SrSn4: A Superconducting Stannide with Localized and Delocalized Bond Character

Article abstract of DOI:10.1021/ic0302128

The title compound is the tin-richest phase in the system Sr-Sn and is obtained by stoichiometric combination of the elements, SrSn₄ peritecticly decomposes under formation of SrSn₃ and Sn at 340 °C. The structure determined from a single crystal shows a new structure type with a novel structure motive in tin chemistry. It can be described by a corrugated, distorted quadratic net of tin atoms as the only building unit. The nets intersect at common Sn atoms, and the resulting channels host the Sr atoms, The structure can alternatively be described as an intergrowth structure of the AlB₂-type and W-type. The atoms that are connected by the two shortest Sn-Sn distances (2,900 and 3.044 A) form a two-dimensional net consisting of hexagons of tin atoms. The hexagons have boat conformation in contrast to the rather similar α-As structure type, where hexagons have a chair conformation. Further tin atoms connect the two-dimensional net of Sn hexagons. Temperature-dependent magnetic susceptibility measurements show that SrSn₄ is superconducting with T_c = 4.8 K at 10 G. LMTO band structure and density of states calculations verify the metallic behavior of SrSn₄. An analysis of the electronic structure with the help of the electron localization function (ELF) shows that localized covalent bonds beside delocalized bonds coexist in SrSn₄.

Full text of DOI:10.1021/ic0302128

Inorg. Chem. 2003, 42, 8748−8754
SrSn4: A Superconducting Stannide with Localized and Delocalized
Bond Character
Stefan Hoffmann and Thomas F. Fa1ssler*
Eduard-Zintl Institute of Inorganic and Physical Chemistry, Technical UniVersity of Darmstadt,
Petersenstrasse 18, D-64287 Darmstadt, Germany
Received June 26, 2003
The title compound is the tin-richest phase in the system Sr−Sn and is obtained by stoichiometric combination of
the elements. SrSn4 peritecticly decomposes under formation of SrSn3 and Sn at 340 °C. The structure determined
from a single crystal shows a new structure type with a novel structure motive in tin chemistry. It can be described
by a corrugated, distorted quadratic net of tin atoms as the only building unit. The nets intersect at common Sn
atoms, and the resulting channels host the Sr atoms. The structure can alternatively be described as an intergrowth
structure of the AlB -type and W-type. The atoms that are connected by the two shortest Sn−Sn distances (2.900
2
and 3.044 Å) form a two-dimensional net consisting of hexagons of tin atoms. The hexagons have boat conformation
in contrast to the rather similar R-As structure type, where hexagons have a chair conformation. Further tin atoms
connect the two-dimensional net of Sn hexagons. Temperature-dependent magnetic susceptibility measurements
show that SrSn4 is superconducting with T ) 4.8 K at 10 G. LMTO band structure and density of states calculations
c
verify the metallic behavior of SrSn4. An analysis of the electronic structure with the help of the electron localization
function (ELF) shows that localized covalent bonds beside delocalized bonds coexist in SrSn4.
chains 1∞[Sn2-].4 Exclusively two-fold bonded Sn atoms are
Introduction
expected from the 8 - N rule; thus, the compounds fulfil
the valence rules. In these so-called Zintl phases all valence
One of the most prominent candidates to study the
relationship between atomic structure and physical properties
electrons are assigned to the tin atoms and the electrons are
is the element tin, since tin has the outstanding property that
localized in either two-center-two-electron (2c-2e) bonds
there exist a metallic and nonmetallic allotrope almost equal
in stability. The difference of the thermodynamically more
stable R form compared to the â form is only 2 kJ mol-1.
This fact manifests also in binary phases of tin and
electropositive metals. On the tin-rich side compounds form
structures that are typical for intermetallic compounds, but
there exist also examples which form structures that are
between these atoms (tin-tin separation of 2.9 Å) or in form
of lone pairs at the tin atoms. At higher tin content A3Sn5
phases are known for AE ) Sr and Ba. The tin atoms form
five-atom clusters with weak interactions between them
(shortest intercluster tin-tin contacts are 3.397 and 3.694 Å
for AE ) Sr and Ba, respectively). In this case the Zintl
concept can be extended by the introduction of the electron-
typical for valence compounds and furthermore compounds
counting scheme according to Wade rules. Electron transfer
that are intermediate between the two classes and which
from the AE atoms to tin leads to the polyanion [Sn5]6-,
possess both structural motives.1-3
which indeed has an arachno-type structure and derives from
Several binary phases of tin and alkaline earth metals (AE)
a pentagonal bipyramid by removing two adjacent cluster
are known: The AESn phases for AE ) Ca to Ba crystallizes
vertexes.5 However, band structure calculations show that
with CrB structure type. The structure contains linear zigzag
these compounds are metallic. At higher tin content the
8
binary phases SrSn3,6 BaSn3,7 and BaSn5 are structurally
* To whom correspondence should be addressed. E-mail: faessler@
ac.chemie.tu-darmstadt.de. Fax: 0049-6151-166029.
