A series of new alkali metal indium iodates with isolated or extended anions

Article abstract of DOI:10.1039/c0dt01272f

Systematic explorations of new phases in the A^I-In ^III-I^V-O system by hydrothermal reactions led to five new compounds, namely, AIn(IO₃)₄ (A = Li, Na), Rb ₃In(IO₃)₆ an

Full text of DOI:10.1039/c0dt01272f

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PAPER
A series of new alkali metal indium iodates with isolated or extended anions†
Bing-Ping Yang, Chuan-Fu Sun, Chun-Li Hu and Jiang-Gao Mao*
Received 22nd September 2010, Accepted 5th November 2010
DOI: 10.1039/c0dt01272f
Systematic explorations of new phases in the AI–InIII–IV–O system by hydrothermal reactions led to five
new compounds, namely, AIn(IO3)4 (A = Li, Na), Rb3In(IO3)6 and A2HIn(IO3)6 (A = Rb, Cs). The
structure of AIn(IO3)4 (A = Li, Na) contains one-dimensional [In(IO3)4]- chains separated by Li+ or
Na+ cations. In both compounds, each In3+ cation is octahedrally coordinated by six IO3 anions,
-
neighboring In3+ cations are interconnected by bidentate bridging iodate anions into 1D chains. The
structures of Rb3In(IO3)6 and A2HIn(IO3)6 (A = Rb, Cs) all feature isolated [In(IO3)6]3- anions with
alkali metal ions (and H+ ions) as spacers. Both optical diffuse reflectance spectrum measurements and
band structure calculations based on DFT methods indicate that LiIn(IO3)4, NaIn(IO3)4, and
Rb2HIn(IO3)6 are insulators.
Introduction
Experimental
Noncentrosymmetric (NCS) oxide materials are of current in-
terest owing to their important properties such as second-
harmonic generation (SHG), piezoelectricity, ferroelectricity, and
pyroelectricity.1 Metal iodates can form a diversity of unusual and
NCS structures because of the presence of stereochemically active
lone-pair electrons on I(V) atom which could lead to an asymmetric
coordination polyhedron.2–4 The introduction of d0 transition
metal ions such as Ti4+, V4+, Nb5+ and Mo6+, which are also
susceptible to second-order Jahn–Teller (SOJT) distortion, into
the metal iodates, led to a number of novel quaternary iodates,5–10
among which a few compounds have been found to display
large SHG responses.5–7 Recently, we found that the combination
of two types of lone-pair cations such as lead(II) and iodate
anions can also afford new second-order NLO compounds.11
As for metal iodates of the group 13 metals, M(IO3)3 (M =
Al, Ga, In) that crystallized in a polar space group P63 have
been reported.12 Al(IO3)3·2HIO3·6H2O,13 Al(IO3)2(NO3)(H2O)6,14
AlH2(IO3)5(H2O)6,15 Al(IO3)3(H2O)8,16 In(IO3)3(H2O)2,17 and a-
Materials and instrumentation
All of the chemicals were analytically pure from commercial
sources and used without further purification. Microprobe el-
emental analyses were performed on a field emission scanning
electron microscope (FESEM, JSM6700F) equipped with an
energy dispersive X-ray spectroscope (EDS, Oxford INCA). The
X-ray powder diffraction data were collected on a Panalytical
X’pert Pro MPD or RIGAKU DMAX2500PC diffractometer
using graphite-monochromated Cu–Ka radiation in the 2q range
of 5–65◦ with a step size of 0.02◦ or 0.05◦. TGA and DTA studies
were all carried out with a NETZSCH STA 449C instrument.
The sample and reference (Al2O3) were enclosed in a platinum
crucible and heated at a rate of 15 ◦C min-1 from room temperature
to 1000 ◦C under a nitrogen atmosphere. The IR spectra were
recorded on a Magna 750 FT-IR spectrometer as KBr pellets in
the range of 4000–400 cm-1. The UV-Vis absorption and optical
diffuse reflectance spectra were measured at room temperature
with a PE Lambda 900 UV-Vis spectrophotometer in the range
of 2500–200 nm. BaSO4 plate was used as a standard (100%
reflectance). The absorption spectrum was calculated from a
reflectance spectrum using the Kubelka–Munk function:20 a/S =
(1 - R)2/2R, where a is the absorption coefficient, S is the
scattering coefficient which is practically wavelength independent
when the particle size is larger than 5 mm, and R is the reflectance.
