A crystalline, large-pore, microporous semiconductor

Article abstract of DOI:10.1002/anie.200603075

Tubes, dumbbells, and pyramids: A crystalline, large-pore, microporous semiconductor, |Rb₁₈| [Sb₃₆O₅₄][(SbTe ₃)₂(Te₂)₆] (Rb-CTH-1), which consists of an 18-ring {Sb₁₈O₂₇} tubular unit surrounded by an intricate, ordered arrangement of trigonal-pyramidal {SbTe ₃}^3- ions and Te₂^2- dumbbells and lined with Rb⁺ ions, has been prepared (see picture; turquoise R). This semiconductor has a band gap of approximately 850 meV. (Figure Presented).

Full text of DOI:10.1002/anie.200603075

Communications
DOI: 10.1002/anie.200603075
Semiconductor Materials
A Crystalline, Large-Pore, Microporous Semiconductor**
Alexander Shulman and Anders E. C. Palmqvist*
Crystalline, microporous semiconductors are of interest for a
variety of potential applications, such as chemical sensors,
electro-optics, thermoelectrics, and low-dimensional electron-
ics. Over the past two decades research has been devoted to
discovering naturally occurring and synthetic microporous
materials with semiconducting properties. Simon et al., for
example, have reported a synthetic cetineite analogue with
the composition j K6(H2O)6 j [Sb12O18][SbSe3]2, which was
described to be the first crystalline, microporous material of
this family of materials to feature a photo-semiconducting
host structure and a band gap of 2060 meV.[1]
Avariety of additional synthetic cetineite-type phases also
reveal optical band gaps in the visible range and electrical
semiconductivities.[2] Experimental results[1,3] and theoretical
computations[3] have demonstrated that the optical band gaps
of the members of the cetineite family with the general
formula A6[Sb12O18][SbX3]2 (A = K or Na, X = S or Se) vary
with the composition of the crystal and tend to decrease with
increasing atomic number of both the alkali metal and the
chalcogen. It was suggested that these properties may be
attributed to an overlap of the sp, dxz, and dyz orbitals of the Sb
atoms of chemically different subunits (namely the {Sb12O18}
tubular unit and the {SbS3}3ꢀ or {SbSe3}3ꢀ pyramids) in an
alternating sequence along the crystallographic c axis.[3,4]
example of a second-order zeoate according to the recent
suggestion for the extension of the nomenclature of micro-
porous materials.[6] The strongly reduced crystal chemical
formula of K-SBC-1 is j (j K6(H2O)6 j [Sb12O18])3 j [Te36], and
this crystalline microporous material is a narrow-band-gap
semiconductor with a band gap of only 250 meV and a highly
anisotropic and relatively large electrical conductivity.[5]
As a result of our recent investigations of the related
rubidium antimony(III) oxide telluride system, we now report
a new crystalline, large-pore, microporous semiconductor.
The structure, referred to as Rb-CTH-1, is related to K-SBC-1
and the cetineite and exhibits a number of fascinating
structural features and properties. For instance, isolated Rb
ions are hosted by a unique 18-ring {Sb18O27} tubular unit
which, in turn, is surrounded by an intricate, ordered network
built up from trigonal pyramidal {SbTe3}3ꢀ anions and Te2
2ꢀ
dumbbells. The overall reduced crystal chemical formula of
Rb-CTH-1 is j Rb18 j [Sb36O54][(SbTe3)2(Te2)6] and the struc-
ture is classified as a first-order zeoate following the most
recent suggestion for this new nomenclature.[6] In addition,
Rb-CTH-1 has a band gap of 850 meV, which makes it
interesting for applications within the domain of microporous
semiconductors.
