RbIO3 and RbIO2F2: Two Promising Nonlinear Optical Materials in Mid-IR Region and Influence of Partially Replacing Oxygen with Fluorine for Improving Laser Damage Threshold

Article abstract of DOI:10.1021/acs.chemmater.5b04511

Laser damage threshold (LDT) has been considered as a key index, apart from the nonlinear optical (NLO) effect, to characterize the performance of a mid-infrared NLO material. This paper reports and compares the properties of RbIO₃ and RbIO₂F₂ as potential nonlinear optical (NLO) materials to be used in the mid-IR region. RbIO₃ is a known compound, and its powder SHG (second harmonic generation) effect is carefully measured in this work for the first time to be as strong as 20 times that of KDP, and its powders show a high damage threshold (LDT) of 125 MW/cm². In order to investigate the influence on the properties by partially replacing oxygen with fluorine atoms, the new compound RbIO₂F₂ is synthesized and characterized. Although it also shows relatively strong powder SHG responses 4 times that of KDP, the LDT value of the powders is improved to 156 MW/cm². The transparent region and the thermal stability of two compounds are also measured with satisfactory results. The electronic structure and the properties of the materials are also investigated by the theoretical approach. All these indicate that RbIO₃ and RbIO₂F₂ are both promising candidates for NLO materials to be used in the mid-IR region and that partially replacing oxygen with fluorine in the molecule can improve the laser damage threshold of the material.

Full text of DOI:10.1021/acs.chemmater.5b04511

Article
pubs.acs.org/cm
RbIO3 and RbIO2F2: Two Promising Nonlinear Optical Materials in
Mid-IR Region and Influence of Partially Replacing Oxygen with
Fluorine for Improving Laser Damage Threshold
Qi Wu,†,‡ Hongming Liu,† Fangchao Jiang,† Lei Kang,§ Lei Yang,§ Zheshuai Lin,*,§ Zhanggui Hu,§
Xingguo Chen,† Xianggao Meng,⊥ and Jingui Qin*,†
†Department of Chemistry, Wuhan University, Wuhan 430072, China
‡Department of Chemistry, Hubei Normal University, Huangshi, Hubei 435002, China
§Beijing Center for Crystal R&D, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
⊥College of Chemistry, Central China Normal University, Wuhan 430079, China
S
* Supporting Information
ABSTRACT: Laser damage threshold (LDT) has been considered as a key
index, apart from the nonlinear optical (NLO) effect, to characterize the
performance of a mid-infrared NLO material. This paper reports and
compares the properties of RbIO3 and RbIO2F2 as potential nonlinear optical
(NLO) materials to be used in the mid-IR region. RbIO3 is a known
compound, and its powder SHG (second harmonic generation) effect is
carefully measured in this work for the first time to be as strong as 20 times
that of KDP, and its powders show a high damage threshold (LDT) of 125
MW/cm2. In order to investigate the influence on the properties by partially
replacing oxygen with fluorine atoms, the new compound RbIO2F2 is
synthesized and characterized. Although it also shows relatively strong powder
SHG responses 4 times that of KDP, the LDT value of the powders is
improved to 156 MW/cm2. The transparent region and the thermal stability of
two compounds are also measured with satisfactory results. The electronic structure and the properties of the materials are also
investigated by the theoretical approach. All these indicate that RbIO3 and RbIO2F2 are both promising candidates for NLO
materials to be used in the mid-IR region and that partially replacing oxygen with fluorine in the molecule can improve the laser
damage threshold of the material.
matchable.” However, its specific SHG intensity or LDT value
has not been reported yet.34 Therefore, we synthesize this
compound and systematically measure its SHG (second
harmonic generation) effect, LDT, and thermal properties.
