An order-disorder ferroelectric host-guest inclusion compound

Article abstract of DOI:10.1002/anie.201307690

The host-guest complex [(DIPA)([18]crown-6)](ClO₄) (1; DIPA=2,6-diisopropylanilinium) was constructed and found to undergo a sequence of phase transitions (Ibam-Pbcn-Pna2₁) at T₁=278K and T₂=132K, respectively. Systematic characterizations, such as differential scanning calorimetry, heat capacity, temperature-dependent dielectric constant, and P-E hysteresis loop, reveal that the centrosymmetric-to-polar phase transition at T₂ is a paraelectric-to-ferroelectric transition. The symmetry breaking was also confirmed by temperature-dependent second-harmonic generation effect and X-ray powder diffraction. The ferroelectric mechanism is attributable to the linear motion of the perchlorate counterions accompanied by the order-disorder transition of the [18]crown-6 molecules and the anions. Copyright

Full text of DOI:10.1002/anie.201307690

.
Angewandte
Communications
DOI: 10.1002/anie.201307690
Molecular Ferroelectrics
An Order–Disorder Ferroelectric Host–Guest Inclusion Compound**
Yi Zhang, Heng-Yun Ye, Da-Wei Fu, and Ren-Gen Xiong*
Abstract: The host–guest complex [(DIPA)([18]crown-6)]-
(ClO4) (1; DIPA = 2,6-diisopropylanilinium) was constructed
and found to undergo a sequence of phase transitions (Ibam–
Pbcn–Pna21) at T1 = 278 K and T2 = 132 K, respectively.
Systematic characterizations, such as differential scanning
calorimetry, heat capacity, temperature-dependent dielectric
constant, and P–E hysteresis loop, reveal that the centrosym-
metric-to-polar phase transition at T2 is a paraelectric-to-
ferroelectric transition. The symmetry breaking was also
confirmed by temperature-dependent second-harmonic gener-
ation effect and X-ray powder diffraction. The ferroelectric
mechanism is attributable to the linear motion of the perchlo-
rate counterions accompanied by the order–disorder transition
of the [18]crown-6 molecules and the anions.
undergo rotational/hopping motion like a molecular rotator
or local wobbling/twisting motion, producing an order–
disorder type ferroelectric phase transition. This has been
evidenced in the first molecular pendulum-like ferroelectrics
[(C7H10NO)([18]crown-6)][BF4] and [(C7H10NO)([18]crown-
6)][ReO4] (C7H10NO = 4-methoxyanilinium cation).[6] The
origin of their ferroelectricity is due to the order–disorder
transition of the pendulum-like motions of the guest 4-
methoxyanilinium cation.
Herein, we report a new host–guest ferroelectric com-
pound [(DIPA)([18]crown-6)](ClO4) (1; DIPA = 2,6-diiso-
propylanilinium; Scheme 1). Structurally, compound 1 is
M
olecular ferroelectrics have recently attracted much
attention from researchers in the fields of chemistry, physics,
and materials science.[1–6] Their high performance is compa-
rable to that of pure inorganic ferroelectrics, such as large
spontaneous polarization and high Curie temperature,[4] as
well as new ferroelectric physics, such as charge-transfer along
p···p-stacked supramolecular networks of electron donors
and acceptors leading to spontaneous polarization.[7] In
contrast to inorganic ferroelectric oxide compounds, molec-
ular ferroelectrics have advantages in many aspects, such as
light weight, nontoxicity, easy processability, structural tuna-
bility, and multifunctionality. A key problem in developing
ferroelectrics is to construct new types of compounds, which
needs deep understanding of the origin of ferroelectricity. Up
to now, in molecule-based ferroelectrics, the most commonly
employed mechanism is of order–disorder type. One example
is hydrogen-bonded organic ferroelectrics such as [(daba-
co)ReO4] in which the order–disorder transition of the
protons induce the occurrence of polarization.[1] Other
examples include order–disorder-type ferroelectrics, and
especially organic salt-type, with relatively larger organic
and/or inorganic groups.[2–6]
Scheme 1. The structural formula of compound 1.
analogous to [(C7H10NO)([18]crown-6)][BF4].[6a] However, it
shows a different phase transition behavior and the origin of
ferroelectricity is found not from motional changes of the
DIPA guest cation but from the order–disorder transition of
the [18]crown-6 host molecule and perchlorate counterion.
