Acentric polyborate, Li3[B8O12(OH) 3], with a new type of anionic layer and Li atoms in the cavities

Article abstract of DOI:10.1021/ic302230z

Single crystals of Li₃[B₈O₁₂(OH) ₃] were synthesized under hydrothermal conditions using a complex initial composition that included SiO₂. The crystal structure is acentric, space group P62c, with a = 8.9301(2) A and c = 15.9962(4) A, and Z = 4. New double three-membered rings of hexaborate blocks and monoborate triangles are connected into a polyborate anionic layer. Although the new structure is similar to double ring silicate Na₃Y[Si ₆O₁₅], detailed comparison of the anionic groups demonstrates principal differences between silicates and borates. Sodium silicate was proposed as an intrinsic fast ion conductor, and for the new borate Li ionic conductivity is very probable, as Li atoms in it are in positions similar to the Na atoms in the silicate.

Full text of DOI:10.1021/ic302230z

Article
pubs.acs.org/IC
Acentric Polyborate, Li3[B8O12(OH)3], with a New Type of Anionic
Layer and Li Atoms in the Cavities
Elena L. Belokoneva* and Olga V. Dimitrova
Department of Crystallography, Geological Faculty, Lomonosov Moscow State University, Leninskije Gory 1, Moscow, 119991
GSP1, Russia
ABSTRACT: Single crystals of Li3[B8O12(OH)3] were
synthesized under hydrothermal conditions using a complex
initial composition that included SiO2. The crystal structure is
acentric, space group P62c, with a = 8.9301(2) Å and c =
̅
15.9962(4) Å, and Z = 4. New double three-membered rings
of hexaborate blocks and monoborate triangles are connected
into a polyborate anionic layer. Although the new structure is
similar to double ring silicate Na3Y[Si6O15], detailed
comparison of the anionic groups demonstrates principal
differences between silicates and borates. Sodium silicate was proposed as an intrinsic fast ion conductor, and for the new borate
Li ionic conductivity is very probable, as Li atoms in it are in positions similar to the Na atoms in the silicate.
heavy atoms possessing lone pairs such as Pb and Bi. New Li-borate
INTRODUCTION
■
was crystallized at the weight ratio PbCO3:Bi2O3:LiIO3:B2O3:SiO2 =
1:1:2:3:1, which corresponds to 0.5 g (∼1.9 mmol) of PbCO3, 0.5 g
(∼1.1 mmol) of Bi2O3, 1.0 g (∼5.5 mmol) of LiIO3, 1.5 g (∼21.6
mmol) of B2O3, and 0.5 g (∼8.3 mmol) of SiO2. The ratio of solid and
liquid phases was 1:5. The synthesis was performed at 270 °C under a
70 atm pressure. The experiment was carried out in a standard
autoclave (volume 5−6 cm3) lined with Teflon. The temperature was
restricted by the kinetics of the hydrothermal reactions and the
instrumental capabilities, respectively. The duration of the experiments
(18−20 days) corresponded to the completion of the reaction. Final
cooling after synthesis to room temperature was done in 24 h. Grown
crystals were isolated by filtering the stock solution and washing with
hot water. All products of crystallization were colorless. The
complexity of the system led to five phases, determined by
morphology analysis, X-ray diffraction data on single crystals and
powders, composition determination, and nonlinear optical properties
measurements. The first three phases contained Bi/I or Pb/I atoms
(Leo 1420 VP + INCA350 analytical scanning electronic microscope)
and demonstrated different nonlinear optical properties. A recently
discovered phase, BiO(IO3), with a very high optical nonlinearity4 was
repeatedly synthesized in our experiment as a white polycrystalline
powder. The identity was confirmed by composition (Bi, I),
comparative strong SHG signal, and powder X-ray diffraction data.
Crystals of known centrosymmetric iodate Pb(IO3)2 and its new
acentric modification (the results will be presented elsewhere) were
identified by composition analysis, unit cell measurements on single
crystals, and SHG measurements.
Compounds containing Li atoms are known as functional
materials in electrochemistry (Li batteries). They are also used
as components of nonlinear optical devices: among Li-borates
LBO-LiB3O5 is used in lasers for frequency conversion.1 There
are other polar Li-borates, e.g., Li3B5O8(OH)2 or Li2B4O7,2 that
are promising materials for nonlinear optical, acoustic, and
thermo luminescence applications.3 These properties had
induced interest in searching for new acentric or polar
compounds in borates with Li atoms, which are relatively
rare. New borates may be obtained under different reaction
conditions: flux or hydrothermal. In the hydrothermal method
widely used for borate synthesis initial composition may
contain a boron component added with the only lithium-
containing component or a complicated multicomponent
mixture, which makes such compositions similar to the natural
ones.
