Tuning of the Critical Temperature in Magnetic 2D Coordination Polymers by Mixed Crystal Formation

Article abstract of DOI:10.1021/acs.cgd.8b00245

Reaction of Co(NCS)₂ and Ni(NCS)₂ with 4-acetylpyridine under different conditions leads to the formation of mixed crystals of the layered compound with the composition [Co_xNi_1-x(NCS)₂(4-acetylpyridine)₂]_n (x = 0.15 0.3, 0.5, and 0.7). Mixed crystal formation was investigated by a combination of X-ray powder diffraction (XRPD), atomic absorption spectroscopy (AAS), simultaneous scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), differential scanning calorimetry (DSC), and magnetic and heat capacity measurements. Dependent on the synthetic method, homo- or heterogeneous mixed crystals were obtained as already indicated by XRPD, where significant differences to the pattern of physical mixtures of the homometallic counterparts [M(NCS)₂(4-acetylpyridine)₂]_n with M = Co or Ni) with the same Co/Ni ratio are observed. Mixed crystals can also be obtained from physical mixtures under thermodynamic control, indicating that they are more stable than the homometallic compounds. This is further supported by AAS, which indicates that the solubility of the mixed crystals is lower than that of the homometallic compounds. Magnetic and heat capacity measurements show a linear increase of the critical temperature of magnetic ordering with increasing Ni content and also confirm that homogeneous samples were obtained.

Full text of DOI:10.1021/acs.cgd.8b00245

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Article
Tuning of the critical temperature in magnetic 2D
coordination polymers by mixed crystal formation
Carsten Wellm, Micha# Rams, Tristan Neumann, Magdalena Ceglarska, and Christian Näther
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00245 • Publication Date (Web): 26 Mar 2018
Downloaded from http://pubs.acs.org on March 27, 2018
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Tuning of the critical temperature in magnetic 2D
coordination polymers by mixed crystal formation
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Carsten Wellm,† Michał Rams,‡ Tristan Neumann,† Magdalena Ceglarska,‡ and Christian Näther*,†
†Institut für Anorganische Chemie, ChristianꢀAlbrechtsꢀUniversität zu Kiel, MaxꢀEythꢀStraße 2, 24118
Kiel, Germany.
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‡Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30ꢀ348 Kraków, Poland.
Abstract: Reaction of Co(NCS)2 and Ni(NCS)2 with 4ꢀacetylpyridine under different conditions leads to
the formation of mixed crystals of the layered compound with the composition [CoxNi1ꢀx(NCS)2(4ꢀ
acetylpyridine)2]n (x = 0.15 0.3, 0.5 and 0.7). Mixed crystal formation was investigated by a combination of
Xꢀray powder diffraction (XRPD), atomic absorption spectroscopy (AAS), simultaneously scanning
electron microscopy (SEM) and energy dispersive Xꢀray spectroscopy (EDX), differential scanning
calorimetry (DSC) as well as magnetic and heatꢀcapacity measurements. Dependent on the synthetic
method, homoꢀ or heterogenous mixed crystals were obtained as already indicated by XRPD, where
significant differences to the pattern of physical mixtures of the homometallic counterparts [M(NCS)2(4ꢀ
acetylpyridine)2]n with M = Co or Ni) with the same Co:Ni ratio are observed. Mixed crystals can also be
obtained from physical mixtures under thermodynamic control, indicating that they are more stable than the
homometallic compounds. This is further supported by AAS, which indicates that the solubility of the
mixed crystals is lower than that of the homometallic compounds. Magnetic and heatꢀcapacity
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measurements show a linear increase of the critical temperature of magnetic ordering with increasing Ni
content and also confirm that homogenous samples were obtained.