(1) Scha¨fer, H.; Eisenmann, B.; Mu¨ller, W. Angew. Chem., Int. Ed. Engl.
1997, 36, 2683; 1973, 85, 742-760.
(2) Corbett, J. D. In Chemistry, Structure, and Bonding of Zintl Phases
and Ions; Kauzlarich, S. M., Ed.; VCH Publishers: Weinheim,
Germany, 1996; p 139.
characterized. They are metallic and even superconducting
(4) Eckerlin, P.; Meyer, H. J.; Wo¨lfel, E. Z. Anorg. Allg. Chem. 1955,
307, 145.
(5) Zu¨rcher, F.; Nesper, R.; Hoffmann, S.; Fa¨ssler, T. F. Z. Anorg. Allg.
Chem. 2001, 627, 2211-2219.
(3) Fa¨ssler, T. F.; Hoffmann, S. Z. Kristallogr. 1999, 11, 722-734.
8748 Inorganic Chemistry, Vol. 42, No. 26, 2003
10.1021/ic0302128 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/22/2003

Superconducting Stannide SrSn4
Table 1. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for SrSn4
at temperatures below 5.4 K (SrSn3), 2.4 K (BaSn3), and
4.4 K (BaSn5). SrSn3 and BaSn3 can be regarded as
compounds at the borderline of structures, which derive from
closest packing of spheres and Zintl phases with localized
chemical bonds. The structures can be described as distorted
hexagonal or cubic closest atom packing as it is known for
a variety of intermetallic compounds of the composition
M′M3. However, formal electron transfer from AE to tin
atoms leads to the formula [Sn3]2- and gets another perspec-
tive on the structure. The closest packing contains trimember
rings of tin atoms, and the structures can be broken down to
the polyanionic Zintl-anionic unit [Sn3]2- with an aromatic
2π-electron system. Interactions between these units through
their π-electron systems are strong, and almost equal intra-
and inter-ring contacts are observed. Finally, in the tin-richest
phase BaSn5 tin atoms form graphite-like layers (honey-
combs). Two such layers built a slab of hexagonal prisms,
which are centered by additional Sn atoms. The central tin
atom has 12 nearest neighbors of tin atoms and represents
the first case of a tin atom with such high coordination
number. The slabs are separated by Ba atoms above and
below the center of each tin hexagon.8
atom
site
x
y
z
Ueq
Sn(1)
Sn(2)
Sn(3)
Sr(1)
8f
4c
4a
4c
0
0
0
0
1959(1)
5902(1)
0
447(1)
16(1)
17(1)
19(1)
14(1)
1
/
4
0
1
3882(1)
/
4
Siemens SMART system with Mo KR radiation and a CCD
detector. The structures were solved by direct methods (SHELXS-
86) and difference Fourier analysis and refined using least-squares
cycles based on F2 (SHELXTL). Further details are listed in Table
3.
Magnetic Measurements. Magnetic susceptibility measurements
were performed using a SQUID magnetometer (Quantum Design
MPMS 5S). Suprasil quartz capillaries (5 mm diameter) served as
sample holders. The sample was cooled in the absence of a magnetic
field checked before by an external Hall probe. After the introduc-
tion of a 10 G field, data were recorded while the sample was
warmed (“shielding”) and then cooled (“Meissner”). The magne-
tization at different external fields was determined at 1.75 K. The
shielding fraction was determined from the slope of the curve in
the linear part. The corrections of the magnetic data were made to
account for demagnetization effects by assuming a spherical shape
for the superconducting particles with demagnetization factor n )
1/3.
During our systematic investigation of the Sr/Sn phase
system we were now able to determine the structure and the
physical properties of the tin-richest phase in the Sr-Sn
system SrSn4.
Electronic Structure Determination. The electronic structure
was calculated with the local density-functional approach and the
linear muffin-tin orbital (LMTO) method in the atomic sphere
approximation (ASA) using the tight-binding (TB) program TB-
LMTO-ASA.10 The exchange correlation potential was parametrized
according to Barth and Hedin.11 The radii of the muffin-tin spheres
and empty spheres were determined after Jepsen and Andersen.12
For the calculation s, p, and “down folded” d-partial waves for Sn
and s, d, and “down-folded” p-partial waves for Sr were used.