Dark-red, acicular crystals of Rb-CTH-1 with a length of
0.1–2 mm and a diameter of approximately 0.03–0.1 mm
(shown in Figure 1) were formed over seven days by
solvothermal treatment of a reagent mixture with the molar
ratio Te/Sb/Rb2O/C3H7NH2/H2O 1:0.682:0.774:0.68:19.35 at
453 K. The yield of product ranged from 1–60 vol% depend-
ing on the orientation of the autoclave in the oven. Syntheses
in vertically positioned autoclaves typically produced
extremely low yields of Rb-CTH-1 of approximately
Recently,
a crystalline, microporous, semiconducting
antimony(III) oxide telluride known as K-SBC-1 has been
synthesized.[5] K-SBC-1 is structurally related to the cetineite
family as it shares the {Sb12O18} tubular unit, but it differs in
that it has a tellurium tubular unit instead of the network of
pyramidal {SbX3}3ꢀ anions enclosing the {Sb12O18} units. The
structure of K-SBC-1 is also of interest from a structural
classification point of view since it consists of guests within a
host within a second host and thus appears to be the first
[*] Prof. A. E. C. Palmqvist
Department of Chemical and Biological Engineering
Applied Surface Chemistry and Competence Centre for Catalysis
Chalmers University of Technology, 41296 Göteborg (Sweden)
Fax: (+46)31-160-062
E-mail: adde@chalmers.se
A. Shulman
Department of Chemical and Biological Engineering
Applied Surface Chemistry
Chalmers University of Technology
41296 Göteborg (Sweden)
[**] Financial support from the Swedish Research Council and the Carl
Trygger Foundation is greatly appreciated. Dr. Vratislav Langer,
Göteborg, and Dr. Jacob Overgaard, Aarhus, are acknowledged for
technical assistance during the single-crystal diffraction data
collection and absorption correction of the obtained datasets. Dr.
Ezio Zanghellini, Göteborg, is acknowledged for his assistance
during the DRIFT analysis. Prof. Susan Jagner, Göteborg, and Dr.
Rasmus Damgaard Poulsen, Aarhus, are thanked for valuable
discussions.
Figure 1. Photographs of collections of Rb-CTH-1 crystals.
718
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Chemie
1 vol%. Such low yields appear to be linked to the fact that
the interfacial area between the gas, liquid, and solid phases,
and therefore the contact area available for secondary
nucleation, are lower under these conditions than in horizon-
tally positioned autoclaves. Syntheses in the latter yielded up
to 60 vol% of Rb-CTH-1. The main impurities were identi-
fied as Sb2Te3, Sb0.405Te0.595, Te2O5, and traces of Te(OH)6.
The crystal structure of Rb-CTH-1 is presented in
Figure 2. Rb-CTH-1 grows in the hexagonal space group
P63/mmc (no. 194) with hexagonal axes, and a = b =
21.8578(2) and c = 9.7599(2) Š at 295 K. The structure con-
tains a unique 18-ring {Sb18O27} tubular unit (see Figure 2a,b)
with a ring width of 13.12 ” 13.49 Š (assuming a van der Waals
radius, r(O), of 1.35 Š). The unit resembles the {Sb12O18} 12-
ring tubular unit in K-SBC-1 and the cetineites in that it is
built up from pyramidal {SbO3} groups linked together
through common oxygen atoms and hosts alkali-metal ions,
in this case rubidium ions. The distance between the Sb and O
atoms in the {SbO3} groups ranges from 1.965 to 2.018 Š.
The walls of the {Sb18O27} unit consist of {Sb6O6} rings.
Unlike K-SBC-1, where all the {Sb6O6} rings are alike, Rb-
CTH-1 features two different types of {Sb6O6} rings. Ring A
has a ring width of 2.88 ” 2.88 ” 2.98 Š (r(O) = 1.35 Š) and
consists of the following antimony atoms named in clockwise
order: SB2, SB2, SB1, SB1, SB1, and SB1. Ring B has a ring
width of 2.75 ” 3.01 ” 3.01 Š (r(O) = 1.35 Š) and is built up
from the following antimony atoms, also listed clockwise:
SB1, SB1, SB2, SB1, SB1, and SB2. These dimensions are
comparable in size to the {Sb6O6} rings in K-SBC-1, which
have a ring width of 2.73 ” 2.88 ” 2.99 Š.[5] The 6-rings
coordinate the rubidium ions in a manner similar to crown
ethers, with the six distances between the rubidium ion and
the oxygen atoms ranging from 2.986 to 3.049 Š for ring A
and from 2.946 to 3.406 Š for ring B.
