Cs4B4O3F10: First Fluorooxoborate with [BF4] Involving Heteroanionic Units and Extremely Low Melting Point

Article abstract of DOI:10.1002/chem.202101321

Herein, a new congruently melting mixed-anion compound Cs₄B₄O₃F₁₀ has been characterized as the first fluorooxoborate with [BF₄] involving heteroanionic units. Compound Cs₄B₄O₃F₁₀ possesses two highly fluorinated anionic clusters and therefore its formula can be expressed as Cs₃(B₃O₃F₆) ? Cs(BF₄). The influence of [BF₄] units on micro-symmetry and structural evolution was discussed based on the parent compound. More importantly, Cs₄B₄O₃F₁₀ shows the lowest melting point among all the available borates and thus sets a new record for such system. This work is of great significance to enrich and tailor the structure of borates using perfluorinated [BF₄] units.

Full text of DOI:10.1002/chem.202101321

Communication
doi.org/10.1002/chem.202101321
Chemistry—A European Journal
Cs4B4O3F10: First Fluorooxoborate with [BF4] Involving
Heteroanionic Units and Extremely Low Melting Point
Ming Xia,[a, b] Miriding Mutailipu,*[a, b] Fuming Li,[a, b] Zhihua Yang,[a, b] and Shilie Pan*[a, b]
rates have been synthesized and characterized with novel
Abstract: Herein, a new congruently melting mixed-anion
compound Cs4B4O3F10 has been characterized as the first
structures and properties in a few short years,[2a,b,3–6] which
provides new fertile fields for expanding the solid state
fluorooxoborate with [BF4] involving heteroanionic units.
chemistry and functionality of borate system.[7,8] For example,
Compound Cs4B4O3F10 possesses two highly fluorinated
Li2B3O4F3 and BiB2O4F are the only fluorooxoborates with
anionic clusters and therefore its formula can be expressed
fluorinated anionic chains, that is, 1[B3O4F3]1 and 1[B6O10F2]1
as Cs3(B3O3F6)·Cs(BF4). The influence of [BF4] units on micro-
chains.[5d,e] AB4O6F (A=NH4, Na, Rb, Cs) and MB5O7F3 (M=Ca, Sr,
symmetry and structural evolution was discussed based on
Mg) series are the most promising candidates for deep-UV
the parent compound. More importantly, Cs4B4O3F10 shows
nonlinear optical applications.[4] More importantly, several
the lowest melting point among all the available borates
unprecedented heteroanionic units have been observed in
and thus sets a new record for such system. This work is of
fluorooxoborate system like [B4O6F4],[5a] [B5O9F3],[4e,f] [B5O10F],[5c]
great significance to enrich and tailor the structure of
[B7O13F2],[5b] and [B10O12F13] groups,[6a] which is found in no other
borates using perfluorinated [BF4] units.
related oxyfluorides.
However, the number of fluorooxoborates is extremely
limited when compared with pure borates (more than 3900)
and only less than 50 anhydrous cases belong to this species.[2a]
The introduction of multiple anions can facilitate the discovery
of novel compounds with extended solid state chemistry and
unique properties, which has led to the significant advances in
wide fields such as chemistry, energy, and materials, etc.[1]
Oxyfluoride is such a case, which can be regarded as the
derivative of partly substituting oxygen with fluorine.[1c] To an
extent, this process not only offers a new handle to regulate the
properties, but also provides novel chromophores to construct
the high-performing optical materials.[1]
Among them, the introduction of fluorine into borates to
construct fluorooxoborates is proved to be an effective
approach to extend the solid state chemistry and to push the
current limitations of borate-based optical materials.[2a,b] The
fluorinated anionic units in fluorooxoborates are beneficial for
broadening the energy bandgap, reducing the symmetry,
improving the hyperpolarizability, and enhancing the polar-
izability anisotropy owing to large electronegativity of fluorine
when compared with oxygen.[2,3] Therefore, many fluorooxobo-
As yet, the existence of perfluorinated [BF4] units in fluoroox-
oborates remains experimentally unknown, although this is very
common in tetrafluoroborates where their anionic frameworks
solely consist of boron-fluorine anions.[9a,b] It should be noted
that the perfluorinated [BF4] units can be seen as the derivatives
of [BO4] units where all the oxygen are completely replaced by
fluorine, which show largest energy bandgap in BÀ O/F modules
according to the theoretical analysis.[2a] In addition, unlike [BO4]
unit, the [BF4] unit can only be in isolated configuration and is
beneficial for reducing the dimension of anionic framework.
