Dielectric properties induced by the framework in the hybrid organic-inorganic compounds M(dca)2pyz M = Fe, Co and Zn

Article abstract of DOI:10.1016/j.poly.2015.12.027

In this work we prepare and study the M(dca)₂pyz compounds, where dca = polar dicyanamide N(CN)₂^- ligand, pyz = pyrazine (C₄H₄N₂) ligand and M = Fe²⁺, Co²⁺ and Zn²⁺ that, according to DSC display a first order phase transition, at T_t ～ 270 K (Fe), ～250 K (Co) and ～240 K (Zn). Very interestingly these compounds, which consist of a rather robust framework with incorporated polar dca ligands, and which does not host guest species in the cavities of the structure, display a dielectric anomaly associated to such phase transition. This latter in fact corresponds to the structural transition from the disordered HT-phase (space group Pnma) to the ordered LT-phase (space group P2₁/n). From a detailed analysis of the dipole distributions in each of these crystal structures we conclude that the polar dca ligands, which bridge two transition metal cations, display a cooperative electrical arrangement, in which the dipoles of each of the two interpenetrated networks show a ferroelectric (FE) arrangement; nevertheless, the relative antiparallel orientation of both networks finally results in net antiferroelectric (AFE) arrangements both in the LT- and HT-polymorphs. Very interestingly, the dynamic disorder of the dca ligands at T_t gives rise to the observed dielectric anomaly. These materials constitute, therefore, an example of hybrid organic-inorganic compounds where the dielectric response directly arises from the framework -and not from polar guest species- which is a very interesting and novel approach in the search for materials in which the dielectric and magnetic properties can couple more strongly.

Full text of DOI:10.1016/j.poly.2015.12.027

Accepted Manuscript
Dielectric properties induced by the framework in the hybrid organic-inorganic
compounds M(dca) pyz M=Fe, Co and Zn
2
A. García-Fernández, J.M. Bermúdez-García, S. Castro-García, R. Artiaga, J.
López-Beceiro, M.A. Señarís-Rodríguez, M. Sánchez-Andújar
PII:
S0277-5387(15)00777-9
DOI:
http://dx.doi.org/10.1016/j.poly.2015.12.027
POLY 11726
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
28 September 2015
14 December 2015
Please cite this article as: A. García-Fernández, J.M. Bermúdez-García, S. Castro-García, R. Artiaga, J. López-
Beceiro, M.A. Señarís-Rodríguez, M. Sánchez-Andújar, Dielectric properties induced by the framework in the
hybrid organic-inorganic compounds M(dca) pyz M=Fe, Co and Zn, Polyhedron (2015), doi: http://dx.doi.org/
2
1
0.1016/j.poly.2015.12.027
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Dielectric properties induced by the framework in the hybrid organic-inorganic
compounds M(dca)
2
pyz M=Fe, Co and Zn
1
1
1
2
A. García-Fernández , J. M. Bermúdez-García , S. Castro-García , R. Artiaga , J.
2
1
1
López-Beceiro , M. A. Señarís-Rodríguez and M. Sánchez-Andújar*
1
QuiMolMat Group, Department of Fundamental Chemistry, Faculty of Science and
CICA, University of A Coruña, Campus A Coruña, 15071 A Coruña, Spain.
2
Department of Industrial Engineering II, University of ACoruña. Campus Ferrol,
15403 Ferrol, Spain.
*corresponding author: Tel.: (+34) 981167000 e-mail: msanchez@udc.es
Abstract
In this work we prepare and study the M(dca) pyz compounds, where dca= polar
2
-
2+
2+
dicyanamide N(CN)
2
ligand, pyz=pyrazine (C
4
H
4
N
2
) ligand and M= Fe , Co and
∼270 K (Fe),
250 K (Co) and ∼240 K (Zn). Very interestingly these compounds, which consist of a
2
+
Zn that, according to DSC display a first order phase transition, at T
t
∼
rather robust framework with incorporated polar dca ligands, and which does not host
guest species in the cavities of the structure, display a dielectric anomaly associated to
such phase transition. This latter in fact corresponds to the structural transition from the
disordered HT-phase (space group Pnma) to the ordered LT-phase (space group P2 /n).
1
From a detailed analysis of the dipole distributions in each of these crystal structures we
conclude that the polar dca ligands, which bridge two transition metal cations, display a
cooperative electrical arrangement, in which the dipoles of each of the two
interpenetrated networks show a ferroelectric (FE) arrangement; nevertheless, the
relative antiparallel orientation of both networks finally results in net AFE arrangements
both in the LT- and HT- polymorphs. Very interestingly, the dynamic disorder of the
t
dca ligands at T gives rise to the observed dielectric anomaly. These materials
constitute, therefore, an example of hybrid organic-inorganic compounds where the
dielectric response directly arises from the framework –and not from polar guest
species–, which is a very interesting and novel approach in the search for materials in
which the dielectric and magnetic properties can couple more strongly.
1