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INTRODUCTION
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In recent years, the development of strategies for a rational synthesis of new coordination polymers with
defined magnetic properties has attracted much attention.1ꢀ8 In most cases homometallic compounds were
prepared with a variety of ligands that can mediate magnetic exchange differently.1ꢀ8 The magnetic
properties of coordination compounds can also be modified by the synthesis of bimetallic compounds,
which is in most cases more difficult, but can be achieved using ligands, that can coordinate to different
metal cations.9ꢀ11 A very effective strategy consists of the reaction of predefined metal containing building
blocks that are linked by other metal cations into coordination polymers of different dimensionality.12ꢀ24
Compared to this, only a very few papers report on the synthesis of mixed crystals of paramagnetic metal
cations as an additional tool to modify or tune the magnetic properties of coordination compounds in more
detail and some selected examples are given in the reference list.25ꢀ29 This is surprising, because this
synthetic strategy is used for decades in inorganic solid state chemistry e.g. for optimization of MnꢀZn soft
ferrites 30, bandgap tuning in AlxGa1−xAs semiconductors 31, inducing highꢀtemperature superconductivity
in La2−xSrxCuO4 32 or colossal magnetoresistance in La1ꢀxSrxMnO3.33
There are surely many reasons for such a situation. First of all the synthesis of pure samples of
coordination compounds is sometimes difficult to achieve, because in solution different compounds might
be in equilibria, which can lead to the formation of mixtures of different compounds. The situation is more
complicated when different but chemically very similar metal cations are used because also physical
mixtures of the homometallic compounds can form. However, if mixed crystals may have been obtained,
the question arises, if their metal ratio corresponds to that used in the synthesis, which is not necessarily the
case. It is also of importance to investigate if “homogenous” mixed crystals are obtained or if the metal to
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metal ratio varies for different crystals. Finally, if metal cations are used that are neighbors in the periodic
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table, mixed crystal formation is more difficult to prove.
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In this context it is noted that we are interested in coordination polymers based on 3d transition metal
cations and thioꢀ or selenocyanate anions for several years.34ꢀ39 Even if the magnetic exchange of these
ligands is usually weaker than that of, e.g. azides, these pseudohalide anions show a variety of coordination
modes and thus, can form structures of different dimensionality.40ꢀ56 With monodentate coꢀligands, like
simple pyridine derivatives substituted in 4ꢀposition chain structures are usually observed, in which the
metal cations are linked by pairs of anionic ligands into chains.57 Dependent on the nature of the coꢀligand,
linear or zigꢀzagꢀlike chains can form, in which the metal cations are differently coordinated including a
trans or a cisꢀcisꢀtransꢀcoordination at the metal center.58 Finally, if other pyridine derivatives are used,
also 2D thiocyanate networks can be obtained, in which the metal cations are linked by the anionic ligands
into layers.59ꢀ63
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This is observed, e.g., in the crystal structure of compounds with the composition [M(NCS)2(4ꢀ
acetylpyridine)2]n for M = Co (1ꢀCo) or M = Ni (1ꢀNi) as ligand, for which two isomers exists. The
thermodynamic stable form at roomꢀtemperature consists of M(NCS)2 dimers that are linked by single
anionic ligands into layers that are parallel to the a/bꢀplane, whereas the metastable isomer forms linear
chains (Fig. 1).62, 63 In the layered compounds dominating ferromagnetic interactions are observed for 1ꢀCo
and 1ꢀNi and both of these compound show magnetic ordering at a critical temperature, which is much
lower for the Co compared to the Ni compound.61, 62
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Figure 1. View of the M(NCS)2 coordination network in [M(NCS)2(4ꢀacetylpyridine)2]n (1ꢀCo and 1ꢀNi).
The 4ꢀacetylpyridine ligands are omitted for clarity (M = orange; C = black; N = blue; S = yellow).
Based on these results the question arise whether homogenous mixed crystals with the composition
[CoxNi1ꢀx(NCS)2(4ꢀacetylpyridine)2]n (1ꢀCoxNi1ꢀx) can be prepared, if they still show a phase transition and
if this is the case, if the critical temperature can be tuned as a function of the actual Co:Ni ratio. The mixed
crystal formation and homogeneity of the samples prepared by different routes was investigated by a
combination of atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM) with
simultaneous energy dispersive Xꢀray spectroscopy (EDX), differential scanning calorimetry (DSC), Xꢀray
powder diffraction (XRPD) as well as magnetic and heatꢀcapacity measurements. Both 2D compounds are
isotypic and, because both metal cations are neighbored in the periodic table, only very small differences in
their XRPD pattern are expected, which is already indicated by their calculated pattern (Table S1 and
Figure S1 in the SI). Therefore, the powder pattern of the mixed crystals were always compared with that
of physical mixtures of 1ꢀCo and 1ꢀNi with the same Co:Ni ratio.
