A Series of Mixed-Metal Germanium Iodates as Second-Order Nonlinear Optical Materials

Article abstract of DOI:10.1021/acs.chemmater.8b00630

A series of new compounds in the A/Ae-Ge⁴⁺-IO₃ system, namely, A₂Ge(IO₃)₆ (A = Li, Na, Rb, or Cs) and BaGe(IO₃)₆(H₂O), have been obtained by introducing GeO₆ octahedra into a ternary metal iodate system. The structures of all five new compounds feature a zero-dimensional [Ge(IO₃)₆]^2- anion composed of a GeO₆ octahedron connecting with six IO₃ groups, the alkali or alkali-earth cations acting as spacers between these anions and maintaining charge balance. Interestingly, the isomeric Li₂Ge(IO₃)₆ and Na₂Ge(IO₃)₆ are noncentrosymmetric (NCS), whereas the isostructural Rb₂Ge(IO₃)₆ and Cs₂Ge(IO₃)₆ are centrosymmetric. BaGe(IO₃)₆(H₂O) is the first NCS alkali-earth germanium iodate reported. Powder second-harmonic generation (SHG) measurements show that Li₂Ge(IO₃)₆, Na₂Ge(IO₃)₆, and BaGe(IO₃)₆(H₂O) crystals are phase-matchable and display very large SHG signals that are approximately 32, 15, and 12 times that of KH₂PO₄, respectively, under 1064 nm radiation and 2, 0.8, and 0.8 times that of KTiOPO₄, respectively, under 2.05 mm laser radiation. The compounds show high thermal stability and a large laser damage threshold, indicating their potential applications as nonlinear optical (NLO) materials in visible and infrared spectral regions. Measurement of optical properties, thermal analysis, and theoretical calculations of the SHG origin have been performed. Our studies indicate that introducing non-second-order Jahn-Teller-distortive MO₆ octahedra into metal iodate systems can also lead to good mixed-metal iodate NLO materials.

Full text of DOI:10.1021/acs.chemmater.8b00630

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A Series of Mixed-metal Germanium Iodates as Second-Order NLO Materials
Fei-Fei Mao, Chun-Li Hu, Jin Chen, and Jiang-Gao Mao
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00630 • Publication Date (Web): 13 Mar 2018
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Chemistry of Materials
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A Series of Mixed-metal Germanium Iodates as Second-Order
NLO Materials
Fei-Fei Mao,†, ‡ Chun-Li Hu,† Jin Chen†, ‡ and Jiang-Gao Mao *,†
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† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou, Fujian 350002, P. R. China
‡ University of Chinese Academy of Sciences, Beijing 100039, P. R. China
ABSTRACT: A series of new compounds in A/Ae-Ge4+-IO3 system, namely, A2Ge(IO3)6(A = Li, Na, Rb, Cs) and BaGe(IO3)6(H2O)
have been obtained by introducing GeO6 octahedra into ternary metal iodate system. The structures of all five new compounds feature
zero-dimensional [Ge(IO3)6]2- anion composed of a GeO6 octahedron connecting with six IO3 groups, the alkali or alkali-earth cations
acting as spacers between these anions and keeping charge balance. Interestingly, the isomeric Li2Ge(IO3)6 and Na2Ge(IO3)6 are
noncentrosymmetric (NCS), whereas the isostructural Rb2Ge(IO3)6 and Cs2Ge(IO3)6 are centrosymmetric (CS). BaGe(IO3)6(H2O) is
the first NCS alkali-earth germanium iodate reported. Powder second harmonic generation (SHG) measurements show that
Li2Ge(IO3)6, Na2Ge(IO3)6, and BaGe(IO3)6(H2O) crystals are phase-matchable and display very large SHG signals of about 32, 15
and 12 times that of KH2PO4 (KDP) under 1064 nm radiation, respectively, and 2, 0.8 and 0.8 times that of KTiOPO4 (KTP) under
2.05 mm laser radiation. The compounds show high thermal stability and large laser damage threshold (LDT), indicating their
potential applications as NLO materials in visible and IR spectral regions. Optical properties measurement, thermal analysis, as well
as theoretical calculations on SHG origin have been performed. Our studies indicate that introducing non second-order Jahn-
Teller(SOJT) distortive MO6 octahedra into metal iodate systems can also lead to good mixed-metal iodates NLO materials.
INTRODUCTION
Cs),29,30 etc. It is interesting to find that, among these
compounds, A2Ti(IO3)6(A = Li, Na) exhibit very strong SHG
effect though the out-of-center distortion of TiO6 octahedron is
very small.22 The well alignment of the iodate groups leads to
the “addition” of the polarization of lone pairs, which results in
the large SHG signals. Even for the other compounds with much
more notable distortion of the MO6 octahedra, the lone pairs on
iodate groups seem to make major contributions to the SHG
efficiency. Thus controlling the proper alignment of the iodate
groups to enhance the net polarization is the key point in
synthesize NLO iodate material. Introducing MO6 octahedra
into iodates can be a good way to “guide” the alignment of the
iodate units, leading to NCS compounds with large polarization
and thus strong SHG effect.