Although unique in its configuration, the outer network
consists of well-known building blocks. The trigonal pyrami-
dal {SbTe3}3ꢀ anions are found, for example, in clathrates, such
as Na3SbTe3 and K3SbTe3,[8] and interestingly enough,
[7]
pyramidal {SbS3}3ꢀ and {SbSe3}3ꢀ anions have been found in
the cetineite structure,[1–4] although the {SbTe3}3ꢀ anion has
not. The Sb Te distance in the {SbTe3}3ꢀ anions in Rb-CTH-1
ꢀ
is 2.726 Š, which is somewhat shorter than those in Na3SbTe3
ꢀ
ꢀ
and K3SbTe3 (2.783 and 2.787 Š, respectively). The Te Sb Te
angle in the {SbTe3}3ꢀ anions in Rb-CTH-1 is 95.458 (100.08 in
Na3SbTe3 and 101.98 in K3SbTe3). In agreement with the
investigation of bond and atomic valences in chalcogen
Figure 2. Structure of Rb-CTH-1: a) Projection onto the (001) plane
showing isolated Rb+ ions hosted by a {Sb18O27} tubular unit which, in
turn, is placed within a complex network consisting of trigonal
compounds,[9] the value of the Te Sb Te angle suggests that
the effective valence of the Sb atom lies close to + 3. The
distortion from the regular tetrahedral angle (109.58) can thus
be assigned to the effect of the lone electron pair (LEP) at
each Sb3+ site, as previously discussed in the literature.[7]
ꢀ
ꢀ
pyramidal {SbTe3}3ꢀ anions and Te2 dumbbells; b) a side front view
2ꢀ
of a {Sb18O27} tubular unit with Rb+ ions as guests and the outer
arrangement of disordered trigonal pyramidal {SbTe3}3ꢀ anions and
Te2 dumbbells. Rb+ ions, {SbTe3}3ꢀ pyramids, and Te22ꢀ dumbbells
2ꢀ
have been removed in the lower part of the figure for clarity; this
enables the location of the assigned atoms and rings in the Sb6O6
units to be seen more clearly. Alternative positions for disordered
atoms are marked with an asterisk; c) a side view of a network
ꢀ
ꢀ
Similar to the Sb S and Sb Se bonds in the cetineites, the
ꢀ
Sb Te bond in Rb-CTH-1 is expected to be rigid due to its
considerable degree of covalency. Structure refinements
reveal an orientational disorder of the {SbTe3}3ꢀ anions. The
{SbTe3}3ꢀ anions in the tubular interstices are stacked either
parallel or antiparallel to [001], and all the pyramids in a
single interstice are expected to have the same orientation in
the c direction since the strong repulsive forces between the
containing trigonal pyramidal {SbTe3}3ꢀ anions and Te2 dumbbells
2ꢀ
surrounding the {Sb18O27} tubular unit. The {Sb18O27} unit and the
disordered atoms/groups have been removed for clarity.
Angew. Chem. Int. Ed. 2007, 46, 718 –722
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Communications
LEPs at the Sb atoms would prevent them from having
Unlike K-SBC-1 and the cetineites no ordered guest
species other than the metal counter cations are encountered
inside the antimony(III) oxide tubular unit of Rb-CTH-1,
thereby suggesting that this structure may have a larger
sorption capacity. However, some residual electron density
found inside the {Sb18O27} tubular unit may be evidence for
the presence of disordered water or template molecules
residing inside the tube. Owing to the larger tube diameter in
Rb-CTH-1 compared to the cetineites and to K-SBC-1, the
mobility of these molecules inside the tubes is expected to be
higher in the former, which causes these molecules to be
disordered in the 1D channels of the structure.
opposite orientations. The orientation of the {SbTe3}3ꢀ anions
is expected to vary randomly from one interstice to another.
