BaPbSi2O6·BaSO4: The first mixed anionic compound synthesized via BaSO4 salt-inclusion

Article abstract of DOI:10.1039/c4ce00527a

Exploring mixed anion materials is an ongoing challenge because of their important characteristics. However, there are no effective ways to guarantee the connection modes of anion groups and the structures of mixed anion materials so far. We have develope

Full text of DOI:10.1039/c4ce00527a

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BaPbSi2O6·BaSO4: the first mixed anionic
compound synthesized via BaSO4 salt-inclusion†
Cite this: CrystEngComm, 2014, 16,
Lingyun Dong,ab Shilie Pan, a Ying Wang,ab Hongwei Yu,ab Qiang Bian,ab
5993
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a
a
a
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*
Zhihua Yang, Hongping Wu and Min Zhang
Received 14th March 2014,
Accepted 14th April 2014
DOI: 10.1039/c4ce00527a
www.rsc.org/crystengcomm
Exploring mixed anion materials is an ongoing challenge because
of their important characteristics. However, there are no effective
ways to guarantee the connection modes of anion groups and the
structures of mixed anion materials so far. We have developed
BaSO4 as an anion group provider and structure template to
synthesize mixed anion materials.
noncentrosymmetric (NCS) materials by introducing relatively
rigid MO4 (M = P and Al) tetrahedra into the B–O framework,
such as Na3Cd3B(PO4)4 (ref. 5) and NaBa4(AlB4O9)2Br3.6 Some
compounds also have mixed SO4 and SiO4 groups, such as
Ca5(SiO4)4SO4 (ref. 7) and Ag6(SO4)(SiO4).8 Because of the
interesting structures and potential applications of these
materials containing mixed anionic groups, we focus on the
subset of mixed anionic compounds. Regarding the synthesis
method, S.-J. Hwu et al. firstly recognized salt-inclusion syn-
thesis using metal halides in phosphates and arsenates.9
Later compounds contained metal halides, which adopt new
structure types and exhibit fascinating physical properties,10
also proving that salt-inclusion is a valid tool for a broad
range of synthetic chemistry.11 These discoveries have
opened doors to novel material synthesis via salt-inclusion
chemistry (SIC). By a broad definition, salt-inclusion solids
(SISs) are compounds made of a hybrid framework exhibiting
integrated covalent and ionic lattices.12 According to the tra-
ditional definition of ionic compounds, sulfates would be a
metallic cation and SO4 anion by ionization. Consequently,
BaSO4 could be seen as an ionic lattice in the structure, and
used as a molten-salt, which plays the role of a structure tem-
plate. In addition, sulfates will provide SO4 tetrahedra, which
could be combined with other anionic groups, showing fasci-
nating structural and physical properties associated with
their features. Based on the above idea, the basic compound,
BaPbSi2O6, which we firstly synthesized by solid-state reac-
tion at 845 °C, melts in the orthorhombic BaSO4 salt, crystal-
lizing in the Pnma space group,13 and we have obtained a
complex silicate and sulfate compound, BaPbSi2O6·BaSO4, for
the first time. Because of the BaSO4 salt-templating effect,
BaPbSi2O6·BaSO4 crystallizes in the Pnma space group, which
is different from that of the basic compound, BaPbSi2O6.
To the best of our knowledge, the 1∞[Si2O6] chains, in which
the periodic four SiO4 tetrahedra are connected through
corners forming open rings, have not been found in any other
complex silicate and sulfate compounds. Here, the synthesis
The design and synthesis of mixed anionic materials has
attracted much attention not only because of their varied
forms and novel structures, but also because of their
multifunctional physical and chemical properties.1 For
example, A[ZnBP2O8] (A = NH4, K, Rb and Cs) show zeolite-
like microporous structures,2 and boron phosphate, BPO4,
has been proved to be a promising deep UV nonlinear optical
(NLO) material.3 The combination of a B4O10 group and SiO4
1h
group in Cs2B4SiO9 further expands the structural diversity
of mixed anionic compounds, and also generates a potential
deep-UV NLO material with an extremely short UV cut-off
wavelength and a relatively large NLO response. The first
borosulfate, K5[B(SO4)4],1f in which a central borate tetrahe-
dron shares all its corners with four sulfate tetrahedra, gives
a glance into the exciting new world of borosulfates. More
recently, a new family of alkali borosulfates were synthesized
which exhibit an interesting framework with mixed anionic
groups.4 Very recently, our group has discovered some
a Key Laboratory of Functional Materials and Devices for Special Environments
of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices,
Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing
Road, Urumqi 830011, China. E-mail: slpan@ms.xjb.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: Experimental section;
crystal data and structure refinements; atomic coordinates, equivalent
isotropic displacement parameters (Å2) and bond valence sum; selected bond
lengths; experimental and calculated XRD patterns; DSC curves; IR spectra;
UV-vis-NIR diffuse reflectance spectra of BaPbSi2O6 and BaPbSi2O6·BaSO4.