18
K3In(IO3)6 are also structurally non-centrosymmetric. Cen-
trosymmetric In(IO3)3(H2O)3, In(OH)(IO3)2·H2O and b-In(IO3)3
have also been reported.4,19 Our current research interests focus
on the effects of the ionic size of alkaline metal and the In/IO3
molar ratio on the structures of compounds formed in the alkali-
In-iodate system. Five new compounds, namely, AIn(IO3)4 (A =
Li, Na), Rb3In(IO3)6 and A2HIn(IO3)6 (A = Rb, Cs) were isolated.
They exhibit three types of structures. Herein, we report their
syntheses, crystal structures and characterizations.
Preparation of LiIn(IO3)4, NaIn(IO3)4, Rb3In(IO3)6,
Rb2HIn(IO3)6 and Cs2HIn(IO3)6
State Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002,
PR China
† Electronic supplementary information (ESI) available: Simulated and
experimental XRD patterns, IR. See DOI: 10.1039/c0dt01272f
These five compounds were synthesized by a similar method.
A mixture of alkali metal salts, In2O3 and H5IO6 in water was
sealed in an autoclave equipped with a Teflon liner (23 mL)
and heated at 200 ◦C for 4 days, followed by slow cooling
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Table 1 Crystal data and structural refinements for the five compounds
Compound
LiIn(IO3)4
NaIn(IO3)4
Rb3In(IO3)6
Rb2HIn(IO3)6
Cs2HIn(IO3)6
Formula
FW
LiInI4O12
821.36
P1
NaInI4O12
837.41
P21/c
7.265(4)
4.957(2)
15.515(9)
90
101.404(9)
90
547.7(5)
2
5.078
13.531
1.133
Rb3InI6O18
1420.63
P1
Rb2HInI6O18
1336.17
P1
Cs2HInI6O18
1431.05
P1
¯
¯
¯
¯
Space group
˚
a/A
4.889(3)
6.534(4)
8.568(5)
85.586(15)
89.911(14)
88.827(15)
272.8(3)
1
4.999
13.540
1.121
0.0296, 0.0737
0.0313, 0.0750
7.076(3)
7.091(4)
10.728(6)
88.222(18)
71.488(15)
72.749(14)
486.3(4)
1
4.851
18.280
1.055
0.0406, 0.1013
0.0463, 0.1136
7.753(3)
7.776(3)
7.776(3)
96.790(15)
98.096(14)
98.948(14)
453.7(3)
1
4.890
16.926
1.035
0.0590, 0.1264
0.0811, 0.1454
7.873(5)
7.918(5)
7.938(5)
96.877(11)
98.695(6)
98.091(7)
479.2(5)
1
4.959
14.723
1.124
0.0215, 0.0477
0.0251, 0.0491
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
m(Mo-Ka)/mm-1
Goodness-of-fit on F2
R1, wR2 [I > 2s(I)]a
R1, wR2(all data)
0.0189, 0.0426
0.0210, 0.0435
ꢀ
ꢀ
ꢀ
ꢀ
1/2
R1 = ꢀFo| -|Fcꢀ/ |Fo|, wR2 = { w[(Fo)2 - (Fc)2]2/ w[(Fo)2]2}
.