Keywords: hybrid organic-inorganic materials, interpenetrated frameworks, phase
transitions, dielectric properties, multiferroic materials.
Introduction
During the last years, the hybrid organic-inorganic frameworks with perovskite
structure have attracted a great deal of attention and have made remarkable strides in
different fields, such as photovoltaic solar cells [1], magnetic [2], dielectric [3] and even
multiferroic materials [4].
Within this latter group, the hybrid perovskite compounds with cooperative electric and
magnetic order continues to attract a lot of attention in view of the potential applications
of such materials in many electric and electronic devices, like capacitors, temperature
sensing, data storage, mechanical actuators, and so on [5,6].
Among them, the dense metal–organic frameworks (MOF) of formula [A][M(HCOO)
where A = mid-sized alkylammonium cations, such as methylammonium,
ethylammonium, dimethylammonium, hydrazinium, formamidinium,
3
]
(
2
+
2+
2+
2+
2+
cyclotrimethyleneammonium and imidazolium and M = Mn , Fe , Co and Ni )
are specially interesting as type I-multiferroics: they combine structural flexibility,
relatively good stability, low density and cooperative magnetic and electrical properties
[
7-13]. These latter arise from different active ”subsystems”: i) A rather robust
framework based on [BO ] octahedra, which are connected through the short organic
formate ligands, and that displays cooperative magnetism at low temperatures (T : 8-36
6
C
K)[2]. ii) A polar mid-sized alkylammonium cations, that occupy the A-sites of the
perovskite architecture, are linked to the framework via hydrogen bonds, and typically
experience an order/disorder process that give rise to dielectric transitions [3,4,12] and
even improper ferroelectricity [14]. In view of the different origins of the magnetic and
ferroelectric order, weak direct magnetoelectric (ME) coupling should be expected in
these systems.
Therefore, it would be very interesting to find alternative hybrid organic-inorganic
materials where the dielectric and magnetic properties are driven by the same active
subsystem as this would, in principle, facilitate a stronger coupling between the two
ferroic properties.
2