Experimental Section
Synthesis. SrSn4 was synthesized as a pure phase via combina-
tion of the elements. Stoichiometric amounts of strontium (ALFA
99.9%, distillation prior to use) and tin (Aldrich, 99.5%) were heated
in a sealed niobium ampule at 200 °C/h to 900 °C. The sample
were held at this temperature for 3 h, quenched to room temperature,
and annealed at 315 °C for 22 days. The X-ray diffraction diagram
of a powdered sample shows the presence of SrSn4 as single phase.
Results
Structure. The histogram of Sn-Sn distances shows a
gap between 3.530 and 4.161 Å. The group of shorter Sn-
Sn contacts below 3.530 Å is further divided into three
subgroups. Shortest contacts occur between Sn(1) atoms
(2.90 and 3.04 Å) and are in the range of atom-atom separa-
tions of elemental R-Sn (2.81 Å) and â-Sn (3.02 and 3.18
Å). Longer contacts of 3.29 Å appear between Sn(2) and
Sn(1) or Sn(2) and Sn(3) atoms, and the longest contacts
listed in Table 2 are in the range from 3.42 to 3.51 Å. Taking
into account all Sn-Sn distances up to 3.3 Å a three-dimen-
sional network of Sn atoms results as shown in Figure 1.
The structure can completely be described by a corrugated,
distorted quadratic net of tin atoms as the only building unit
as shown in Figure 1b. Tin-tin atom separations within the
net are in the range from 3.044 to 3.302 Å and occur between
all three different types of Sn atoms. The nets lie with their
mean planes parallel to the a axes of the orthorhombic unit
cell. The two perpendicular planes {021} and {042h} intersect
The DTA of the product indicates the presence of small amount
of the eutectic of Sn-SrSn4 (endothermic effect at 220 °C).
Furthermore an endothermic effect with a Tonset ) 340 °C is
observed. According to Marshall et al. this effect corresponds to
the decomposition of SrSn4.9 The complete melting of the sample
is accompanied by a broad effect in the range between 500-600
°C. On cooling the reversed processes are observed and the powder
diffraction diagram of the product after the DTA experiment shows
lines of SrSn3,6 SrSn4, and Sn.
X-ray Structure Determination. Single crystals of SrSn4 for
diffraction studies were selected from the reaction of Sr and Sn in
the ratio 1:8. A sealed niobium ampule was heated at 150 °C/h to
500 °C, held there for 20 h, and cooled to room temperature at the
same rate. The product was annealed over 2 months in a sealed
steel ampule at 320 °C. The ingot was ductile and contains cuboid
crystals of SrSn4. Crystals were mounted in glass capillaries under
the microscope in a drybox. Diffraction data were collected on a
(6) Fa¨ssler, T. F.; Hoffmann, S. Z. Anorg. Allg. Chem. 2000, 626, 106-
112.
(7) Fa¨ssler, T. F.; Kronseder, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
2683. 1997, 109, 2800.
(8) Fa¨ssler, T. F.; Hoffmann, S.; Kronseder, C. Z. Anorg. Allg. Chem.
2001, 620, 2486-2492.
(9) Marshall, D.; Chang, Y. A. J. Less-Common Met. 1981, 78, 139-
145.
(10) van Schilfgarde, M.; Paxton, T. A.; Jepsen, O.; Andersen, O. K.; Krier,
G. In Programm TB-LMTO; Max-Planck-Institut fu¨r Festko¨rperfors-
chung: Stuttgart, Germany, 1994.
(11) Barth, U.; Hedin, L. J. Phys. Chem. 1972, 5, 1629.
(12) Jepsen, O.; Andersen, O. K. Z. Phys. B 1995, 97, 35.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8749

Hoffmann and Fa1ssler
Figure 1. Various details of the crystal structure of SrSn4: (a) all Sn-Sn
contacts up to 3.3 Å are shown as solid lines, unit cell outlined; (b)
corrugated net of distorted squares of Sn(1), Sn(2), and Sn(3) atoms; (c)
net of Sn(1) atoms (all distances in Å; cf. also Table 2).
Table 2. Selected Interatomic Distances in Å with Standard Deviation
in Units of the Last Decimal in Parentheses
Figure 2. Coordination polyhedra of (a) Sr(1), (b) Sn(2), and (c) Sn(3).