Crystals of 1 were obtained by slow evaporation of an
acetone
solution
containing
2,6-diisopropylaniline,
[18]crown-6, and HClO4 at room temperature. Phase purity
of 1 was confirmed by powder X-ray diffraction and IR
analysis (Supporting Information, Figures S1 and S2a).
The phase transition behavior of 1 was firstly evidenced by
heat capacity (Cp) and differential scanning calorimetry
(DSC; Figure 1). The DSC curves clearly show two phase
transitions at 278 K (T1) and 132 K (T2), respectively. The
phase above T1 is designated as the high-temperature phase
(HTP), the phase between T1 and T2 as the intermediate-
Host–guest compounds are a very promising class of
molecular ferroelectrics, for example, [18]crown-6-based
compounds.[5,6] Generally, the [18]crown-6 molecule acts as
a host of which the six O atoms afford lone-pair electrons to
anchor the guest ammonium cation. The guest ion may
[*] Dr. Y. Zhang, Dr. H.-Y. Ye, Dr. D.-W. Fu, Prof. R.-G. Xiong
Ordered Matter Science Research Center
Southeast University, Nanjing 211189 (P.R. China)
E-mail: xiongrg@seu.edu.cn
[**] This work was supported by the Project 973 2014CB848800, the
National Natural Science Foundation of China (21290172,
21101025 and 91222101).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307690.
Figure 1. Heat capacity and DSC measurements of 1.
2114
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 2114 –2118

Angewandte
Chemie
temperature phase (ITP), and the phase below T2 as the low-
temperature phase (LTP). The broad peaks and the narrow
temperature hysteresis (2 K) of the thermal anomalies
indicate that both phase transitions are of second-order
type. Entropy changes (DS) accompanying the phase tran-
sitions at around T1 and T2 are about 5.216 JmolÀ1 KÀ1 and
2.585 JmolÀ1 KÀ1, respectively. According to the Boltzmann
equation, DS = RlnN, where R is the gas constant and N is the
ratio of the number of respective geometrically distinguish-
able orientations, the values of N(T1) and N(T2) are calculated
as 1.87 and 1.36, respectively, indicating the disorder–order
feature of the two phase transitions. The Cp measurement of
1 was taken in two temperature ranges around the phase
transition points T1 and T2. The Cp curves also obviously show
two thermal anomalies at around 278 K and 132 K, matching
well with the DSC measurement.
Variable-temperature X-ray diffraction analysis of 1 con-
firms it undergoes two sequential structural phase transitions
with space group Ibam in HTP, Pbcn in ITP, and Pna21 in
LTP.[8] At room temperature, the crystal structure consists of
host [18]crown-6 molecules and guest DIPA cations with
ClO4À as counterpart anions (Figure 2a). The DIPA cation is
À
situated in the cavity of the [18]crown-6 molecule by N H···O
hydrogen-bonding interactions. The H-bonded DIPA···[18]-
crown-6 moiety is located at the mirror site. The intra-
molecular mirror plane of the DIPA cation is superimposed
with the crystallographic mirror plane, whereas the mirror
symmetry of the [18]crown-6 molecule is satisfied by the
orientational disorder. Different from the common 2D sym-
metrical disk-shaped structure,[9] the structure of the
[18]crown-6 molecule is boat-shaped owing to the steric
hindrance between the skeleton of the [18]crown-6 molecule
and the two isopropyl groups of the DIPA cation. The
alternated DIPA cations and [18]crown-6 molecules form
columns along the b-axis. The tetrahedral ClO4À is disordered
over two orientations; the Cl atom lies on the twofold rotation
axis along the b-axis direction. The packing view of 1 at 293 K
along the b-axis is illustrated in the Supporting Information,
Figure S3.
At 173 K, the ITP remains centrosymmetric.[8] A common
origin and axial directions similar to those of the HTP was
chosen. The asymmetric unit contains one DIPA cation, one
[18]crown-6 molecule, and two half-perchlorate anions, as
shown in Figure 2b. The [18]crown-6 molecule becomes
Figure 2. Asymmetric units of 1 in a) the high-temperature phase
(HTP, 293 K), b) the intermediate-temperature phase (ITP, 173 K), and
c) the low-temperature phase (LTP, 93 K). C gray, H pale blue, N dark
blue, O red, Cl green. The parts with pink-colored bonds are generated
by symmetry operation in (a) and (b). H atoms bonded to the C
atoms are omitted for clarity. The red arrow in (c) indicates the
direction of spontaneous polarization (Ps).