Noncentrosymmetric (NCS) materials have attracted
extensive academic interest due to their potential application as
nonlinear optical (NLO) materials as well as functional
materials with ferroelectric, pyroelectric, and piezoelectric
properties.1-3 Due to their frequency conversion function, NLO
materials have been widely applied in photoelectric and laser
technologies.4-7 It is commonly recognized that ideal second
order NLO materials should better feature large second
harmonic generation (SHG) efficiency, eligible phase matching
angle and birefringence, high laser damage threshold (LDT),
wide transparency window, good thermal stability, non-
hygroscopicity and a facile synthetic route.8-12
During the past decades, metal iodate systems have been
extensively investigated since the stereochemically active lone-
pair electrons on IO3 unit can induce the formation of NCS or
polar materials with excellent SHG properties when the lone-
pairs are properly aligned.13-16 A number of iodates with large
SHG signals have been reported, exemplified by α-LiIO3,17, 18
NaI3O8,19 BiO(IO3),20 and Bi(IO3)F2.21 During the past decades,
great efforts have been made in the syntheses of metal iodates
containing octahedrally coordinated d0 transition metal(TM)
cations.22-39 It is based on the idea of forming NLO materials by
the synergistic effect of the second-order Jahn-Teller (SOJT)
distortion of both the lone-pair electrons on iodate groups and
d0 TM cations. A large number of mixed-metal iodates
containing d0 TM cations with large SHG responses have been
successfully obtained, such as A2Ti(IO3)6(A = Li, Na),22
NaVO2(IO3)2(H2O),24 A(VO)2O2(IO3)3(A = K, Rb, Cs, NH4),25,
26 Zn2(VO4)(IO3),27 BaNbO(IO3)5,28 AMoO3(IO3)(A = Li, K, Rb,
Later the phases of A2Sn(IO3)6(A = Li, Na) were isolated by
replacing the Ti4+ with Sn4+.40 Even though the second-order
Jahn Teller (SOJT) distortive TiO6 octahedron is replaced by
the non SOJT-distortive SnO6 octahedron, their structures stay
unchanged. Results of SHG measurements show that the SHG
intensities of A2Sn(IO3)6(A = Li, Na)( ~15, 12×KDP) are only
slightly weaker than those of A2Ti(IO3)6(A = Li, Na) ( ~17,
14×KDP). This may be due to the larger ionic radius of Sn4+
than that of Ti4+, which elongates the M-O bonds and give a rise
to the unit cell volumes of A2Sn(IO3)6(A = Li, Na), leading to
lower density of polar IO3 units and thus slightly weakened
SHG intensity. Among the post-transitional main group cations
(In3+、Ga3+、Ge4+、Sn4+、Sb5+) that tend to adopt octahedral
geometry, Ge4+ have the smallest ionic radius. The small ionic
radius of Ge4+ can lead to more compact unit cell and higher
density of the polar units in the resultant compounds and thus
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higher SHG intensity. The introduction of GeO6 octahedra into Li, Na, Cs) (0.8 mmol), GeO2 (0.3 mmol), I2O5 (1.2 mmol), and
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metal iodates may also lead to other interesting physical
properties of the materials. Up to date, only K2Ge(IO3)6 was
reported in the mixed-metal germanium iodate system.41 As
H2O (2 mL) in 23 mL Teflon-lined autoclave. Single crystals of
Rb2Ge(IO3)6 were prepared from a mixture of Rb2CO3 (0.8
mmol), GeO2 (0.3 mmol), I2O5 (2.5 mmol), and H2O (2 mL) in
23 mL Teflon-lined autoclave. Single crystals of
̅
K2Ge(IO3)6 crystallizes in the space group of R 3 , it is
BaGe(IO3)6(H2O) were prepared from
a
mixture of
centrosymmetric (CS) and SHG-inert. Herein, our explorations
of new compounds in the A/Ae-Ge4+-IO3 systems led to five
new germanium iodates, namely, A2Ge(IO3)6(A = Li, Na, Rb,
Cs) and BaGe(IO3)6(H2O), among which, Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O) are noncentrosymmetric
and display very large SHG signals of about 32, 15 and 12 times
that of KDP. Herein we report their syntheses, crystal structures,
optical properties, SHG effects, LDT values as well as
theoretical calculations.
Ba(IO3)2·H2O (0.5 mmol), GeO2 (0.3 mmol), I2O5 (1.2 mmol),
and H2O (2 mL) in 23 mL Teflon-lined autoclave. The
autoclaves were heated to 230 ºC in 6 h and held for 4 days, and
then cooled to 30 ºC at a rate of 3 ºC/h. Colorless prismatic
crystals of Li2Ge(IO3)6 and Na2Ge(IO3)6, and block-shaped
crystals of Rb2Ge(IO3)6, Cs2Ge(IO3)6 and BaGe(IO3)6(H2O)
were collected in high yields of ca. 85% (based on Ge). Powder
XRD analyses confirmed the purities of the products (Figure
S1). EDS elemental analyses gave average A/Ge/I molar ratios
of 2.1: 1.0: 5.9, 2.0: 1.0: 5.8, 2.0: 1.0: 5.9, respectively for Na,
Rb and Cs compounds, and Ba/Ge/I molar ratio of 1.1: 1.0: 5.9
for BaGe(IO3)6(H2O), which are in good agreement with those
determined from single crystal X-ray diffraction studies (Figure
S2).
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EXPERIMENTAL SECTION
Materials and Methods. Li2CO3 (99+%), Na2CO3 (99+%),
Rb2CO3 (99+%), Ba(IO3)2·H2O (99+%), Cs2CO3 (99+%), GeO2
(99+%), I2O5 (99.0%), were used as purchased from Shanghai
Reagent Factory. Powder X-ray diffraction (XRD) patterns
were recorded on Rigaku MiniFlex II diffractometer with
graphite-monochromated Cu−Kα radiation in the 2θ range of 5–
65º. Microprobe elemental analyses were performed on a field
emission scanning electron microscope (FESEM, JSM6700F)
with energy dispersive X-ray spectroscope (EDS, Oxford
INCA). Infrared (IR) spectra were recorded on Magna 750 FT-
IR spectrometer in the form of KBr pellets in 4000–400 cm–1.