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EXPERIMENTAL SECTION
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Reagents. Co(NCS)2, Ba(NCS)2·3H2O and 4ꢀacetylpyridine were obtained from Alfa Aesar,
Ni(SO4)2·6H2O was purchased from Merck. All chemicals and solvents were used without further
purification. Ni(NCS)2 was prepared by a reaction of equimolar amounts of NiSO4·6H2O and
Ba(NCS)2·3H2O in water. The resulting precipitate of BaSO4 was filtered off, and the solvent was removed
completely using a rotary evaporator leading to a green residue of Ni(NCS)2. The purity was checked by
XRPD.
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Synthesis of 1ꢀCo. Crystalline powders were obtained by stirring a mixture of Co(NCS)2 (175mg, 1 mmol)
and 4ꢀacetylpyridine (222 µL, 2 mmol) in ethanol (1.5 mL) for 2 d. The purity was checked by XRPD and
by IR spectroscopy (Figure S2 and S3).63
Synthesis of 1ꢀNi. Crystalline powders were obtained by stirring a mixture of Ni(NCS)2 (175mg, 1 mmol)
and 4ꢀacetylpyridine (222 µL, 2 mmol) in ethanol (4 mL) for 2 d. The purity was checked by XRPD and by
IR spectroscopy (Figure S4 and S5).62
Synthesis of 1ꢀCoxNi1ꢀx. Crystalline powders were obtained by dissolving Co(NCS)2 and Ni(NCS)2
completely in the desired ratio in ethanol. Afterwards a stoichiometric amount of 4ꢀacetylpyridine was
added to the clear solution and the reaction mixture was stirred for 2 d.
Elemental Analysis. CHNS analysis was performed using a EURO EA elemental analyzer, fabricated by
EURO VECTOR Instruments and Software.
Scanning electron microscopy (SEM) and energy dispersive Xꢀray spectroscopy (EDX). These
measurements were performed using a Philips ESEM XL 3 with an EDAX New XLꢀ30 Detector, at an
acceleration voltage of 20 kV.
Atomic absorptions spectroscopy (AAS). The AAS experiments were performed with a Perkin Elmer
Aanalyst 300. Each sample was dissolved in water with 2.5 mL HNO3 for 100 mL analyte.
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IR spectroscopy. The IR data were measured using a Genesis Series FTIR Spectrometer with WINFIRST
control software from ATI Mattson.
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Differential scanning calorimetry (DSC). The DSC experiments were performed using a DSC 1 star
system with STARe Excellence software from MettlerꢀToledo AG under dynamic nitrogen atmosphere.
The instrument was calibrated using standard reference materials.
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Magnetic measurements. All magnetic measurements were performed using a Physical Property
Measurement System (PPMS) from Quantum Design, which was equipped with a 9 T magnet, and using
QD MPMSꢀ5XL squid magnetometer. The data were corrected for core diamagnetism.
Specific heat measurements. Specific heat was measured by the relaxation technique using a Quantum
Design PPMS. Powder samples were pressed into pellets. Apiezon N grease was used to ensure thermal
contact of the samples with the calorimeter. The heat capacity of the grease was measured before each run
and subtracted.
XꢀRay Powder Diffraction (XRPD). The XRPD measurements were performed by using a Stoe
Transmission Powder Diffraction System (STADI P) with CuKα radiation that was equipped with a linear
positionꢀsensitive MYTHEN detector from STOE & CIE. All measurements were performed with Zn as
internal standard.
RESULTS AND DISCUSSION
Synthetic investigations. To investigate if one can differentiate between the homometallic compounds by
XRPD, physical mixtures of 1ꢀCo and 1ꢀNi in different Co:Ni ratios with additional Zn as internal standard
were measured. As expected, most reflections overlap but the 043 reflections observed at 2ꢀTheta = 25.02°
for 1ꢀCo and at 25.18° for 1ꢀNi are successfully resolved (Figure S6). Moreover, the intensity ratio between
the 043 reflections for these compounds nicely changes with the Co:Ni ratio and even metal contents of
15% for Co can be proven. We also have investigated these physical mixtures by IR spectroscopy to check
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if two bands for the CN stretching vibrations are observed, but they are not successfully resolved and thus,
do not allow to prove mixed crystal formation (Figure S7). Finally, these mixtures were also investigated
by DSC measurements, and the comparison with the homometallic counterparts shows an additional
thermal event that can be traced back to the different decomposition temperatures of the Co and the Ni
compound (Figure S8). For mixed crystals only one peak would be expected.
