EPR of Structural Phase Transition in Manganese- and Copper-Doped

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EPR of Structural Phase Transition in Manganese and Copper Doped Formate Framework of [NH(CH)NH][Zn(HCOO)] 3

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Mantas Šim#nas, Aneta Ciupa, Miroslaw Robert Maczka, Georg Völkel, Andreas Pöppl, and Juras Banys J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07389 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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The Journal of Physical Chemistry

EPR of Structural Phase Transition in Manganese and Copper Doped Formate Framework of [NH3(CH2)4NH3][Zn(HCOO)3]2 Mantas Šim˙enas,∗,† Aneta Ciupa,‡ Mirosław Mączka,‡ Georg Völkel,¶ Andreas Pöppl,¶ and J¯uras Banys† Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania, Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box-1410, PL-50-950 Wroclaw 2, Poland, and Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany E-mail: mantas.simenas@ff.vu.lt Phone: +370 5 2234537. Fax: +370 5 2234537

∗

To whom correspondence should be addressed Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius, Lithuania ‡ Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box-1410, PL-50-950 Wroclaw 2, Poland ¶ Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany †

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Abstract Electron paramagnetic resonance (EPR) and pulse electron-nuclear double resonance (ENDOR) spectroscopy is applied to investigate the structural phase transition and the low temperature phase in manganese and copper doped [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 dense metal-organic framework (MOF). Continuous-wave (CW) EPR measurements indicate successful incorporation of Mn2+ and Cu2+ ions at the Zn2+ lattice sites. Pulse ENDOR spectrum reveals at least four different proton species in the vicinity of the Mn2+ center showing excellent agreement with the X-ray diffraction experiments. Temperature dependent CW EPR spectra demonstrate that Mn2+ and Cu2+ local paramagnetic probes are sensitive to the phase transition in the studied compounds. The analysis of the temperature dependence of the Cu2+ hyperfine coupling parameter reveals a first-order phase transition at Tc = 235 K into an antiferroelectric phase which is close to the tricritical point. The obtained logarithmic divergence of the linewidth of the Mn2+ EPR spectrum indicates an order-disorder type phase transition.

Introduction Metal-organic frameworks (MOFs) are extensively studied highly porous compounds which consist of metal centers (nodes) and organic linker molecules. 1,2 A huge diversity of these building components allows fabrication of many different MOF structures with various topologies providing a wide spectrum of properties and functionalities. 3–5 The high surface area of these materials makes them attractive for applications related to the selective gas adsorption, 6–8 storage 9 and separation. 10 In addition to these functions, some MOFs exhibit peculiar magnetic, 11,12 optical, 13 electric 14 and dielectric 15–17 phenomena. Divalent metal formates with general formula [A][M(HCOO)3 ]n (M2+ = Zn2+ , Mn2+ , Fe2+ , Co2+ , Cu2+ , Ni2+ and Mg2+ ) constitute a dense MOF family with a rich ferroelectricityrelated behavior that originates from the cooperative ordering of An+ molecular cations during a structural phase transition. The most thoroughly studied members for n = 1 contain 2


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(CH3 )2 NH2+ and NH4+ cations. 18–22 The [NH4 ][Zn(HCOO)3 ] compound was proven to be ferroelectric, while the precise nature of the low temperature phase in [(CH3 )2 NH2 ][M(HCOO)3 ] is still obscure. In comparison, compounds for which n = 2 are much less studied, although their properties seem to match n = 1 analogues. 23–25 For example, [(pnH22+ )2 (H2 O)][Mg(HCOO)3 ]2 and [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOFs exhibit structural phase transition during which an enormous 36-fold increase in the unit cell volume was observed. 26,27 The structure of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 framework is based on the niccolite topology. 27 In this MOF a NH3 (CH2 )4 NH32+ (1,4-butyldiammonium) cation is contained within a cavity formed by two one-corner-missing twinned cuboids (see Figure 1 for the low-temperature structure). Corners of the cuboid represent ZnO6 octahedra which are interconnected via HCOO– (formate) linkers. NH3+ groups form N-H· · · O H-bonds with oxygen atoms from these linkers. Above the phase transition temperature (Tc ≈ 235 K) the NH3 (CH2 )4 NH32+ cation is trigonally distorted and the space group of the structure is trigonal P ¯31c. However, below this temperature the cations partly order and the structure changes to R¯3c. The disorder remains down to 100 K until all cations freeze to one orientation. 27

(a)

(b)

Zn O

C

H

N

Figure 1: (a) Cavity of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOF at low temperature. (b) Topological view of the same cavity with sticks as the HCOO– linkers. Radii of atoms in the NH3 (CH2 )4 NH32+ cation are enlarged for clarity. Structure taken from ref. 27

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In addition to X-ray diffraction (XRD) experiments, the [NH3 (CH2 )4 NH3 ][M(HCOO)3 ]2 MOF family was investigated using several other experimental tools. Heat capacity and differential scanning calorimetry studies revealed structural phase transitions in Mn, Mg, Cu, Co and Zn frameworks. 25,27 Dielectric spectroscopy indicated that the type of these transitions depends on the M2+ metal cation. 23,24,27 Dielectric spectra of Mn, Mg, Cu and Co compounds suggested paraelectric to ferroelectric phase transition, while Co and Zn members showed paraelectric to antiferroelectric transition as indicated by the jump in the dielectric permittivity at Tc . It is claimed that the frameworks with Fe and Ni form a dipolar glass state. Pyro-current measurements supported the ferroelectric type behavior in [NH3 (CH2 )4 NH3 ][Mn(HCOO)3 ]2 compound. 25 In addition, at low temperature the magnetic members of this family simultaneously showed magnetic long-range order making them good candidates for single-phase multiferroics. 28 Despite these studies, the observed phase transitions and the phases at low temperature lack more detailed picture. Among many characterization techniques, magnetic resonance tools such as electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) proved to be highly effective for characterization of gas adsorption, 29–31 structural changes 32,33 and magnetic properties 34 in various MOFs. In addition, due to the ability to probe the close vicinity of a paramagnetic center, EPR spectroscopy proved to be adventagous for investigation of dynamics and local structural changes occurring during structural phase transitions. 35,36 EPR allows to investigate the local polarization in ferroelectric or anitiferrolectric powders, while most of other methods measuring macroscopic polarization require electrical electrodes of the samples which adds more complexity to the experiments. Many MOF and ferroelectric compounds do not contain intrinsic paramagnetic centers and thus the usual procedure is to dope them with paramagnetic probes whose concetration shall be as low as possible to minimize changes of the ordering process due to the doping. Usually transition metal ions having similar ionic radius and the same charge as the intrinsic metal nodes are used for doping. 37 As revealed in our recent study, substitution of Zn2+ ions with Mn2+ probes allowed us to suc-