UV-vis-NIR spectra in 200–2500 nm were collected on
PerkinElmer Lambda 900 UV-Vis-NIR spectrophotometer. By
using Kubelka-Munk function,42 reflectance spectra were
converted into absorption spectrum. Thermogravimetric
analyses (TGA) and differential scanning calorimetry (DSC)
were performed with NETZCH STA 449F3 unit under N2
atmosphere, at a heating rate of 10 °C/min. Powder SHG
measurements were carried out with Q-switch Nd:YAG laser
generating radiations at 1064 nm according to Kurtz and Perry
method.43 A2M(IO3)6(A=Li, Na; M=Ti, Sn) crystals were
Single Crystal Structure Determination. Single-crystal
X-ray diffraction data were collected on an Agilent
Technologies SuperNova dual-wavelength CCD diffractometer
with Mo Kα radiation (λ = 0.71073 Å) at 293 K. Data reduction
was performed with the program CrysAlisPro, and absorption
correction based on the multi-scan method was applied.45 The
structures were solved by direct method and refined by full-
matrix least-squares fitting on F2 using SHELXL–97.46 All of
the non-hydrogen atoms were refined with anisotropic thermal
parameters. The H atom associated with the water molecule in
BaGe(IO3)6(H2O) is located at geometrically calculated position
and refined with isotropic thermal parameters. The structures
were checked for missing symmetry elements using PLATON
and none was found.47 The Flack parameters were refined to
0.22(14), 0.03(7) and 0.43(3) for NCS Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O), respectively, indicating the
existence of racemic twinning in Na2Ge(IO3)6 and
BaGe(IO3)6(H2O).48,49 Crystallographic data and structure
refinements of the five compounds are given in Table 1, and
selected bond distances are listed in Table S1.
40
obtained hydrothermally according to the references.22,
Crystalline A2M(IO3)6(A=Li, Na; M=Ge, Ti, Sn) and
BaGe(IO3)6(H2O) samples were sieved into distinct particle-
size ranges (25-45, 45-53, 53-75, 75-105, 105-150, 150-210
µm). Sieved KH2PO4 (KDP) and KTiOPO4 (KTP) samples in
the same particle-size ranges were taken as references for SHG
measurement under 1064 nm and 2.05 µm laser radiation.
Crystalline samples in the particle-size range of 150-210 µm
were used for SHG measurements. The laser-induced damage
threshold (LDT) measurements were performed on crystalline
samples of A2M(IO3)6(A=Li, Na; M=Ge, Ti, Sn),
BaGe(IO3)6(H2O) and α-LiIO3 in the particle-size range of 150-
210 µm with AgGaS2 sample in the same particle-size range as
the reference, under 1064 nm laser source (10 ns, 1Hz). The
laser spot has a diameter of 3.6mm. The energy of the laser
emission was gradually increased until the samples turned black
in color. 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).44
Computational Descriptions. Single-crystal structural
data of Li2Ge(IO3)6, Na2Ge(IO3)6 and BaGe(IO3)6(H2O) were
used for the theoretical calculations. The electronic structures
and optical properties were performed using a plane-wave basis
set and pseudo-potentials within density functional theory (DFT)
implemented in the total-energy code CASTEP.50,51 For the
exchange and correlation functional, we chose Perdew–Burke–
Ernzerhof (PBE) in the generalized gradient approximation
(GGA).52 The interactions between the ionic cores and the
electrons were described by the norm-conserving
pseudopotential.53
The
following
valence-electron
configurations were considered in the computation: Li-2s1, Na-
2p63s1, Ba-5s25p66s2, Ge-4s24p2, I-5s25p5, and O-2s22p4. The
numbers of plane waves included in the basis sets were
determined by a cutoff energy of 750eV. Monkhorst-Pack k-
point sampling of 3 × 3 × 4 and 3 × 3 × 2 was used to perform
numerical integration of Brillouin zone for A2Ge(IO3)6(A = Li,
Na), and BaGe(IO3)6(H2O), respectively. Other parameters and
convergent criteria were set as the default values of CASTEP
code. During the optical property calculations, approximately
Syntheses. Single crystals of A2Ge(IO3)6 (A = Li, Na, Cs)
were obtained hydrothermally from a mixture of A2CO3 (A =
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Chemistry of Materials
Table 1. Crystallographic Data and for A2Ge(IO3)6(A=Li, Na, Rb, Cs) and BaGe(IO3)6(H2O).
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formula
Fw
crystal system
space group
T (K)
Li2Ge(IO3)6
1135.87
hexagonal
P63 (No. 173)
293(2)
Na2Ge(IO3)6
1167.97
hexagonal
P63 (No. 173)
293(2)
Rb2Ge(IO3)6
1292.93
Cs2Ge(IO3)6
1387.81
BaGe(IO3)6(H2O)
1277.35
trigonal
R3(No. 146)
293(2)
11.3707(4)
11.3707(4)
11.3223(7)
120.000
trigonal
trigonal
̅
̅
R3 (No. 148)
R3 (No. 148)
293(2)
293(2)
a (Å)
b (Å)
c (Å)
9.1900(6)
9.1900(6)
5.2393(5)
120.000
383.21(5)
1
9.4604(4)
9.4604(4)
5.3260(3)
120.000
412.81(3)
1
11.3256(10)
11.3256(10)
11.4380(14)
120.000
1270.6(2)
9
11.5185(10)
11.5185(10)
11.6063(14)
120.000
1333.6(2)
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γ (°)
9
V (Å3)
1267.77(10)
3
0.71073
5.019
15.152
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Z
λ (Mo-Ka) (Å)
0.71073
4.922
14.183
0.71073
4.698
13.221
0.71073
5.069
18.545
0.71073
5.184
Dc (g/cm−3)
μ (mm−1)
16.266
GOF on F2
Flack factor
R1, wR2 [I >2σ(I)]
R1, wR2 (all data)
1.302
0.22(14)
0.0401, 0.1472
0.0403, 0.1472
1.128
0.03(7)
0.0296, 0.0683
0.0324, 0.0700
1.071
--
0.0314, 0.0697
0.0357, 0.0741
0.921
--
0.0309, 0.0779
0.0338, 0.0803
1.009
0.43(3)
0.0225, 0.0482
0.0230, 0.0485
R1 = Σ||Fo| -|Fc||/Σ|Fo|, wR2 = {Σw[(Fo)2 -(Fc)2]2/Σw[(Fo)2]2}1/2
.