Solid lines represent Sn-Sn contacts up to 3.53 Å (for distances see Table
2).
atom-atom
atom-atom
contact
dists
noc
contact
dists
noc
Sn(1)- Sn(1)
Sn(1)-Sn(1)
Sn(1)-Sn(2)
Sn(1)-Sn(3)
Sn(1)-Sr(1)
Sn(1)-Sr(1)
2.900(1)
3.044(1)
3.287(1)
3.417(1)
3.434(1)
3.642(1)
1
2
2
1
2
1
Sn(3)-Sn(2)
Sn(3)-Sn(1)
Sn(3)-Sr(1)
Sn(3)-Sn(3)
3.302(1)
3.417(1)
3.496(1)
3.530(1)
4
2
4
2
formed by the atoms connected by the two shortest Sn-Sn
distances (2.900 and 3.044 Å). Those Sn atoms form a two-
dimensional net consisting of six-membered rings of tin
atoms. These rings of Sn(1) atoms have a boat conformation
(Figure 1c) in contrast to the rather similar R-As structure
type, where hexagons take a chair conformation. Sn(2) and
Sn(3) atoms connected the two-dimensional net of Sn(1)
atoms (Figure 1a).
further contacts g4.161(1)
further contacts G4.618(1)
Sn(2)-Sn(1)
Sn(2)-Sn(3)
Sn(2)-Sr(1)
Sn(2)-Sr(1)
3.287(1)
3.302(1)
3.510(1)
3.550(1)
4
4
1
2
Sr(1)-Sn(1)
Sr(1)-Sn(3)
Sr(1)-Sn(2)
Sr(1)-Sn(2)
Sr(1)-Sn(1)
3.434(1)
3.496(1)
3.510(1)
3.550(1)
3.642(1)
4
4
1
2
2
further contacts g4.259(1)
further contacts g4.618(1)
Further, the structure can be rationalized in terms of rather
similar coordination polyhedra of the atoms Sr, Sn(2), and
Sn(3). In all cases the structure motive is a distorted quadratic
prism. The first coordination sphere of Sr is a distorted
quadratic prism of Sn atoms, elongated along the a direction
(Figure 2a). The distances between the central Sr atom and
the corners of the distorted prism are 3.434(1) Å for Sr(1)-
Sn(1) and 3.496(1) Å for Sr(1)-Sn(3). Sn-Sn contacts along
the edges are longer than 3.530 Å. Three faces along the
height of the prism are capped by Sn(2) atoms with a Sn-
Sn distance of 3.287 and 3.302 Å; the fourth face is bridged
by a dumbbell of Sn(1) atoms at a distance of 3.044 Å. This
Sn2 unit is part of the net of Sn(1) atoms shown in Figure
1c. Interestingly, the atoms Sn(2) and Sn(3) have a rather
similar coordination polyhedron (Figure 2b,c, respectively).
Sn(2) is surrounded four Sn(1) and four Sn(3) atoms. Sr
atoms cap two opposite faces of the distorted quadratic prism
Table 3. Data Collection and Refinement Details
empirical formula
fw
SrSn4
562.38
Cmcm, 4
space group, Z
unit cell params (Å)
V (Å3)
a ) 4.6179(9), b ) 17.372(4), c ) 7.060(1)
566.4(2)
293(2)
2975
601 (Rint ) 0.099)
R1 ) 0.044, wR2 ) 0.110
1.203
T (K)
reflcns collcd
indpndt reflcns
R (all data)
goodness of fit
at the Sn(2) atoms (Figure 1a). The resulting channels are
oriented parallel to the a axes and host the Sr atoms.
Consequently, the Sr atoms form linear chains with a Sr-
Sr atom separation having the length of the a axes. Due to
the corrugation of the net, short Sn(1)-Sn(1) contacts of
2.900 Å are formed. Alternatively, the structure can be
described based on a substructure of Sn atoms, which is
8750 Inorganic Chemistry, Vol. 42, No. 26, 2003

Superconducting Stannide SrSn4
Figure 4. (a) LMTO-band structure and (b) density of states plot. The
Fermi level is taken as point of zero energy.
undergoes the superconducting transition. The assumption
of the demagnetization factor might be the reason of the
deviation from the expected value of 100%.
A small kink in the cooling curve (arrow in Figure 3a) at
4.4 K is observed and could arise from a very small amount
of a superconducting impurity. This kink is also observed
during temperature-dependent measurements at fields of 20,
40, and 60 G and disappears at higher fields. Due to different
transition temperatures of Sn, SrSn3, and Nb3Sn, these phases
can be excluded as possible superconducting impurities. Thus
another unknown superconducting phase exists in the Sr-
Sn-Nb phase system.
