhedra. and Si4 with two Rh neighbors by 0.2 electron. Taking into
The bond angles Si1–Si5–Si1 and Si2–Si5–Si2 were determined account the site multiplicities, these results suggest in total a
1
.3+
1.3−
to be only ≈72°. The arrangement of these unusual building balance of [Ba ][Rh
2
Si
9
]
.
elements is shown in Fig. 3b.
The analysis of the ELI yields predominantly two-center
Si–Si and Si–Rh bonds (Fig. 5). The Si5–Si1, Si5–Si2 and Si1–
Si2 bonds have two electrons in their basins, but the electron
3
.3 Electronic structure and chemical bonding
Band structure calculations were performed using the experi- counts of the bonds involving Si3 and Si4 atoms are each
mentally determined structural data. BaRh Si is found to be higher than two: 2.3 e for Si3–Si3, 2.5 e− for Si4–Si4 and
−
2
9
−
semiconducting with a band gap of 0.12 eV. The total density 2.2 e for Si3–Si4. This latter group of bonds can be described
of states (DOS) and some selected partial DOS are shown in by quasi-two-center bonds with additional Rh atom contri-
Fig. 4. The Rh1 and Rh2 atoms as well as the Si1 and Si2 butions of approximately 0.2 electron in each case: Rh2 con-
atoms have very similar partial DOS; therefore only Rh1 and tributes to the Si3–Si3, Rh1 to the Si4–Si4, and both Rh1 and
Si1 contributions are presented. States between ≈−12.5 eV and Rh2 to the Si3–Si4 interaction. The Si1–Rh and Si2–Rh bond
−
7 eV are dominated by Si 3s states, while the hybridization of basins each have 1.7 electrons. The Si3–Rh1 (Si4–Rh1) and
Rh 4d and Si 3p states is more pronounced between −5 and Si3–Rh2 (Si4–Rh2) bond basins contain 0.8 (1.7) and 1.6 (1.0)
1 eV. Population analysis based on the projected DOS reveals electrons, respectively, emphasizing the slightly different
−
that Ba 5d states are occupied by 0.65 electron and the Rh 4d bonding behaviors of the Si3 and Si4 atoms. In Si–Rh bonds,
basis functions by 8.0 electrons. A more accurate and basis set the Rh atoms contribute around 35% of the total electron
independent method to obtain the effective charge of atoms is counts implying moderately polar covalent bonds. For com-
the QTAIM approach. Here we find that the Rh atoms gain one parison, in the clathrate I phase with an Rh–Si framework,
electron extra while the Ba atoms lose 1.3 electrons. The Ba
8
Rh
x
Si46−x−y
□
y
, the Rh atoms contribute to the Si–Rh bonds
4
silicon atoms in BaRh Si show a non-uniform charge distri- by 40%. Although the Rh atoms are 4-coordinated with Si in
2
9
2 9
bution. The Si5 atoms with only Si near neighbors are nega- the clathrate but 6-coordinated in BaRh Si , the number of
tively charged by 0.6 excess electron, while Si1 and Si2 with electrons in Rh atom basins is essentially the same. Fig. 5c
Fig. 5 (a) ELI-D with an isosurface value of υ = 1.271 in the first coordination shell of the Rh2 atoms. (b) ELI-D with an isosurface value of υ = 1.515
4 3
in the Si and Si Rh rings revealing the covalent Si–Si and Si–Rh interactions. (c) ELI-D distribution in a plane perpendicular to [001] (z = 0.25)
showing the structuring of the penultimate shells of Ba and Rh atoms and direct Rh–Si interactions.
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shows the structuring of the ELI distribution in the penulti-
mate shells of Ba (n = 5) and Rh (n = 4) atoms. The observed
deviation from the spherical distribution, which would be
expected for chemically inert core shells, indicates that elec-
trons belonging to these shells (mainly Ba 5d and Rh 4d) par-
ticipate in chemical bonding. Apart from this, Ba atom
interactions with the Si–Rh substructure are mostly of ionic
character.
3
.4 Physical properties
BaRh Si shows a negative, weakly temperature-dependent
2
9
magnetic susceptibility which is well described by χ(T) = χ +
0
C/T. Here, χ
0
is the diamagnetic temperature-independent
intrinsic susceptibility of the compound and C/T is a paramag-
netic Curie term due to impurities and charged defects. A fit
−
6
−1
−5
results in χ = −210(10) × 10 emu mol and C = 8 × 10
0
emu K mol− ; the latter would correspond to a concentration
1
of hypothetical S = 1/2 impurities of 0.02%. The diamagnetism
of BaRh Si is stronger than the sum of the closed-shell dia-
2 9
magnetic increments which is calculated to be χ ≈ −126 ×
dia
−
−
6
6
−1
2+
−6
−1 24
4+
1
0
emu mol (Ba : −32 × 10 emu mol
emu mol
,
Rh : −18 ×
−1 25
−6
−1
1
0
,
and α-Si: −6.4 × 10 emu mol , ref. 26).