◦
to room temperature at a rate of 6 C h-1. The loaded com-
positions are Li(CH3COO)·2H2O (0.0765 g, 0.75 mmol), In2O3
(0.0694 g, 0.25 mmol), H5IO6 (0.9118 g, 4 mmol) and H2O
(5 mL) for LiIn(IO3)4; NaCl (0.0438 g, 0.75 mmol), In2O3
(0.0694 g, 0.25 mmol), H5IO6 (0.4558 g, 2 mmol), and H2O
(2 mL) for NaIn(IO3)4; Rb2CO3 (0.1732 g, 0.75 mmol), In2O3
(0.0694 g, 0.25 mmol), H5IO6 (0.6838 g, 3 mmol), and H2O (2 mL)
for Rb3In(IO3)6; RbCl (0.1814 g, 1.5 mmol), In2O3 (0.0694 g,
0.25 mmol), H5IO6 (0.6838 g, 3 mmol), and H2O (2 mL) for
Rb2HIn(IO3)6; and CsCl (0.1684 g, 1 mmol), In2O3 (0.0694 g,
0.25 mmol), H5IO6 (0.6838 g, 3 mmol), and H2O (2 mL) for
Cs2HIn(IO3)6. The initial and final pH values are 2.0 and 2.0,
0.7 and 0.7, 0.6 and 0.7, 0.3 and 0.4, 0.2 and 0.3, respec-
tively, for LiIn(IO3)4, NaIn(IO3)4, Rb3In(IO3)6, Rb2HIn(IO3)6 and
Cs2HIn(IO3)6. Crystals of Rb3In(IO3)6 were obtained in a low yield
(ca 20%) along with RbIO3 and In(IO3)3 crystals as impurities. For
the preparation of Rb2HIn(IO3)6, when Rb2CO3 was used as the
starting material instead of RbCl and the initial pH value was
adjusted to 0.5 by adding hydrochloric acid, both Rb2HIn(IO3)6
and Rb3In(IO3)6 were formed in the final products. Colorless
crystals of Cs2HIn(IO3)6 were obtained in a ca 83% yield along
with very small amounts of unidentified impurities. Crystals of
LiIn(IO3)4, NaIn(IO3)4, and Rb2HIn(IO3)6 were collected in a yield
of 46%, 64%, and 58% based on In2O3, respectively as single phases
(Fig. S1†). Results of microprobe elemental analyses on several
single crystals of each compound gave average molar ratios of A:
In: I of 1.0 : 1.1 : 3.9, 2.9 : 1.0 : 5.6, 2.0 : 1.0 : 6.1, and 2.1 : 1.0 : 5.9
respectively for NaIn(IO3)4, Rb3In(IO3)6, Rb2HIn(IO3)6 and
Cs2HIn(IO3)6, which are in good agreement with those determined
from single-crystal X-ray structural studies. A lot of effort includ-
ing changing the molar ratio of the starting materials, reaction tem-
perature and the pH value of the reaction media was made in order
to prepare pure phase of Rb3In(IO3)6 and Cs2HIn(IO3)6, but were
unsuccessful. Hence their physical properties were not measured.
ation (l = 0.71073 A) at 293 K. The data sets were corrected
for Lorentz and polarization factors as well as for absorption
by SADABS program.21 All five structures were solved by the
Patterson method and refined by full-matrix least-squares fitting
on F2 by SHELX-98.21 All non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms in A2HIn(IO3)6
(A = Rb, Cs) were not refined due to the difficulty in determination
of its exact location. But we expect that the proton should be lo-
cated on an inversion center. Crystallographic data and structural
refinements for compounds are summarized in Table 1. Important
bond lengths are listed in Table 2. Further details of the crystal
structure studies can be obtained from the Fachinformation-
szentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany
(Fax: (49) 7247808666; e-mail: crysdata@fiz-karlsruhe.de), on
quoting the depository numbers CSD 422056–422060.
˚
Computational descriptions
Single-crystal structural data of the first three compounds were
used for the theoretical calculations. The other two compounds
were not calculated theoretically due to the difficulty in deter-
mination of the exact location of hydrogen atoms. The ab initio
band structure calculations and density of states (DOS) were
performed by using the computer code CASTEP.22 The ultrasoft
pseudopotentials were chosen to treat the interactions between
the core and valence electrons.23 The exchange–correlation energy
was calculated using the Perdew–Burke–Ernzerh of modification
to the generalized gradient approximation (GGA).24 The number
of plane waves included in the basis set is determined by a kinetic-
energy cutoff of 380 eV for LiIn(IO3)4, 370 eV for NaIn(IO3)4
and 340 eV for Rb3In(IO3)6. The numerical integration of the
Brillouin zone is performed by using 5 ¥ 4 ¥ 3, 4 ¥ 5 ¥ 2 and 4 ¥ 4 ¥
2 Monkhorst–Pack k-point sampling for LiIn(IO3)4, NaIn(IO3)4
and Rb3In(IO3)6, respectively. The following orbital electrons
were treated as valence electrons: Li-1s22s1, Na-2s22p63s1, Rb-
4s24p65s1, In-4d105s25p1, I-5s25p5, O-2s22p4. The other calculating
parameters used in the calculations and convergent criteria were
set by the default values of the CASTEP code.