̅
150 ~170 and 490 empty bands were involved to ensure the
convergence of linear optical properties and SHG coefficients
for A2Ge(IO3)6(A = Li, Na), and BaGe(IO3)6(H2O), respectively.
The other parameters and convergent criteria were the default
values of CASTEP code. The calculations of second-order NLO
properties were based on length-gauge formalism within the
independent-particle approximation.54 We adopted the Chen’s
static formula, which was derived by Rashkeev et al.55 and later
improved by Chen’s group.56 The static second-order NLO
susceptibility can be expressed as
and Cs2Ge(IO3)6 crystalize in centrosymmetric space group R3
(No. 148). It is interesting that the four stoichiometrically
identical compounds display two different structure types. The
structures of A2Ge(IO3)6 (A=Li, Na, Rb, Cs) and
BaGe(IO3)6(H2O) all feature a zero-dimensional [Ge(IO3)6]2-
anion composed of a GeO6 octahedron connecting with six IO3
groups, with alkali or alkali-earth cations filling in the inter
anionic spaces and keeping charge balance (Figures 1 and 2).
The acentric phases A2Ge(IO3)6 (A = Li, Na) are isostructural
with A2M(IO3)6 (M = Ti, Sn)A2Ti(IO3)6 (M = Ti, Sn).22, 40 Their
asymmetric units contain one alkali metal, one Ge, one I, and
six O atoms. The alkali metal and Ge atoms are lying on the
threefold axis, while other atoms occupy the general sites. The
alkali metal atom is in pseudo-octahedral coordination
environment with six O atoms, with the bond distances in the
ranges of 2.06(2)-2.27(7) Å for Li-O and 2.371(7)-2.335(7) Å
for Na-O bond, respectively. The Ge atom is octahedrally
coordinated by six oxygen atoms from six iodate groups in a
unidentate fashion (Figure 1a). The Ge–O distances range from
1.995(11) to 2.033(12) Å, which is slightly longer than those
χαβγ = χαβγ(VE) + χαβγ(VH) + χαβγ(two bands)
where χαβγ(VE) and χαβγ(VH) give the contributions to χαβγ from
virtual-electron processes and virtual-hole processes,
respectively; χαβγ(two bands) is the contribution to χαβγ from the
two-band processes. The formulas for calculating χαβγ(VE),
χαβγ(VH), and χαβγ(two bands) are given in Ref. 56.
RESULTS AND DISCUSSION
A series of alkali and alkaline earth metal germanium
iodates, namely, A2Ge(IO3)6 (A = Li, Na, Rb, Cs) and
BaGe(IO3)6(H2O), have been isolated through hydrothermal
reactions. It is interesting to note that, for the sythesis of
RbGe(IO3)6, more excess amount of I2O5 is needed compared
with those for A2Ge(IO3)6 (A = Li, Na, Cs). Otherwise, only
RbIO3 crystals can be obtained. The structures of the five new
compounds all feature a 0D [Ge(IO3)6]2- anion composed of
GeO6 octahedron corner-sharing with six IO3 groups, with
alkali or alkali-earth cations as the spacers and keeping charge
balance. Remarkably, Li2Ge(IO3)6, Na2Ge(IO3)6, and
BaGe(IO3)6(H2O) crystals are phase-matchable and display
very large SHG signals of about 32, 15 and 12 times that of
KH2PO4 (KDP) under 1064 nm radiation, and they also exhibit
high thermal stability and LDT values.
41
reported in centrosymmetric K2Ge(IO3)6. Comparing the Ge-
O bonds in A2Ge(IO3)6 (A = Li, Na) with M-O bonds in
A2M(IO3)6 (M = Ti, Sn), we can see that Ge-O bonds (1.995 to
2.033 Å) are slightly shorter than Ti-O (2.028 to 2.056 Å) and
Sn-O bonds (2.07 to 2.14 Å) due to the smallest ionic radius of
Ge4+ (Table S2). The I5+ atoms are coordinated by three oxygen
atoms in a IO3 trigonal pyramidal geometry, which is very
common in metal iodates.23-28 The I-O bond lengths are in the
range of 1.766(11) to 1.879(6) Å. Bond valence calculations
gave values of 0.94 (Li), 1.35 (Na), and 5.09-4.91 (I),
respectively, revealing that alkali metal and I are in oxidation
states of 1+ and 5+, respectively.57,58 Likewise, the
centrosymmetric phases A2Ge(IO3)6 (A = Rb, Cs) are
isostructural with A2Ti(IO3)6 (A = Rb, Cs),22 and A2Sn(IO3)6 (A
= Rb, Cs),40 K2Ge(IO3)6.41 Their asymmetric units contain one
alkali metal, one Ge, one I, and six O atoms. The alkali metal
cation lies on the threefold axis, Ge atom occupying sites with
Structural Description.