Electronic Structure. The LMTO-band structure (Figure
4a) of SrSn4 shows bands with a large dispersion crossing
the Fermi level EF revealing the metallic property. As in other
superconducting binary intermetallic compounds A/AE-Sn/
Pb (A ) alkali metal) EF cuts the density of states at
the flank of a local maximum (Figure 4b).13 The relatively
high density of states at EF arises from flat band sections at
EF along the symmetry lines ·-Τ and close to Γ at Y-Γ-
S.
Figure 3. Magnetism of SrSn4 at low temperatures: (a) diamagnetic
shielding (O) and Meissner effect (b) at 10 G (the arrow marks a small
kink); (b) magnetization at 1.75 K (demagnetization factor n ) 1/3).
that have mixed atoms sites and the face of Sn(3) atoms.
Further capping with Sn(2) atoms is due to translation
symmetry along the a direction. The coordination polyhedron
of Sn(3) is a distorted quadratic prism of Sn(2), and Sr atoms
as shown in Figure 2c. The two opposite faces parallel to
the ab plane are capped with Sn(3) atoms, and two opposite
faces parallel to the ac plane are capped with Sn(1) atoms.
At longer distances two further faces are capped with Sn(3)
atoms. The coordination polyhedra are also shown in Figure
6 and will be discussed in more detail below.
Magnetic Properties. The recorded curves for the low-
field magnetic susceptibility show the characteristics of a
superconductor (Figure 3a). An external field of 10 G applied
to a sample, which was cooled in a zero field regime, causes
a negative value for the magnetic susceptibility (“shielding”).
The pronounced diamagnetic property of SrSn4 vanishes at
higher temperatures with the transition temperature of 4.8
K (10 G). During the cooling of the sample in the external
field a jump of the susceptibility is observed again, but the
value of the shielding curve at the same temperature is not
reached (full circles in Figure 3a). This behavior is charac-
teristic for type II superconductors. The large value of the
magnetization at 1.75 K shows that the whole sample is in
the superconducting state. This was confirmed by measuring
the magnetization in dependence of the external field at 1.75
K (Figure 3b). The slope of the linear part gives a shielding
fraction of 123% assuming a demagnetization factor of 1/3,
which shows that the whole sample and not an impurity
A topographical analysis of the electron density distribu-
tion using the electron localization function (ELF), which
originally was introduced for the deduction of the shell
structure of atoms from the electron density,14 gives also
insight in chemical bonding in intermetallic compounds. ELF
values are scaled between 0 and 1 with the meaning that
high ELF values and local ELF maxima correspond to areas
and centers of localized electrons, respectively, and low ELF
values define the spatial area around these maxima. The
resulting partition of space into areas of high electron
localization can be interpreted in terms of bonding and
nonbonding electron pairs and their three-dimensional shapes.
Thus spatial domains of localized electrons become visible.15-18
(13) Fa¨ssler, T. F. Chem. Soc. ReV. 2003, 32, 80-86.
(14) Becke, A. D.; Edgecombe, E. J. Chem. Phys. 1990, 92, 5397.
(15) Savin, A.; Becke, A. D.; Flad, J.; Nesper, R.; von Schnering, H. G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2683; 1991, 103, 421.
(16) Silvi, B.; Savin, A. Nature 1994, 371, 683.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8751

Hoffmann and Fa1ssler
diagram of Figure 5c the ELF maximum is located between
the two Sn(1) atoms and corresponds to a covalent bond. If
the ELF is calculated from the total electron density and not
only from the valence electron density generally the same
picture results (Figure 3d and 3e). In agreement with previous
results6 appears the bonding region 1 more compressed and
with lower ELF values as compared to ELF calculated from
the valence electrons. The compression of the bond region
is a result of repulsive interactions of 1 with core electron
domains (core shells). There is also a lone pair 2 located at
the Sn(1) atom, which points toward the Sr atom. The lone
pair corresponds with respect to its direction and shape to a
lone pair typically located at three-bonded atoms as e.g. the
lone pair at the phosphorus atom in PF3.13 The bonding region
between two Sn(1) atoms, which are separated by a distance
of 3.044 Å (Figure 1), is visible from the two-dimensional
representation parallel to the ab plane shown in Figure 1
and 5g. The ELF maximum of the bond region appears along
the Sn(1)-Sn(1) vector. The Sn(1) atoms in Figure 1 and
5g lie above and below the intersecting plane, thus the plane
shown in Figure 5f and 5g intersects the Sn(1)-Sn(1) bond.