With this in mind, Cs4B4O3F10, the first fluorooxoborate with
[BF4] involving heteroanionic units has been synthesized, which
is the second case of possessing two types of isolated
fluorinated anionic clusters. Symmetry analysis confirms the
roles of perfluorinated [BF4] units on symmetry breaking based
on its parent compound, Cs3(B3O3F6). More importantly, the title
compound exhibits congruent melting behavior and therefore
millimeter scale single crystals can be easily grown. Besides, the
more detailed experimental and theoretical analyses have also
been studied to clarify the structure-property relationship.
Cs4B4O3F10 crystals can be grown by high-temperature
solution method in a closed or open system using the B2O3 flux
or under its stoichiometry (see detail in the following exper-
imental section). Polycrystalline sample of Cs4B4O3F10 was
obtained by solid-state reaction or by grinding the above as-
grown single crystals (Figure 1a). The single crystal and powder
XRD analyses confirmed the crystal structure and phase purity,
respectively (Figure 1a). Figure 1b presents the thermal gravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC), in which there are two endothermic peaks at about 293
[a] M. Xia, Prof. M. Mutailipu, F. Li, Prof. Z. Yang, Prof. S. Pan
CAS Key Laboratory of Functional Materials and Devices for Special
Environments
Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences
Xinjiang Key Laboratory of Electronic Information Materials and Devices
40-1 South Beijing Road, Urumqi 830011 (P.R. China)
E-mail: miriding@ms.xjb.ac.cn
slpan@ms.xjb.ac.cn
[b] M. Xia, Prof. M. Mutailipu, F. Li, Prof. Z. Yang, Prof. S. Pan
Center of Materials Science and Optoelectronics Engineering
University of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
°
and 322 C on the heating curve without any weight loss on the
TG curve. But there are no any detectable exothermic peaks on
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101321
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Figure 2. The melting points of available congruently melting borate. All the
compounds are listed in Table S7 in the Supporting Information.
Cs4B4O3F10 crystallizes into monoclinic crystal system with a
centrosymmetric space group P21/c (No. 14; Table S1, Support-
ing Information).[10] As portrayed in Figures 3a and 3b, the
structure of Cs4B4O3F10 features two alternately pseudo-layers
composed of [B3O3F6] and [BF4] clusters with the sequence of
À AA’BB’AA’À in (011) plane. These pseudo-layers are further
stacked along the [100] direction with the monovalent Cs
cation-based polyhedra located in the interlayer. The asymmet-
ric unit of Cs4B4O3F10 contains four Cs, four B, three O, and ten F
symmetrically independent atoms, respectively. (see Table S2,
Supporting Information). All the B atoms have only one
coordination model, four-coordinated tetrahedra with different
fluorinated types of [B(1,2,3)O2F2] and [B(4)F4] units (Figure 3a
and Table S2, Supporting Information). The BÀ O and BÀ F bond
lengths in [BO2F2] tetrahedra vary from 1.412(6) to 1.431(6) Å
and 1.431(5) to 1.461(6) Å, respectively, whereas the BÀ F bond
lengths in [B(4)F4] units change from 1.339(8) to 1.377(7) Å
(Tables S3 and S4, Supporting Information). These values agree
well with those of other reported borates and
fluorooxoborates.[2a,7,9] Every three [BO2F2] units polymerize into
a three-membered [B3O3F6] ring by sharing the bridged O(1,2,3)
atoms. Both [B3O3F6] and [BF4] units are in isolated arrangement
without further linking with the neighboring B atoms, and such
a configuration is extremely rare in fluorooxoborates system
and is only observed in isomorphic A10B13O15F19 (A=K, Rb)[6a]
with isolated [B10O12F13] and [B3O3F6] units. Here we present the
second example of possessing two different fluorinated BÀ O/F
anionic units. To our knowledge, Cs4B4O3F10 is the first
fluorooxoborate with perfluorinated [BF4] units and its formula
can be expressed as Cs3(B3O3F6)·Cs(BF4) according to its ionic
salt nature, and therefore Cs4B4O3F10 powder can also be
obtained by mixing Cs3(B3O3F6) and Cs(BF4). For the cationic
part, Cs(1) and Cs(2,3,4) are coordinated with O and F atoms to
form [Cs(1)F6O4], [Cs(2)F8O2], and [Cs(3,4)F9O] polyhedra (Ta-
ble S3, Supporting Information), respectively, which are further
interconnected with [B3O3F6] and [BF4] units to form the final
framework, shown in Figure 3b.