Among these five compounds, Li2Ge(IO3)6 and
Na2Ge(IO3)6 crystalize in polar space group P63 (No. 173) and
BaGe(IO3)6(H2O) in polar R3 (No. 146), whereas Rb2Ge(IO3)6
̅
3 symmetry, and the remaining atoms located at the general
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sites. The Rb+ or Cs+ ion is ten-coordinated by ten O atom, with
the bond distances in the ranges of 2.897(5)-3.056(4) Å for Rb-
O and 3.038(5)-3.162(4) Å for Cs-O bond, respectively. Each
Ge atom also coordinates with six oxygen atoms from six iodate
groups. In the structure of BaGe(IO3)6(H2O), the polarizations
of six IO3 groups in a [Ge(IO3)6]2+ unit compensate each other
along a and b-axis but constructively add along c-axis. Thus,
macroscopic dipole moments along c-axis, are favorable for the
formation of polar structure and SHG response. This polarity
shift from polar to non-polar with the increase of the alkali
metal cation size can be explained referring to the previous
study.22 In order to maintain the octahedral environment around
Li+ and Na+ cations in the noncentrosymmetric structure, the
IO3 groups connecting with Ge4+ have to align in a parallel
fashion. For Rb+ and Cs+ cations with larger size, higher
coordination environment is preferred, thus, leading to the anti-
parallel alignment of the IO3 groups.
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Thermal Analyses. Thermogravimetric analyses (TGA)
indicate that A2Ge(IO3)6 (A=Li, Na, Rb, Cs) show good thermal
stability and exhibit similar thermal behavior. Samples of
A2Ge(IO3)6 (A=Li, Na, Rb, Cs) display two steps of weight
losses in the temperature region of 30 ~ 800 °C and are
thermally stable up to 416, 410, 460, and 414°C, respectively
(Figure S3). The thermal stability of A2Ge(IO3)6 (A = Li, Na,
Rb, Cs) seems at the same level with that of A2M(IO3)6 (A = Li,
Na, Rb, Cs; M = Ti4+, Sn4+) (stable up to ~400 °C ). For
Li2Ge(IO3)6, the sample quickly lost weight in the temperature
region of 416 ~ 497 °C and then showed one small step of
weight loss in the temperature region of 497 ~ 615 °C.
Endothermic peaks were observed at 474 and 562°C in the DSC
curve. For Na2Ge(IO3)6, the sample showed one sharp step of
weight loss in the temperature region of 410 ~ 490 °C, followed
by one mild step of weight loss in the temperature region of 490
~ 604 °C. Endothermic peaks were observed at 443 and 543°C
in the DSC curve. For Rb2Ge(IO3)6, the sample exhibited a large
weight loss in the temperature region of 460 ~ 525 °C and then
a small step of weight loss in the temperature region of 525 ~
Figure 1. Views of (c) the coordination geometry around Ge4+
in Li2Ge(IO3)6, (b) the structure of Li2Ge(IO3)6 along c-axis, (c)
he coordination geometry around Ge4+ in Rb2Ge(IO3), and (d)
the structure of Rb2Ge(IO3)6 along c-axis.
groups in a unidentate fashion into a GeO6 octahedron (Figure
1c). The Ge–O distances are in the range of 1.886(4)-1.875(4)
Å, which is close to the Ge-O distance (1.901 Å) in the reported
K2Ge(IO3)6.41 The Ge-O distances (1.886-1.875 Å) in
A2Ge(IO3)6 (A = Rb, Cs) are also shorter than the Ti-O distances
reported in A2Ti(IO3)6 (A = Rb, Cs) (1.949-1.943 Å) and Sn-O
distances in A2Sn(IO3)6 (A = Rb, Cs) (2.038-2.037 Å) (Table
S2). The I-O bond lengths are in the range of 1.784(4) to
1.886(4) Å. Bond valence calculations gave values of 1.26 (Rb),
1.24(Cs), 4.91-5.02 (I), and 4.13-4.25 (Ge), respectively,
revealing that Rb (or Cs), I and Ge are in oxidation states of 1+,
5+ and 4+.57, 58
For the polar BaGe(IO3)6(H2O), its asymmetric unit
contains one Ba, one Ge, two I, and seven O atoms. Ba, Ge and
̅
O(1W) atoms are lying on the 3 axis, while the remaining
atoms occupy the general sites. The Ba atom is ten-coordinated
by nine iodate oxygen atoms and an aqua ligand with Ba-O
bond distances in the range of 2.635(10)-3.024(5) Å (Figure 2a).
The Ge atom is octahedrally coordinated by six iodate groups
in a unidentate fashion (Figure 2b). The Ge–O distances fall in
the range from 1.855(5) to 1.896(5) Å. The I-O bond lengths
are in the range of 1.781(5) to 1.880(5) Å. Bond valence
calculations gave values of 2.11, 4.98-5.01, and 4.26 for Ba, I
and Ge, respectively, revealing that Ba, I and Ge are in
oxidation states of 2+, 5+ and 4+.57, 58
Comparing the NCS phases with the CS ones, it is notable
that the structures of the [Ge(IO3)6]2+ units are different to some
extent, deciding the polarity of the structure of the compounds.
In the structure of Li2Ge(IO3)6 and Na2Ge(IO3)6, the six IO3
groups in a [Ge(IO3)6]2+ unit lie in a parallel manner towards c-
axis thus the lone pairs are in a perfect parallel alignment which
leads to the favorable “addition” of the polarizations of the IO3
Figure 2. Views of (a) the coordination geometry around Ba2+
in BaGe(IO3)6(H2O) (b) he coordination geometry around Ge4+
in BaGe(IO3)6(H2O), (c) he structure of BaGe(IO3)6(H2O) along
c-axis.
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Figure 3. UV-vis-IR spectra of A2Ge(IO3)6 (A = Li, Na, Rb, Cs) and BaGe(IO3)6(H2O).