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To determine the solubility of both compounds, which might influence the mixed crystal formation, a
suspension of 1ꢀCo and 1ꢀNi was stirred in ethanol, from which no solvate can form, filtered off and the
metal content was determined by AAS in the filtrate. This shows that the solubility of 1ꢀCo (6.18 g/L) is
significantly higher than that of 1ꢀNi (1.15 g/L) (Table S2). Immediate XRPD measurements of the filtered
residue prove that no other crystalline phase has formed, which would influence the outcome of this
experiment.
In the beginning it was tried to prepare mixed crystals following the procedure for the synthesis of the
homometallic compounds, where the metal thiocyanate were added simultaneously to a small amount of
solvent.62, 63 Therefore, a mixture of Co(NCS)2, Ni(NCS)2 and 4ꢀacetylpyridine was stirred in different
molar ratios in a small amount of ethanol and the residues that formed within 2 d were investigated by
XRPD. It is noted that, e.g., water cannot be used as solvent, because hydrates with the composition
M(NCS)2(H2O)2(4ꢀacetylpyridine)2 (M = Co, Ni) will form.63, 64 For these samples AAS proves, that the
Co:Ni ratio of the mixed crystals exactly corresponds to that used in the synthesis (Table S3). XRPD
measurement shows that the 043 reflection of this batch is located exactly between that of the physical
mixture, indicating for mixed crystal formation (Figure S9). However, the full width at half maximum
(FWHM) is much broader than that of the homometallic compounds, indicating that crystals of a variety of
compositions were obtained (Figure S9). Scanning electron microscopy (SEM) of these mixed crystals and
the homometallic compounds indicates, that the broadening cannot be traced back to large differences in
the particle size (Figure S10). The formation of inhomogeneous samples is further confirmed by EDX
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measurements of selected crystals, that show significant deviations from the expected Co:Ni ratio (Table
S4) and this is also in agreement with magnetic and heat capacity measurements (see below). The
formation of inhomogeneous samples might originate from the different solubility and dissolution rate of
the metal salts in ethanol and the fact that using small amounts of solvent the metal salts are not completely
dissolved. Moreover, the sequence of adding the reactants is also of importance, because if, e.g. Ni(NCS)2
is added first to the mixture with ligand before Co(NCS)2, Niꢀrich particles can precipitate in the beginning
and would become Coꢀrich with increasing reaction time. This would also explains that after complete
precipitation the ratio between Co and Ni will correspond to that used in the synthesis. For the synthesis of
the homometallic compounds it is unimportant and if only a very small amount of solvent is used it is of
advantage, because the products are obtained in high yield.
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However, concerning the stability of mixed crystals one would assume that they are more stable than the
homometallic counterparts because of entropy and thus, one would expect formation of homogenous
samples with increasing reaction time. Therefore, we prepared a suspension of equivalent amounts of 1ꢀCo
and 1ꢀNi with large excess of solid in ethanol, which was stirred for 30 d. Investigations of the residue by
AAS reveal, that mixed crystals with x close to 0.5 are obtained, which according to XRPD measurements
should be homogenous (Figure S11). For such a sample the Co and Ni concentration in a saturated solution
was measured by AAS using the same procedure as that used to determine the solubility of the individual
compounds. These values are lower than that of the homometallic compounds, which also points to a
higher stability (Table S5).
Based on the above described results we prepared mixed crystals by adding 4ꢀacetylpyridine to clear
solutions of Co(NCS)2 and Ni(NCS)2 in ethanol at roomꢀtemperature but also at elevated temperatures.
Independent of the reaction time or temperature, always samples were obtained, in which the Co:Ni ratio
corresponds exactly to that used in the synthesis, as proven by AAS measurements (Table S6) EDX
measurements of selected crystals indicate the formation of homogenous samples (Table S7). This is
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further supported by DSC measurements where in contrast to the physical mixtures only one thermal event
is observed (Figure S12). Mixed crystal formation is also indicated by XRPD measurements, where two
peaks are observed for the 043 reflection in the physical mixtures, whereas only one peak is visible for the
mixed crystals (Figure 2 and Figure S13).