At Sn(2) a torus-type three-dimensional domain 3 is visible
(Figure 5a). Distinct ELF maxima lie between the eight
atom-atom connections to nearest Sn atoms (Sn(1) 4×, Sn-
(3) 4×). The electron localization domain is thus of non-
bonding type.16 From the three-dimensional representation
in Figure 1a it is obvious that the maximum 1 is shifted
slightly away from the Sn(1)-Sn(1) vector. The shift of the
center of this bonding electron pair domain can be understood
if we consider weak repulsive interactions with the lone-
pair type region 3 of the Sn(2) atom, which lie on the
opposite site with respect to the Sn(1)-Sn(1) bond vector.
A similar torus-type region 4 is also observed at Sn(3),
which has a quadratic-planar nearest neighbor coordination
to four Sn(2) atoms. The orientation of the torus is
perpendicular to the plane through neighboring Sn(2) atoms
and slightly tilted in that way that the repulsion between other
localization domains is minimized.
Figure 5. (a) Three-dimensional ELF representation calculated from the
valence electron density. Orientation of the unit cell as in Figure 1a. The
surface corresponds to the constant ELF value of 0.77. (b-e) Two-
dimensional ELF represented on a cross section as indicated in Figure 6a
perpendicular to the b direction and parallel to the Sn(1)-Sn(1) bond vector
with Sn-Sn distance of 2.900 Å. (b) Density of pixels represents the valence
electron density and the color of each pixel corresponds to a specific ELF
value as indicated in the color bar at the bottom. (c) Contour line diagram
of ELF e 0.85 in line of decent steps of 0.05, and section as in Figure 6b.
(d) Representation as in Figure 5b, calculated with an all electron model.
(e) Contour line diagram of ELF e 0.50 in line of decent steps of 0.05 of
an all electron model, and section as in Figure 6c. (f, g) Two-dimensional
ELF of an all-electron model represented on a cross section through the
center of the Sn(1)-Sn(1) bond vector with Sn-Sn distance of 3.044 Å
and parallel to the ab plane. Contour line diagram of ELF e 0.50 in line of
decent steps of 0.05.
The calculation has been carried out with only valence
electrons or including all electrons for ELF analysis. The
results are represented as a three-dimensional (3D) function
of constant ELF value in Figure 5a. A two-dimensional (2D)
intersection can be represented either as a pixel image (Figure
5b,d, f), where the density of pixels reflects the electron
density and the color of each pixel shows the ELF value
according to the color bar in Figure 5b, or as contour line
diagram (Figure 5c,e,g). In the case of an all-electron cal-
culation the shell structure of the core electrons is visible in
the 2D images (Figure 5d,f). In case of the inclusion of only
valence electrons deep blue areas appear at the atom site.
Figure 5a represents a 3D image of the ELF of a valence
electron model. The bonding region 1 corresponds to a
localized bond between nearest Sn(1) atoms. This bond
region is also shown in the two-dimensional intersection
perpendicular to the b direction of the unit cell. In the color-
coded representation of Figure 5b and the contour line
Discussion
The search for familiar structures that are related to SrSn4
reveals five examples (Cr7BC4, DyFe2SiC, UFeS3, VCr2C2,
and Rh3Te3) with the space group Cmcm, the combination
of Wyckoff positions fc2a, and the Pearson symbol oS20.19
None of them shows a distinct structural similarity to SrSn4.19
However, if we regard SrSn4 as a structure consisting of slabs
of face sharing tetragonal prisms as shown in Figure 6, which
alternate along b with the net of 3-fold bonded Sn(1) atoms
(Figures 6a and 1c), we can trace the structure back to ones,
which contain rather similar building blocks of face sharing
tetragonal prisms. The structures PdBi, Mn2AlB2, MoAlB,
and Ru3Al2B2 contain similar slabs of face sharing tetragonal
prisms. Those can further be described as intergrowth
structures (IGS) of AlB2-type and W-type slabs with a
(19) Parthe´, E.; Gelato, L. M.; Chabot, B. A.; Penzo, M.; Cenzual, K.;
Gladyshevskii, R., Eds. TYPIX-Standardized Data and Crystal Chemi-
cal Characterization of Inorganic Structure Types; Springer-Verlag:
Heidelberg, Germany, 1993; Vol. 1.
(17) Savin, A.; Nesper, R.; Wengert, S.; Fa¨ssler, T. F. Angew. Chem., Int.
Ed. Engl. 1997, 36, 2683; 1997, 109, 1892.