624 °C. Endothermic peaks were observed at 499 and 607°C in
the DSC curve. For Cs2Ge(IO3)6, the sample quickly lost weight
in the temperature region of 414 ~ 492 °C and then slowly lost
one small step of weight loss in the temperature region of 492
~ 607 °C. Endothermic peaks were observed at 483 and 602°C
in the DSC curve. Sample of BaGe(IO3)6(H2O) is thermally
stable up to 396 °C and displays three steps of weight losses in
the temperature region of 30 ~ 800 °C. After dehydration at
around 396 ~ 404 °C, the sample quickly lost weight in the
temperature region of 404 ~ 464 °C and then slowly lost weight
in the temperature region of 464 ~ 653 °C. Endothermic peaks
were observed at 397, 444 and 639°C in the DSC curve. The
residuals at 500°C are amorphous and at 800°C should be
Ba5(IO6)2 and GeO2 based on PXRD studies (Figure S4). The
weight losses correspond to the release of H2O, I2 and O2, and
the experimental and calculated values are given in Figure S3.
transparent in the region of 0.32 – 10.8, 0.26 – 10.8, 0.30 – 11.7,
0.30 – 12.1, and 0.30 – 2.7 μm, respectively. Optical diffuse-
reflectance spectra indicate a wide band gap of 3.86, 4.60, 4.06,
4.12, and 4.06 eV for A2Ge(IO3)6 (A=Li, Na, Rb, Cs) and
BaGe(IO3)6(H2O), respectively, confirming their wide-band-
gap semiconductor nature.
SHG Measurements. As Li2Ge(IO3)6, Na2Ge(IO3)6 and
BaGe(IO3)6(H2O) crystallize in noncentrosymmetric space
groups, powder SHG measurements were conducted using both
KDP and KTP samples as references. In the visible and near-IR
region, powder SHG measurements revealed that Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O) samples are phase-
matchable and display very large SHG signals of about 32, 15,
and 12 times that of KDP sample under 1064 nm laser radiation,
respectively, and 2, 0.8 and 0.8 times that of KTP sample under
2.05 mm laser radiation (Figure 4). Their SHG responses are
much higher than most of metal iodates reported, eg. BiO(IO3)
(12.5×KDP),20 Bi(IO3)F2(11.5×KDP),21 NaVO2(IO3)2(H2O)
(20×KDP),24 BaNbO(IO3)5(14×KDP),28 α-AgI3O8(9×KDP) and
β-AgI3O8(8×KDP).61
Optical Measurements. IR spectra reveal that A2Ge(IO3)6
(A=Li, Na, Rb, Cs) and BaGe(IO3)6(H2O) are transparent in the
region of 2.5 – 10.8 μm (4000 – 922 cm–1), 2.5 – 10.8 μm
(4000 – 923 cm–1), 2.5 – 11.7 μm (4000 – 843 cm–1), 2.5 –
12.1 μm (4000 – 828 cm–1), and 2.5 – 2.7 μm (4000 – 3650 cm–
1), respectively (Figure S5). Absorption bands observed in the
range of 3000 - 3650 cm–1 and 1380-1650 cm–1 can be attributed
to O–H vibration for BaGe(IO3)6(H2O). Absorption bands
observed at 878 and 782 cm–1, 872 and 797 cm–1, 814 and 797
cm–1, 818 and 801 cm–1 and 881 and 765 cm–1can be assigned
to Ge – O vibration for A2Ge(IO3)6 (A=Li, Na, Rb, Cs) and
BaGe(IO3)6(H2O), respectively.59, 60 Other bands observed at
756, 737, 688, 515 and 431 cm–1, 755, 739, 688, 508 and 429
cm–1, 768, 704 and 466 cm–1, 777, 762 and 460 cm–1, and 760,
737 and 495 cm–1can be assigned to the I – O vibration.17-20
It is meaningful to discuss the SHG intensity shift among the
isostructural A2M(IO3)6 (A = Li, Na; M = Ge, Ti, Sn). Taking
Li2M(IO3)6 (M = Ge, Ti, Sn) for example, with the decrease of
the M4+ cation size from Sn4+ to Ge4+( Sn4+> Ti4+> Ge4+), the M-
O bond distances shortened consequently, which leads to the
shrink of the unit cell volume from Li2Sn(IO3)6 to Li2Ge(IO3)6
(Li2Sn(IO3)6 > Li2Ti(IO3)6 > Li2Ge(IO3)6) (Table S2). Since the
numbers of IO3 units per unit cell are identical for Li2M(IO3)6
(M = Ge, Ti, Sn), the density of the polar IO3 unit increases from
Li2Sn(IO3)6 to Li2Ge(IO3)6 (Li2Ge(IO3)6> Li2Ti(IO3)6
>
Li2Sn(IO3)6). This tendency are in accordance with the SHG
intensities observed (Li2Ge(IO3)6 > Li2Ti(IO3)6 > Li2Sn(IO3)6).
Similar phenomenon was also observed in the series of
Na2M(IO3)6 (M = Ge, Ti, Sn). As the M4+ cation size
downgrades, the unit cell volume reduces, leading to a upgrade
UV-vis absorption spectra reveal absorption edges of 321,
267, 305, 300 and 304 nm for A2Ge(IO3)6 (A = Li, Na, Rb, Cs)
and BaGe(IO3)6(H2O), respectively (Figure 3). Thus,
A2Ge(IO3)6 (A = Li, Na, Rb, Cs) and BaGe(IO3)6(H2O) are
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Figure 4. Oscilloscope traces of the SHG signals (150 – 210 mm) of Li2Ge(IO3)6, Na2Ge(IO3)6 and BaGe(IO3)6(H2O) at 1064 nm
(a) and 2.05 μm (c). Plots of measured SHG intensity versus particle sizes of Li2Ge(IO3)6, Na2Ge(IO3)6 and BaGe(IO3)6(H2O) under
laser radiation at λ = 1064 nm (b) and 2.05 μm (d). KDP and KTP samples serve as the references for λ = 1064 nm and 2.05 μm
laser radiation, respectively.
in the density of polar unit and thus enhanced SHG intensity.