X-Ray crystallography
Crystallographic data were collected on a SCXmini CCD diffrac-
tometer equipped with a graphite-monochromated Mo-Ka radi-
1056 | Dalton Trans., 2011, 40, 1055–1060
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˚
Table 2 Important bond lengths (A) for five compounds
Results and discussion
LiIn(IO3)4
Crystal structure descriptions
Li(1)–O(1) ¥2
Li(1)–O(6) ¥2
Li(1)–O(4) ¥2
I(1)–O(2)
I(1)–O(3)
I(1)–O(1)
2.104(4)
2.126(4)
2.327(4)
1.790(4)
1.816(4)
1.855(4)
In(1)–O(1) ¥2
In(1)–O(5) ¥2
In(1)–O(4) ¥2
I(2)–O(6)
I(2)–O(4)
I(2)–O(5)
2.127(4)
2.133(4)
2.161(4)
1.788(4)
1.814(4)
1.836(4)
The structures of AIn(IO3)4 (A = Li, Na) feature one-dimensional
[In(IO3)4]- chains separated by Li+ or Na+ cations.
¯
LiIn(IO3)4 crystallizes in a triclinic P1. The In(III) cation is in
a slightly distorted octahedron, being coordinated by six oxygen
atoms from six iodate groups with In–O bond (2 ¥ In(1)–O(1)
˚
˚
˚
2.127(4) A, 2 ¥ In(1)–O(5) 2.133(4) A, 2 ¥ In(1)–O(3) 2.161(4) A).
Both I5+ cations are coordinated by three oxygen atoms in a
distorted trigonal-pyramidal geometry. I–O distances are in the
NaIn(IO3)4
˚
range of 1.788(4)–1.855(4) A.
Na(1)–O(6) ¥2
Na(1)–O(1) ¥2
Na(1)–O(4) ¥2
I(1)–O(2)
I(1)–O(3)
I(1)–O(1)
2.263(3)
2.452(3)
2.484(3)
1.805(3)
1.810(3)
1.835(3)
In(1)–O(4) ¥2
In(1)–O(1) ¥2
In(1)–O(5) ¥2
I(2)–O(6)
I(2)–O(5)
I(2)–O(4)
2.116(3)
2.140(3)
2.181(3)
1.779(3)
1.836(3)
1.840(3)
The interconnection of indium(III) atoms via bidentate bridging
I(2)O3 groups results in a chain along the a-axis, the I(1)O3 groups
are attached monodentately on both sides of the chain (Fig. 1a).
The polarizations of IO3 groups cancel each other, hence the
compound is non-polar.