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cessfully investigate a structural phase transition in [(CH3 )2 NH2 ][Zn(HCOO)3 ] framework. 38 An alternative divalent paramagnetic probe ion could be Cu2+ . In this work we report the EPR and pulse ENDOR study of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOF powder doped with tiny amount of paramagnetic Mn2+ and Cu2+ ions. A small level of doping minimizes the magnetic dipolar and exchange interactions between these ions allowing to obtain well resolved EPR and ENDOR powder patterns. Temperature dependent continuous wave (CW) EPR spectra of Mn2+ and Cu2+ indicate that these probes are indeed sensitive to the structural phase transition in [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 compound. Different spectral features of Mn2+ and Cu2+ powder patterns permit further characterization of the observed phase transition. Mn2+ ion probes reveal the nature and dynamics of the phase transition, while Cu2+ ions provide further information about the critical behavior of the phase transition. Pulse ENDOR spectroscopy is used to investigate the local environment of Mn2+ center. The obtained structural information is compared with the XRD data.

Experimental Details Sample Preparation ZnCl2 (99%, Fluka), MnCl2 (99%, Sigma-Aldrich), CuCl2 (99%, Sigma-Aldrich), 1,4-diaminobutane (99%, Sigma-Aldrich), methanol (99.8%, Sigma-Aldrich) and formic acid (98%, Fluka) were commercially available and used without further purification. [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 : 0.1 Mn2+ mol% (1) and [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 : 1 Cu2+ mol% (2) crystals were obtained by a slow diffusion method. In a typical experiment, 15 mL methanol solution containing 10 mmol of 1,4-diaminobutane and 80 mmol of formic acid was placed at the bottom of a glass tube (9 mm inner diameter). On this solution, 25 mL methanol solution containing 1.998 mmol of ZnCl2 and 0.002 mmol of MnCl2 (1.98 mmol of ZnCl2 and 0.02 mmol of CuCl2 ) or was gently added. The tube was sealed and kept undisturbed. Col5



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orless crystals were harvested after 5 days, washed 3 times with methanol and dried at room temperature. Different levels of Mn2+ and Cu2+ doping were used to ensure satisfactory EPR signal-to-noise ratio in each sample.

EPR Spectroscopy All EPR measurements were done on powder samples of compounds 1 and 2. X-band (∼9.5 GHz) continuous wave (CW) and pulse EPR experiments were performed with Bruker ELEXSYS E580 spectrometer. Bruker EMX 10-40 spectrometer was used to measure CW EPR spectra at Q-band (∼34 GHz) frequency. For CW EPR experiments at room temperature we used microwave power of 1 mW. To avoid saturation effects, measurements at low temperature were carried out at 0.05 mW microwave power. The strength and frequency of the modulating field were accordingly 5 G and 100 kHz. To account for possible changes of the dielectric permittivity at the phase transition point, measurements of the EPR signal intensity were performed using a double cavity resonator. Pulse EPR experiments were performed with a typical nonselective

π 2

microwave pulse length of t π2 = 16 ns. For pulse

ENDOR experiments the Mims ENDOR sequence 39 was used with the microwave and radiofrequency pulse lengths of t π2 = 16 ns and tRF = 10 µs, respectively. Pulse delay between the first and the second microwave pulses was τ = 104 ns and 130 ns. All simulations of EPR spectra were performed using EasySpin 5.0.19 simulation package 40 implemented in Matlab environment.

Results and Discussion EPR of 1 The Q-band CW EPR spectra of 1 measured above and below the phase transition temperature are presented in Figure 2 (see Supporting Information (SI) for spectra at X-band frequency). The detected patterns are typical for Mn2+ ions in the 3d5 electron configura6


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tion (6 S5/2 electronic ground state). 41 The total electron spin of this state is S = 5/2 which results in five fine structure (fs) transitions (∆mS = ±1, where mS is the magnetic electron spin quantum number). If the so called zero-field splitting (zfs) of Mn2+ center is nonzero, these transitions should have different resonance fields. 35 The fs transitions are further split into six lines originating from the hyperfine (hf) interaction between unpaired electrons and 55

Mn nucleus (nuclear spin I = 5/2). Both presented spectra consist of the well resolved hf

lines of the central fs transition (mS = − 12 ↔ + 12 ), while the so called outer fs transitions (mS = ± 32 ↔ ± 12 , mS = ± 52 ↔ ± 32 ) are unresolved and appear as broad spectral wings (see Figure 2b). This indicates nonzero, broadly distributed values of parameters describing the zfs. 42 The fs is slightly better resolved in the spectrum recorded at 160 K, however, the lines are still too broad to precisely determine the zfs parameters. We note that the spectrum merely changed as we cooled the sample to 15 K.