Likewise, when the M4+ cation is set, Li2M(IO3)6 exhibits larger
SHG response than Na2M(IO3)6, which can as well be explained
by the smaller cation size of Li+ than that of Na+.
Na2Ge(IO3)6, the c-component of local dipole moment of I(1)O3
is calculated to be 15.43 D, which compares well to that of the
IO3 groups in Na2Ti(IO3)6 (15.05 D) and Na2Sn(IO3)6 (15.28 D).
Each unit cell contains six I(1)O3 and the c-component adds up
to a net dipole moment of 101.5 and 92.6 D for Li2Ge(IO3)6 and
Na2Ge(IO3)6, respectively, significantly larger than the isomeric
titanium and tin iodates with the calculated values of 90.2 - 92.6
D. For BaGe(IO3)6(H2O) in the polar space group of R3, the
local dipole moments of I(1)O3 and I(2)O3 were calculated to
be 1.65 and 9.94 D, respectively. Each unit cell contains 18
I(1)O3 and 18 I(2)O3, and the c-component adds up to a net
dipole moment of 208.6 D. Taking into consideration the
similar unit-cell volumes, the dipole moments per unit volume
of Li2Ge(IO3)6, Na2Ge(IO3)6 and BaGe(IO3)6(H2O) are 0.265,
0.224 and 0.165 D·Å -3, respectively, which is considerably
larger than those of the reported polar iodates with strong SHG
effect, e.g. Bi(IO3)F2 (0.135 D·Å -3)8 and K2Au(IO3)5 (0.114
D·Å -3).68 Dipole moments calculation reveals that the proper
alignment of IO3 units leads to the strong SHG responses of the
compounds. The largest SHG effect of Li2Ge(IO3)6 can be
attribute to the more polarizable IO3 units, as well as the
smallest unit cell volume leading to largest density of IO3 units.
It is expected that the SHG efficiencies of Li2Ge(IO3)6 and
BaGe(IO3)6(H2O) would be even stronger without the racemic
twinning problem.
To estimate the direction and magnitude of the dipole
moments, calculations of the dipole moments based on the
geometric structure were performed on A2Ge(IO3)6 (A=Li, Na,
Rb, Cs) and BaGe(IO3)6(H2O) and the titanium iodates and tin
iodates using the method reported earlier (Table S3). 7, 22, 62-67
̅
For the phases crystalizing in the space group R3, the MO6
(M=Ge, Ti, Sn) octahedra are non-distortive and the
polarization of the iodate groups cancel out each other due to
the centrosymmetric space group, leading to a net dipole
moment of 0 D for a unit cell. For the noncentrosymmetric
phases, the calculated dipole moments for MO6 (M=Ge, Ti, Sn)
octahedra are very small and can be neglected. Since
Li2Ge(IO3)6 and Na2Ge(IO3)6 crystalize in P63 space group and
BaGe(IO3)6(H2O) in R3 space group, the a-, and b-component
of the polarizations from IO3 units within one unit cell are
cancelled out completely, while the c-component is positive and
adds up to a net dipole moment. For Li2Ge(IO3)6, the c-
component of the local dipole moment of I(1)O3 is calculated to
be 16.92 D, which compares favorably to that of the IO3 groups
in Li2Ti(IO3)6 (15.13 D) and Li2Sn(IO3)6 (15.02 D). For
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LDT Measurements. As A2Ge(IO3)6 (A = Li, Na) and
Theoretical Studies. To reveal the SHG effect origins of
the title compounds, theoretical calculations have been
performed employing DFT methods. The calculated results on
band structures (Figure S6) indicate that due to the structural
similarity, the band structures of Li2Ge(IO3)6 and Na2Ge(IO3)6
are very similar to each other: the bands in both VB and CB are
very flat, but the bands in CB are looser than those in VB. We
also find that BaGe(IO3)6(H2O) is an indirect band gap
compound with the valence band maximum (VBM) and the
conduction band minimum (CBM) at G and A points,
respectively. The band structures calculations also give the
band gaps of 3.176 eV, 3.312 eV and 3.047 eV for Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O), respectively, which are
smaller than the experimental values [Li2Ge(IO3)6: 3.86 eV;
Na2Ge(IO3)6: 4.6 eV; BaGe(IO3)6(H2O): 4.06 eV]. The band
gap underestimation by theoretical calculations should be
attributed to the limitation of the exchange and correlation
function of GGA-PBE. Hence, to accurately describe the optical
properties, the scissors of 0.684 eV, 1.288 eV and 1.013 eV
have been adopted to match the experimental gaps in the
following analyses for Li2Ge(IO3)6, Na2Ge(IO3)6 and
BaGe(IO3)6(H2O), respectively.
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BaGe(IO3)6(H2O) possess large band gaps, LDT measurements
were performed using AgGaS2 as the reference. Results reveal
large LDT values of 76.63, 68.77, and 81.54 MW/cm2, which
are about 42, 38, 39, 25 and 45 times higher than that of AgGaS2
(1.81 MW/cm2). The LDT values of the three mixed-metal
germanium iodates are at the same level with those of
A2M(IO3)6(A=Li, Na; M=Ti, Sn)(Table S4), and are
comparable to α-LiIO3 with a large SHG response (~300×α-
SiO2) and high LDT value of 104.53 MW/cm2 (54×AgGaS2).
Hence, A2Ge(IO3)6 (A = Li, Na) and BaGe(IO3)6(H2O) are a
promising candidate for high-power NLO applications in the
visible to IR window.
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The partial density of states is a powerful technique to
assign the band structure and understand the bonding
interactions in the crystal. As shown in Figure S7, the PDOS
diagrams of the three compounds behave very similar, so we
take Li2Ge(IO3)6 as an example to describe them in detail. We
can see that the electronic states of I and Ge atoms are fully
overlapped with those of O atoms in the whole energy region,
showing the strong bonding interactions of I-O and Ge-O bonds
in the compound. The peaks below -9.0 eV in the VB region are
mainly originated from the O-2s and I-5s5p states; the VB
region near Fermi level (-8.0 eV ~ 0 eV) is contributed by O-2p
and I-5s5p, mixed by a little amount of Ge-4s4p and Li-2s states.