Rb3In(IO3)6
Rb(1)–O(3) ¥2
Rb(1)–O(5) ¥2
Rb(1)–O(4) ¥2
Rb(1)–O(1) ¥2
In(1)–O(7) ¥2
In(1)–O(1) ¥2
In(1)–O(4) ¥2
I(1)–O(2)
I(1)–O(1)
I(1)–O(3)
I(2)–O(6)
I(2)–O(4)
2.781(8)
2.817(9)
3.171(7)
3.204(7)
2.127(6)
2.157(6)
2.180(6)
1.795(6)
1.857(6)
1.970(10)
1.786(7)
1.845(6)
1.957(11)
Rb(2)–O(9)
Rb(2)–O(5)
Rb(2)–O(9)
Rb(2)–O(6)
Rb(2)–O(2)
Rb(2)–O(6)
Rb(2)–O(3)
Rb(2)–O(8)
I(3)–O(7)
2.818(7)
2.928(8)
2.973(7)
2.978(7)
2.978(7)
3.052(7)
3.104(9)
3.105(7)
1.829(6)
1.797(7)
1.822(6)
I(3)–O(9)
I(3)–O(8)
I(2)–O(5)
Rb2HIn(IO3)6
Rb(1)–O(6)
Rb(1)–O(1)
Rb(1)–O(3)
Rb(1)–O(9)
Rb(1)–O(4)
Rb(1)–O(7)
Rb(1)–O(5)
Rb(1)–O(2)
Rb(1)–O(8)
I(3)–O(9)
2.921(11)
2.944(10)
2.941(13)
2.988(11)
2.999(10)
3.016(10)
3.040(12)
3.139(11)
3.149(11)
1.792(10)
1.805(10)
In(1)–O(4) ¥2
In(1)–O(1) ¥2
In(1)–O(7) ¥2
I(1)–O(3)
I(1)–O(1)
I(1)–O(2)
I(2)–O(6)
I(2)–O(5)
I(2)–O(4)
I(3)–O(7)
2.124(11)
2.130(11)
2.125(11)
1.799(10)
1.815(10)
1.830(10)
1.784(11)
1.819(10)
1.832(9)
Fig. 1 (a) View of the one-dimensional ribbon in compound LiIn(IO3)4.
(b) View of the 2D layer parallels the ab plane in compound LiIn(IO3)4. (c)
A packing diagram for compound LiIn(IO3)4 projected along the a-axis.
The following colors correspond to these atoms: indium, blue; iodine,
purple; oxygen, red; lithium, gray.
1.836(12)
Neighbouring 1-Dimensional chains are further interconnected
by lithium(I) ions into a 2D layer parallel to the ab plane (Fig. 1b).
I(3)–O(8)
Neighbouring layers are held together by weak van der Waals
+
˚
forces (Fig. 1c). The interlayer distance is 2.79 A. Each Li
Cs2HIn(IO3)6
cation is octahedrally coordinated by six oxygen atoms from six
IO3 groups in a unidentate fashion with Li–O distances in the
Cs(1)–O(4)
Cs(1)–O(8)
Cs(1)–O(5)
Cs(1)–O(7)
Cs(1)–O(1)
Cs(1)–O(3)
Cs(1)–O(9)
Cs(1)–O(6)
Cs(1)–O(2)
I(3)–O(8)
3.043(4)
3.068(4)
3.088(4)
3.090(4)
3.107(4)
3.137(4)
3.209(4)
3.252(4)
3.268(4)
1.778(4)
1.815(4)
In(1)–O(7) ¥2
In(1)–O(1) ¥2
In(1)–O(4) ¥2
I(1)–O(3)
I(1)–O(2)
I(1)–O(1)
I(2)–O(5)
I(2)–O(4)
I(2)–O(6)
I(3)–O(7)
2.118(3)
2.135(3)
2.140(4)
1.791(4)
1.796(4)
1.815(3)
1.784(4)
1.808(3)
1.868(4)
1.816(3)
˚
range of 2.104(4)–2.327(4) A. The IO3 groups adopt two types
of coordination modes. I(1)O3 is bidentate, it bridges with one In
atom and one Li atom by using two oxygen atoms whereas I(2)O3
group is tetradentate, bridging with two In and two Li atoms, two
oxygen atoms are unidentate whereas the third one is bidentate
(Fig. 1b). Bond valence calculations indicate that the Li, In and
I atoms are in an oxidation state of +1, +3, and +5, respectively.