(a) 260 K

160 K

Simulation

(b) 260 K Simulation

160 K

1150 1175 1200 1225 1250 1275 B (mT) Figure 2: Normalized Q-band EPR spectra of 1 measured at 260 K and 160 K. Simulated 160 K spectrum is presented in blue. Emphasis on (a) central and (b) outer fs transitions. We simulated X- and Q-band spectra recorded at 160 K to roughly estimate the unre-

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solved fs. The spin Hamiltonian used for simulation is 41

HMn = βe BgS + SAI + SDS,

(1)

where the first term describes the electron Zeeman interaction characterized by the electron g-tensor g. B denotes the external magnetic field and βe is Bohr magneton. The second term takes into account the hf interaction, while the last term describes the fs. A and D are the hf and fs tensors, respectively. D is parameterized by D (axial) and E (orthorhombic) zfs parameters. The simulated spectra (see Figure 2 and Figure S1 (SI)) agree sufficiently well with the experiment. For simulations the isotropic g- and A-tensors were used with the corresponding components g = 2.0016(1) and Aiso = −262(1) MHz. The determined value of Aiso indicates Mn-O coordination 43 suggesting that Mn2+ ions successfully replaced Zn2+ and formed MnO6 octahedra. The axial zfs parameter and its distribution used for simulation are |D| = 300(50) MHz and ∆D = 250(70) MHz, respectively. The orthorhombic zfs parameter E was set to zero. In our recent study we observed a similar value and distribution of D in the high temperature phase of manganese doped [(CH3 )2 NH2 ][Zn(HCOO)3 ] MOF. 38 In this compound the broadening mechanism was assigned to fast motion of (CH3 )2 NH2+ cations. Below the phase transition point the motion of these cations slows down significantly resulting in a well resolved fs. Temperature dependence of D clearly indicated a first-order phase transition in [(CH3 )2 NH2 ][Zn(HCOO)3 ]. 38 Note that in the low-temperature phase of [(CH3 )2 NH2 ][Zn(HCOO)3 ]: Mn2+ MOF all MnO6 octahedra are expected to be the same and thus the observation of a well resolved fs was anticipated. In contrast, XRD data 27 for undoped [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOF reveals seven differently distorted ZnO6 octahedra in the low temperature phase. In the high temperature phase two different ZnO6 octahedra are found. Thus, a broad distribution of D in 1 could result from a combined effect of NH3 (CH2 )4 NH32+ motion and different MnO6 geometries.

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In this case the expected change in the spectrum due to the ordering of NH3 (CH2 )4 NH32+ cations could be masked by a distribution of D originating from different Mn2+ environments. Note that NH3 (CH2 )4 NH32+ cation is much bigger than (CH3 )2 NH2+ and its characteristic motional time is expected to be much slower. We cannot eliminate a possibility that even in the high temperature phase this time is much slower than the time scale of our experiment and thus the motion of the cation barely affects the distribution of D. We also performed two-pulse echo-detected field-sweep pulse EPR experiments (see Figure S2 in SI). The obtained spectra perfectly agree with the CW EPR measurements verifying that Mn2+ probes are well distributed in the sample. Otherwise, the regions with a high local Mn2+ concentration would not contribute to the echo-detected spectra due to much faster relaxation. This would cause a difference between echo-detected and CW EPR spectra. Surprisingly, the Hahn echo in 1 was observable up to about 230 K indicating very well isolated Mn2+ centers. However, we did not detect any pulse response above this temperature preventing direct relaxation time measurements of Mn2+ centers at the phase transition. Using the saturation recovery and the two-pulse echo decay experiments we determined the longitudinal relaxation time T1 and the phase memory time Tm at 200 K to be 260 ns and 37 ns, respectively (measurement position in the Mn2+ powder spectrum is indicated in Figure S2). The spectral resolution of the CW and echo-detected EPR spectra is insufficient to resolve the hf interactions between paramagnetic Mn2+ centers and ligand nuclei such as 1 H. Therefore, we performed Mims ENDOR experiments to investigate the local environment of Mn2+ ions in the low-temperature structure of 1. The obtained 1 H pattern (Figure 3) recorded at 361.8 mT, τ = 104 ns and 15 K (measurement field is indicated in Figure S2) shows at least four different proton species. The corresponding hf splittings at 90◦ edge singularities in the powder pattern are AH ⊥ = 0.7 MHz, 1.1 MHz, 1.9 MHz and 2.3 MHz. Note that due to the low signal to noise ratio, the splittings at 0◦ edge singularities were not resolved. Essentially the same ENDOR pattern was obtained for τ = 130 ns (not shown). In our analysis we assumed no orientation selectivity, since measurements at several other field

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positions yielded the same pattern. We can assume that the isotropic part of the proton hf 2+ coupling tensors is negligible (AH is occupied iso = 0), since the first coordination shell of Mn

by oxygen. In such case the point dipole approximation is valid and thus one can use the 44 following expression of AH ⊥ to obtain Mn-H distance dMn-H :

AH ⊥ =

µ0 gMngH βe βn , 4π d3Mn-H

(2)

where gMn and gH are g-factors of Mn2+ and proton. µ0 and βn denote vacuum permeability and nuclear magneton, respectively.

0.7 MHz 1.1 MHz 1.9 MHz 2.3 MHz

-2

-1

0 1 ν - ν H (MHz)

2

Figure 3: X-band 1 H Mims ENDOR spectrum of 1 recorded at 15 K at 361.8 mT and τ = 104 ns. Four different 1 H couplings are indicated by the arrows. The observed splittings correspond to Mn-H distances of dMn-H = 0.48 nm, 0.42 nm, 0.35 nm and 0.32 nm, respectively. Careful analysis of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOF structure 27 suggests that the most strongly coupled protons originate from the formate HCOO– linkers which join metal centers. For these protons Zn-H distance obtained from XRD analysis varies from 0.30 nm to 0.31 nm. Weaker couplings correspond to protons from NH3 (CH2 )4 NH32+ molecular cation. XRD revealed dZn-H = 0.36 nm for protons bonded to nitrogen which is in a perfect agreement with dMn-H = 0.35 nm obtained from ENDOR spectrum. We assigned protons at distances of 0.42 nm and 0.48 nm to methylene groups. A 10