À
ordered and loses the mirror symmetry. The ClO4 anions
remain on the special position with twofold axis symmetry.
À
One ClO4 anions becomes ordered, and the other remains
disordered over two staggered orientations with a common
À
O Cl bond. This bond is superimposed on the crystallo-
graphic twofold axis. Except the ordering, the crystal struc-
ture is similar to the RTP (Supporting Information, Figures S3
and S4).
fraction of the domain with present absolute structure being
0.35(4), that of the corresponding inverted structure being
0.65(4). The presence of ferroelectric domain structure is the
result of minimizing the free energy of the ferroelectric
crystal.[12] The asymmetric unit contains two molecular units.
The obvious structural change is the further ordering of the
disordered ClO4À anion in the ITP. The ordering leads to the
single orientation of this anion (Figure 2c). In the ITP, the
At 93 K, the LTP assumes a polar space group. The
relationship between the two cells at 93 and 173 K is a93k
ꢀ
c
173k, b93k ꢀ b173k, and c93k ꢀ a173k. The structure is refined with
a racemic twinning model with the Flack parameter[10] of
0.65(4). For ferroelectric crystals, the domain structure is
identical with the twinning structure.[11] The refinement
reveals the LTP consists of 1808 ferroelectric domains, the
À
crystallographic twofold axis symmetry of this ClO4 anion
along the b-axis is satisfied by the twofold orientational
Angew. Chem. Int. Ed. 2014, 53, 2114 –2118
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2115

.
Angewandte
Communications
disorder (Figure 2b), while in the LTP the intra-anionic
twofold axis is not superimposed with crystallographic two-
fold axis, and the crystallographic twofold axis symmetry
along the b-axis is broken. The check of additional symmetry
of the crystal structure with PLATON[13] reveals that the
structure has pseudo center of symmetry. That means that
except for the ordering of the ClO4À anions, the ferroelectric
phase at 93 K just shows small structural change from that of
paraelectric phase at 173 K. It appears that the order–
À
disorder transition of the [18]crown-6 and/or ClO4 anions
play an important role in the phase transitions. The difference
between the three temperature structures can be seen in the
Supporting Information, Figures S3–S5.
The space group of 1 changes from Ibam to Pbcn to Pna21
during the two-phase-transition process. In the HTP, 1 adopts
a centrosymmetric space group Ibam belonging to a nonpolar
point group D2h. The ITP changes into a lower symmetric
space group Pbcn. In this phase transition, symmetry breaking
does not occur and the symmetric elements remain
unchanged. In the LTP, 1 still belongs to an orthorhombic
crystal system but with a noncentrosymmetric space group
Pna21 (polar point group C2v). Symmetry breaking occurs
with an Aizu notation of mmmFmm2. The eight symmetric
elements (E, C2, C2’, C2’’, i, sh, sv, s’v) in the paraelectric phase
(ITP) are halved into four (E, C2, sv, s’v) in the ferroelectric
phase (LTP), a typical second-order phase transition feature
according to Landau phase transition theory.[14] According to
the Curie symmetry principle, the space group in the ferro-
electric phase is a subgroup of the one in the paraelectric
phase (primary phase), that is, Pna21 is a subgroup of Pbcn
whose maximal nonisomorphic subgroups include Pna21,
Pnc2, Pca21, P21212, P21/c, and P2/c, while the nonisomorphic
subgroups of Ibam also cover Pbcn. Therefore, Pbcn probably
can be taken as a modulated commensurate phase.
Figure 4. a) Dielectric constants of 1 measured along the c-axis with
frequencies of 500 Hz to 1 MHz. (b) Electric hysteresis loop of
1 measured at various temperatures. Inset: Ps versus T curve of 1.