The calculated total bond valences for Li(1), In(1), I(1), and I(2)
are 0.888, 3.153, 4.888 and 4.984, respectively.25
I(3)–O(9)
The structure of compound NaIn(IO3)4 is similar to that
of compound LiIn(IO3)4 though NaIn(IO3)4 crystallizes in a
monoclinic system with space group P21/c. In–O distances
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˚
˚
range from 2.116(3) A to 2.161(4) A. I–O distances are in the
Rb–O bond distances are slightly longer than the K–O bond
+
18
˚
˚
range of 1.779(3)–1.840(3) A. Na cations are also octahedrally
lengths in K3In(IO3)6 (2.66–3.13 A). Bond valence calculations
coordinated by six oxygen atoms from six IO3 groups in a
unidentate fashion with Na–O distances in the range of 2.263(3)–
indicate that the Rb, In and I atoms are in an oxidation state of
+1, +3, and +5, respectively. The calculated total bond valences
for Rb(1), Rb(2), In(1), I(1), I(2), and I(3) are 1.260, 1.140, 3.036,
4.297, 4.427 and 4.936, respectively. The large deviation of the
bond valence of I(1) and I(2) atoms from their expected value
+5 indicates that the intermolecular interactions I–O distances
˚
2.484(3) A. In LiIn(IO3)4, the chains run down the a-axis, whereas
in NaIn(IO3)4, the chains are along the b-axis. Neighbouring 1D
chains are further interconnected by sodium(I) ions into a 2D layer
parallel to the ab plane and neighbouring layers are held together
by weak van der Waals forces. The interlayer distance is 2.96 A.
(2.519–3.046 A) should also be considered (See Table S1†).26 With
˚
˚
Bond valence calculations indicate that the Na, In and I atoms are
in an oxidation state of +1, +3, and +5, respectively. The calculated
total bond valences for Na(1), In(1), I(1), and I(2) are 1.240, 3.114,
4.927 and 4.916, respectively.
Rb3In(IO3)6 is isostructural with K3In(IO3)6 reported
previously.18 The structure of Rb3In(IO3)6 features isolated
[In(IO3)6]3- anions that are separated by Rb+ cations (see Fig. 2).
these weak bonds included the calculated total bond valences of
I(1) and I(2) are 4.756 and 4.862, respectively.
Rb3In(IO3)6 crystallizes in the centrosymmetric space group P1.
Hence it is non-polar and the polarizations of the IO3 groups
cancel each other.
¯
Rb2HIn(IO3)6 and Cs2HIn(IO3)6 are isostructural and crys-
¯
tallized in the triclinic P1. Their structures also feature isolated
[In(IO3)6]3- anions as in Rb3In(IO3)6. However one of three alkali
metal cations lying on the inversion center is replaced by a proton.
In Rb2HIn(IO3)6, the In–O and I–O bond distances range from
˚
˚
2.124(11) to 2.130(11) A and from 1.784(11) to 1.836(12) A,
respectively. In Cs2HIn(IO3)6, the In–O and I–O bond distances
˚
˚
range from 2.118(3) to 2.140(4) A and from 1.778(4) to 1.868(4) A,
respectively (see Table 2). These bond distances are comparable
to those in Rb3In(IO3)6. The Rb+ and Cs+ cations are in nine-
fold coordination environments with bond distances in the range
˚
of 2.921(11)–3.149(11) and 3.043(4)–3.268(4) A. Bond valence
calculations indicate that the Rb/Cs, In and I atoms are in an
oxidation state of +1, +3, and +5, respectively. For Rb2HIn(IO3)6,
the calculated total bond valences for Rb(1), In(1), I(1), I(2),
and I(3) are 1.194, 3.272, 4.954, 4.998 and 5.006, respectively.
For Cs2HIn(IO3)6, the calculated total bond valences for Cs(1),
In(1), I(1), I(2), and I(3) are 1.314, 3.232, 5.144, 4.902 and 5.115,
respectively.
Since the lone pairs on IO3 polyhedra are located trans to each
other (see Fig. 3), their local dipole moments cancelled each other,
rendering the materials non-polar.
Fig. 2 (a) [In(IO3)6]3- unit in Rb3In(IO3)6. (b) A packing diagram for
compound Rb3In(IO3)6 projected along the a-axis. The following colors
correspond to these atoms: indium, blue; iodine, purple; oxygen, red;
rubidium, gray.