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good correspondence between ENDOR and XRD studies confirms that Mn2+ is incorporated at the Zn2+ sites allowing to exploit these probes to study phase transition in 1. Additionally, ENDOR analysis verifies the proton positions in the structure obtained by XRD 27 which are often difficult to determine precisely by XRD based methods. X-band CW spectra recorded at different temperatures were used to determine a temperature dependence of the peak-to-peak EPR linewidth Γpp of the low-field hf line (magnetic nuclear spin quantum number mI = − 52 ) from the central fs transition. Γpp was obtained by calculating the first-derivative of the CW spectra and finding the values of the magnetic field at which the derivative is zero. The obtained results are presented in Figure 4a. We observe an overall increase of the linewidth with rising temperature. In addition, an anomalous temperature dependence of Γpp is observed at the phase transition point Tc = 235 K. It is known that the linewidth of an unsaturated purely homogeneous EPR line is inversely proportional to the transverse relaxation time T2 of the paramagnetic species (Γpp ∼ 1/T2 ) 41 . Note that the determined value of Tm is too long to account for the width of about 1.2 mT indicating that we observe inhomogeneous EPR lines. Nevertheless, the observed anomaly implies a decrease of T2 at the phase transition temperature, since broadening of the homogeneous lines of the individual spin packets would also increase the observed inhomogeneous EPR linewidth. Such an anomalous behavior of Γpp is caused by a critical slowing down of the order-parameter fluctuations 45 and it is usually encountered while studying phase transition phenomena in TGS, 46 TSCC, 45,47 SrTiO3 48 and other similar systems. 49 An anomaly of T2 is frequently complemented with a change of T1 , 50,51 however, as already mentioned, due to too fast relaxation at Tc the direct pulse EPR measurements of relaxation times were not possible in 1. Note that a recent CW EPR study of [(CH3 )2 NH2 ][Mn(HCOO)3 ] MOF clearly revealed a first-order phase transition manifesting as a sudden decrease of the EPR linewidth at the transition point. 52 This observation was related to diminishing crystal field distribution caused by ordering of (CH3 )2 NH2+ cations. 52 In contrast, the temperature dependence of Γpp in 1 is much more gradual.

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( a)

160

1.20

140

Γpp,0 4.0

4.5 -1 1000/T (K )

0.024

1.24

Γpp,c (mT)

IEPR (arb. u.)

180

Γpp (mT)

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(b)

0.016 0.008 0

1.16 5.0

6

12 18 24 30 T−Tc (K)

Figure 4: (a) Temperature dependence of the peak-to-peak linewidth Γpp (circles) and the EPR signal intensity IEP R (squares) of 1. Error bars are smaller than the data points. Solid curve marks the expected linewidth behavior in the absence of phase transition. (b) Logarithmic temperature dependence of Γpp,c for T > Tc . Solid line is guide for eyes. The observed anomaly of Γpp was further investigated by subtracting the temperature dependence of the inhomogeneous linewidth (Γpp,0) in the absence of the critical slowing down of the order parameter fluctuations. Γpp,0 was obtained by fitting a general exponential decay to experimental data measured sufficiently far away from the phase transition point. The observed gradual increase of Γpp,0 with the increasing temperature might indicate a motional effect which is not related to the phase transition. 53 As illustrated in Figure 4b, the obtained critical part of homogeneous linewidth Γpp,c = Γpp − Γpp,0 follows the logarithmic temperature dependence at T > Tc . The same behavior of EPR linewidth was observed in manganese doped TSCC which exhibits an order-disorder ferroelectric phase transition caused by hydrogen bonding. 47,54 Such a logarithmic temperature dependence of the spinlattice relaxation time is also frequently encountered in nuclear magnetic resonance studies of various H-bonded ferroelectrics. 55–57 The logarithmic divergence indicates an order-disorder transition influenced by the anisotropic long-range dipolar forces. Such an uniaxial ordering is often observed for H-bonded ferroelectric or antiferroelectric phases. 58 Recently some of us performed a Monte Carlo study of the phase transition in [(CH3 )2 NH2 ][Mn(HCOO)3 ] MOF which indeed revealed that the H-bonds between the (CH3 )2 NH2+ cations and the

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framework is the main ordering mechanism in this system. 59 We note that the obtained EPR data does not necessarily imply the antiferro- or ferroelectric ordering in 1. Obviously, additional measurements of electric polarization hysteresis loops would help to reveal the true nature of this phase transition. However, as shown below, the EPR investigation of a second paramagnetic probe ion, Cu2+ , provides further evidence that the phase transition at Tc leads to an ordering into an antiferroelectric phase. In addition, we obtained EPR signal intensity IEP R at different temperatures by double integrating the spectra. This dependence is also presented in Figure 4a. IEP R deviates from the expected Currie law 41 behavior (IEP R ∼ 1/T ) showing a small maximum (only few percent of the total intensity) at the phase transition point. A similar but much more pronounced behavior of the EPR intensity was observed in the Cu2+ doped ferroelectric (NH4 )2 SO4 compound. 53 It was assigned to a decrease of T1 at the phase transition point. Note that the phase transition in 1 is related to the motional effects and thus a change of the line shape at Tc is expected 41 . Thus, the observed anomaly might also originate from the numerical integration error of different line shapes. To account for possible changes of the Q-factor of the resonator, the measurements of IEP R were performed using a double cavity resonator with a reference sample.

EPR of 2 The EPR investigation of the Mn2+ probe ions reveals details about the dynamics of the phase transition at Tc but do not give more insight about its nature. Thus, alternative paramagnetic Cu2+ probe ions were introduced into [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 resulting in sample 2. The X-band CW EPR spectra of 2 recorded at 237 K and X-band frequency is presented in Figure 5. It displays a typical anisotropic Cu2+ powder pattern with four well resolved hf lines at lower magnetic field values. The hf splitting originates from the interaction between unpaired electrons (S = 21 ) and nuclei of

63

Cu and

65

Cu isotopes (both possessing I = 32 ).

