Pb2B5O9I: An Iodide Borate with Strong Second Harmonic Generation
Yi-Zhi Huang,† Li-Ming Wu,† Xin-Tao Wu,† Long-Hua Li,† Ling Chen,*,† and Yong-Fan Zhang*,‡
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China, and Department of Chemistry,
Fuzhou UniVersity, Fuzhou, Fujian 350002, P. R. China
Received July 8, 2010; E-mail: chenl@fjirsm.ac.cn
Abstract: The combination of lone-pair effects on Pb2+ cations
and the smaller electronegativity of I- anions into the pentaborate
framework generates a phase-matchable material, Pb2B5O9I, with
the largest powder SHG response among borates, about 13.5
times that of KDP (KH2PO4), and transparency over the near-UV
to middle-IR region. DFT calculations on electronic structure and
cutoff-energy-dependent SHG coefficients confirm these origins.
Borates have long been a remarkable source of nonlinear optical
(NLO) materials,1 and some examples, such as �-BaB2O4 (BBO) and
LiB3O5 (LBO), have been commercially manufactured and used
worldwide. The asymmetric electronic distributions on the distorted
planar anions of [B3O6]3- and [B3O7]5- are responsible for the large
second harmonic generation (SHG) in these, respectively. The design
and construction of new inorganic NLO materials often utilize MOn
polyhedra as effective noncentrosymmetric (NCS) building units. These
may contain second-order Jahn-Teller (SOJT) distorted cations such
as d0 transition metal ions,2,3 p-cations with stereochemically active
lone pairs (quoted as lone-pair effects subsequently),4-7 or d10 cations
with large polar displacement in a few cases.8,9 The combination of
diverse functional building units can produce materials with high NLO
performance; for example, powdered Cd4BiO(BO3)3 shows an SHG
response about six times that of KH2PO4 (6× KDP), representing the
largest powder NLO coefficient among borates to date.8 In such a
compound, the combinations of the polar displacement of d10 Cd2+ ion,
lone-pair effects on Bi3+, and π-delocalization of BO3 are believed to be
the origin of the strong SHG activity. In this communication, we report
the discovery of Pb2B5O9I, a phase-matchable iodide borate with an SHG
intensity measured on ground crystals approximately 13.5 times that of
KDP and about twice as large as that of Cd4BiO(BO3)3.8
Although lighter halide pentaborates M2B5O9X (M ) Ca, Sr, Ba,
Pb, Eu; X ) Cl, Br) are known, the iodide, Pb2B5O9I, has not been
obtained because of synthetic difficulties.10-12 The sharp increase of
the SHG activity along isostructural Ca < Sr < Ba < Pb for M and Cl
< Br for X suggested that the key SHG factors come from not only
the lone pairs on Pb2+ but also the bonding of the halogen anions.
Consequently, the unknown iodide pentaborate, Pb2B5O9I, would be
expected to exhibit the strongest SHG activity among M2B5O9X. In
contrast, the theoretical calculations of SHG coefficients utilizing
Phillips-Van Vechten-Levine-Xue bond theory indicated that the
planar BO3 triangle is the sole functional group affecting the NLO
properties of the chloride and bromide pentaborates.10 Here, we report
the synthesis of Pb2B5O9I and density functional theory (DFT)
calculations that indicate that its remarkably large SHG intensity arises
from all three components, the I- anion, the lone 6s2 pair on Pb2+,
and the borate groups.
The prismatic colorless or pale yellow Pb2B5O9I crystals (Figure
S1) together with a second phase, brown lamellar PbB4O7 (ICSD
203201) were synthesized in a solid-state reaction of PbI2/PbO/
B2O3 in an evacuated silica tube (Supporting Information). The
purity of the handpicked Pb2B5O9I crystals was confirmed by the
XRD pattern (Figure S2a). The isostructural Pb2B5O9I also crystal-
lizes in space group Pnn2 (No. 34).