Figure 1. (a) Experimental and calculated XRD patterns of Cs4B4O3F10, insert
gives the photograph of as-grown single crystals. (b) TG-DSC curves of
Cs4B4O3F10.
the cooling curve because of the high viscosity of Cs4B4O3F10
compound. Analysis of the powder XRD pattern of the solidified
melt indicates that the entire residues exhibit a diffraction
pattern identical to that of the initial polycrystalline samples,
suggesting that Cs4B4O3F10 is a congruently melting compound.
Thus, the two endothermic peaks might refer to the phase
transition and melting point, respectively. Unfortunately, we
have not obtained this low temperature phase, even though
the quenching method was used. The reason may be that the
phase transition is reversible. Even so, bulk Cs4B4O3F10 single
crystals could, in principle, be grown under its stoichiometry
with Czochralski and Kyropoulos techniques. More importantly,
Cs4B4O3F10 shows the lowest melting point among all the
available borates and thus sets a new record for borate system
with congruently melting behavior. The melting points of all
the available congruently melting borate are listed and
summarized in Figure 2 based on the related literature data.
Obviously, compounds with congruently melting behavior are
extremely rare and their proportion is less than ~3% in borate
system. Such a low melting point might come from the smaller
formation energies of Cs4B4O3F10 (À 5.91 eV) when compared
with CsB4O6F (À 7.59 eV), which requires lower energy to break
the chemical bonds in related structures.
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Figure 3. (a) The geometric configuration of [BF4], [BO2F2], and [B3O3F6] units. (b) Crystal structure of Cs4B4O3F10 viewed in the (001) plane. (c and e) The anionic
framework of Cs3(B3O3F6) and Cs4B4O3F10. (d and f) The space group diagrams which show the positions of the symmetry elements of Pbcn and P21/c within a
single unit cell.
As was mentioned, the formula of Cs4B4O3F10 can also be
written as Cs3(B3O3F6)·Cs(BF4), thus, in this part, the influence of
[BF4] units on micro-symmetry and also the structural evolution
from Cs3(B3O3F6) to Cs3(B3O3F6)·Cs(BF4) were discussed. In
contrast to Cs4B4O3F10, Cs3B3O3F6 crystallizes into the orthorhom-
bic crystal system with the space group Pbcn (No. 61) and its
structure is only composed of [B3O3F6]-based pseudo-layers that
are separated by Cs+ cations.[6b] As shown in Figures 3d and 2f,
the space group diagram of P21/c indicates that Cs4B4O3F10 has
the micro-symmetry elements including 21 helical axis and slip
plane (SP) c, whereas Cs3B3O3F6 possesses three micro-symmetry
elements, namely the mutually perpendicular SPs, namely b, c
and n. In fact, the anionic part of Cs3B3O3F6 ([B3O3F6]) and
Cs4B4O3F10 ([B3O3F6] and [BF4]), is sufficient to present the
different micro-symmetric elements between them, thus their
cations are omitted here for clarity. These projections of micro-
symmetry elements were visually displayed in Figure S1 in the
Supporting Information except the SP n, since it is a 3D micro-
symmetric element here. Taking the anionic framework of
Cs3B3O3F6 as the prototype, the strong polar [BF4] units are
embedded into its lattice, resulting in the strong steric
hindrance of two adjacent [B3O3F6] rings. In order to relieve the
internal stress caused by [BF4] units, the [B3O3F6] rings undergo
a greater distortion, which can be verified by the increased
(Figure S1, Supporting Information). Similar to Cs3B3O3F6, the
[B3O3F6] ring remains similar coordination configuration and
thus a SP c micro-symmetry element has been reserved in
Cs4B4O3F10. Therefore, the perfluorinated [BF4] tetrahedra can
increase the distortion of [B3O3F6] rings because of the steric
hindrance, which leads to the reduction of local symmetry from
Pbcn (No. 61) in Cs3B3O3F6 to P21/c (No. 14) in Cs4B4O3F10. This
clearly indicates that the perfluorinated [BF4] units can be used
to tailor the structure of borates.
The UV-vis-NIR diffuse reflectance spectrum data (Figure S2,
Supporting Information) were collected based on the as-
prepared polycrystalline sample, which indicates that Cs4B4O3F10
crystal can be transparent into deep-UV spectral region below
200 nm. Such a short cutoff edge of Cs4B4O3F10 attributes to the
following two main factors from the viewpoint of micro-
structure: (1) All the compositions including Cs, B, O, and F in
Cs4B4O3F10 are free of d–d or f–f electronic transitions; (2) The
non-bonding states of O atoms in anionic framework are
effectively removed by the introduction of fluorine with the
largest electronegativity. The IR spectrum and the related
absorption peaks are presented in Figure S3 and Table S5 in the
Supporting Information, which further confirms the existence of
the BÀ F bonds in the structure of Cs4B4O3F10 with the formation
of [BF4] and [BO2F2] tetrahedra. In order to further explore the
band structures of Cs4B4O3F10, the theoretical calculations were
carried out based on DFT method, which shows that Cs4B4O3F10
is an indirect compound with the valence band maximum and
conduction band minimum located in the different points. The
calculated bandgaps using GGA and HSE06 methods are about
5.206 and 6.580 eV for Cs4B4O3F10, indicating that it can be
transparent into the deep-UV spectral region (Figure 4a).