Moreover, we are especially interested in exploring the Rb−I−
O−F system by partially replacing O2− of RbIO3 with F− owing
to the fact that the strongest electronegativity of F− is helpful to
expand the band gap so as to lead to higher LDT. A compound
in the Rb−I−O−F system as NLO material has not been
investigated. As a result, a new compound RbIO2F2 is
synthesized by hydrothermal reaction. Indeed, this compound
exhibits wider band gap and higher LDT compared with the
parent compound RbIO3 and shows good mid-IR NLO
performance. This paper reports the synthesis, crystal structure,
optical properties, and the theoretical study of these two
compounds. RbIO3 and RbIO2F2 have shown very large SHG
effects (20 and 4 times KDP, respectively), high LDT (125 and
INTRODUCTION
■
For decades, great efforts have been made to explore new
nonlinear optical (NLO) materials due to their important
applications for laser frequency conversion, optical parameter
oscillators(OPOs), signal communication, and so forth,1
especially in the mid-IR atmospheric window with wavelengths
of 3−5 μm. However, the application of currently commer-
cialized NLO crystals in the mid-IR region such as AgGaS2,2
AgGaSe2,3 and ZnGeP24 have been heavily limited for their low
laser damage threshold(LDT). To search for new mid-IR NLO
materials with high LDT and good comprehensive properties
has become one of the greatest challenges in this field.5−16
Many metallic iodates compounds have been studied for
NLO properties in the IR region for quite a long time.17−29 α-
LiIO3 and KIO3 have been reported to be nonlinear optical
materials in the past.30,31 RbIO3 is a known compound whose
crystal structure was reported by Alcock with a non-
centrosymmetric structure.32 In the literature,33 it was
mentioned that “Powder measurements on a number of iodates
such as KIO3, LiIO3, TlIO3, and RbIO3 have shown that these
materials also have large nonlinear coefficients and are phase
Received: November 23, 2015
Revised: February 1, 2016
© XXXX American Chemical Society
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156 MW/cm2), which is 24 and 30 times that of AgGaS2 (5.2
MW/cm2) in the same test conditions and wide transparent
wavelength regions (up to 13 and 12 μm), with a relatively high
thermal stability (550 and 400 °C). The results indicate that
RbIO3 and RbIO2F2 are both new candidates for NLO
materials in the IR region. More importantly, the compound
RbIO2F2 obtained by introducing F− into RbIO3 may open a
way for exploring new NLO materials with higher LDT and
good comprehensive properties in A−I−O−F systems.
Table 1. Crystallographic Data for RbIO3 and RbIO2F2
formula
RbIO3
260.37
RbIO2F2
282.37
FW
T (K)
296(2)
0.71073
colorless
R3m
298(2)
0.71073
colorless
Pca21
8.567(4)
6.151(3)
8.652(4)
90
wavelength (Å)
crystal color
space group
a (Å)
6.413(2)
6.413(2)
7.854(2)
90
b (Å)
c (Å)
EXPERIMENTAL SECTION
■
α (deg)
β (deg)
γ (deg)
Reagents. All starting materials were commercially available from
Sinopharm with analytical grade and used without further treatment.
Synthesis of RbIO3. RbIO3 was synthesized by hydrothermal
reactions in this work. The mixture of Rb2CO3 (1 mmol), HIO3 (2
mmol), and H2O(3 mL) in an autoclave equipped with a Teflon linear
(23 mL) was heated at 230 °C for 83 h, followed by slow cooling to
room temperature at a rate of 1.5 °C/h. The reaction product was
washed with cold water and dried in air. A colorless block crystal of
RbIO3 was obtained as single phases (0.4820 g, yield is 93%) (Figure
S1).
90
90
120.00
279.73(14)
3
90
volume (Å3)
455.9(4)
4
Z
dcacl (g cm−3
)
4.637
4.114
496
F(000)
342
reflections collected
independent reflections
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
min/max Δρ/eÅ−3
748
4416
226
1350
0.0730/0.1923
0.0730/0.1923
−5.632/5.059
0.0215/0.0492
0.0229/0.0500
−0.823/0.786
Synthesis of RbIO2F2. RbIO2F2 was synthesized by hydrothermal
reaction of a mixture of RbIO3(3 mmol) and hydrofluoric acid solution
(3 mL) in an autoclave equipped with a Teflon linear (23 mL). The
mixture was heated at 230 °C for 24 h and then slowly cooled to room
temperature at a rate of 3 °C/h. After filtration, the filtrate was slowly
volatilized at room temperature. A few days later, large colorless
massive crystals were harvested with the different size up to 10 × 10 ×
3 mm3 (0.6800 g, yield is 80%) (Figure S2).