In this context, the goal of this paper is to demonstrate that the framework of hybrid
organic-inorganic materials can be responsible for both the magnetic and the dielectric
properties in such type of materials.
For this purpose, we have looked up the literature in the search for a compound with the
following features: a robust 3D framework with incorporated polar ligands and which
does not host guest species in the cavities of the structure.
2
We have focused our attention in the M(dca) pyz series of compounds, where members
2
+
2+
2+
2+
2+
2+
2+
-
with M = Mn , Fe , Co , Ni , Zn and Cu dca= polar dicyanamide N(CN)
ligand and pyz=pyrazine (C ) ligand have been obtained and described [15]. From
the structural point of view, these compounds contain two interpenetrating 3D ReO
2
4
4 2
H N
3
related networks. Bidentate dca ligands, which have been widely used to synthesize a
vast family of coordination polymers compounds [16,17], bridge the transition metal
2
cations to form square-grid-like M(dca) sheets, with the pyrazine ligands linking these
sheets together to form the 3D networks, see Fig. 1 and more details at Fig. S1 of
Supporting Information (S.I.) [15].
It is known that at room temperature, the crystal structure of these compounds is
orthorhombic with space group Pnma and contains dynamic disorder of the dca and
pyrazine ligands, see Fig. 1a and Fig. S1 of S.I.) [15]. Meanwhile, at lower temperatures
1
the structure is monoclinic (space group P2 /n) and the dca and pyrazine ligands are
ordered, see Fig. 1b and Fig. S1 of S.I.) [15], even if the temperature and characteristics
of such phase transition has not been reported yet for any of those compounds.
As for their magnetic properties, a weak antiferromagnetic coupling between the
2
+
2+
2+
2+
transition metal cations has been detected in the Mn -, Fe -, Co - and Ni -
compounds, with evidence of long range antiferromagnetic order below 2.7 K in the
case of the Mn-one [15].
Here and for the first time, we study the dielectric response and thermal behavior of the
2
+
2+
2+
2+
2
M(dca) pyz compounds with M = Fe , Co and Zn . As we will show below, we
have found that these compounds display a dielectric anomaly at the same temperature
at which the structural phase transition takes place. So, both phenomena are associated.
The obtained results support our initial hypothesis that dielectric and magnetic order can
be both induced in the framework of hybrid organic-inorganic materials, which is a very
3

interesting and novel approach to try to set a stronger coupling between the magnetic
and dielectric properties.
Experimental Section
Materials
Iron (II) perchlorate hexahydrate Fe(ClO
hexahydrate Co(NO ·6·H (Aldrich 98%), zinc (II) nitrate hexahydrate
Zn(NO ·6·H O (Aldrich 98%), sodium dicyanamide NaN(CN) (Aldrich 96%) and
pyrazine CH (Aldrich 96%) were commercially available and used as purchased
4
)
2
·6·H O (Merck 98%), cobalt (II) nitrate
2
3
)
2
2
O
3
)
2
2
2
4
4 2
H N
without further purification. A reagent amount of deionised water was also used in the
synthesis.
Synthesis
2
+
2+
2+
2+
The M(dca)
according to the procedure described in the literature [15]. In a typical experiment, a
warm aqueous solution (5 mL) of M(NO ·xH O (0.6 mmol) was added to a warm
2
pyz where M = Fe , Co and Zn compounds were synthesized
3
)
2
2
aqueous solution (10 mL) of Na(dca) (1.2 mmol) and pyrazine (0.6 mmol). Fine crystals
with needle morphology were obtained on cooling. They were washed with ethanol and
dried at room temperature. They were found to be stable in air.
Elemental analysis
Elemental chemical analyses for C, N and H were carried out using a FLASHEA1112
(
ThermoFinnigan) Analyzer.
For these compounds: Fe-compound calculated C: 35.85, H: 1.50, N: 41.81; found C:
5.95, H: 1.25, N: 40.68, Co-compound calculated C: 35.44, H: 1.49, N: 41.33; found
3
C: 35.57, H: 1.21, N: 40.15, Zn-compound calculated C: 34.62, H: 1.45, N: 40.37;
found C: 34.70, H: 1.15, N: 39.06.
Scanning Electron Microscopy
The morphology of the samples was studied by Scanning Electron Microscopy (SEM),
in a JEOL 6400 microscope.
Powder X-ray diffraction
4