The experimental spectrum was simulated using the following spin Hamiltonian with 13



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Simulation 237 K 237 K

220 K 260

200

280 B(mT)

250

300

300 B (mT)

350

400

Figure 5: X-band CW EPR spectrum of 2 measured at 237 K and its simulation. Comparison between Azz hf splitting recorded at 237 K and 220 K is presented in the inset. included electron Zeeman and hf interactions:

HCu = βe BgS + SAI.

(3)

Here g and A are the g- and hf tensors of copper center. Other symbols have their usual meanings. The best simulation (see Figure 5) was obtained using orthorhombic g-tensor with principal values gxx = 2.079(3), gyy = 2.15(1) and gzz = 2.403(1). The hf tensor components Axx and Ayy were unresolved, while Azz was found to be 371(1) MHz. Such values of gzz and Azz are frequently observed for copper in octahedral environment. 60 The observed orthorhombic g-tensor is in agreement with the XRD results which show that ZnO6 octahedra are slightly distorted. 27 As the sample was cooled, the shape of the spectrum remained unchanged. However, as shown in Figure 6a, we observed a complex behavior of Azz near the phase transition temperature. Above 237 K Azz increased with increasing temperature, whereas the value of this parameter gradually decreased from 371(1) MHz at 237 K to 362(1) MHz at 220 K (see inset in Figure 5). As the temperature was further lowered, Azz steadily increased again

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to 380(1) MHz at 87 K. The value of gzz parameter was constantly rising from 2.401(1) at 267 K to 2.412(1) at 87 K without any significant anomaly within the accuracy of our measurements (Figure S3, SI). Lattice contraction might be one effect leading to such a temperature dependence of gzz if the shrinking of the lattice would result in a reduction of the axial elongation of the Cu-O octahedrons. But we may not rule out other causes such as e.g. a dynamic Jahn-Teller effect which provides a comparable temperature dependencies of gzz . 61 However, a detailed analysis of such a temperature dependence is beyond the scope of this paper. All these findings demonstrate that Cu2+ ions were also successfully incorporated at the Zn2+ sites into the [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 framework. Furthermore, the decrease of Azz on cooling in the temperature interval between 237 K and 220 K show that these probe ions are susceptible to the phase transition in 2. Note that no change of the hf interaction of Mn2+ probes was observed in 1.

384

16

(a)

(b)

372

ΔAzz

366 360

100

150 200 T (K)

12 8 4 0

250

(ΔAzz)2 (MHz2)

378

ΔAzz (MHz)

Azz (MHz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 100 50 0

100

200

220 T (K)

240

150 200 T (K)

250

Figure 6: Temperature dependence of (a) copper hf parameter Azz and (b) ∆Azz . The square of ∆Azz is indicated in the inset. The error of Azz is less than 1 MHz. This phase transition was further investigated by analyzing the temperature dependence of Azz . We obtained a critical component ∆Azz of the hf interaction by subtracting a line from the Azz data (see Figure 6a and b). According to Shang et al. 27 [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 framework exhibits an antiferroelectric ordering below Tc ≈ 235 K. Thus, a critical EPR parameter such as the copper hf splitting here measures the equilibrium values of the two 15



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spontaneous sublattice polarizations Pa,0 and Pb,0 , which have different signs but same magnitude. The EPR parameter can show either a linear (∆Azz,a ∼ Pa,0 and ∆Azz,b ∼ Pb,0 ) 2 or a quadratic dependence (∆Azz ∼ Pa(b),0 ) on the sublattice polarizations depending on

the local symmetry of the paramagnetic probe ion. In the former case a splitting into two hf branches ∆Azz,a and ∆Azz,b would be observed whereas the latter case leads just to a shift of the resonance lines measured by ∆Azz which is the situation observed here for 2. Therefore, the order parameter η and the absolute value of the equilibrium values of the sublattice polarizations must be proportional to the square root of the critical component √ η ∼ |Pa,0 | = |Pb,0| ∼ ∆Azz . The presented temperature dependence is typical for a first-order phase transition into an antiferroelectric phase. According to Kittel’s model of antiferroelectricity 36 the free energy density in case of a first-order antiferroelectric phase transition depends on the two collinear sublattice polarizations Pa and Pb and is given by g(T, Pa, Pb ) = g0 (T ) + f (Pa2 + Pb2 ) + g ′Pa Pb + h(Pa4 + Pb4 ) + j(Pa6 + Pb6 ) + · · ·

(4)

with f = 1/2g ′ +λ(T −T0 ) and g > 0, j > 0, h < 0 and g ′ > 0. Pa,0 and Pb,0 are then obtained by the minimization condition of the free energy the paraelectric and 2 ∆Azz ∼ Pa,0



−h  1+ = 3j

∂g ∂Pa(b)

= 0 and yield |Pa,0 | = |Pb,0 | = 0 in

s



3jλ 1 − 2 (T − T0 ) h

(5)

in the antiferroelectric phase. Here the temperature T0 with T0 < Tc defines the stability limit. The solid line in Figure 6b represents a satisfying fit of the experimental data using Eq. 5 with parameter ratios h/λ = −0.044, h/j = −0.95 (in their corresponding units) and T0 = 235 K. The analysis shows clearly that the experimentally obtained ∆Azz values 2 and consequently also Pa(b),0 follow the temperature dependence as expected for a first-order

antiferroelectric phase transition although the jump in the critical component at Tc is only 16


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weakly expressed here, but, nevertheless, it has a finite value and can at least quantitatively explain the jump of the dielectric permittivity at Tc . 27 This indicates that the absolute value of the parameter h must be small and the phase transition appears to be close to a tricritical point where h = 0 holds. 36,62 Indeed, a linear dependence ∆A2zz ∼ (Tc − T ) can be found for T < T0 as indicated by the straight line in the inset of Figure 6b. This implies that |Pa(b),0 | ∼ (T0 − T )1/4 as obtained from Eq. 5 in the limit h → 0. Note that we also performed measurements of the EPR signal intensity of 2 at different temperatures. The obtained dependence (see Figure S4 in SI) shows a very tiny increase of IEP R at the phase transition demonstrating similar behavior as observed for 1.