In CB, the electronic states are comprised of the unoccupied I-
5s5p, O-2p, Ge-4s4p and Li-2s orbitals. For Li2Ge(IO3)6, the
highest VB is dominated by the 2p nonbonding states of O
atoms, while the lowest CB is the empty I-5p, O-2p and some
Ge-4s orbitals, so the band gap of Li2Ge(IO3)6 is determined by
I, O and Ge atoms.
The SHG coefficients of three polar crystals were also
calculated. The calculated largest SHG tensors d31 are 1.90 × 10-
8
esu, 1.09 × 10-8 esu and 1.02 × 10-8 esu for Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O), respectively, which are
somewhat smaller than the corresponding measured values (32,
15 and 12 times that of KDP for Li2Ge(IO3)6, Na2Ge(IO3)6 and
BaGe(IO3)6(H2O), respectively).
Li2Ge(IO3)6 and Na2Ge(IO3)6 are isostructural and have the
same anionic groups, but what causes such a difference in SHG
responses between them? Naturally, we examined whether the
band gap effect controls the SHG difference. As described
above, the experimental band gaps of Li2Ge(IO3)6 and
Na2Ge(IO3)6 are 3.86 and 4.6 eV, respectively. If Li2Ge(IO3)6
had a gap of 4.6 eV, the calculated d31 would be sharply
decreased from 1.897 × 10−8 to 1.235 × 10−8 esu; and if
Na2Ge(IO3)6 had a gap of 3.86 eV, the calculated d31 would be
largely increased from 1.090 × 10−8 to 1.633 × 10−8 esu;
obviously, the re-evaluated SHG coefficients are very close to
those of the other compounds. Therefore, we can believe that
Figure 5. The SHG density for Li2Ge(IO3)6 (a: VB, b: CB),
Na2Ge(IO3)6 (c: VB, d: CB) and BaGe(IO3)6(H2O) (e: VB, f:
CB).
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
the large SHG difference between Li2Ge(IO3)6 and Na2Ge(IO3)6
is mainly caused by their difference in band gaps.
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To investigate the origin of such strong SHG effects, it is
desirable to show clearly which energy levels of electronic
states contribute to the SHG effect, so we perform the spectral
decomposition of the largest tensor d31 for Li2Ge(IO3)6,
Na2Ge(IO3)6 and BaGe(IO3)6(H2O), as plotted in the
bottommost panels of Figure S7. For the isostructural
Li2Ge(IO3)6 and Na2Ge(IO3)6, the most contributed regions are
just in the upper part of VB (-5.0 ~0 eV) and the bottommost
CB (< 7.5 eV), the corresponding electronic states are mainly
the O-2p and I-5p states in VB and the unoccupied I-5p, Ge-4s
and O-2p states in CB. While for BaGe(IO3)6(H2O), the SHG-
contributed energy region is from -3.0 eV to 7.0 eV, which also
corresponds to O-2p, I-5p and Ge-4s states. Furthermore, SHG
density analyses have been performed to accurately and
intuitively show which orbitals give contributions to SHG for
these compounds. As indicated in Figure 5, for the three
compounds, the main contributions in VB comes from the O-2p
nonbonding states, especially the terminal O atoms, while the
SHG contributions in CB are mainly at the unoccupied O-2p
and I-5p states.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mjg@fjirsm.ac.cn
Notes
The authors declare no competing financial interest
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ACKNOWLEDGMENT
Our work has been supported by the National Natural Science
Foundation of China (91622112, 21701173), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB20000000), and the 100 Talents Project of Fujian Province.
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Integral of SHG density over VB and CB shows that the
strong SHG responses of Li2Ge(IO3)6, Na2Ge(IO3)6 and
BaGe(IO3)6(H2O) originate from IO3 groups dominantly, and
the calculated contribution percentages of Li+/Na+/Ba2+, GeO6
and IO3 groups are <0.01%, 8.68% and 91.25% for Li2Ge(IO3)6,
0.14%, 8.10% and 91.67% Na2Ge(IO3)6, and 0.83%, 7.59% and
90.59% for BaGe(IO3)6(H2O). The results imply that in the
three compounds, except for IO3 groups, GeO6 groups play an
insignificant part in SHG process, while the alkali metal and
alkali-earth metal cations give tiny contributions that are
negligible.
CONCLUSIONS
In summary, by introducing GeO6 octahedra into metal
iodates, five new mixed-metal iodates in the A/Ae-Ge4+-IO3
system, namely, A2Ge(IO3)6(A = Li, Na, Rb, Cs) and
BaGe(IO3)6(H2O), have been obtained. The NCS A2Ge(IO3)6(A
= Li, Na) exhibit large SHG responses and LDT values, wide
transmittance window, and high thermal stabilities, which make
them promising new NLO crystals in visible and IR spectral
regions. BaGe(IO3)6(H2O) cystals are potential NLO material in
the visible window. Results of our work indicate that
introducing non-SOJT-distortive MO6 octahedra can also be a
facile strategy to design NCS mixed-metal iodates with large
SHG effect and potential application.
‐
ASSOCIATED CONTENT
Supporting Information. X–ray crystallographic files in CIF
format, check reports, simulated and experimental XRD patterns,
TGA and DSC curves, IR and UV spectra, crystallographic data
and structure refinements, selected bond distances, calculated state
energy of the L-CB and H-VB, calculated band structure, DOS and
PDOS. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Accession Codes
CCDC 1819449−1819453 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
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