Within the framework of a systematic search of new
promising borates at hydrothermal conditions, here we report
the synthesis of a new acentric Li-borate with a unique anionic
radical, its crystal structure investigation and crystal chemistry
analysis, determination of the position in a system, comparison
with Na,Y-silicates, and prediction of possible ion-conducting
properties.
Transparent or semitransparent crystals of the new phase were
found among crystallization products as a satellite phase. They were
regularly faced by trigonal prisms (110) and pinacoids (001). All small
crystals were practically of equal dimensions, with a horizontal edge
common for prisms and pinacoids of ∼0.18 mm and a prism height of
∼0.06 mm. Such an ideal form allowed us to separate the new phase,
but the amount of crystals and their dimensions were sufficient neither
EXPERIMENTAL DETAILS
■
Hydrothermal Synthesis. New borate single crystals were
obtained under hydrothermal conditions. As a rule in our synthesis
we model processes taking place in the geological environment. It is
known that in nature the presence of the SiO2 component is very
common. Therefore, a significant amount of SiO2 was included in the
batch in order to increase the viscosity of the water solution. Thus, the
borosilicate system was chosen for synthesis. The initial content was
complex because we tried to test the common crystallization of Li-
containing components together with the components containing
Received: October 12, 2012
Published: March 11, 2013
© 2013 American Chemical Society
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Article
for X-ray powder diffraction nor for measurements of any properties
including second-harmonic generation. Colorless massive aggregates
found in the products together with the prisms were crystals of
sassolite H3BO3, identified by unit cell measurements on single
crystals. The compositions of trigonal prismatic crystals as well as
sassolite blocks were confirmed by EDX carried out on a Jeol JSM-
6480LV scanning electronic microscope. The test revealed the
presence of any heavy elements, which meant both phases were
borates with the only light elements. Thus, in the crystallization
products we obtained the borate part without heavy metals (two
phases: new borate and sassolite) and a non-borate, namely, an iodate
part with lead and bismuth (three phases). The new borate was the
object of investigation.
to a hexagonal (trigonal) system and had no analogous compounds in
the ICSD database. Experimental data were collected at room
temperature using an Xcalibur S diffractometer equipped with a
CCD area detector using graphite-monochromated Mo Kα radiation.
Full spheres of data were collected in omega scan mode for the
compound with an exposure time of 1.5 s per frame and with control
frame measurements during the process of data collection to check the
stability of all measurements. The absorption of the new borate was
negligible: μ = 0.198 1/cm (calculated in Table 1). The data were
integrated using the CrysAlis program with the intensities corrected
for Lorentz and polarization factors. A possible space group suggested
by the CrysAlis system was P63mc: the extinctions of reflections hhl: l
= 2n were practically observed, but several 00l reflections did the
follow the rule l = 2n and the 63-axis. The Patterson function Puvw
calculated for the P3 space group revealed the horizontal mirror plane
mz, which was also contradictory with the P63mc space group. The
structure was solved by direct methods using SHELXS-975 under the
assumption that it is a new modification of sassolite, thus using in the
calculations the initial formula H3BO3 and the least symmetrical P3
space group. The direct method gave a good solution (in contrast to
P63mc) with ∼27 O and B atoms, identified by interatomic distances.
The list of atoms was added to final 20 O and 12 B atoms in a series of
Fourier syntheses.
Single-Crystal X-ray Diffraction. A transparent crystal in the
form of a regular trigonal prism with an edge size of 0.175 and a height
of 0.075 mm was glued on a glass fiber for single-crystal X-ray
determination study. The new borate unit cell (Table 1) corresponded
Table 1. Crystal Data and Structure Refinement for
Li3B8O12(OH)3
formula
fw (g/mol)
T (K)
Li3B8O12(OH)3.
347.3
293
B atoms had both triangle and tetrahedral coordination. The
analysis of the structure drawn in ATOMS6 revealed a higher
symmetry than P3: it was the mz plane, proposed before, the vertical c-
glade plane, and the resulting 2-fold axis on coordinate positions with
cryst syst
space group
a (Å)
hexagonal
P62c
̅
8.9301(2)
8.9301(2)
15.9962(5)
1104.73(5)
4
b (Å)
the final group P62c. The origin on the space group was shifted from
̅
c (Å)
V (Å3)
the first to the final models to satisfy the correct setting of the space
group with fewer independent atoms than before. The residual
electron density showed two additional electron density peaks
coordinated by O atoms with the distances corresponding to Li−O.