This structural peculiarity is also observed in the ceti-
neites.[10,11]
The Te atoms in the {SbTe3}3ꢀ anions do not lie on special
positions—the deviation of the split position of the Sb and Te
atoms in the {SbTe3}3ꢀ anion from the mirror plane parallel to
(001) is 1.18 and 0.24 Š, respectively.
2ꢀ
Te2 dumbbells are also encountered in a variety of
structures, such as K-SBC-1,[5] ThTe2I2,[12] and BaBiTe3.[13] The
2ꢀ
ꢀ
Te Te distance in a Te2 dumbbell is 2.727 Š in Rb-CTH-1,
which is somewhat shorter than the values of 2.784 Š found in
K-SBC-1[5] and 2.864 Š in elemental tellurium. The Te atom
that lies closest to the {SbTe3}3ꢀ anion in the Te22ꢀ dumbbells
in Rb-CTH-1 also exhibits orientational disorder, with a
deviation of the split position from the mirror plane parallel
to (001) of 0.191 Š. This disorder appears to be caused by
repulsion between the LEP on the Sb atom in the anion and
the electron cloud of the Te atom in the dumbbell. Hence, the
orientation of the dumbbell in the c direction is expected to
be similar to that of the neighboring {SbTe3}3ꢀ anion.
When projected normal to (001), the arrangement is seen
to be composed of layers, each of which consists of six
dumbbells and three {SbTe3}3ꢀ anions (see Figure 2c). This
network is clearly not closed since the separations between
the building blocks of the network exceed the van der Waals
In agreement with the IUPAC nomenclature for porous
solids, Rb-CTH-1 should be classified as a microporous
material similar to the structurally related cetineites and K-
SBC-1.[6,14–16] It is, however, still unclear whether the struc-
tural integrity would remain intact upon removal of the guest
species and hence it is uncertain whether Rb-CTH-1 would
exhibit any porosity in the more applied meaning of the word,
that is, the ability to demonstrate reversible sorption and ion-
exchange.
Generally, it is difficult to distinguish between tellurium
and antimony in the {SbTe3}3ꢀ anions by X-ray diffraction
alone because of the similar scattering cross-sections for X-
ꢀ
rays for these elements. However, the Sb Te bond distance
and the Te Sb Te angle in the {SbTe3}3ꢀ anions confirm that
the position at the top of the pyramid is in fact occupied by an
antimony atom. Furthermore, simple electron-counting argu-
ments strongly support that the crystal chemical formula for
Rb-CTH-1 is j Rb18 j [Sb36O54][(SbTe3)2(Te2)6]. The Sb in the
{Sb18O27} tubes is expected to have the same oxidation state as
in K-SBC-1,[5] that is + 3, which means that the {Sb18O27} tubes
are charge neutral. Tellurium is known to have mixed
oxidation states in polytellurides.[17] The average Te oxidation
state in K-SBC-1, for instance, is ꢀ0.5,[5] whereas it is
ꢀ
ꢀ
ꢀ
bond distance of 4.12 Š for a Te Te interaction.
The Te22ꢀ dumbbells and the {SbTe3}3ꢀ anions are located
on the opposite sides of the {Sb6O6} rings of the {Sb18O27} tube
walls to the rubidium ions. An interesting feature of the Rb-
CTH-1 structure is the position of the {SbTe3}3ꢀ anions and
2ꢀ
the Te2 dumbbells with respect to the two types of {Sb6O6}
rings and to the two crystallographically different kinds of
2ꢀ
rubidium ions. Apparently, the Te2 dumbbells lie on the
opposite side of {Sb6O6} rings of type A from RB2 cations,
whereas the {SbTe3}3ꢀ anions lie opposite to {Sb6O6} rings of
type B, which coordinate RB1 cations. The two Te atoms in a
dumbbell are not symmetrically located with respect to the
center of the ring of type A, with a separation between these
Te atoms and the rubidium cation of 4.353 and 4.944 Š,
respectively.
expected to be ꢀ11= in Rb-CTH-1.