symmetric structure (Pbcn) above 132 K to a noncentrosym-
metric one (Pna21) below 132 K. Above 132 K, no SHG signal
is observed, indicating the ITP and HTP are centrosymmetric,
consistent with the space groups Ibam and Pbcn. The value of
the temperature-dependent effective second-order nonlinear
optic susceptibility c(2) behaves like spontaneous polarization
(Ps; inset of Figure 4b), consistent with the Landau relation-
ship c(2) = 6e0bPs, where b is almost independent of the
temperature. Both the variations of Ps and c(2) in the vicinity
of T2 are continuous, revealing a second-order phase tran-
sition.[16]
The second harmonic generation (SHG) effect in crystals
is an effective way to test symmetry-breaking during phase
transitions because noncentrosymmetric solids are SHG-
active. Therefore, we performed SHG measurement to
confirm the centrosymmetric-to-noncentrosymmetric phase
transition.[15] As shown in Figure 3, 1 begins to be SHG-active
and shows a clear peak at 532 nm (frequency doubling) below
132 K, corresponding to the phase transition from a centro-
The temperature dependence of the real part of the
complex relative dielectric permittivity (e’) of 1 was measured
along the three crystallographic axes on the single-crystal
sample and on powdered sample with various frequencies
(Figure 4a; Supporting Information, Figure S6). The crystal
faces of the LTP are selected in the measurement. As shown
in Figure 4a, the temperature dependent dielectric permit-
tivity along the c-axis also exhibits two dielectric anomalies at
around 278 and 132 K upon cooling, consistent with the
results of Cp and DSC measurements. At around 278 K, the
dielectric constants only show small anomalies from about 5
to 5.6. In contrast to this, large dielectric anomalies were
observed at around 132 K with the peak value of about 35,
corresponding to an enhancement of five times, which is the
characteristic feature of paraelectric–ferroelectric phase tran-
sitions. The reciprocal of dielectric constants around the
Figure 3. Temperature dependence of the second-order nonlinear effec-
tive coefficient of 1. Inset: Intensity of SHG as a function of wave-
length at different temperatures.
2116
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 2114 –2118

Angewandte
Chemie
0.71073 ꢀ). Data collection, cell refinement, and data reduction was
ferroelectric phase transition point can be well fitted with the
Curie–Weiss law, e = C/(TÀT0), where C is the Curie–Weiss
constant and T0 is the Curie temperature. The fitted
parameters at different frequencies indicate that the value
of T0 decreases with the increase of frequency and is almost
equal to T2 at 1 MHz, suggesting the phase transition is of
a second-order type (Supporting Information, Table S1). The
value of Cpara and Cferro decreases with increasing frequency.
The ratio of Cpara to Cferro is in the range of 2.69–2.41 and
smaller than 4, also indicating a second-order phase transi-
tion.
performed using Rigaku CrystalClear 1.3.5. The structure of 1 was
solved by direct methods and refined by the full-matrix method based
on F2 using the SHELXL97 software package. All non-hydrogen
atoms were refined anisotropically and the positions of all hydrogen
atoms were generated geometrically. CCDC 891944, 891945, 891946
for 1 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
X-ray powder diffraction was measured on a Rigaku DMX/2000
X-ray diffraction instrument. Specific heat analyses were carried out
on a Quantum Design PPMS. Differential scanning calorimetry
experiments were performed on a NETZSCH DSC 200 F3 under
a nitrogen atmosphere in aluminum crucibles with the heating and
cooling rate of 10 KminÀ1. For dielectric, ferroelectric, and pyro-
electric measurements, the samples were made with single crystals cut
into the form of thin plates perpendicular to the crystal axes. Silver
conductive paste deposited on the plate surfaces was used as the
electrodes. Complex dielectric permittivity was measured with an
Agilent 4284A impedance analyser at the frequency range from 20 Hz
to 1 MHz with an applied electric field of 0.5 V. Dielectric hysteresis
loops were recorded on a Radiant Precision Premier II. Pyroelectric
properties were measured with an electrometer/high resistance meter
(Keithley 6517B). For SHG experiments, an unexpanded laser beam
with low divergence (pulsed Nd:YAG at a wavelength of 1064 nm,
5 ns pulse duration, 1.6 MW peak power, 10 Hz repetition rate) was
used. The instrument model is FLS 920, Edinburgh Instruments and
the low temperature system is 10–325 K, DE 202, while the laser is
Vibrant 355 II, OPOTEK. The numerical values of the nonlinear
optical coefficients for SHG were determined by comparison with
a KDP reference.
Polarization as a function of the applied electric field of
1 was measured with E//c at different temperatures in
a cooling run. The P–E loop is nearly a straight line without
obvious hysteresis above 130 K. At 130 K, just below T2, Ps
occurs and the characteristic dielectric hysteresis loop
appears. Upon further cooling, Ps increases gradually and
reaches saturation at about 108 K with a saturation polar-
ization of 0.35 mCcmÀ2. The inset of Figure 4b represents the
temperature dependence of Ps deduced from the P–E
hysteresis loops. Pyroelectric measurement is also an effective
method to characterize the temperature dependence of Ps.