Powder X-ray diffraction (PXRD) at room temperature was performed in a Siemens
D-5000 diffractometer using CuKα radiation (λ=1.5418 Å).
For the Fe- and Co-samples the typical holder was used for the experiments.
Nevertheless, in the case of the Zn-sample, and in view of the very strong preferential
orientation effects it displays (see below) a special procedure had to be used: first of all
the sample had to be grinded during longer time (30 min) prior to deposit it onto a
special sample holder, where the powder was sandwiched between two glasses to carry
out the PXRD experiments.
The obtained PXRD patterns were analyzed by Le Bail profile analysis using the
GSAS software [18]. The peak shapes were described by a pseudo-Voigt function and
the background was modelled with a 6-term polynomial function.
Thermal studies
Thermogravimetric analyses (TGA) were carried out in a TGA-DTA Thermal
Analysis SDT2960 equipment. For these experiments approximately 27 mg of sample
were heated at a rate of 5 K/min from 300 K to 1200 K using corundum crucibles under
a flow of dry nitrogen.
Differential scanning calorimetric (DSC) analyses were carried out in a TA
Instruments MDSC Q-2000 equipped with a liquid nitrogen cooling system, by heating
and cooling the sample during several cycles at 10 K/min up to 380 K under a nitrogen
atmosphere.
Dielectric measurements
To measure the dielectric properties, pellets were prepared by cold pressing the obtained
samples under a pressure of 10 tons that was maintained during 5 minutes. In addition,
and taking into account that the compactness of the pellets can affect the dielectric
properties their densities were determined by using the Archimedes method.
r r r
The complex dielectric permittivity (ε = ε’ - iε’’ ) of the so obtained cold-press
pelletized samples was measured as a function of frequency and temperature with a
parallel-plate capacitor coupled to a Solartron1260A Impedance/Gain-Phase Analyzer,
capable to measure in the frequency range from 10 µHz up to 32 MHz using an
amplitude of 2 V. The capacitor was mounted in a Janis SVT200T cryostat refrigerated
with liquid nitrogen, and with a Lakeshore 332 incorporated to control the temperature
5

from 78K up to 400K. The data were collected on heating and before carrying out the
measurements, the pellets were kept for two minutes at each temperature so as to reach
the thermal equilibrium.
2
Pellets with an area of approximately 530 mm and a thickness of approximately 1 mm
were prepared to fit into the capacitor, and silver paste was painted on their surfaces to
ensure a good electrical contact with the electrodes.
All the dielectric measurements were carried out in a nitrogen atmosphere where several
cycles of vacuum and nitrogen gas were performed to ensure that the sample
environment is free of water.
The impedance analysis software SMART (Solartron Analytical) was used for data
acquisition and processing. Impedance complex plane plots were analyzed using the
LEVM program, a particular program for complex nonlinear least squares fitting [19].
Results and discussion
General characterization
According to elemental chemical analyses for C, N and H, all the obtained materials
have the expected chemical composition.
Although at first glance all the obtained crystals have a needle-like morphology, a more
detailed analysis by SEM shows that these needles are in fact formed by stacking of
sheets as it can be seen in Fig. 2, sheets that are very thin in the case of the Zn-sample.
These compounds were obtained as single phase materials, as confirmed by comparison
of their experimental powder X-ray diffraction (PXRD) patterns at room temperature
with those simulated from the corresponding single crystal X-ray diffraction data
(
SCXRD), see Fig. 3. We have to note that the intensity of several peaks in the
experimental PXRD patterns do not match with the intensity of the simulated patterns.
We suggest that this difference is due to the presence of preferential orientations in the
samples, specially in the case of the Zn sample, related to the peculiar morphology of
the crystals. In fact, in the case of this latter, and in order to try to minimize such effects
in the PXRD patterns we had to follow a special procedure, to record them, as indicated
in the Experimental section. In that context, it should be noted that the use of a special
6