3
Similar to the case of cetineites[1–4,10] and K-SBC-1,[5] the
ꢀ
{Sb18O27} tubes are presumably stabilized by the strong Sb O
bonds, whereas the overall structure is held together by ionic
forces between the negatively charged {SbTe3}3ꢀ pyramids and
2ꢀ
Te2 dumbbells in the tubular interstices, and the rubidium
counter cations lined on the inside of the {Sb18O27} tubes.
As it is structurally and compositionally similar to the
members of the cetineite family and to K-SBC-1, Rb-CTH-1
was expected to exhibit semiconducting properties. The
diffuse reflectance spectrum of Rb-CTH-1 is shown in
Figure 3. A series of bands are found below 250 meV
associated with the optical vibrational modes of the crystal
structure. A broad band centered around 405 meV is asso-
ciated with OHstretching modes, which are apparently due to
adsorbed molecular water in the sample structure. The
measurements demonstrate that Rb-CTH-1 has a forbidden
band below 660 meV, where photons are absorbed solely by
exciting the vibrational modes of the system. Above approx-
imately 660 meV, the steep rise in the absorption of the
sample is evidence for the presence of a conduction band.
Some spurious features are observed at about 350–360 and
790 meV. These arise from CHcontamination of the beam
The {SbTe3}3ꢀ anions lie symmetrically with respect to the
center of an {Sb6O6} ring of type B. As a consequence of the
orientational disorder exhibited by the {SbTe3}3ꢀ anions, the
rubidium cation RB1 located on the opposite side of the
{Sb6O6} ring of type B may occupy two positions, each of
which lies symmetrically with respect to the mirror plane
parallel to (001), with a separation of 0.264 Š from the latter.
The position occupied by the cation is clearly directed by the
orientation of the {SbTe3}3ꢀ anion. Apparently, the stacking of
the anions parallel to [001] causes the rubidium cations to
occupy a position above the mirror plane, whereas for
antiparallel stacking of the anions the coordinated rubidium
cations occupy positions below the mirror plane. The distance
between the two Te atoms pointing inward to the rubidium
cation located on the opposite side of the {Sb6O6} ring is
5.348 Š.
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Angewandte
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(99.999%, crushed pieces, freshly ground), tellurium (99.999%,
crushed pieces, freshly ground), n-propylamine (98%), rubidium
hydroxide (99.9% as 50 wt% solution in water), and deionized water.
All reagents used, except for the deionized water, were purchased
from Aldrich.
The procedure used for preparation of Rb-CTH-1 was as follows.
The reagents were weighed and placed in a 45-mL, Teflon-lined,
stainless-steel autoclave (Parr Instrument Corp.). The autoclave was
vigorously shaken for approximately 1 min and then placed in an
oven at 453 K. The autoclave was typically quenched in air at ambient
temperature after seven days of heat treatment. The liquid phase was
separated from the solid phase by filtration, and the solid product was
washed with ethanol and dried in air. Rb-CTH-1 formed as dark-red,
acicular crystals, which were manually separated from the rest of the
solid material and placed in sealed glass vials. The yield of Rb-CTH-1
in the product mixture was estimated visually.
Figure 3. The diffuse reflectance spectrum of an Rb-CTH-1 sample
ground with KCl showing the logarithm of the inverse reflectance as a
function of photon energy.