Figure S7 clearly shows that above T2, Ps is zero, while below
T2, it increases gradually and reaches an maximum value of
0.42 mCcmÀ2. The behavior of the temperature-dependent Ps
is consistent with that deduced from P–E hysteresis loop
measurement (inset of Figure 4b). According to Landau
phase transition theory, Ps can be calculated by the formula
Ps = (2e0CDS)1/2, where Ps, e0, DS, and C stand for sponta-
neous polarization, vacuum permittivity, entropy change, and
Curie–Weiss constant, respectively. If DS = 2.585 JmolÀ1 KÀ1
and C = 150.4 K at 500 Hz are put into the equation, Ps is
estimated to be 0.398 mCcmÀ2, which is fairly consistent with
the experimental result. Meanwhile, with the point charge
model (Supporting Information, Figure S8), the estimated
value (0.1 mCcmÀ2) is in the same order of magnitude as that
from the experiment (Supporting Information, Table S2).
In summary, this work has demonstrated that the host–
guest compound [(DIPA)([18]crown-6)](ClO4) undergoes
a paraelectric-to-ferroelectric phase transition at 132 K,
characterized by X-ray diffraction measurement, thermal
analysis, and dielectric and second harmonic generation
experiments. Origin of the ferroelectricity is attributable to
the synergistic order–disorder transitions of the host
[18]crown-6 molecule and perchlorate counterion rather
than the rotation of the guest DIPA cation, which is different
from the known mechanism in other host–guest ferroelec-
trics.[6] It points out a new way to explore and design novel
molecule-based ferroelectric compounds.
Received: September 1, 2013
Revised: December 3, 2013
Published online: February 4, 2014
Keywords: dielectric constant · ferroelectrics · host–
.
guest compounds · order–disorder · phase transitions
´
[1] a) A. Katrusiak, M. Szafranski, Phys. Rev. Lett. 1999, 82, 576 –
´
579; b) A. Katrusiak, M. Szafranski, G. J. McIntyre, Phys. Rev.
Lett. 2002, 89, 215507; c) S. Horiuchi, Y. Tokura, Nat. Mater.
2008, 7, 357 – 366.
[2] a) S. Ohkoshi, H. Tokoro, T. Matsuda, H. Takahashi, H. Irie, K.
Hashimoto, Angew. Chem. 2007, 119, 3302 – 3305; Angew. Chem.
Int. Ed. 2007, 46, 3238 – 3241; b) D.-W. Fu, W. Zhang, H.-L. Cai,
Y. Zhang, J.-Z. Ge, R.-G. Xiong, S. D. Huang, N. Takayoshi,
Angew. Chem. 2011, 123, 12153 – 12157; Angew. Chem. Int. Ed.
2011, 50, 11947 – 11951; c) Z. H. Sun, T. L. Chen, J. H. Luo, M. C.
Hong, Angew. Chem. 2012, 124, 3937 – 3942; Angew. Chem. Int.
Ed. 2012, 51, 3871 – 3876.
[3] a) W. Zhang, H.-Y. Ye, H.-L. Cai, J.-Z. Ge, R.-G. Xiong, S. P. D.
Huang, J. Am. Chem. Soc. 2010, 132, 7300 – 7302; b) G.-C. Xu, W.
Zhang, X.-M. Ma, Y.-H. Chen, L. Zhang, H.-L. Cai, Z.-M. Wang,
R.-G. Xiong, S. Gao, J. Am. Chem. Soc. 2011, 133, 14948 – 14951.
[4] a) D.-W. Fu, W. Zhang, H.-L. Cai, J.-Z. Ge, Y. Zhang, R.-G.
Xiong, Adv. Mater. 2011, 23, 5658 – 5662; b) D. W. Fu, H. L. Cai,
Y. M. Liu, Q. Ye, W. Zhang, Y. Zhang, X. Y. Chen, G.
Giovannetti, M. Capone, J. Y. Li, R. G. Xiong, Science 2013,
339, 425 – 428.
Experimental Section
All of the reagents and solvents in the synthesis were of reagent grade
and used without further purification. Evaporation of a acetone
solution (50 mL) containing 2,6-diisopropylaniline (1.77 g, 0.01 mol),
[18]crown-6 (2.64 g, 0.01 mol), and HClO4 (1.4 g, 0.01 mol) results in
the formation of 1 as colorless crystals. IR: n˜ = 3128 (vs, NH3+), 1467
(s, C = C), 1109 cmÀ1 (vs, ClO4À).