Single Crystal Structure Determination. The crystal data
collection were performed at 298 (K) using a Bruker SMART
APEX diffractometer equipped with a CCD detector using graphite-
monochromated Mo Kα (λ = 0.71073 Å) radiation. The transparent
single crystals of RbIO3 and RbIO2F2 with dimensions of ca. 0.08 ×
0.06 × 0.02 mm3 and 0.06 × 0.05 × 0.02 mm3 were mounted on the
glass fiber with epoxy for structure determination. The software
package SAINT PLUS35 was used for data set reduction and
integration for RbIO3 and RbIO2F2. The crystal structures were
solved by direct methods with SHELXS-97 and refined by full-matrix
least-squares on F2 with SHELXL-97.36 The final refined crystallo-
graphic data and structure refinement information are summarized in
Table 1. The atomic coordinates and equivalent isotropic displacement
parameters are shown in Tables S1 and S2, and some selected
important bond lengths and angles are listed in Tables S3−S6 in the
Supporting Information. The CCDC numbers for RbIO3 and RbIO2F2
are 1435343 and 1435342, respectively.
Powder X-ray Diffraction. X-ray diffraction patterns of
polycrystalline material was obtained at room temperature on Bruker
D8 Advanced diffractometer with Cu Kα1 radiation (λ = 1.54186 Å) in
the range of 10−70° at a scanning rate of 0.5°/min. The purities of
RbIO3 and RbIO2F2 were confirmed by XRD studies, and the XRD
pattern matches the one calculated from single-crystal XRD analysis
very well (see Figure S3 and S4).
Energy Dispersive X-ray Spectroscopy (EDS) Measurements.
Energy dispersive X-ray spectroscopy (EDS) was performed on a FEI
Quanta 200 scanning electron microscope (SEM) equipped with an X-
ray spectroscope. The energy-dispersive spectrometry (EDS)
elemental analyses on several single crystals for the two compounds
gave average Rb/I/O/and Rb/I/O/F molar ratios of approximately
0.9:1:3 and 1.1:1:2.4:2 for RbIO3 and RbIO2F2, which is in good
agreement with those determined from single-crystal X-ray structure
analyses (Figure S5 and S6).
used as the standard (100% reflectance), on which the finely ground
samples from the crystals were coated. The absorption spectrum was
calculated from reflectance spectrum using the Kubelka−Munk
function:37 α/S = (1 − R)2/2R, where α is the absorption coefficient,
S is the scattering coefficient, and R is the reflectance.
NLO Property and LDT Measurement. Powder second-
harmonic generation (SHG) signals were measured using the method
adapted from Kurtz and Perry.38 A pulsed Q-switched Nd:YAG laser
was utlilized to generate fundamental 1064 nm light with a pulse width
of 10 ns. The samples were pressed between two glasses. To make
relevant comparisons with known SHG materials, microcrystalline
KH2PO4 (KDP) served as the standard. The powders RbIO3,
RbIO2F2, and KDP were ground and sieved into the same particle
size (100−125 μm). Since phase-matchable SHG materials are very
important owing to their applications, we searched for the relation-
ships between SHG efficiencies and particle sizes. Polycrystalline
samples were ground and sieved into the following particle size ranges:
20−40, 40−60, 60−80, 80−100, 100−125, 125−150, 150−200, 200−
300, 300−400, and 400−500 μm. The laser-induced damage threshold
(LDT) tests were performed on powder samples which were obtained
by grinding the single crystal, with the laser source (1064 nm, 5 ns, 1
Hz). The two compounds and AgGaS2 were ground and sieved into
the same particle size range (100−200 μm). The energy of the laser
emission was gradually increased until the color of the samples
changed.10−12 It should be noted that the LDT measurement using
powder samples is feasible since each crystallite has a diameter much
larger than the wavelength of the incident laser. Thus, each crystallite
behaves as a macroscopic bulk material with the similar multiphoton
absorption (a main process for LDT as the laser pulse width is shorter
than 50 ps).39
Thermogravimetric Analysis. The thermogravimetric analysis
(TGA) was carried out with a Netzsch STA 449c analyzer instrument.