(18) Bader, R. F. W. Coord. Chem. ReV. 2000, 197, 71-94.
8752 Inorganic Chemistry, Vol. 42, No. 26, 2003

Superconducting Stannide SrSn4
Figure 6. Structural relationship between SrSn4 and Mn2AlB2: (a) structure of SrSn4 with slabs of coordination polyhedrons of Sn(3) atoms; (b) projection
of the atom sites in SrSn4 along a with dashed lines parallel to c indicate the corrugation of the slab; (c) projection of Mn2AlB2 along a with highlighted
slabs of AlB2- and W-types. The heights of the atom sites along a are given at the bottom. See also Table 1.
variable thickness of the W-type slab.19 Further inspection
shows that Mn2AlB220 has a comparable W-type slab of three
atom layers and that the AlB2-type slab is quite similar to
the observed net of Sn(1) atoms in SrSn4 (Figure 1c). Both
structures are shown in Figure 6b and 6c as a projection along
the a axis. The structures are generated by two different
layers of atoms: The W-type slab in Mn2AlB2 consists of
[AlMn4Mn4] polyhedra (Figure 6c). In SrSn4 the correspond-
ing part is build up by [SnSn4Sr4] polyhedra, which form a
slightly corrugated slab in contrast to the flat one in Mn2-
AlB2 (Figure 6a and 6b). In SrSn4 the a1/c1 ratio of such a
distorted quadratic prism (with a1 and c1 being prism edges
along the a and c direction, respectively) is 1.31. Therefore
this part of the structure can also be regarded as a part of
the Cu3Au-type slab and formal composition “SrSn3”
()Sn(3)Sn(2)4/8Sr(1)4/8). The second type of slab in Mn2-
AlB2 is derived from the AlB2 structure, if the structure is
cleaved perpendicular to the planar boron atom layers in such
a way that infinite B atom zigzag chain are formed. The
resulting chains run parallel to the a axis as shown in Figure
6c. A similar slab in SrSn4 can be derived from the AlB2-
type. As indicated by arrows in Figure 6c, small distortions
of the B atom zigzag chains lead to the formation of a B-atom
layer similar to the net of Sn(1) atoms in SrSn4 (Figures 1c
and 6b). Corresponding B atom layers are observed in
RuB2,21,22 and interestingly, RuB2 is also superconducting.23
The formal composition of a three-
dimensional structure derived from the AlB2-type slab in
SrSn4 is “SrSn5” ()Sn(1)2/2Sn(2)2/8Sr2/8). Hence the com-
parison with the known compound BaSn5 suggests itself. As
shown earlier, the structure of BaSn5 can also be rationalized
as superstructure of AlB2.8 As a result SrSn4 can be derived
from one to one intergrowth structures of the composition
SrSn3 and SrSn5.
The analysis of the structure of SrSn4 using the approach
of the intergrowth structure concept24,25 offers an easy way
for the representation of SrSn4 although the strong conditions
for the definition of slabs do not hold for this compound.
The large advantage of this view is the structural relationship
to compounds and a possible classification of the new
structure type.
Another way of the extrusion of characteristic structural
features is the analysis of the structure in terms of localized
chemical bonds. At a first glance the net of Sn(1) hexagons
in Figure 1c can be regarded as an analogue to the structure
of neighboring group-15 elements. Considering only the
shortest Sn-Sn contacts (2.900(1) and 3.044(1) Å) a net of
Sn(1) atoms with exclusively three bonded (3b) atoms results.
This description is an agreement with the ELF calculations,
which show two types of localized bonds between Sn(1)
atoms and lone pairs located at the Sn(1) atoms. Assuming
that the valence electrons of strontium are exclusively
transferred to these Sn(1) atoms, Sn(1) atoms are singly
(23) Vandenberg, J. M.; Matthias, B. T.; Corenzwit, E.; Barz, H. Mater.
Res. Bull. 1975, 10, 889-894.
(24) Parthe´, E.; Chabot, B. A.; Cenzual, K. Chimia 1985, 39, 164-174.
(25) Grin, Y. In Modern PerspectiVes in Inorganic Crystal Chemistry;
Parthe´, E., Ed.; Kluwer Academic Publisher: Dordrecht, The Neth-
erlands, Boston, MA, London, 1992; Vol. 382, pp 77-96.
(20) The structure of Mn2AlB2 was first described in C22227 and later
corrected to Cmmm. 28
(21) Aronsson, B.; Stenberg, E.; Aselius, J. Nature 1962, 195, 377-378.