Previous studies have shown that the electronic states near the
°
distorted dihedral angles from <B1B2B1À F4 > =98.73 in
Cs3B3O3F6 to <B2B1B2À F3 > =110.77° in Cs4B4O3F10. At the same
time, the number of crystallographic positions of B atoms in
[B3O3F6] rings increase from two to three, indicating that the
insert of [BF4] units into the lattice will reduce the local
symmetry of [B3O3F6] rings. Thus, the [BF4] units break the
original micro-symmetry operation of SP b in Cs3B3O3F6, then a
new micro-symmetry element 21 helical axis is generated
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tube was flame-sealed under 10À 3 Pa. The tube was heated to
°
380 C in 3 h, and held at this temperature for 24 h, and then
°
°
cooled to 30 C with a rate of 1.0 C /h. The single crystals of
Cs4B4O3F10 crystals can also be synthesized by a high-temperature
solution reaction with spontaneous nucleation technique in an
open system. A mixture of CsF, CsBF4, and H3BO3 was weighted
under its stoichiometry. The samples were mixed in an agate
mortar, and then moved to a platinum crucible, which were placed
in the furnace assembled by us. The samples were gradually heated
°
°
at the speed of 50 C/h from room temperature to 350 C and held
°
°
at this temperature for 24 h, then cooled to 100 C at a rate of 1 C/
°
h, after that, cooled to the room temperature at a rate of 5 C/h.
Figure 4. (a) Electronic band structure of Cs4B4O3F10 based on the results of
GGA method. (b) The total and partial density of states of Cs4B4O3F10, in
which EF refers to Fermi level with energy of 0 eV.
During the process of spontaneous crystallization, small single
crystals were formed in the platinum crucible. The polycrystalline
samples of Cs4B4O3F10 were synthesized via conventional solid-state
reactions in an open system. The stoichiometric compositions are
as follows: CsBF4 (6.9 mmol, 1.521 g), CsF (6.9 mmol, 1.051 g), and
H3BO3 (6.9 mmol, 0.428 g). The mixtures were ground thoroughly,
Fermi level play a dominant role on the optical properties. Thus,
we portrayed the density of states and it shows that the
uppermost part of valence bands from À 1.5 to 0 eV is
essentially dominated by O 2p and F 2p states and a tiny
contribution from Cs 6s orbitals can also be observed. Whereas
for the bottom of conduction band, it is mainly composed of Cs
6s, Cs 6p and B 2p orbitals (Figure 4b). And the orbitals of F
atoms are all far from the bottom of conduction band,
indicating that the F atom with large electronegativity is
beneficial for increasing the minimum of conduction band.
When taken together, we can conclude that the CsÀ O and BÀ O/
F interactions play a decisive role in determining the bandgap
of Cs4B4O3F10. The calculated energy bandgap of Cs4B4O3F10
(6.58 eV) is smaller than that of Cs3B3O3F6 (7.16 eV)[6b] using the
same HSE06 method, which can be explained by the stronger
ionic character of Cs components in Cs4B4O3F10 than that in
Cs3B3O3F6. The involved states of Cs at conduction band make
the minimum value shift to near the Fermi level, which will
further decrease the bandgap.
°
placed in platinum crucibles, and gradually heated to 250 C and
held at this temperature in air for 24 h.
Characterization: Powder XRD data were collected with a Bruker
D2 PHASER diffractometer (Cu Kα radiation with λ=1.5418 Å, 2θ=
°
°
10 to 60 , scan step width=0.02 , and counting time=1 s/step).