Crystal samples were added into an Al2O3 crucible and heated from
room temperature to 800 °C at a rate of 10 K/min under flowing
nitrogen gas.
Computational Methods. The first-principles calculations at the
atomic level for the RbIO3 and RbIO2F2 crystals were performed by
the plane-wave pseudopotential method40 implemented in the
CASTEP program41 based on density functional theory (DFT).42
The exchange−correlation (XC) functionals were described by the
local density approximation (LDA).43 The ion−electron interactions
were modeled by the ultrasoft pseudopotentials44 for all constituent
elements. In this model, Rb 4s24p65s1, O 2s22p4, I 5s25p5, and F 2s22p5
are treated as the valence electrons, respectively. A kinetic energy
Infrared Spectrum and UV−vis Diffuse Reflectance Spec-
trum. The IR spectra were carried out on a NICOLET 5700 Fourier-
transformed infrared (FTIR) spectrophotometer in the 4000−700
cm−1 region (2.5−14 μm) using the attenuated total reflection (ATR)
technique with a germanium crystal. The UV−vis diffuse reflectance
spectra were performed on a Varian Cary 5000 UV−vis−NIR
spectrophotometer in the region 200−800 nm. A BaSO4 plate was
B
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Figure 1. (a) Ball-and-stick diagrams of RbIO3. (b) Ball-and-stick diagrams of RbIO2F2.
cutoff of 400 eV and Monkhorst−Pack k-point meshes45 spanning less
than 0.04/Å3 in the Brillouin zone were chosen to ensure the sufficient
accuracy of the present purposes.
RESULTS AND DISCUSSION
■
Synthesis Method. In this work, RbIO3 was synthesized by
a hydrothermal reaction which is different from the solution
method in the literature.46 RbIO2F2 is a new compound, and
we also used a hydrothermal synthetic method for its
preparation, mainly in the hope to get larger size crystalline
product directly in hydrothermal reaction.
Crystal Structure. The crystal structure of RbIO3
determined in this work is essentially the same as that reported
in the literature,30 and the cell parameters are similar. RbIO3
crystallizes in the acentric space group R3m. Its structure is very
simple. It contains I5+ cation with a lone pair of electrons.
Figure 1a clearly shows that each I atom is coordinated with
three O atoms to form a distorted trigonal-pyramidal
Figure 2. FTIR spectra for RbIO3 and RbIO2F2 (4000−700 cm−1
environment[IO3]−, All the I−O bond lengths in the [IO3]−
anion group are the same (1.73(3)Å). All the [IO3]− anion
groups are arranged in the same direction. It is such
arrangement that results in a macroscopic net polarization
leading to large SHG responses. On the other hand, RbIO2F2
crystallizes in the acentric space group Pca21. It also contains
I5+ cation with a lone pair of electrons. From Figure 1b, one
may see that each I atom is linked to two O atoms and two F
atoms to form a distorted tetrahedron anion group [IO2F2]−.
Two I−F bond lengths (1.985(3) and 2.017(3)Å) are longer
than two I−O lengths (1.764(4) and 1.777(3)Å). The F2−I−
F1 bond angle is almost 180° (177.79(13)°), and thus, the
dipole moments of I−F1 and I−F2 nearly cancel each other.
Analogously, a half number of O1−I bonds are almost parallel
to the a-axis, whereas the other half of O1−I bonds are almost
antiparallel to the a-axis; therefore, all O1−I bonds together
contribute almost nothing to the net dipole moment in the
[IO2F2]− group. In comparison, all O2−I bonds are almost
parallel to the c-axis, and the result is a net dipole moment in
the whole crystal structure along this direction (see black arrow
in Figure 1b). This net polarization leads to a moderate SHG
response in RbIO2F2.
region).
results are consistent with the IR absorption data for other
metallic iodate NLO materials.10,17,47 Although there would
have some deviation from the measurement on single crystals,
the IR absorption edge of powder samples can at least provide
the preliminary evidence to determine the transparent region of
materials. Our measured data clearly indicate that both RbIO3
and RbIO2F2 are transparent in the mid-IR region from 3 to 5
μm. The UV−vis diffuse reflectance spectra of RbIO3 and
RbIO2F2 show that the absorption edge in the UV side is about
310 and 295 nm, indicating that the band gaps of two
compounds can be calculated to be 4.0 and 4.2 eV (Figure 3),
respectively. The result is consistent with our design strategy
that introducing F− into RbIO3 can expand the bang gap so as
to improve the LDT.