sample holder for that purpose help indeed to reduce the preferential orientation effects,
even if it also resulted in a decrease in the intensity of the diffraction peaks , as it can be
seen in Fig. 3 and S2 of S.I..
In addition, LeBail refinement of the obtained PXRD patterns indicates that at room
temperature the three compounds show orthorhombic symmetry and space group Pnma.
The obtained refinements are shown in Fig S2 and Table 1 summarizes the cell
parameters obtained from such LeBail refinements for each compound. It should be
indicated that these are in full agreement with those previously reported in the literature
[
15].
On the other hand, and in the case of the pellets obtained by cold-pressing and that will
be used for the dielectric measurements, their experimental densities are in all cases
3
very similar to the theoretical values, ∼ 1.7 g/cm , that expressed as relative densities
corresponds to∼ 90 %.
Thermogravimetric analyses (TGA)
To study the thermal stability of the obtained materials, we have carried out
thermogravimetric analyses under N atmosphere. The thermal decomposition of the
2
three compounds undergoes through a similar two-step weight loss, see Fig. 4.
The first weight loss (of 30.5-31.8 % depending on the compound) is due to the loss of
all pyrazine ligands from the material. Meanwhile, the second weight loss (of 41-45.5
%, depending on the compound) is due to the partial removal of the dca ligands.
At temperatures above 1100º C the materials have completely decomposed into Co, Fe
and ZnO, respectively, as revealed by the study of the XRPD patterns of the ashes
obtained after the ATG experiment.
Differential scanning calorimetric (DSC)
As we have indicated in the Introduction, it has been reported that these compounds
exhibit two polymorphs: a room temperature one and a low temperature one. Here, we
have undertaken a DSC study to characterize the transition from one to another.
7

As shown in Fig. 5, according to the DSC results the three M(dca)
display an endothermic peak on heating (that is seen at T ~277 K for M=Fe, ~ 250 K
for M=Co and ~243 K for M=Zn) whose associated exothermic peak appears on cooling
at T ~268 K when M=Fe, ~234 K when M=Co and ~238 K when M=Zn. Such
2
pyz compounds
t
t
transition seems to correspond to the structural phase transition from the orthorhombic
polymorph (HT-phase) to the monoclinic one (LT-phase) [15].
The observed thermal hysteresis (see Fig. 5) implies that the transition is first order, as
thermal hysteresis is thermodynamically forbidden in a second order transition [20].
-1
-1
-1
The changes in the enthalpy ∆H (J mol ) and the entropy ∆S (J mol K ) were
determined from the area under the heat flow/time curves. The results, which depend on
-1
M as shown in detail in table S1 of S.I., yield enthalpies ∆H ~1100-1340 J mol and
-
1
-1
entropies ∆S ~3.14-4.88 J mol K .
Taking into account that for an order-disorder transition ∆S = R ln(N), where R is the
gas constant and N is the ratio of the number of configurations in the disordered and
ordered system, a value of N ∼ 0.6 is calculated, for these compounds see table S1 of
S.I..
In view of the LT- and HT-crystal structures [15], see Fig. 1, this value is much smaller
than the one expected for this order-disorder phase transition, which implies at least
fourfold order/disorder of the C-atoms of pyrazine and twofold order/disorder of the C-
and N-amide atoms of the dca anions.
Also, the DCS results indicate that the order-disorder process takes place within a rather
broad temperature range and is not an abrupt transition, as it is the case in other hybrid
inorganic-organic materials, as for example in the [(CH
3
)
2
NH
2
][Mn(HCOO) ]
3
compound [21].
Dielectric properties
’
The temperature dependence of the real part of the complex dielectric permittivity, ε
the so-called dielectric constant) of these compounds was measured on a
polycrystalline pellet at different frequencies. The obtained results are shown in Fig. 6.
r
,
(
8