Initial components in our experiment, composition of crystals, and
height of the peaks allowed us to introduce Li atoms into the model in
special positions. Taking into account the multiplicity of the common
Z
3
ρcalcd (g/cm )
2.088
μ (mm−1
)
0.198
F(000)
676
θ range (deg)
reflns collected
indep reflns
2.63−32.59
22 081
and special positions in the space group P62c occupied by basal O, B,
̅
1343
and Li atoms, the charges in the resulting chemical formula were
balanced by introduction of one O6 atom to the (OH) group:
Li12B32O48(OH)12 or Li3B8O12(OH)3 with Z = 4. The analysis of
polyhedral connections and the Pauling valence sum on oxygens in the
structure confirmed this assumption. The final structural model,
corresponding to the given formula, was refined using the least-squares
procedure in the anisotropic approximation of thermal displacement
for all atoms and by refinement of the weighting scheme (SHELXL-
97).7 It was found (Flack test) that the crystal is a racemic twin (−1 0
0/0 −1 0/0 0 −1/), which was introduced into the refinement.
Crystallographic data, atomic coordinates, and selected bonds are
presented in Tables 1, 2, and 3.
R(int)
0.1094
completeness to θ = 32.59 (%)
data/restraints/params
GOF
97.7
1383/0/86
1.115
a
R1, wR2 [I >2σ(I)]
0.0515, 0.1070
0.0634, 0.1119
a
R1, wR2 (all data)
Δρmax and Δρmin (e·Å−3
)
0.419 and −0.363
a
R(F) = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo − Fc2)2/
2
2
2
2
∑w(Fo )2]1/2 for Fo > 2σ(Fo ).
a
4
Table 2. Atomic Coordinates (×10 ) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for Li3B8O12(OH)3
atom
O(1)
Wyckoff position and point symmetry
x
y
z
Ueq
6h, m
12i, 1
12i, 1
6h, m
12i, 1
12i, 1
12i, 1
2b, −6
6h, m
12i, 1
6h, m
6g, 2
0553(2)
03200(19)
82668(18)
0562(3)
19385(17)
7808(2)
1564(3)
0
6144(3)
60817(18)
6388(2)
8827(3)
49711(17)
5873(2)
6349(3)
0
1/4
7.7(4)
11.1(3)
12.6(3)
13.1(4)
8.5(3)
O(2)
O(3)
O(4)
O(5)
O(6)−OH
B(1)
40329(8)
17839(8)
1/4
33888(8)
46509(9)
33422(13)
1/4
24.3(4)
8.1(4)
B(2)
9.9(9)
B(3)
9415 (4)
8809(3)
3081(7)
5704(11)
6946(4)
6118(3)
9686(7)
5704(11)
1/4
9.1(5)
B(4)
39562(14)
1/4
11.4(4)
17.3(11)
Li(1)
Li(2)
1/2
67(3)
a
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.
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3-fold axis into double three-membered rings via three pairs of
tetrahedral apexes due to loss of one triangle in every
hexaborate block (Figure 1b). This complex ring was never
found in borates before. It is described as 3×{6:[3T+3Δ]−1Δ}
= 15:[9T+6Δ] for a new nonperiodical unit: ring
[B15O24(OH)6]9− (triangle apexes in a new hexaborate block
combination are presented by OH groups). An additional
independent B-triangle (monoborate unit) connects the
complex hexaborate rings into a layer with a new configuration,
and the borate is classified as a polyborate, because two
different types of initial blocks are presented here: hexaborate
and monoborate (Figures 1b, 2a). The B atom in the
Table 3. Selected Interatomic Distances for Li3B8O12(OH)3
atoms
bonds (Å)
atoms
B(3) tetrahedron
1.429(3) B(3)−
bonds (Å)
atoms
bonds (Å)
B(1) tetrahedron
Li(1) tetrahedron
B(1)−
1.449(2) Li(1)−
1.981(6)
O(5)
O(3)×2
O(4)
B(1)−
1.435(2) B(3)−
1.466(4) Li(1)−
2.016(4)
2.108(5)
2.355(6)
O(5′)
O(4)
O(5)×2
B(1)−
1.499(2) B(3)−
1.508(4) Li(1)−
O(3)×2
O(2)
O(1)
B(1)−
1.580(2) B(4) triangle
Li(1)−
O(1)
O(1)
B(4)−
1.346(3) Li(2) tetrahedron
1.371(3) Li(2)− 1.892(6)
O(3)
B(2) triangle
B(4)−
O(2)
O(6)×2
2.136(8)
B(2)−
1.369(2) B(4)−
1.373(2) Li(2)−
O(2)×2
O(4)×3
O(6)
RESULTS AND DISCUSSION
Structure. There are four symmetrically independent B
atoms in the structure (space group P62c); two of them are in
■
̅
triangle coordination and other two are tetrahedral with typical
bond lengths. Boron polyhedra are condensed into a polyborate
anionic radical, composed of a new hexaborate block complex
unit, joined into a new type of layer by additional B-triangles.