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Figure 2. XRPD pattern of 1ꢀCo, 1ꢀNi and 1ꢀCoxNi1ꢀx (x = 0.15, 0.3, 0.5, and 0.7; black) and of physical
mixtures of 1ꢀCo and 1ꢀNi with the same Co:Ni ratio (red). For the full range see Figure S13.
It is also visible that, with increasing Ni content, the 043 reflection shifts to higher Bragg angles. If the
FWHM of the reflections of the mixed crystals is compared with that of the physical mixture and that of an
inhomogenous sample as described above, it is strongly indicated that by this procedure homogenous
samples are obtained.
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For these mixed crystals the unit cell volume was determined from Pawley fits using XRPD patterns
measured at roomꢀtemperature, which shows a linear increase with increasing Ni content. This nicely
proves that Vegard's law is valid, which is expected for homogenous mixed crystals (Figure 3).66, 67
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Figure 3. Unit cell volume as function of the Co and Ni content in 1ꢀCoxNi1ꢀx (x = 0, 0.15, 0.3, 0.5, 0.7 and
1.0).
Magnetic and heatꢀcapacity measurements. To investigate the influence of mixed crystal formation on
the magnetic properties, the temperature dependence of the susceptibility
χ was measured at 100 Oe for 1ꢀ
CoxNi1ꢀx but also for 1ꢀCo and 1ꢀNi (Figure 4). For 1ꢀNi the susceptibility increases significantly starting at
about 10 K and at about 2 K nearly saturation is observed, which is in agreement with its ferromagnetic
behavior, already reported in literature.61 With decreasing Ni content the temperature at which the
susceptibility starts to increase moves to lower temperatures and one can anticipate that saturation might be
reached at below 2 K. For 1ꢀCo the
χ vs. T curve looks like that for a paramagnet, which can be traced back
to the fact that magnetic ordering occurs at a temperature that is below the lowest temperature available in
T vary between 1.21 cm3 K mol−1 for
the used magnetometer (Figure 4). The roomꢀtemperature values of
χ
the Ni and 3.35 cm3 K mol−1 for the Co compound, which is in the range typically observed for these
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cations in an octahedral coordination.61,
For all compounds the magnetic behavior changes
continuously as function of the actual Co:Ni ratio (Figure S14 and S15).
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Figure 4. Temperature dependence of the magnetic susceptibility at 100 Oe measured for 1ꢀCo, 1ꢀNi and 1ꢀ
CoxNi1ꢀx (x = 0.15, 0.3, 0.5, and 0.7). Only the low temperature range is shown.
The field dependence of the magnetization of 1ꢀNi measured at 2.1 K shows a behavior typical for a
ferromagnet with nearly saturation at high fields (Figure S16). With increasing Co content the
magnetization is modified in the range 0ꢀ15 kOe, indicating that significant anisotropy is introduced even
by x = 0.15 admixture of Co. The saturation magnetization for the mixed crystals is in between that of the
homometallic counterparts and increases with increasing Co content (Figure S16). To get some idea how
the critical temperature changes as function of the actual Co:Ni ratio, the first derivative of χ(T) was plotted
as function of temperature, which shows that the critical temperature shifts to lower temperatures with
increasing Co content (Figure 5).
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Figure 5. First derivative of the dc susceptibility for 1ꢀCo, 1ꢀNi and 1ꢀCoxNi1ꢀx (x = 0.15, 0.3, 0.5, and 0.7)
measured as function of temperature at 100 Oe. Please note that for 1ꢀCo the critical temperature is at too
low temperatures.
To prove the magnetic ordering for the mixed crystals, heat capacity measurements down to 0.4 K were
performed, from which the critical temperatures might be determined more precisely. For all samples a
lambdaꢀtype peak is observed, which points to a second order phase transition (Figure 6).
Figure 6. Temperature dependence of the lowꢀtemperature specific heat C measured for 1ꢀCo, 1ꢀNi and 1ꢀ
CoxNi1ꢀx (x = 0.15, 0.3, 0.5, and 0.7).