-
Each In3+ cation is bonded to six oxygen atoms from six IO3
Fig. 3 (a) [In(IO3)6]3- unit in Cs2HIn(IO3)6. (b) A packing diagram for
compound Cs2HIn(IO3)6 projected along the a-axis. The following colors
correspond to these atoms: indium, blue; iodine, purple; oxygen, red;
caesium, gray.
group in a distorted octahedral environment with In–O bond (2 ¥
˚
˚
In(1)–O(7) 2.127(6) A, 2 ¥ In(1)–O(1) 2.157(6) A, 2 ¥ In(1)–
˚
O(4) 2.180(6) A), which is close to those in K3In(IO3)6 (2.127
5+
˚
to 2.215 A). Each I cation is bonded to three oxygen atoms in a
trigonal pyramidal coordination environment with bond distances
˚
ranging from 1.786(7) to 1.970(10) A, which are similar to the I–
18
˚
O distances (1.794 to 1.852 A) in K3In(IO3)6. There are two
Thermogravimetric analyses (TGA)
crystallographically unique Rb+ cations in the asymmetric unit
of Rb3In(IO3)6. Both of them are eight-coordinated by oxygen
atoms with the Rb–O bond distances in the range of 2.781(8)–
Thermal analyses for LiIn(IO3)4, NaIn(IO3)4, and Rb2HIn(IO3)6
were performed in N2 from 30 to 1000 ◦C (Fig. S2†). In the
temperature range of 330 to 420 ◦C, all three compounds start
˚
˚
3.204(7) A for Rb(1) and 2.818(7)–3.105(7) A for Rb(2). These
1058 | Dalton Trans., 2011, 40, 1055–1060
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to decom◦pose. All of them exhibit two steps of weight losses
Theoretical studies
◦
(330–600 C and 600–900 C). The first one and the second one
To further understand the chemical bonding in these compounds,
band structure as well as DOS calculations for LiIn(IO3)4,
NaIn(IO3)4 and Rb3In(IO3)6 based on the DFT method were made
by using the computer code CASTEP.22
overlapped, during which the iodate anions are decomposed to
I2 and O2, and further decomposition of the compounds. The
observed total weight losses at 900 ◦C are 88.4%, 95.3%, and
85.7% respectively for LiIn(IO3)4, NaIn(IO3)4, and Rb2HIn(IO3)6.
The final residues were not characterized due to their melting with
the TGA bucket made of Al2O3 under such high temperature.
The calculated band structures of three compounds along high-
symmetry points of the first Brillouin zone are plotted in Fig. 6.
For LiIn(IO3)4, the lowest energy of conduction band (L-CB) is
between Q and Z points and the highest energy of valence band
(H-VB) is between G and F, thus it is an indirect band-gap crystal
with a band gap of 3.74 eV. NaIn(IO3)4 is also an indirect band
gap crystal with the L-CB between A and B and the H-VB at B
point, and its band gap is 3.15 eV. Rb3In(IO3)6 is a direct band-
gap crystal (from Z to Z) with a band gap of 2.45 eV. (Table S2†)
The calculated band gaps are much smaller than the experimental
values (4.27 eV for LiIn(IO3)4 and 4.25 eV for NaIn(IO3)4). It
is well known that the DFT-GGA does not accurately describe
the eigenvalues of the electronic states, causing the quantitative
underestimation of band gaps.27
IR, UV-Vis absorption and diffuse reflectance spectra.
IR spectra of AIn(IO3)4 (A = Li, Na) and Rb2HIn(IO3)6 indicate
that they are transparent in the range of 4000–1200 cm-1 (2.5–
8.3 mm). The IR absorption bands at 600–836 cm-1 are due to the
I–O vibrations, whereas, those at 400–490 cm-1 are assigned to the
In–O vibrations (Fig. S3†). These assignments are consistent with
those previously reported for other metal iodates and IR spectrum
of In2O3.7
UV absorption spectra of LiIn(IO3)4, NaIn(IO3)4, and
Rb2HIn(IO3)6 show little absorption in the range of 420–2500 nm
(0.42–2.5 mm) (Fig. 4). Hence, all three compounds are transparent
in the range of 0.42–6.6 mm.
Fig.
6
Band structures for LiIn(IO3)4 (a), NaIn(IO3)4 (b), and
Rb3In(IO3)6 (c). The Fermi level is set at 0 eV.