[5] T. Akutagawa, H. Koshinaka, D. Sato, S. Takeda, S.-I. Noro, H.
Takahashi, R. Kumai, Y. Tokura, T. Nakamura, Nat. Mater. 2009,
8, 342 – 347.
Variable-temperature X-ray diffraction analysis was carried out
on 1 using a Rigaku CCD diffractometer with MoKa radiation (l =
[6] a) D.-W. Fu, W. Zhang, H.-L. Cai, Y. Zhang, J.-Z. Ge, R.-G.
Xiong, S. D. Huang, J. Am. Chem. Soc. 2011, 133, 12780 – 12786;
Angew. Chem. Int. Ed. 2014, 53, 2114 –2118
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2117

.
Angewandte
Communications
b) D.-W. Fu, H.-L. Cai, S.-H. Li, Q. Ye, L. Zhou, W. Zhang, Y.
Zhang, F. Deng, R.-G. Xiong, Phys. Rev. Lett. 2013, 110, 257601.
[7] A. S. Tayi, A. K. Shveyd, A. C.-H. Sue, J. M. Szarko, B. S.
Rolczynski, D. Cao, T. Jackson Kennedy, A. A. Sarjeant, C. L.
Stern, W. F. Paxton, W. Wu, S. K. Dey, A. C. Fahrenbach, J. R.
Guest, H. Mohseni, L. X. Chen, K. L. Wang, J. F. Stoddart, S. I.
Stupp, Nature 2012, 488, 485 – 489.
different temperatures are shown in the Supporting Information,
Figure S2.
[9] T. Akutagawa, T. Nakamura, Dalton Trans. 2008, 6335 – 6345.
[10] H. D. Flack, Acta Crystallogr. 1983, A39, 876 – 881.
[11] J. Fousek, V. Janovec, J. Appl. Phys. 1969, 40, 135 – 142.
[12] M. E. Lines, A. M. Glass, Principles and Applications of Ferro-
electrics and Related Materials, Clarendon, Oxford, 1977, pp. 94 –
102.
[8] Crystal data for 1 at 293 K: [(C12H20N)(18C6)](ClO4), Mr =
542.05, orthorhombic, Ibam, a = 17.083(3), b = 17.615(4), c =
19.139 (4) ꢀ, V= 5759(2) ꢀ3, Z = 8, 1cald = 1.25 MgmÀ3, R1 (I >
2s(I)) = 0.0790, wR2 (all data) = 0.2338, m = 0.184 mmÀ1, S =
1.050. 1 at 173 K: orthorhombic, Pbcn, a = 16.874 (4), b =
[13] G. M. Sheldrick, Acta Crystallogr. 2008, A64, 112 – 122.
[14] K. Aizu, Phys. Rev. B 1970, 2, 754 – 772.
[15] a) H.-L. Cai, W. Zhang, J.-Z. Ge, Y. Zhang, K. Awaga, T.
Nakamura, R.-G. Xiong, Phys. Rev. Lett. 2011, 107, 147601; b) Y.
Zhang, W. Zhang, S.-H. Li, Q. Ye, H.-L. Cai, F. Deng, R.-G.
Xiong, S. D. Huang, J. Am. Chem. Soc. 2012, 134, 11044 – 11049.
[16] a) T. Hang, W. Zhang, H.-Y. Ye, R.-G. Xiong, Chem. Soc. Rev.
2011, 40, 3577 – 3598; b) W. Zhang, R.-G. Xiong, Chem. Rev.
2012, 112, 1163 – 1195.
17.507 (4), c = 18.943 (5) ꢀ, V= 5596(2) ꢀ3, Z = 8, 1cald
=
1.287 MgmÀ3, R1 (I > 2s(I)) = 0.0591, wR2 (all data) = 0.1677,
m = 0.190 mmÀ1, S = 1.158. 1 at 93 K: orthorhombic, Pna21, a =
18.831 (4), b = 17.464 (3), c = 16.720 (3) ꢀ, V= 5498.6 (3) ꢀ3,
Z = 8, 1cald = 1.310 MgmÀ3, R1 (I > 2s(I)) = 0.0374, wR2 (all
data) = 0.1051, m = 0.193 mmÀ1, S = 1.116. XRD patterns at
2118
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 2114 –2118