The term “hexaborate” was introduced by Christ and Clark,8 as
well as other terms: “penta”-, “tetra”-, “tri”-, “di”-, “mono”-
borates depending of the type of fundamental building blocks
formed by B-polyhedra. A hexaborate block contains six boron
polyhedra and is built of three BO4 tetrahedra (T) connected
through one central oxygen atom and three BO3 triangles (Δ)
attached to three terminal O atoms, with a notation after Christ
and Clark of 6:[3Δ+3T]. The bridge atoms are O2− and the
apical groups are (OH)−. The hexaborate block
[B6O7(OH)6]4+ with the symmetry 3m (Figure 1a) is present
Figure 2. Crystal structure of Li3B8O12(OH)3 in two projections: (a)
ab-projection with one layer (complex hexaborate ring is shown by the
circle); (b) side projection with two layers (Li atoms and Li−O bonds
are given in ball-and-stick presentation).
Figure 1. Structural blocks: isolated hexaborate block 6:[3T+3Δ] =
[B6O7(OH)6]2− (a); complex double hexaborate ring 3×{6[3T+3Δ]−
1Δ} = [B15O24(OH)6]9− of B(1), B(2), and B(4) polyhedra with
additional B(3) triangle for condensation into the layer (b); double
“dreier” ring 6:[6T] = [Si6O15]6− in Na,Y-silicate (c).
monoborate triangle unit is coordinated by apical oxygens of
new hexaborate rings on special position 2b, 0 0 1/4 with the
symmetry −6, and it is simply added to the radical. The crystal
chemical formula of the new layer is [B16O24(OH)6]6−∞ ∞ with
in the mineral macallisterite, Mg2[B6O7(OH)6]·9H2O, and its
varieties are condensed into a ribbon in the mineral aristarainite
and into layers in many natural and synthetic compounds.9
Another type of strongly corrugated hexaborate layer in
Lu(BO2)3 has been obtained at high pressure and high
temperature.10 This layer is built of hexaborate blocks that
contain only tetrahedrally coordinated boron atoms (which is
typical at high pressure; see publications of Huppertz’s group)
with the same topology as that for six polyhedra. Framework
structures for hexaborates may be considered as well. They are
built of typical layers via their condensation with the known or
predicted varieties; they are built of blocks condensed directly
by an R-lattice via vertices.11 In the new Li-borate structure
three hexaborate blocks in a vertical orientation are linked by a
the notation {3×{6:[3T+3Δ]−1Δ}+1Δ}∞ ∞.
The borate structure Li3B8O12(OH)3 has a remarkable
similarity to the silicate Na3YSi6O15 despite the difference in
the chemical formula.12 Both compounds share many structural
features. The silicate structure is orthorhombic, but is close to
pseudohexagonal. A unique double “dreier” ring [Si6O15]6−
built only of tetrahedra is discovered in the silicate (Figure 1c).
The new hexaborate ring is an “extension” of the silicate ring
because it contains as a core the double “dreier” ring,
topologically equal to that the silicate. Every vertical pair of
tetrahedra in the borate ring adds a third tetrahedron (to the
central bridge oxygen) and two triangles (to the apexes)
(Figure 1b). The new, enlarged ring with the symmetry point
group 3 has the formula [B15O24(OH)6]9−, in contrast to
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[Si6O15]6−. The topology of the new borate ring is not possible
in the silicate because the charge of the central atom Si4+ in the
tetrahedra is too high, and only two tetrahedra may condense
on one bridge oxygen. In contrast to that up to four tetrahedra
with B3+ in the center may condense on one bridge oxygen in
borates (cubic boracite’s structure).11 In the silicate, the rings
are on two levels in the unit cell along the axis b = 15.25 Å, and
in the new borate the layers with the rings are also on two levels
along axis c = 15.99 Å (Figure 2b).
effectiveness in inducing crystallization in glass-forming
systems, and that led to a double “dreier” ring, found only in
this silicate. We suppose that for synthesis of a new borate
relative to a silicate with the unique ring, it is important to
introduce the silicon component and to increase the viscosity of
the water solution.