CCDC 969031, 969032 for BaPbSi2O6 and BaPbSi2O6·BaSO4, respectively. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ce00527a
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of BaPbSi2O6·BaSO4 via BaSO4 salt-inclusion is reported. The
crystal structures and UV-vis-NIR diffuse reflectance spectra
of BaPbSi2O6 and BaPbSi2O6·BaSO4 are presented. Theoretical
calculations based on density functional theory (DFT) are
also performed on the reported materials.14
Firstly, a polycrystalline powder sample of the basic
compound, BaPbSi2O6, was prepared by standard solid-state
reaction. Then, analytical grade orthorhombic BaSO4 was
added into BaPbSi2O6. A pure polycrystalline sample of
BaPbSi2O6·BaSO4 was synthesized in air by standard solid-
state reaction, according to the following formula:
The asymmetric unit of the basic compound, BaPbSi2O6,
contains one crystallographically independent Ba atom,
one independent Pb atom, one independent Si atom and
three independent O atoms. In the structure, the only
crystallographically independent Si atom coordinates to four O
atoms to form SiO4 tetrahedra, which are further connected
through corner-sharing, forming 1∞[Si2O6] chains with open
rings (Fig. 2a). The 1∞[Si2O6] chains are stacked with a coplanar
orientation along the [100] direction and the Si–O bond lengths
are in the range from 1.603(4) to 1.629(3) Å, which are consis-
tent with those observed in other compounds.15 The BaO10
polyhedra and PbO4 tetrahedra are connected alternatively
to form the 2∞[BaPbO6] layer (Fig. 2b). Then, the 1∞[Si2O6]
chains are bridged with the adjacent layers, forming the
whole structure of BaPbSi2O6 (Fig. 2c).
Generally, a highly electronegative element (e.g., O, F, Cl)
tends to form ionic bonding (electrostatic interactions) with
a highly electropositive element (e.g., alkali metal, alkaline-
earth metal and rare-earth metals).16 The ∞2 [BaPbO6] layers
are not firm enough because of the weaker ionic bonds.
Hence the 2∞[BaPbO6] layers can be easily disconnected, and
the [100] slab of orthorhombic BaSO4 (Fig. 2d) can insert into
the 2∞[BaPbO6] layers, forming the stronger 2∞[BaPbSO10] layers
(Fig. 2e). As shown in Fig. 2e, the 1∞[BaSO4] chains insert into
the 2∞[BaPbO6] layer along the [010] direction. Obviously, the
“salt templating” effect has resulted in a new centric solid.
This work affirms the utility of molten BaSO4 salt for the
high-temperature synthesis of mixed anionic materials.
The S–O bond lengths are in the range from 1.447(8) to
1.475(5) Å, which are consistent with the orthorhombic BaSO4
and other compounds.17 There are two crystallographically
independent Ba atoms, and both of them are in eleven-
coordination environments with the Ba–O bond distances
ranging from 2.342(11) to 3.285(11) Å. The results of bond
valence calculations18 for the two compounds (Ba, 1.940–2.052;
Pb, 2.256–2.274; Si, 4.125–4.250; S, 6.109) indicate that the
Ba, Pb, Si and S atoms are in oxidation states of +2, +2, +4
and +6, respectively.19
BaPbSi2O6 + BaSO4 → BaPbSi2O6·BaSO4.
The experimental X-ray diffraction (XRD) patterns are in
agreement with the calculated patterns based on the single-
crystal crystallographic data of BaPbSi2O6 and BaPbSi2O6·BaSO4,
respectively (Fig. S1, ESI†).
Colorless block crystals of BaPbSi2O6 and BaPbSi2O6·BaSO4
were grown by a spontaneous crystallization method. As
we expected, the structure of BaPbSi2O6·BaSO4 can be viewed
as an intergrowth of a BaPbSi2O6 [001] slab and a [100] slab
of the orthorhombic sulfate, BaSO4 (Fig. 1). Fig. 2 shows
ball-and-stick diagrams of BaPbSi2O6 and BaPbSi2O6·BaSO4.
Fig. 1 Structure design of BaPbSi2O6·BaSO4.
The 1∞[Si2O6] chains with four SiO4 tetrahedra forming
open rings are not found in any other sulfosilicates, from the
ICSD data. The adjacent SiO4 tetrahedra are pulled closely to
hold the smaller PbO4 tetrahedra, forming the open rings.
For example, from the formulae of BaPbSi2O6 and BaSiO3
(which can be seen as Ba2Si2O6),20 the Pb2+ cation partially
replaces the Ba2+ cation in BaSiO3. The 1∞[Si2O6] chains
exhibit different bond tendencies owing to the different
cation size and the coordination environment, resulting in
1∞[Si2O6] chains with a repeat interval of two SiO4 tetrahedra
in BaSiO3 and 1∞[Si2O6] chains with open rings formed by four
SiO4 tetrahedra in BaPbSi2O6 (Fig. S2, ESI†).