A first general remark is that these compounds present a low dielectric constant value,
which is frequency independent, except for the case of the Fe-compound, which
displays a small frequency dependence for T>280 K.
’
r
Most interestingly, in the three compounds the ε vs T curves display an anomaly, very
close to the temperature at which the structural phase transition takes place, see Fig. 6.
’
Also, in the temperature interval corresponding to the LT-phase, the ε
r
values are very
small at low temperature and increase with temperature, showing a rather sharp rise
close to the phase transition temperature (T ). Just above T , and in the case of the Co-
and Zn- compounds the dielectric constant goes through a broad maximum, and then the
ε’
values begin to slightly decrease. Meanwhile, in the case of the Fe-compound, a
t
t
r
sharper rise in the dielectric constant is observed above the transition temperature in the
HT-phase.
’
It should also be indicated that the observed anomalies in the ε
r
vs T curves are not as
sharp as in other hybrid materials with dielectric properties [3,4,12]. This observation
agrees with the DSC results, which indicate that the phase transition in the M(dca) pyz
M = Fe , Co and Zn ) compounds is not abrupt and that the order-disorder process
2
2
+
2+
2+
(
is taking place over a broad temperature range.
Taking into account that the presence of extrinsic factors such electronic and/or ionic
conductivity could result in the appearance of artifacts in the dielectric measurements,
we have made additional studies to make sure that the observed signal really comes
from the sample.
2
2,23
For this purpose, and to avoid misinterpreting results
we have performed impedance
complex plane (Z'' vs Z') analysis of the data obtained at different temperatures. A
typical impedance complex plane plot for these samples in the whole temperature range
100<T(K)<350 K is shown in Fig. S3 of S.I.. This plot shows a single large arc that can
be modeled by an equivalent circuit containing two elements connected in parallel: a
resistance (R), and a capacitance (C). As this arc intercepts zero and the order of
-1
magnitude of its capacitance is of pFcm and the resistance is of giga Ω, it is associated
2
2
with the material bulk response. Taking into account that this is the only contribution
present, we conclude that the observed dielectric response of these compounds is purely
intrinsic and they are insulator in the whole studied temperature interval. This result
9

confirms that the observed dielectric transition is real and associated to the material´s
bulk response.
To understand the dielectric response of these compounds we have analysed in detail
the characteristics of their LT- and HT-crystal structures, paying special attention to the
spatial distribution of the dipole moments associated to the polar ligands.
For that purpose, we have focused in the arrangement of the polar dca ligand, which
displays an electric dipole moment of µ=0.53 D, see Fig. S4 of S.I.. As these
compounds are comprised of two interpenetrating networks, we will first refer to the
dipole arrangement of one of the networks to subsequently check the arrangement of the
whole structure.
In both the HT-phase and LT-phase, the dipole associated to the dca ligand displays a
cooperative arrangement. Very interestingly, and as it can be seen in Fig. 7, the dipoles
of each network show a ferroelectric (FE) arrangement along one of the crystal axis
(
that is the a-axis in the case of the HT-phase, that in fact correspond to the b-axis in the
LT-polymorph).
Nevertheless, the antiparallel arrangement of the two interpenetrated FE networks
finally gives rise to a net long-range antiferroelectric (AFE) order in both the HT- and
LT-polymorphs of these compounds, see Fig. 7.
And the main different between the AFE arrangements in the two polymorphs relies on
their “static” (in the LT-phase) or “dynamic” (in the HT-phase) characteristics. Such
“static” vs “dynamic” nature is related to the order-disorder process that is experienced
by the polar dca ligands at the structural phase transition.
In that context, the “dynamic” disorder of the dca ligands can sporadically break up the
antiparallel arrangement of the electrical dipoles and be responsible for the dielectric
anomaly observed at T
t
. We have to note that this dielectric response has certain
similarities with the behavior reported for the (C
3
N
2
H
5
)[Mn(HCOO)
3
] metal–organic
+
framework with perovskite–like structure [13]. In this compound, the C
3
N
2
H
5
imidazolium cations, which are guest cations located in the cavities of the framework,
also display an antiferroelectric order that thermal energy seems to be able to
sporadically break the antiparallel arrangement of the electrical dipoles, giving rise to
’
the anomaly inε
r
vs T curve, in that case at T ∼ 200K.
1
0