Fig.
4
UV absorption spectra for LiIn(IO3)4, NaIn(IO3)4 and
Rb2HIn(IO3)6.
The bands can be assigned according to the total and partial
densities of states (DOS) as plotted in Fig. S4.† Since all three
compounds display similar characteristics in DOSs, we will take
Rb3In(IO3)6 as an example to discuss them in detail. The bottom-
most VB region around -23 eV comes from Rb-4s states, and the
bands between -21 and -16 eV are mainly originated from O-2s
states, mixing with small amounts of I-5s, 5p states. Isolated In-4d
states contribute to the peak near -15 eV. The bands from -11.6
to -7.7 eV are composed of Rb-4p, I-5s and O-2s, 2p states. In the
Fermi level regions, namely, -7.5–0 eV in VB and 2.0–6.5 eV in
CB, the overlap of O-2p and I-5p indicate the covalent interactions
between them. In addition, the CB from 6.5 to 11 eV arises from
I-5s, Rb-5s, 5p and some In-5s, 5p states.
Optical diffuse reflectance spectra studies indicate that
LiIn(IO3)4, NaIn(IO3)4, and Rb2HIn(IO3)6 are insulators with
optical band gaps of 4.27, 4.25, and 4.07 eV, respectively (Fig. 5).
Population analyses allow for a more quantitative bond analysis.
The calculated bond orders of In–O, I–O, Li–O, Na–O and Rb–O
bonds are 0.25–0.32 e, 0.25–0.45 e, 0.10–0.16 e, 0.09–0.14 e and
0.02–0.13 e, respectively (covalent single-bond order is generally
1.0 e), so we can say that I–O and In–O bonds have more covalent
Fig. 5 UV-visible diffuse reflectance spectra for LiIn(IO3)4, NaIn(IO3)4
and Rb2HIn(IO3)6. (a) The original spectra. (b) Partial enlarged drawing
of UV-visible diffuse reflectance spectra.
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character, while A–O (A = Li, Na, Rb) bonds exhibit obvious
ionicity. Also it is obvious the covalent strength of A–O bonds
follows the order of Li–O > Na–O > Rb–O as expected. For all
three compounds, the shorter In–O bonds generally have the larger
bond orders than those of longer bonds (see Table S3†).
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Conclusions
In summary, five new indium(III) iodates, namely, AIn(IO3)4
(A = Li, Na), Rb3In(IO3)6 and A2HIn(IO3)6 (A = Rb, Cs), have
been synthesized and characterized. They exhibit three kinds of
structure types. 1D structure is preferred by the smaller cations, Li+
and Na+, whereas 0D structures are found for the larger ones, K+,
Rb+, and Cs+. Because of the ionic size of Li+ and Na+, (relatively)
small coordination environments are preferred. In LiIn(IO3)4 and
NaIn(IO3)4, the Li+ and Na+ cations are in pseudo-octahedral
coordination environments (Fig. S5†). The large ionic sizes of K+,
Rb+, and Cs+, require larger coordination numbers. In K3In(IO3)6
and Rb3In(IO3)6, the alkali cations are in eightfold coordination
environments whereas in Rb2HIn(IO3)6 and Cs2HIn(IO3)6, they
are 9-coordinated. It is also worthy to point out that Rb2HIn(IO3)6
preferred a low pH value (~0.3) and Rb3In(IO3)6 is formed under
a high pH value (>0.5). Hence both the pH value of the reaction
media and the size of the alkali cation have strong effects on the
chemical compositions and structures of the compounds formed
in the A–In–I–O systems. Since all structures are centrosymmetric,
these compounds are not SHG active, but this does not rule out
the possibility to find SHG materials in these group 13 iodates.
Our future research efforts will be devoted to further studies on
the syntheses, crystal structures and optical properties of other
related phases.
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Acknowledgements
This work was supported by National Natural Science Foundation
of China (Nos. 21003127, 20731006, and 20825104).
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1060 | Dalton Trans., 2011, 40, 1055–1060
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The Royal Society of Chemistry 2011
©