As shown in Fig. S3, ESI,† only one clear endothermic
peak at 1048 °C and one sharp endothermic peak at 1174 °C
are observed in the DSC heating curves for BaPbSi2O6
and BaPbSi2O6·BaSO4, respectively, which suggests that after
introducing stronger covalent bonds in the SO4 group,
BaPbSi2O6·BaSO4 has higher thermal stability than BaPbSi2O6.
Fig. 2 Ball-and-stick diagrams of BaPbSi2O6 and BaPbSi2O6·BaSO4.
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The pure polycrystalline samples of the two compounds were
put in a platinum crucible, heated to 1050 °C, and then slowly
cooled down to room temperature. Analysis of the residue of
BaPbSi2O6 in the platinum pan revealed that BaPbSi2O6
decomposed to BaSi2O5 and SiO2 (Fig. S4a, ESI†). However,
BaPbSi2O6·BaSO4 did not decompose (Fig. S4b, ESI†), which
further suggests that BaPbSi2O6·BaSO4 has higher thermal
stability than BaPbSi2O6. Next, BaPbSi2O6·BaSO4 in the platinum
crucible was heated to 1400 °C. Analysis of the residue in the
platinum pan revealed that BaPbSi2O6·BaSO4 decomposed
to BaSi2O5, BaSO4 and SiO2, as well as some unidentified
products (Fig. S4b, ESI†). Therefore, it is necessary to use
the flux method to grow large single crystals of the two
compounds.
In order to specify and compare the coordination of
silicon and sulfur in both compounds, the IR spectra were
measured, and they display similar features (Fig. S5, ESI†).
The main IR absorption region between 1200–450 cm−1
reveals several absorption bands on account of the stretching
and bending vibrations of the Si–O and S–O groups, which
are similar to those of other metal silicates and sulfates.1f,h,18
The IR spectra further confirm the existence of SiO4 and SO4
tetrahedra, which are consistent with the results obtained
from the single-crystal X-ray structural analysis of the two
compounds.
Fig. S6, ESI† shows the optical diffuse reflectance spectra
of the two compounds, which were converted from UV-vis-NIR
diffuse reflectance spectra using the Kubelka–Munk function.21
The UV cut-off edges of the two compounds are all below
300 nm. From the F(R) versus E(eV) plots, the band gap
of BaPbSi2O6 is about 3.27 eV, and the band gap of
BaPbSi2O6·BaSO4 is about 3.30 eV. That means that the
introduction of the SO4 group has not apparently influenced
the band gap, which will be explained well using the theoreti-
cal calculations as discussed below.
The band structures of BaPbSi2O6 and BaPbSi2O6·BaSO4
along the high symmetry points of the first Brillouin zone
(BZ) are shown in Fig. S7, ESI.† It can be seen that both com-
pounds are indirect-gap materials. The calculated band gaps
are 3.24 eV for BaPbSi2O6 and 3.29 eV for BaPbSi2O6·BaSO4,
which are in good agreement with the values obtained from
the diffuse-reflectance spectra. The partial density of states
(PDOS) for the two compounds are similar. As shown in
Fig. S8, ESI,† the PDOS can be divided into three major
distinct regions for both compounds. It is worth noting that
the states near the band gap are mainly composed of Ba 5p
and O 2p orbitals below the Fermi level, and Ba 6d and Si 2p
states at the bottom of the conduction bands for both
compounds. Accordingly, the absorption spectra near the UV
cutoff edge can be assigned as charge transfers from the Ba
5p and O 2p states to the Ba 6d and Si 2p states, leading to
the UV cutoff edge of both compounds being located at about
300 nm.
According to the electronic structure calculations, the absorp-
tion spectra near the UV cutoff edge can be assigned as charge
transfers from the Ba 5p and O 2p states to the Ba 6d and Si 2p
states, which means that the introduction of the SO4 group has
not apparently influenced the band gap. This is the first report
of the synthesis of mixed anionic compounds via BaSO4 salt-
inclusion. Further research on the mixed anionic compound
via sulfate salt-inclusion is in progress.
Acknowledgements
This work is supported by the 973 Program of China (grant
no. 2012CB626803), the National Natural Science Foundation
of China (grant no. U1129301, 51172277, 21201176, 21101168,
11104344), the Main Direction Program of Knowledge Inno-
vation of CAS (grant no. KJCX2-EW-H03-03), The Funds for
Creative Cross & Cooperation Teams of CAS, the Major
Program of Xinjiang Uygur Autonomous Region of China
during the 12th Five-Year Plan Period (grant no. 201130111),
the High Technology Research & Development Program of
Xinjiang Uygur Autonomous Region of China (grant no.
201116143, 201315103), and the Science and Technology
Project of Urumqi (grant no. G121130002).
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In conclusion, the synthesis of a novel complex silicate and
sulfate compound, BaPbSi2O6·BaSO4, is a result of the inclu-
sion of the BaSO4 salt en route to the BaPbSi2O6 framework.
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