Thus, the origin of the dielectric anomaly observed in the here studied M(dca)
compounds (M= Fe, Co, Zn), is quite different from that observed in other hybrid
inorganic-organic compounds, such as [A][M(HCOO) ] (where A = mid-sized
2
pyz
3
alkylammonium cations). In those latter the polar arrangement of the hybrid materials is
typically related to order/disorder phenomena involving polar cations or to H-bonded
between the framework and guest molecules, which are located inside the cavities of the
framework. This is a drawback in order to obtain multiferroic materials with strong
magnetoelectric coupling, due to the fact that the polar and magnetic order arise from
different subunits: while the framework induces long range magnetic order and, the
guest molecules provoke the polar arrangement. In the case of the here studied
M(dca) pyz compounds, the dielectric response is directly provoked by their
2
framework, as it is also their magnetic behavior in the case of the compounds containing
transition metal ions with incomplete d orbitals.
This finding is very interesting and paves the way to obtain type II-multiferroic
materials [24,25] based on hybrid organic-inorganic compounds, with a strong coupling
between both functional properties.
Conclusions
2
+
2+
2+
2+
2
We have prepared the M(dca) pyz with M = Fe , Co and Zn hybrid organic-
inorganic compounds as single phase materials. These compounds are thermally stable
up to T≈ 410 K-550 K depending of the transition metal cations, and their thermal
decomposition occurs in two-steps. According to the DSC results, these compounds
display a first order phase transition, that is seen at around ∼270 K, ∼250 K and ∼240 K
for the Fe, Co and Zn compounds, respectively, on heating and ~268 K, ~234 K and
~238 K for the Fe, Co and Zn compounds, respectively, upon cooling. Such phase
transition corresponds to the structural transition from the HT-polymorph, which is
orthorhombic, space group Pnma, and contains dynamic disorder of the dca and
pyrazine ligands, to the LT-polymorph, that is monoclinic, space group P2 /n and where
1
the dca and pyrazine ligands are ordered. Interestingly, these compounds display a
’
dielectric anomaly associated to such phase transition, where the ε
r
values are small at
low temperature and increase with temperature, showing an anomaly close to the phase
transition temperature. From a detailed analysis of the LT- and HT-crystal structures,
we have observed that the polar dca ligands, which bridge two transition metal cations,
1
1

display a cooperative electrical arrangement. In fact, the crystal structure of these
compounds is comprised of two interpenetrated networks, where the dipoles within each
network show a ferroelectric (FE) arrangement. Nevertheless, the antiparallel stacking
of the two interpenetrated FE networks gives rise to a net long-range antiferroelectric
(
AFE) order in these compounds. In this context, the “dynamic” disorder of the dca
ligands in the HT-phase can sporadically break up the antiparallel arrangement of the
’
electrical dipoles, giving rise to the anomaly in ε
r
observed at the phase transition. Very
pyz
interestingly, the origin of such dielectric anomaly observed in the M(dca)
2
compounds, is thus quite different from that observed in other hybrid inorganic-organic
compounds, as in the here presented compounds, both the dielectric and magnetic
response directly come from the framework. This finding is very interesting to obtain
type II-multiferroic materials, with a strong coupling between both functional
properties.
Acknowledgments
The authors are grateful for the financial support from Ministerio de Economía y
Competitividad MINECO (MINECO) ENE2014-56237-C4-4-R and Xunta de Galicia
under the project GRC2014/042.
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1
4

FIGURE CAPTIONS
Fig. 1. Crystal structure of the M(dca) pyz compounds viewed along the c-axis (a) HT-
2
phase and (b) LT-phase. To facilitate the visualization, the unit cells have been
expanded and the [MN ] octahedra of the interpenetrated networks have been labeled
6
with different colors (cyan and red).
Fig. 2. SEM images of the obtained M(dca)
magnifications.
2
pyz materials obtained under different
2
+
2+
Fig. 3.Room temperature PXRD patterns for the as-prepared M(dca) pyz (M:Fe , Co ,
2
2
+
Zn ) compounds and the simulated one based on the single crystal data at room
temperature.
2
+
2+
2+
Fig. 4. TGA curves of the M(dca)
atmosphere.
2 2
pyz (M:Fe , Co , Zn ) materials obtained under N
Fig. 5. DSC results as a function of temperature obtained by heating and cooling the
2
+
2+
2+
2+
2+
2
samplesM(dca) pyz (M:Fe , Co , Zn ) at a rate of 10 K/min. a) M=Fe , b) M=Co
and c) M=Zn .
2
+
Fig. 6. Temperature dependence of the dielectric constant, ε’
r 2
of M(dca) pyz compounds
2
+
2+
measured at different frequencies (f=10–40 kHz), a) M=Fe , b) M=Co and c)
2
+
M=Zn .Additionally, we have highlighted with different colors the temperature
regions, where each of the polymorphs is stable: (cyan) for the HT-phase and (yellow)
for the LT-phase.
Fig. 7. Crystal structure of the HT-phase viewed along the b-axis and of the LT-phase
viewed along the a-axis. To facilitate the visualization, each of the subnetworks has
been drawn in different colors (cyan and red) and the pyzarine ligands have been
removed. The arrows show the electrical dipole of the dca ligands.
1
5