The three-dimensional network was built from [B5O93-]n and
[Pb2O9I15-]n substructures shown in Figure 1. The primary building
units in the [B5O93-]n substructure (Figure 1a) are BO4 tetrahedra and
BO3 triangles connected in a double six-member ring motif (Figure
S3a). Here the BO4 tetrahedra form chains along the c axis via vertex
sharing, which are further linked along both a (via B2O3) and b axes
(via B1O3) so as to define large channels along the c axis. The centers
of such channels are occupied by I1 or I2 anions. The [Pb2O9I15-]n
substructure is made of knitted chains of PbO7I2 monocapped
hexagonal bipyramids via sharing I apexes along a and b axes,
respectively (Figures 1b, S3b-c). The Pb1- and Pb2-polyhedron strings
are condensed via shared I1-O8-O3 and I2-O4-O5 faces and
I1-O7 edges (Figure S4). Compared with the earlier Pb2B5O9X (X
) Cl, Br), the PbO7I2 polyhedra exhibit significantly greater distortions,
such as the angle reduction for I-Pb-I (137° and 152°) with respect
to Cl-Pb-Cl (157° and 163°) and Br-Pb-Br (150° and 160°) as
well as the larger Pb1-O distance ranges (2.50-3.28 Å vs 2.50-2.92
Å for Cl and 2.48-3.02 Å for Br, respectively). The structure
parameters indicate an enhancement of the SOJT distortion in PbO7I2
† Chinese Academy of Sciences.
‡ Fuzhou University.
Figure 1. (a) Crystal structure of Pb2B5O9I viewed down c axis with Pb-O
and Pb-I bonds omitted for clarity. Gray: BO4 tetrahedra; black: BO3
triangles. (b) [Pb2O9I15-]n substructure along c axis. Gray and black: PbO7I2
monocapped hexagonal bipyramids.
Table 1. Optical Properties for Pb2B5O9X (X ) I, Br, Cl)
experimental calculated
X SHGintensitya
transparent
region (µm)
band gap
energy (eV)b
SHG coefficients
(pm/V)c
I 13.5, PM 0.40-6.96 3.33/3.36 16.6/9.4/1.8
Br 4.7, PM 0.38-6.86 3.54/3.54 7.4/2.6/-1.2
Cl 0.7, PM 0.31-6.80 3.72/3.69 4.5/1.0/-1.8
a Relative to KDP (150-210 µm) with λincident ) 1064 nm. PM )
phase-matchable (Figure S5). b Direct/indirect gaps. c Static d15/d24/d33
(d15 ) d31, d24 ) d32) according to the length-gauge formalism.13,14
10.1021/ja106066k XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A
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polyhedra, which arises from the larger electronegativity difference
between the coordinated I- and O2- anions.
The measured and calculated optical parameters for Pb2B5O9X
(X ) I, Br, Cl) are summarized in Table 1 (more details in the
Supporting Information). Powdered Pb2B5O9I shows an SHG
activity of approximately 13.5 × KDP, much greater than those of
the two lighter analogues (Figure S5), a relative smaller band gap
of about 3.1 eV (Figure S6a), and a similar infrared absorption
edge of about 7.0 µm (Figure S6b). From Cl to I, the increasing
SHG and the decreasing band gap are consistent with the calculated
results. An additional calculation also indicates that the strong SHG
intensity of Pb2B5O9I does not originate from just a reduction of
the band gap. If Pb2B5O9I had a band gap of 3.7 eV, i.e., the
calculated value for Pb2B5O9Cl, the calculated d15 would decrease
only to 11.1 pm/V, still significantly greater than the 4.5 pm/V for
Pb2B5O9Cl. Subsequently, the detailed electronic structures around
the Fermi level (EF) were also studied.
The densities of states (DOS) of Pb2B5O9I are shown in Figure
2a with assigned numbers to mark different regions in the valence
bands (VB) and conduction bands (CB) for the sake of clarity.