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This confirms, that all mixed crystals show a longꢀrange ordering as already observed for 1ꢀCo and 1ꢀNi
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and excludes a spinꢀglass behavior. The ac susceptibility shows a maximum of χ'(T) at temperature that
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does not depend on the ac frequency, which is also against a spinꢀglass phase, and in favor of the long
range ordering (Figure S18). As the spinꢀglass ground state would be expected in case of competing FMꢀ
AFM exchange interactions in disordered NiꢀCo lattice, the presence of the second order transition for all
compositions also suggests that all exchange interactions, also between Ni and Co pairs, are ferromagnetic.
From the heatꢀcapacity measurements the values for the critical temperatures were extracted and plotted
as function of the actual Co:Ni ratio, which shows that Tc decreases with increasing Co content almost
linearly (Figure 6). The values obtained from dχ/dT are in very good agreement with that obtained from the
heatꢀcapacity measurements (Figure 6).
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Figure 6. Critical temperature of magnetic ordering for 1ꢀCoxNi1ꢀx (x = 0, 0.15, 0.3, 0.5, 0.7 and 1.0) as
determined from specific heat (red points) and magnetic measurements (black points).
Finally, we also investigated one sample with x = 0.5 by magnetic and heatꢀcapacity measurements for
which XRPD measurements indicate inhomogeneity (see above and Figure S11). In this case one would
expect broad peaks of dχ/dT, because if each crystal has a different Co:Ni ratio it will exhibits his own
transition temperature. This is exactly observed in the magnetic measurements and it is obvious that dχ/dT
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starts to decrease at higher temperatures, which clearly proves that Niꢀrich particles are present (Figure
S19). This is further confirmed by heatꢀcapacity measurements, for which a behavior with at least two
distinct broad maxima is observed (Figure S20).
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CONCLUSION
In the present contribution we have shown that the critical temperatures of coordination compounds can
be tuned by mixed crystal formation. This was proven using layered compounds with the general
composition [M(NCS)2(4ꢀacetylpyridine)2]n (M = Co, Ni), that show dominating ferromagnetic interactions
within the layers and magnetic ordering at low temperatures. For the mixed crystals the same behavior was
observed with the ferromagnetic ordering and no sign for the formation of a spinꢀglass and it was found
that the critical temperature increases linear with increasing Ni content. However, in this case homogenous
samples are needed, because otherwise each of the particles exhibits its own transition temperature leading
to no definite transition, as proven by magnetic and heatꢀcapacity measurements. This cannot be achieved
by simply reacting all reactants in an arbitrary order in a small amount of solvent for a limited time,
because depending on the solubility and dissolution behavior of the reactants and the products as well as
the order in which the reactants are added, the composition of each particle can be different. However, even
from inhomogeneous samples homogenous mixed crystals can form using very long reaction time, which
indicates that the mixed crystals seem to be more stable. To prove mixed crystal formation and the
homogeneity of samples, different methods should be applied and it this case XRPD represents a powerful
tool, even if both compounds are isotypic with cations neighbored in the periodic table and consequently
very similar XRPD patterns.
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ACKNOWLEDGEMENTS
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This project was supported by the Deutsche Forschungsgemeinschaft (Project No. Na 720/6ꢀ1) and the
State of SchleswigꢀHolstein and by National Science Centre, Poland (project no. 2017/25/B/ST3/00856).
We thank Prof. Dr. Wolfgang Bensch for access to his experimental facilities.
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ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI:
AUTHOR INFORMATION
Corresponding Author
*Email: cnaether@ac.uniꢀkiel.de.
Notes
The authors declare no competing financial interest.
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coordination polymers by mixed crystal formation
Carsten Wellm, Michał Rams, Tristan Neumann, Magdalena Ceglarska, and Christian Näther
Mixed crystals of a layered coordination polymer with the composition [CoxNi1ꢀx(NCS)2(4ꢀ
acetylpyridine)2]n were prepared and characterized by XRPD, EDX, AAS, DSC, magnetic and
heat capacity measurements, which show that the critical temperature of magnetic ordering
depends linearly on the actual Co and Ni content.
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Figure for the table of contents
338x190mm (96 x 96 DPI)
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