The single-crystal XRD data were collected on a Bruker D8 Venture
diffractometer using Mo Kα radiation (λ=0.71073 Å) at room
temperature. The intensity, reduction and cell refinements were
carried out on Bruker SAINT.[11] All the structures were solved by
direct method and refined through the full-matrix least-squares
fitting on F2 with OLEX2 software.[12] These structures were verified
by virtue of ADDSYM algorithm from PLATON.[13] Thermal gravimet-
ric analysis (TGA) and differential scanning calorimetry (DSC) were
carried out on a simultaneous NETZSCH STA 449 F3 thermal
analyzer instrument in a flowing N2 atmosphere, the sample was
placed in Pt crucible, heated from 40 to 400 C at a rate of 5 C
minÀ 1. Infrared spectroscopy was carried out on a Shimadzu IR
Affinity-1 Fourier transform infrared spectrometer in the 400–
4000 cmÀ 1 range. UV-vis-NIR diffuse-reflectance spectroscopy data
in the wavelength range of 200–2500 nm were recorded at room
temperature using a powder sample of Cs4B4O3F10 on a Shimadzu
SolidSpec-3700 DUV spectrophotometer.
°
°
In summary, Cs4B4O3F10, the first fluorooxoborate with [BF4]
involving heteroanionic units has been synthesized and charac-
terized. Structurally, Cs4B4O3F10 possesses two highly fluorinated
anionic clusters, that is, [B3O3F6] and [BF4] units, which make it
the second case with two types of isolated fluorinated anionic
clusters. The before and after melting XRD curves of powder
have demonstrated that the title compound is a concurrently
melting compound. The influence of [BF4] units on micro-
symmetry and structural evolution based on the parent
compound of Cs3B3O3F6 indicate that the perfluorinated [BF4]
units can be used to tailor the structure of borates, which is of
great significance to enrich structural chemistry of borates.
Calculation details: The electronic structure and the optical
property were calculated by using the DFT method implemented in
the CASTEP package.[14] During the calculation, the generalized
gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE)
functional was adopted.[15] Under the norm-conserving pseudopo-
tential (NCP), the following orbital electrons were treated as valence
electrons: B:2s22p1, O:2s22p4, F:2s22p4, and Cs:5s25p66s1. The kinetic
energy cutoff of 940 eV was chosen, and the numerical integration
of the Brillouin zone was performed using a 2×3×3 Monkhorst-
Pack k-point sampling. The other calculation parameters and
convergent criteria were the default values of the CASTEP code.
The formation energy can be expressed by the following calculation
formula:
X
Eform ¼ ðEcompound
À
NiEatomÞ=N
Experimental Section
Where Ni is the number of different atoms in the related
compound, Ecompound refers to the energy of corresponding com-
pound, Eatom is the energy of individual atoms, and N is the total
number of atoms in the compound.
Single crystal preparation and synthesis of title compounds:
Cs4B4O3F10 crystals can be grown by high-temperature solution
method in a closed or open system using the B2O3 flux or under its
stoichiometry. Polycrystalline sample of Cs4B4O3F10 was obtained by
solid-state reaction by grinding the above as grown single crystals.
A mixture of CsBF4 (0.201 g, 0.915 mmol), CsF (0.139 g, 0.915 mmol),
B2O3 (0.032 g, 0.459 mmol) and H3BO3 (0.028 g, 0.459 mmol) was
loaded into a tidy silica glass tube (Φ10 mm×100 mm) and the
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Communication
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Chem. Int. Ed. 2018, 57, 6577–6581; Angew. Chem. 2018, 130, 6687–
Acknowledgements
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We gratefully acknowledge the Xinjiang Key Laboratory of
Electronic Information Materials and Devices (2020D04013),
National Natural Science Foundation of China (52002397),
Xinjiang Tianshan Youth Program-Outstanding Young Science
and Technology Talents (2019Q026), the International Partner-
ship Program of CAS (1A1365KYSB20200008), CAS President’s
International Fellowship Initiative (2020DC0006), the Science
and Technology Service Network Initiative of CAS (KFJ-STS-
QYZD-130), and the Western Light Foundation of CAS
(Y92S191301).
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Conflict of Interest
The authors declare no conflict of interest.
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Keywords: Borates · congruently melting · fluorooxoborates ·
heteroanionic units · micro-symmetry
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Manuscript received: April 13, 2021
Accepted manuscript online: May 3, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–6
www.chemeurj.org
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COMMUNICATION
A new congruently melting borate
Cs4B4O3F10 has been synthesized as
the first fluorooxoborate with [BF4]
involving heteroanionic units, which
possesses two highly fluorinated
anionic clusters and therefore its
formula can be expressed as
Cs3(B3O3F6)·Cs(BF4). More importantly,
Cs4B4O3F10 shows the lowest melting
point among all the available borates
and thus sets a new record for such
system.
M. Xia, Prof. M. Mutailipu*, F. Li, Prof. Z.
Yang, Prof. S. Pan*
1 – 6
Cs4B4O3F10: First Fluorooxoborate
with [BF4] Involving Heteroanionic
Units and Extremely Low Melting
Point