The crystal structure of Rb-CTH-1 was determined at 298 K using
a Siemens SMART CCD single-crystal X-ray diffractometer (MoKa
radiation, l = 0.71073 Š). Data collection and integration were
carried out using the SMART software. Averaging and absorption
correction were done using SADABS. Structure solution and refine-
ment were performed using the WinGX v1.70.01 program package.[20]
Rb-CTH-1: hexagonal, P63/mmc (no. 194), a = b = 21.8578(2), c =
splitter in the former case and a small drift of the response of
the germanium diode detector in the latter.
It is not obvious from the spectrum where the steep rise
ends and therefore it is not very clear exactly how large the
band gap is. The rise itself is much broader than 50 meV (kBT
for T of about 300 K), therefore it is intrinsic to the structure
and is not simply a consequence of thermal effects.[18] The
derivative of the steep rise is almost constant over quite a
broad range (more than five times kBT).
Despite the fact that the logarithm of the inverse
reflection does not precisely match the real absorption
spectrum, the former is often used to estimate the latter
when the nature of the sample or the measurement techni-
ques do not allow for collection of the real absorption
spectrum. It has been reported that band-gap energies in
semiconducting materials can be evaluated directly from the
absorption spectrum with an accuracy of 5 meV.[19] Thus, the
band-gap energy in Rb-CTH-1 was estimated to be 850 meV
from the derivative spectrum of the inverse logarithmic
reflectance with respect to the photon energy.
9.7599(2) Š at 298 K. V= 4038.21(1) Š3, 25 ” 30 ” 440 mm, 1calcd
=
3.840 gcmꢀ3, 2qmax = 58.248, l = 0.71073 Š, m = 14.88 mmꢀ1; number
of measured reflections: 55726; number of unique reflections
included in the refinement: 1958; least-squares refinement: Npar
=
66, R(int) = 0.1016, R(s) = 0.0271, R1 = 0.0639 refined against j F j ,
wR2 = 0.1231 and GooF = S = 1.055 refined against j F2 j . Further
details of the crystal structure investigations may be obtained from
the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopold-
shafen, Germany (fax: (+ 49)7247-808-666; e-mail: crysdata@fiz-
karlsruhe.de), on quoting the depository number CSD-416106.
FTIR spectroscopy analysis of Rb-CTH-1 was performed in a
diffuse-reflectance (DR) geometry to investigate the optical proper-
ties of the material. A small amount of Rb-CTH-1 was ground
together with KCl in an agate mortar in a ratio of about 1 to 50 in
volume. A similar sample of pure KCl was used as reference. The
measurements were performed on a Bruker IFS 66v/S FTIR instru-
ment, using a Graseby Specac diffuse reflection unit, model Selector.
The signal in the mid-IR region (650–5700 cmꢀ1 or 80–710 meV) was
detected using a KBr beam-splitter and an MCT detector. The signal
in the near-IR region (5700–10000 cmꢀ1 or 710–1200 meV) was
detected using a quartz beam-splitter and a germanium diode
detector.
In conclusion, a new crystalline, large-pore, microporous
semiconductor (Rb-CTH-1) has been prepared by means of
solvothermal synthesis. Its structure was determined by
single-crystal X-ray diffraction. The material possesses sev-
eral unique structural features, such as an 18-ring {Sb18O27}
tubular unit that was previously unknown in any chemical
compound. The antimony(III) oxide tubular unit is sur-
rounded by an intricate, ordered arrangement consisting of
Received: July 29, 2006
Revised: October 30, 2006
Published online: December 8, 2006
2ꢀ
trigonal pyramidal {SbTe3}3ꢀ anions and Te2 dumbbells.
Keywords: antimony · microporous materials · semiconductors ·
.
tellurium · zeolite analogues
Some positive residual electron density inside the {Sb18O27}
tubular unit presumably arises from disordered guest species
occupying the tube. The nature of these species is difficult to
determine, although the diffuse reflectance spectroscopy
analysis indicates the presence of water in the structure.
DRIFT measurements also reveal a band gap of 850 meV,
thus making Rb-CTH-1 interesting for applications typical for
materials in the domain of crystalline, microporous semi-
conductors.
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