Conclusions We used EPR and pulse ENDOR spectroscopy to study structural phase transition and the low temperature phase in [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 MOF powder doped with small amount of paramagnetic Mn2+ and Cu2+ probe ions. CW and pulse EPR spectra of manganese doped compound revealed sharp, well resolved hf lines demonstrating that the undesired magnetic dipolar and exchange interactions between Mn2+ probes are weak. Simultaneous simulation of X- and Q-band CW EPR spectra showed that Mn2+ ions form MnO6 octahedra. A broad distribution of zfs parameters was also observed indicating different crystal field environments around the Mn2+ centers. X-band 1H Mims ENDOR study of manganese doped MOF shows at least four different proton species in the local environment of Mn2+ probes. The obtained 1H hf couplings were compared with the available XRD data. The most strongly coupled proton was assigned to formate linkers, while the other three protons were found to originate from ammonium and methylene groups of the NH3 (CH2 )4 NH32+ cation confirming successful incorporation of Mn2+ ions at the Zn2+ lattice sites. X-band CW EPR spectra of Cu2+ doped compound revealed well resolved copper hf lines.

17



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Precise simulation of the experimental powder patterns allowed to determine gzz and Azz components of g-tensor and hf tensors. The obtained spin Hamiltonian parameters demonstrated distorted octahedral Cu2+ ion environment and indicated likewise an incorporation of the cupric ion at Zn2+ sites into the oxygen octahedra of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 . Temperature dependence of Azz shows complex behavior which originates from further distortion of octahedra caused by ordering of NH3 (CH2 )4 NH32+ cations at the phase transition temperature Tc = 235 K. It provided detailed insight into the nature of this structural transition. The temperature dependence of the critical part ∆Azz of Azz below Tc resembled a typical order-parameter behavior and confirmed the presence of a first-order phase transition from a paraelectric into an antiferroelectric phase in accordance with previous dielectric measurements. 27 As deduced from ∆Azz the temperature dependence of the absolute value of the equilibrium sublattice polarizations in the antiferroelectric phase displayed a behavior close to a tricritical one in this MOF which was not observed in the dielectric study. EPR spectroscopy of the Mn2+ probe ions provided further information about the phase transition, in particular its dynamics. The results indicate an order-disorder type phase transition in accordance with other studies. The phase transition was related to critical slowing down of cation dynamics which influences the relaxation rate of paramagnetic Mn2+ ion probes. Consequently, the temperature dependent EPR linewidth and intensity show characteristic anomalies where the anomalous behavior of the EPR linewidth was assigned to a decrease of the transverse relaxation time T2 of Mn2+ center at the phase transition point Tc . The obtained logarithmic divergence of EPR linewidth indicates an uniaxial ordering in the antiferroelectric phase of [NH3 (CH2 )4 NH3 ][Zn(HCOO)3 ]2 which has been often found for H-bonded systems, but which could as well remind the chain-like arrangement of the NH3 (CH2 )4 NH32+ cation in the cavities of the metal formate framework.

Acknowledgement This work was supported by the STSM Grant from the COST Action MP1308-TO-BE and 18


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the Deutsche Forschungsgemeinschaft (DFG) within the priority program SPP 1601.

Supporting Information Available Additional EPR spectra and temperature dependence of EPR parameters. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Abrahams, B.; Hoskins, B.; Michail, D.; Robson, R. Assembly of Porphyrin Building Blocks into Network Structures with Large Channels. Nature 1994, 369, 727–729. (2) Meek, S. T.; Greathouse, J. A.; Allendorf, M. D. Metal-Organic Frameworks: A Rapidly Growing Class of Versatile Nanoporous Materials. Advanced Materials 2011, 23, 249– 267. (3) Kitagawa, S.; Kitaura, R.; Noro, S. Functional Porous Coordination Polymers. Angew. Chem. Int. Ed. 2004, 43, 2334–2375. (4) Rosseinsky, M. Recent Developments in Metal-Organic Framework Chemistry: Design, Discovery, Permanent Porosity and Flexibility. Microporous Mesoporous Mater. 2004, 73, 15 – 30. (5) Schoedel, A.; Ji, Z.; Yaghi, O. M. The Role of Metal-Organic Frameworks in a CarbonNeutral Energy Cycle. Nature Energy 2016, 1, 16034. (6) Kuppler, R. J.; Timmons, D. J.; Fang, Q.-R.; Li, J.-R.; Makal, T. A.; Young, M. D.; Yuan, D.; Zhao, D.; Zhuang, W.; Zhou, H.-C. Potential Applications of Metal-Organic Frameworks. Coordination Chemistry Reviews 2009, 253, 3042–3066. (7) Wu, H.; Gong, Q.; Olson, D. H.; Li, J. Commensurate Adsorption of Hydrocarbons and Alcohols in Microporous Metal Organic Frameworks. Chem. Rev. 2012, 112, 836–868.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