b)
a)
M
se
HT-phase
LT-phase
Fig. 1
1
6

Fe-compound
Co-compound
Zn-compound
Fig. 2
1
7

Zn-compound
Co-compound
Fe-compound
Reference
10
15
20
25
30
35
40
45
2
θ (º)
Fig. 3
1
8

1
00
Fe-compound
Co-compound
Zn-compound
8
6
4
2
0
0
0
0
3
00 400 500 600 700 800 900 1000 1100
T (K)
Fig. 4
1
9

a)
0.7
0.6
0.5
Fe-compound
exo
exo
cooling
-
-
-
-
-
0.5
0.6
0.7
0.8
heating
0.9
2
20
240
260
280
300
320
T (K)
b) 0.36
0
0
0
0
.35
.34
.33
.32
Co compound
cooling
exo
endo
-
-
-
-
-
0.28
0.32
0.36
0.40
heating
0.44
2
00
220
240
260
280
300
T(K)
c)0
.52
.50
Zn-compound
cooling
0
exo
endo
-
-
-
-
0.48
0.49
0.50
0.51
2
heating
00 210 220 230 240 250 260 270
T(K)
Fig. 5
2
0

a)
4
4
3
3
3
3
.2
f=10 kHz
f=16 kHz
f=25 kHz
f=40 kHz
.0
.8
.6
.4
.2
Fe compound
LT-phase HT-phase
8
0
120
160
200
240
280
320
360
T (K)
b)
f=10 kHz
f=16 kHz
f=25 kHz
f=40 kHz
3
3
3
3
3
.5
.4
.3
.2
.1
Co compound
LT-phase
HT-phase
8
0
120
160
200
240
280
320
360
T (K)
c)
6
6
6
6
6
6
.7
f=10 kHz
f=16 kHz
f=25 kHz
f=40 kHz
.6
.5
.4
.3
.2
Zn compound
LT-phase
HT-phase
120
160
200
240
280
320
360
T (K)
Fig. 6
2
1

LT-phase
HT-phase
Fig. 7
2
2

TABLE CAPTION
2
+
2+
2+
Table 1.- Cell parameters of the M(dca) pyz (M:Fe , Co , Zn ) compounds and
2
agreement factors obtained from the LeBail refinement of their PXRD patterns at room
temperature.
2
3

2
Compound
a (Å)
b (Å)
c (Å)
R
wp
R
p
χ
Fe
Co
Zn
17.216(2) 7.227(1)
17.043(3) 7.106(1)
17.154(3) 7.201(2)
8.729(1)
8.623(1)
8.695(2)
20.44
19.79
28.24
11.71
12.03
20.68
1.82
1.41
1.33
Table 1
2
4

GRAPHICAL ABSTRATS
Pictogram:
2
5

GRAPHICAL ABSTRATS
Synopsis:
2
The hybrid organic-inorganic M(dca) pyz compounds display a crystal structure
comprised of two interpenetrated networks, where each net displays a ferroelectric (FE)
arrangement of the polar dicyanamide (dca) anions. Both FE subnets showed an
antiferroelectric arrangement. Very interestingly, the polar dca anions are involves at
magnetic and electric arrangement that is interesting for obtain magnetoelectric
coupling.
2
6