(22) Roof, R. B.; Kempter, C. P. J. Chem. Phys. 1962, 37, 1473-1476.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8753

Hoffmann and Fa1ssler
negative charged and are isosteric with Sb atoms. Indeed
form the Sn(1) atoms a substructure similar to those of group-
15 elements (R-As type). As a consequence of singly
negative charged Sn(1) atoms, the remaining Sn atoms have
a formal oxidation state of zero, and in terms of the extended
Zintl-Klemm-Busmann concept the formula of SrSn4 is
[Sr2+][(3b)Sn1-]2[Sn0]2. According to the 8 - N rule Sn
atoms with oxidation state zero should in case of exclusively
covalently bound atoms be 4-fold connected. Since Sn(2)
has eight and Sn(3) six surrounding Sn atoms, delocalized
bonds between Sn(1) and these atoms must be assumed. This
is in agreement with the distance range of 3.3-3.4 Å to
neighboring Sn atoms, that are much longer than expected
for localized Sn-Sn bonds. In this view Sn(2) and Sn(3)
atoms must be regarded as the “metallic part” of the structure,
which connects the covalently bonded nets of Sn(1) atoms.
ELF mirrors this bond characteristic. There are localized
bonds between Sn(1) atoms and lone pairs at these atoms.
Domains of localized electrons at the “metallic” Sn(2) and
Sn(3) atoms have their maxima between the atom-atom
vectors. Such domains are comparable to lone pair regions
in molecules and aresas shown in other casesslocated
energetically close to the Fermi level. Such lone pair domains
are frequently present in polar intermetallic compounds, and
repulsive interactions are discussed to play a crucial role for
the superconductivity of these compounds.8,13
the quadratic net. The Sn-Sn distances in SrSn4 also
compare well to those in BaSn5, where pairs of planar
hexagonal nets of Sn atoms form face-sharing hexagonal
prisms, which are centered by Sn atoms. Distances from the
central Sn atom to 12 atoms of the prism are 3.44 Å and are
longer than the distances from the central Sn(2) atom in the
distorted quadratic prism in SrSn4 (3.287, 3.302 Å). Shorter
distances in SrSn4 are expected because of the lower
coordination number of the centering atom. Sn-Sn distances
along the height of the hexagonal prism in BaSn5 are 2.96
Å, which compares well with the shortest contacts of 2.90
and 3.04 Å in SrSn4. Short distances indicate that these are
the strongest Sn-Sn interactions. The covalent character of
these bonds has been revealed also for BaSn5 by ELF
calculations.
Summary
SrSn4 represents an intermetallic compound that crystal-
lizes in a novel structure type having covalent and metallic
bond characteristics. The presence of lone pair type localiza-
tion domains at the atoms of the “metallic” substructure is
in coincidence with the superconducting property of SrSn4.
This is also observed in other intermetallic compounds.
Acknowledgment. We are grateful to the Verband der
Chemischen Industrie and BMFT for support of this work.
The authors thank R. Nesper, ETH Zurich, for using the
SQUID magnetometer and the Siemens SMART diffracto-
meter and B. Eisenmann, TU Darmstadt, for helpful discus-
sions. Some of the results were presented at the Workshop
on High-temperature Superconductors and Novel Inorganic
Materials MSU-HTSC-VI, Moscow-St. Petersburg, June 24-
30, 2001, and at the ACS conference, New Orleans, LA,
March 23-27, 2003.
Comparable quadratic nets of Sn atoms are also found as
building units in other binary stannides. Slightly corrugated
quadratic nets of Sn atoms as shown in Figure 1b are also
observed in the binary phase NaSn5.26 A framework of 4-fold
bonded Sn atoms connects the coplanar two-dimensional
nets. Again distances from four-connected atoms to other
Sn atoms are ∼2.9 Å, whereas Sn-Sn contacts within the
quadratic nets are longer (3.14 Å). In agreement with the
results here, we observe localized bonds between the 4-fold
connected Sn atoms in NaSn5 and lone pairs at the atoms of
Supporting Information Available: Tables of data collection
and refinement details, positional and thermal parameters, aniso-
tropic displacement parameters, and important distances as well as
an X-ray crystallographic file in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(26) Fa¨ssler, T. F.; Kronseder, C. Angew. Chem., Int. Ed. Engl. 1997, 36,
2683. 1998, 110, 1641.
(27) Becher, H. J.; Krogmann, K.; Peisker, E. Z. Anorg. Allg. Chem. 1966,
344, 140-147.
(28) Cenzual, K.; Gelato, L. M.; Penzo, M.; Parthe´, E. Acta Crystallogr.
1991, B47, 433-439.
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