(8) He, Y.; Zhou, W.; Qian, G.; Chen, B. Methane Storage in Metal-Organic Frameworks. Chem. Soc. Rev. 2014, 43, 5657–5678. (9) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen Storage in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1294–1314. (10) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal-Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869–932. (11) Kurmoo, M. Magnetic Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1353– 1379. (12) Dechambenoit, P.; Long, J. R. Microporous Magnets. Chem. Soc. Rev. 2011, 40, 3249– 3265. (13) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Luminescent MetalOrganic Frameworks. Chem. Soc. Rev. 2009, 38, 1330–1352. (14) Ramaswamy, P.; Wong, N. E.; Shimizu, G. K. H. MOFs as Proton Conductors - Challenges and Opportunities. Chem. Soc. Rev. 2014, 43, 5913–5932. (15) Cui, H.; Wang, Z.; Takahashi, K.; Okano, Y.; Kobayashi, H.; Kobayashi, A. Ferroelectric Porous Molecular Crystal, [Mn3 (HCOO)6 ](C2 H5 OH), Exhibiting Ferrimagnetic Transition. Journal of the American Chemical Society 2006, 128, 15074–15075. (16) Guo, M.; Cai, H.-L.; Xiong, R.-G. Ferroelectric Metal Organic Framework (MOF). Inorganic Chemistry Communications 2010, 13, 1590–1598. (17) Zhang, W.; Xiong, R.-G. Ferroelectric Metal-Organic Frameworks. Chemical Reviews 2012, 112, 1163–1195. (18) Jain, P.; Dalal, N. S.; Toby, B. H.; Kroto, H. W.; Cheetham, A. K. Order-Disorder Antiferroelectric Phase Transition in a Hybrid Inorganic-Organic Framework with the 20


Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Perovskite Architecture. Journal of the American Chemical Society 2008, 130, 10450– 10451. (19) Jain, P.; Ramachandran, V.; Clark, R. J.; Zhou, H. D.; Toby, B. H.; Dalal, N. S.; Kroto, H. W.; Cheetham, A. K. Multiferroic Behavior Associated with an OrderDisorder Hydrogen Bonding Transition in Metal-Organic Frameworks (MOFs) with the Perovskite ABX3 Architecture. Journal of the American Chemical Society 2009, 131, 13625–13627. (20) Xu, G.-C.; Ma, X.-M.; Zhang, L.; Wang, Z.-M.; Gao, S. Disorder-Order Ferroelectric Transition in the Metal Formate Framework of [NH4 ][Zn(HCOO)3 ]. Journal of the American Chemical Society 2010, 132, 9588–9590. (21) Mączka, M.; Gągor, A.; Macalik, B.; Pikul, A.; Ptak, M.; Hanuza, J. Order-Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3 )2 NH2 ][M(HCOO)3 ] and [(CH3 )2 ND2 ][M(HCOO)3 ] (M = Ni, Mn). Inorganic Chemistry 2014, 53, 457–467. (22) Maczka, M.; Pietraszko, A.; Macalik, B.; Hermanowicz, K. Structure, Phonon Properties, and Order-Disorder Transition in the Metal Formate Framework of [NH4 ][Mg(HCOO)3 ]. Inorganic Chemistry 2014, 53, 787–794. (23) Shang, R.; Chen, S.; Hu, K.-L.; Jiang, Z.-C.; Wang, B.-W.; Kurmoo, M.; Wang, Z.-M.; Gao, S. Hierarchical Cobalt-Formate Framework Series with (412 · 63 )(49 · 66 )n (n = 1-3) Topologies Exhibiting Slow Dielectric Relaxation and Weak Ferromagnetism. APL Mater. 2014, 2, 124104. (24) Shang, R.; Xu, G.-C.; Wang, Z.-M.; Gao, S. Phase Transitions, Prominent Dielectric Anomalies, and Negative Thermal Expansion in Three High Thermally Stable Ammonium Magnesium-Formate Frameworks. Chemistry - A European Journal 2014, 20, 1146–1158. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(25) Maczka, M.; Gagor, A.; Costa, N. L. M.; Paraguassu, W.; Sieradzki, A.; Pikul, A. Temperature- and Pressure-Induced Phase Transitions in the Niccolite-Type Formate Framework of [H3 N(CH3 )4 NH3 ][Mn2 (HCOO)6 ]. J. Mater. Chem. C 2016, 4, 3185–3194. (26) Shang, R.; Wang, Z.-M.; Gao, S. A 36-Fold Multiple Unit Cell and Switchable Anisotropic Dielectric Responses in an Ammonium Magnesium Formate Framework. Angewandte Chemie International Edition 2015, 54, 2534–2537. (27) Shang, R.; Chen, S.; Hu, K.-L.; Wang, B.-W.; Wang, Z.-M.; Gao, S. A Variety of PhaseTransition Behaviors in a Niccolite Series of [NH3 (CH2 )4 NH3 ][M(HCOO)3 ]2 . Chemistry - A European Journal 2016, 22, 6199–6203. (28) Eerenstein, W.; Mathur, N. D.; Scott, J. F. Multiferroic and Magnetoelectric Materials. Nature 2006, 442, 759–765. (29) Jee, B.; Eisinger, K.; Gul-E-Noor, F.; Bertmer, M.; Hartmann, M.; Himsl, D.; Pöppl, A. Continuous Wave and Pulsed Electron Spin Resonance Spectroscopy of Paramagnetic Framework Cupric Ions in the Zn(II) Doped Porous Coordination Polymer Cu3-x Znx (btc)2 . The Journal of Physical Chemistry C 2010, 114, 16630–16639. (30) Jee, B.; St. Petkov, P.; Vayssilov, G. N.; Heine, T.; Hartmann, M.; Pöppl, A. A Combined Pulsed Electron Paramagnetic Resonance Spectroscopic and DFT Analysis of the 13