Below EF, the B-O bonding states are dispersed from VB-5 to
VB-2, and Pb 6s states mix with I 5p as well as O 2p over VB-4
to VB-1. Above EF, the prominent characters for each region are
as follows: CB-1, Pb 6p states (with small contribution of I 5p);
CB-2, B-O π-antibonding states on BO3; CB-3, I 5d states; and
CB-4, B-O σ-antibonding states from both BO3 triangles and BO4
tetrahedra (with small I 5d contribution). The lone pairs on Pb2+
are characterized mainly by Pb 6s-O 2p bonding interactions at
VB-4 and (Pb 6s-O 2p)-I 5p antibonding-antibonding interactions
at VB-1. The high-energy VB-1 region dominated by I 5p states is
responsible for the lone-pair effects on Pb2+, which is visualized
by the asymmetric electron distribution around the cations as shown
in Figure 2b (versus the symmetric density at VB-4, Figure S7).
This lone-pair effect is different from that for MOx polyhedra (M
) p metals) in which M ns usually mixes with O 2p and M np.15-18
Obviously, the filled I 5p in Pb2B5O9I better matches the Pb 6s
energy than does the empty Pb 6p. Furthermore, relative to
Pb2B5O9Cl and Pb2B5O9Br (Figure S8), the I 5p in Pb2B5O9I
contributes more significantly to the lone-pair effects on Pb2+ at
VB-1 owing to the smaller electronegativity of I, and I 5d states
disperse much more around the empty B-O states, in both CB-3
and CB-4, because of their strong penetration effect. Note that, from
I to Cl, the B-O states in the electronic structures of Pb2B5O9X
do not show obvious changes.
The local structure contributions of Pb2B5O9I to the overall SHG
efficiency have been estimated by the cutoff-energy-dependent SHG
coefficient according to the length-gauge formalism.13,14 As clearly
shown in Figure 2c, the states at VB-1, CB-3, and CB-4 make the
most significant contributions to the SHG coefficient. Similar
calculations for Pb2B5O9Br and Pb2B5O9Cl (Figure S9) show that
these regions contribute much less to the overall SHG efficiencies.
Since the dominant characters of VB-1, CB-3, and CB-4 regions
are filled I 5p, empty I 5d, and B-O states, respectively, one may
speculate that the electronic transitions from I 5p to I 5d and to
B-O states are reflected in the high SHG efficiency. However, the
following evidence excludes such a speculation. A hypothetical
“Ba2B5O9I” without lone pairs has been built in the same crystal-
lographic structure of Pb2B5O9I. The parallel calculations on
“Ba2B5O9I” generate a very small coefficient, 3.2 pm/V, which is
even smaller than 4.5 pm/V for Pb2B5O9Cl. Obviously, the main
contributor in VB states should come from the lone-pair effect on
Pb2+ at VB-1 instead of I 5p alone. Consequently, the sharp SHG
increase for Pb2B5O9I in comparison with Pb2B5O9Br and
Pb2B5O9Cl originates mainly with the increased cooperation of I-,
Pb2+ lone pairs and B-O groups.
In summary, a new phase-matchable compound Pb2B5O9I with
the largest powder SHG coefficient among borates has been
synthesized and characterized. Theoretical analyses reveal that the
cooperation of I-, lone pairs on Pb2+, and the BO3 and BO4 groups
are responsible for the remarkable SHG response. The growth of
large crystals for further physical property studies is ongoing.
Acknowledgment. This research was supported by the National
Natural Science Foundation of China under Projects (90922021,
90922022, 20773130, 20733003, 20821061, 20973175, 20773024),
the “Knowledge Innovation Program of the Chinese Academy of
Sciences” (KJCX2-YW-H20), and New Century Excellent Talents
at Universities of Fujian Province (HX2006-97).
Supporting Information Available: The cif data, experimental and
theoretical methods, and additional tables and figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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JA106066K
Figure 2. (a) Densities of states of Pb2B5O9I. (b) Asymmetric electron
distribution (>0.06 eV/Å3) around Pb2+ at VB-1 region. (c) The cutoff-
energy-dependent static SHG coefficients for Pb2B5O9I, dashed line: EF;
dotted line: different regions in VB and CB.
B J. AM. CHEM. SOC. 9 VOL. xxx, NO. xx, XXXX
C O M M U N I C A T I O N S
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