Thermogravimetric Analysis. The TGA curves for the
RbIO3 and RbIO2F2 powders in nitrogen are shown in Figure 4.
It is clear that both compounds have a relatively high thermal
stability. RbIO3 is stable up to 550 °C, and RbIO2F2 is 400 °C.
NLO Properties and LDT Measurements. Powder SHG
measurements using 1064 nm laser revealed that RbIO3 and
RbIO2F2 powders exhibit SHG effects of about 20 and 4 times
that of KDP (at the same particle size range of 100−125 μm).
The result that the SHG effect of RbIO2F2 is much weaker than
that of RbIO3 may come from the following reasons: First, the
Infrared Spectrum and UV−vis Diffuse Reflectance
Spectrum. The IR spectra of RbIO3 and RbIO2F2 exhibit wide
transparent range from 2.5 to 13.6 (4000−735 cm−1) and 2.5−
12.3 μm (4000−810 cm−1), respectively (Figure 2). These
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Figure 3. UV−vis absorption spectra for RbIO3 and RbIO2F2.
Figure 5. Dependence of SHG intensity on the particle size of RbIO3,
RbIO2F2, and KDP, respectively.
Table 2. LDT Values for RbIO3, RbIO2F2, and Some
Representative Mid-IR NLO Compounds (Measured in the
Same Condition)
AgGaS2 KIO3 K2BiI5O15 Rb2BiI5O15 RbIO3 RbIO2F2
LDT (MW/
cm2)
5
140
84
72
125
156
compounds are 125 MW/cm2 (RbIO3) and 156 MW/cm2
(RbIO2F2), indicating that both RbIO3 and RbIO2F2 show
much higher LDT than AgGaS2. In particular, the LDT of
RbIO2F2 is higher than RbIO3, KIO3, K2BiI5O15, and
Rb2BiI5O15. Because the above values obtained on the powders
are underestimated, the value on the single crystal should be
higher. The result that the LDT value of RbIO2F2 is higher than
that of RbIO3 is in good agreement with our design strategy by
partially replacing O2− of RbIO3 with F− to improve LDT. The
more accurate LDT and IR absorption measurements on
RbIO3 and RbIO2F2 will be made as the single crystals with
large size and good quality are obtained in the future studies.
Theoretical Results. The first-principles partial density of
states (PDOS) projected on the constitutional atoms were
plotted in Figure 6a,b, respectively, for RbIO3 and RbIO2F2.
Note that the top of the valence band (VB) and the bottom of
conduction band (CB) are mainly occupied by the orbitals of O
(2p), I (5p), and F (2p). Namely, the states on both sides of
the band gap are mainly composed of the orbitals from the
(IO3)− and (IO2F2)− groups in RbIO3 and RbIO2F2. Since the
optical response of a crystal in the visible-IR region originates
mainly from the electronic transitions between the VB and CB
states close to the band gap,48 the (IO3)− or (IO2F2)− anionic
units are considered to mainly determine the optical properties
of the crystal series. Moreover, in RbIO2F2, the energy spanning
of F 2p orbitals is about 5 eV at the top of VB, and they exhibit
quite large hybridization with the orbitals on the neighbor ions.