The electrical conductivity (Fig. 6a) of BaRh Si is of the ther-
2
9
mally-activated type with resistivity values of 0.5 mΩ m at
00 K, 2.7 mΩ m at 77 K, and 15 mΩ m at 4.2 K. The Seebeck
3
coefficient (Fig. 6b) is positive and increases with temperature,
running through a broad maximum of +215 μV K−1 at T ≈
2
50 K. The thermal conductivity κ(T) (Fig. 6c) increases first
rapidly with increasing temperature. In this low-temperature
Fig. 6 Temperature dependence of the transport properties of
α
region it can be described with a power law κ(T) ∝ T with α ≈ BaRh
2
Si
9
: (a) electrical resistivity showing semiconducting behavior;
−
1
(b) thermopower indicating p-type conduction, and (c) thermal conduc-
2
K
.4. Then, κ(T) shows a pronounced maximum of 27 W m
−
1
−1
tivity typical of a well-crystallized semiconducting material.
at T ≈ 40 K followed by a decrease with κ(T) ∝ T . Such a
temperature dependence of the thermal conductivity with a
Debye-law region at low temperatures, a typical phonon peak
and a decrease proportional to 1/T is characteristic of a well-
crystallized material without free charge carriers. The observed
Acknowledgements
2 9
transport properties thus indicate that BaRh Si is a p-type The authors thank the competence groups Metallography and
semiconductor. The regime of intrinsic conduction is expected Structure at MPI CPfS. M. B., H. D. N., and Yu. G. gratefully
for temperatures above those of the present resistivity data. An acknowledge financial support from the Deutsche Forschungs-
estimate of the doping level energy from the data between gemeinschaft (SPP 1415). A. O. thanks Ulrike Nitzsche (IFW
3
30 K and 350 K gives ≈0.06 eV.
Dresden) for help in computational aspects.
5
. Conclusion
References
BaRh Si is a new type of clathrate-like cage structure in which
1 S. Yamanaka, Dalton Trans., 2010, 39, 1901–1915.
2 C. Cros, M. Pouchard and P. Hagenmuller, J. Solid State
Chem., 1970, 2, 570–581.
3 H. G. von Schnering, R. Kröner, H. Menke and K. Peters,
Z. Kristallogr. – New Cryst. Struct., 1998, 213, 677–678.
4 W. Jung, H. Kessens, A. Ormeci, W. Schnelle, U. Burkhardt,
H. Borrmann, H. D. Nguyen, M. Baitinger and Y. Grin,
Dalton Trans., 2012, 41, 13960–13968.
2
9
Ba atoms are encapsulated in an Rh–Si-framework. The frame-
work comprises structure elements with unusually small Si–Si–
Si bond angles. According to quantum chemical calculations,
2 9
BaRh Si is electronically balanced. The Ba atoms and the Si
atoms adjacent to Rh are positively charged, while the Rh
atoms and the remaining Si atoms are negatively charged.
Thermoelectric transport measurements indicate that
BaRh
2
Si
9
is a p-type semiconductor, in agreement with the
5 M. Falmbigl, A. Grytsiv, P. Rogl and G. Giester, Inter-
metallics, 2013, 36, 61–72.
observed diamagnetism and with band structure calculations.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 2140–2146 | 2145

View Article Online
Paper
Dalton Transactions
6
7
J. H. Roudebush, M. Orellana, S. Bux, T. Yi and 15 J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter,
S. M. Kauzlarich, J. Solid State Chem., 2012, 192, 102–108. 1992, 45, 13244–13249.
M. Falmbigl, M. X. Chen, A. Grytsiv, P. Rogl, E. Royanian, 16 M. Kohout, Int. J. Quantum Chem., 2004, 97, 651–658.
H. Michor, E. Bauer, R. Podloucky and G. Giester, Dalton 17 A. Ormeci, H. Rosner, F. R. Wagner, M. Kohout and
Trans., 2012, 41, 8839–8849.
Yu. Grin, J. Phys. Chem. A, 2006, 110, 1100–1105.
8
9
U. Aydemir, C. Candolfi, A. Ormeci, H. Borrmann, 18 M. Kohout, Program DGRID, version 4.6, 2011.
U. Burkhardt, Y. Oztan, N. Oeschler, M. Baitinger, 19 S. Raub and G. Jansen, Theor. Chem. Acc., 2001, 106,
F. Steglich and Yu. Grin, Inorg. Chem., 2012, 51, 4730–4741.
G. Cordier and P. Woll, J. Less-Common Met., 1991, 169, 20 R. F. W. Bader, Atoms in Molecules, A Quantum Theory,
91–302. Clarendon Press, Oxford, 1995.
0 N. Jaussaud, P. Gravereau, S. Pechev, B. Chevalier, 21 D. Langen, S. Schoolaert, H. Ploss and W. Jung, Z. Anorg.
223–232.
2
1
M. Ménétrier, P. Dordor, R. Decourt, G. Goglio, C. Cros and
Allg. Chem., 1997, 623, 1561–1566.
M. Pouchard, C. R. Chim., 2005, 8, 39–46.
22 H. Ploss, Dissertation, Universität zu Köln, 1998.
1
1 C. Candolfi, U. Aydemir, M. Baitinger, N. Oeschler, 23 J. Emsley, The elements, de Gruyter, Berlin, New York,
F. Steglich and Yu. Grin, J. Appl. Phys., 2012, 111, 043706. 1994.
2 L. G. Akselrud, P. Y. Zavalii, Yu. Grin, V. K. Pecharsky, 24 E. S. R. Gopal, Specific Heat at Low Temperatures, Plenum
1
B. Baumgartner and E. Wölfel, Mater. Sci. Forum, 1993,
33–136, 335–340.
3 G. M. Sheldrick, SHELXS-97, Göttingen, 1997.
Press, London, 1966.
25 P. W. Selwood, Magnetochemistry, Interscience, NewYork,
2nd edn, 1956.
1
1
1
4 K. Koepernik and H. Eschrig, Phys. Rev. B: Condens. Matter, 26 S. Hudgens, M. Kastner and H. Fritzsche, Phys. Rev. Lett.,
999, 59, 1743–1757. 1974, 33, 1552–1555.
1
2146 | Dalton Trans., 2014, 43, 2140–2146
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