Crystallographic data for the structure reported in this paper
have been deposited with the Karlsruhe Inorganic Crystallo-
graphic Structural Data Centre as supplementary publication
CSD number 425207 for Li3B8O12(OH)3. Copies of the data
can be obtained free of charge on application to ICSD FIZ
Karlsruhe−Leibnitz Institut fuer Informationsinfrastructur,
Termann-von-Helmholtz Platz 1, 76344 Eggenstein, Leopold-
shafen, CrysDATA@fiz-karlsruhe.de, http://www.fiz-karlsruhe.
de.
Lithium Coordination and Possible Ionic Conductiv-
ity. Li atoms compensate the effective charge of the borate
anionic layer in Li3B8O12(OH)3 and occupy large cavities: cages
inside the layer (Li(1) special position on mirror plane m) and
interlayer space with only Li atoms (Li(2) special position on
the 2-fold axis) (Figure 2b). Li atoms in the first positions have
distorted octahedral coordination with one longer distance
(half-octahedra); the second position has regular tetrahedral
coordination. Li−O distances correspond to the typical ones
(Table 3) found recently in other compounds for four-, five-,
and six-coordinate environments. Thermal vibration parameters
for Li atoms are significantly enlarged especially for Li(2)
(Table 2) and mostly along the c-axis. Nevertheless the
positions are not split. The significant amount of Li atoms in
the structure and their positions in the cavities allow the
proposal that new crystals may possess ionic conductivity,
observed in a number of alkali silicates and phosphates. This
assumption found confirmation by structural comparison
because for silicate Na3YSi6O15 possible conductivity was
discussed and its pathway was predicted. In the silicate, Na
atoms occupy positions between levels with the rings together
with Y-octahedra, and in cages inside levels with rings, exactly
like Li atoms in the new borate. No splitting of Li(1) positions
is found in the borate, in contrast to an equal Na(2) position in
the silicate, but this does not exclude possible Li-ion
transportation, despite that the interlayer space in the borate
contains only Li atoms. As for Na atoms in the silicate, thermal
vibration parameters for both Li atoms are enlarged mainly in
the c-axis direction especially for Li(2) in the interlayer space
(Table 2). A possible pathway could be along the channel in
the c-axis direction, as in silicate, with the greatest contribution
from Li(2) atoms.
AUTHOR INFORMATION
Corresponding Author
*E-mail: elbel@geol.msu.ru. Tel: +7(495) 9394926.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Russian Foundation for Basic Research (RFBR),
funded by Ministry of Science & Education, Russia (Grant 11-
03-00544a), for support. The authors are grateful to Natalie
Zubkova for her aid in collection of experimental diffraction
data, to Elizaveta Koporulina and Vasily Japaskurt for
determination of compositions, to Sergey Stefanovich for the
SHG measurements and consulting, and to Pavel Plachinda for
his help in the production of this paper.
REFERENCES
■
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CONCLUSION
■
Single crystals of the new borate Li3B8O12(OH)3 were
synthesized in a complex borosilicate system. A layered anionic
radical of new type is characterized as a polyborate. It is built
from a new complex hexaborate ring with the notation
3×{6[3T+3Δ]−1Δ} added to a monoborate unit, a boron
triangle that is constructed from the apical oxygens of the
hexaborate rings. The notation for the layer with the crystal
chemical formula [B16O24(OH)6]6−
is {(3×{6[3T+3Δ]−
∞∞
1Δ)+1Δ}∞∞. The new structure is related to the double ring
silicate Na3YSi6O15; however comparison of the anionic groups
demonstrates a principal difference between the silicates and
borates depending on the charge of the atoms in the center of
the tetrahedra. Sodium silicate was proposed as an intrinsic fast
ion conductor, and the borate crystals should also possess ionic
conductivity because Li atoms occupy similar positions in the
borate structure to Na atoms in the silicate. This may be tested
on larger crystals obtained by hydrothermal synthesis without
heavy metal components under similar conditions of pressure
and temperature. It is worthwhile to say that the silicate crystals
were also grown by the hydrothermal method because of its
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