This decreases the energy bandwidth of the VB and enlarges
the energy band gap in RbIO2F2. Therefore, the absorption
edge of RbIO2F2 is blue-shifted compared with RbIO3, which is
also confirmed by the first-principles band gaps for these two
compounds (4.2 eV for RbIO2F2 vs 2.8 eV for RbIO3). In fact,
the enlargement of band gap by incorporation of fluorine has
been observed in UV NLO crystals49 but is directly
demonstrated in the mid-IR NLO materials for the first time
Figure 4. Thermogravimetric analysis curves for RbIO3 and RbIO2F2.
electronic negativity of F is stronger than that of O so the
electronic clouds on the [IO2F2]− anionic groups are less
polarizable than those on the [IO3]− groups. Second, the
[IO3]− groups form trigonal-pyramidals with strong dipolarity,
while the [IO2F2]− groups form distorted tetrahedrons with
smaller dipolarity. Third, all the [IO3]− groups in the crystal of
RbIO3 are arranged in a perfectly parallel direction (see Figure
1a), giving rise to a large net dipolarity, whereas in the crystal
structure of RbIO2F2 (see Figure 1b), the arrangement of the
[IO2F2]− groups is far less perfectly parallel than that of [IO3]−
groups in RbIO3, resulting in a much smaller net dipolarity. As
shown in Figure 5, for two compounds, the powder SHG
intensities at first increase gradually with the increase in particle
size and then reach a plateau. This is a typical curve of the
phase−matchable materials. It is an important factor for the
materials to be practically useful. Following the method in the
literature,10 a preliminary measurement of the LDT has been
performed on powders with the AgGaS2 powders as the
reference at the same measurement conditions. Table 2 lists the
LDT values for RbIO3, RbIO2F2 and some representative mid-
IR NLO compounds (measured in the same condition). The
results show that the LDT values of the AgGaS2, KIO3,
K2BiI5O15, and Rb2BiI5O15 powders are 5.2, 140, 84, and 72
MW/cm2, respectively, while the values for the titled
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(such as O2− anion in a known oxysalt NLO material) with
another anion of stronger electronegativity (such as F−).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.5b04511.
Photograph of the single crystal of RbIO3 and RbIO2F2,
powder X-ray diffraction pattern data, EDX spectrum,
atomic coordinates and equivalent isotropic displacement
parameters and some selected important bond lengths
and angles, the calculated linear and nonlinear optical
properties and CIF files (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jgqin@whu.edu.cn.
*E-mail: zslin@mail.ipc.ac.cn.
Notes
Figure 6. Partial density of states (PDOS) of RbIO3 (a) and RbIO2F2
The authors declare no competing financial interest.
(b).
ACKNOWLEDGMENTS
■
This work was financially supported by the National Basic
Research Program of China (No. 2010CB630701), the Natural
Science Foundation of China. (No. 91022036) and the 863
Program of China (No. 2015AA034203).
in this study. Moreover, it is well-known that energy bandgaps
determine the multiphoton absorption processes, and a larger
bandgap usually corresponds to a larger LDT in the crystal. The
much larger bandgap of RbIO2F2 (>4 eV) than that of AgGaS2
(∼2.6 eV) confirms the conclusion obtained from the powder
LDT measures.
Based on the obtained electronic structure, the SHG
coefficients (dij) of the two crystals are computed by the
scissors-corrected LDA approach,50,51 and the corresponding
powder-SHG (PSHG, ⟨dpowder⟩) effects are obtained according
to the Kurtz−Perry method, which are listed in Table S7.
PSHG effects are 8.5 and 5.5 times larger than that of KDP
(⟨dpowder⟩ = 0.33 pm/V) for RbIO3 and RbIO2F2, respectively,
consistent with those of experimental measurements (20 and 4
times KDP, respectively). In addition, the calculated
birefringence values for both compounds are moderate
(∼0.06), which indicates that their phase-matching capability
for second-order generation in the mid-IR region can be
achieved.
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■
A halogenated oxysalt RbIO3 is synthesized in hydrothermal
reaction and then systematically evaluated for its potential as a
new NLO material in the mid-IR region. Its powders show
strong SHG responses (20 times KDP), high LDT (125 MW/
cm2, about 20 times higher than that of AgGaS2 powders
measured in the same condition), wide transparent region
(0.3−13 μm), and high thermal stability (550 °C). By
introducing F− anion into RbIO3, we successfully obtained a
new material RbIO2F2, which exhibits an even higher LDT
value (156 MW/cm2) than that of RbIO3, while exhibiting
good SHG effect (4 times KDP), wide transparent region (0.3−
12 μm), and high thermal stability (400 °C). All the results
indicate that both RbIO3 and RbIO2F2 are promising NLO
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