CO2 and

CO Adsorption on the Metal-Organic Framework Cu2.97 Zn0.03 (btc)2 . The

13

Journal of Physical Chemistry C 2013, 117, 8231–8240. (31) Šim˙enas, M.; Jee, B.; Hartmann, M.; Banys, J.; Pöppl, A. Adsorption and Desorption of HD on the Metal-Organic Framework Cu2.97 Zn0.03 (Btc)2 Studied by Three-Pulse ESEEM Spectroscopy. The Journal of Physical Chemistry C 2015, 119, 28530–28535. (32) Mendt, M.; Jee, B.; Stock, N.; Ahnfeldt, T.; Hartmann, M.; Himsl, D.; Pöppl, A. Structural Phase Transitions and Thermal Hysteresis in the Metal-Organic Framework

22


Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Compound MIL-53 As Studied by Electron Spin Resonance Spectroscopy. The Journal of Physical Chemistry C 2010, 114, 19443–19451. (33) Sheveleva, A. M.; Kolokolov, D. I.; Gabrienko, A. A.; Stepanov, A. G.; Gromilov, S. A.; Shundrina, I. K.; Sagdeev, R. Z.; Fedin, M. V.; Bagryanskaya, E. G. Structural Dynamics in a "Breathing" Metal-Organic Framework Studied by Electron Paramagnetic Resonance of Nitroxide Spin Probes. The Journal of Physical Chemistry Letters 2014, 5, 20–24. (34) Šim˙enas, M.; Kobalz, M.; Mendt, M.; Eckold, P.; Krautscheid, H.; Banys, J.; Pöppl, A. Synthesis, Structure, and Electron Paramagnetic Resonance Study of a Mixed Valent Metal-Organic Framework Containing Cu2 Paddle-Wheel Units. The Journal of Physical Chemistry C 2015, 119, 4898–4907. (35) Jain, V. K.; Lehmann, G. Electron Paramagnetic Resonance of Mn2+ in Orthorhombic and Higher Symmetry Crystals. Physica Status Solidi (b) 1990, 159, 495–544. (36) Blinc, R.; Žekš, B. Soft Modes in Ferroelectrics and Antiferroelectrics; North-Holland Publishing Company, 1974. (37) Nevjestic, I.; Depauw, H.; Leus, K.; Kalendra, V.; Caretti, I.; Jeschke, G.; Van Doorslaer, S.; Callens, F.; Van Der Voort, P.; Vrielinck, H. Multi-Frequency (S, X, Q and W-band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal-Organic Framework MIL-53. ChemPhysChem 2015, 16, 2968–2973. (38) Šim˙enas, M.; Ciupa, A.; Mączka, M.; Pöppl, A.; Banys, J. EPR Study of Structural Phase Transition in Manganese-Doped [(CH3 )2 NH2 ][Zn(HCOO)3 ] Metal-Organic Framework. The Journal of Physical Chemistry C 2015, 119, 24522–24528. (39) Mims, W. B. Pulsed Endor Experiments. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 1965, 283, 452–457.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(40) Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. Journal of Magnetic Resonance 2006, 178, 42 – 55. (41) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Clarendon Press, Oxford, 1970. (42) Misra, S. K. Estimation of the Mn2+ Zero-Field Splitting Parameter from a Polycrystalline EPR Spectrum. Physica B: Condensed Matter 1994, 203, 193–200. (43) Šimánek, E.; Müller, K. Covalency and Hyperfine Structure Constant A of Iron Group Impurities in Crystals. Journal of Physics and Chemistry of Solids 1970, 31, 1027–1040. (44) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press, 2001. (45) Volkel, G.; Brunner, W.; Windsch, W. Critical Fluctuations in Trissarcosine Calcium Chloride (TSCC) Observed by the Electron Spin Echo (ESE) Method. Solid State Communications 1975, 17, 345 – 348. (46) Nishimura, K.; Hashimoto, T. ESR Investigation of TGS Doped with Cr3+ Ions. Journal of the Physical Society of Japan 1973, 35, 1699–1703. (47) Windsch, W. Paramagnetic Resonance Studies in Ferroelectric Tris-Sarcosine Calcium Chloride. Ferroelectrics 1976, 12, 63–69. (48) von Waldkirch, T.; Müller, K. A.; Berlinger, W. Fluctuations in SrTiO3 near the 105-K Phase Transition. Phys. Rev. B 1973, 7, 1052–1066. (49) Owens, F. J.; Poole, C. P.; Farach, H. A. Magnetic Resonance of Phase Transitions; Academic Press, 1979. (50) Blinc, R.; Prelovšek, P. Electron Spin-Lattice Relaxation Anomaly and the Local Pseudo Freeze-Out of Impurity Dynamics in H-Bonded Ferroelectrics. Solid State Communications 1982, 42, 893–896. 24


Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Volkel, G.; Brunner, W.; Müller, H.-E. The Anomaly in the Electron Spin-Lattice Relaxation of Paramagnetic Impurities at the Ferroelectric Phase Transition. Ferroelectrics 1988, 78, 267–274. (52) Abhyankar, N.; Bertaina, S.; Dalal, N. S. On Mn2+ EPR Probing of the Ferroelectric Transition and Absence of Magnetoelectric Coupling in Dimethylammonium Manganese Formate (CH3 )2 NH2 Mn(HCOO)3 , a Meta-Organic Complex with the Pb-Free Perovskite Framework. The Journal of Physical Chemistry C 2015, 119, 28143–28147. (53) Owens, F. Effect of Ferroelectric Phase Transition of Ammonium Sulphate on EPR of Divalent Copper. Chemical Physics Letters 1976, 38, 106–108. (54) Lippe, R.; Windsch, W.; Volkel, G.; Schulga, W. EPR Investigations of Critical Order Parameter Fluctuations in Ferroelectric Tris-Sarcosine Calcium Chloride (TSCC). Solid State Communications 1976, 19, 587–590. (55) Bonera, G.; Borsa, F.; Rigamonti, A. Nuclear Quadrupole Spin-Lattice Relaxation and Critical Dynamics of Ferroelectric Crystals. Phys. Rev. B 1970, 2, 2784–2795. (56) Tatsuzaki, I.; Sakata, K.; Todo, I.; Tokunaga, M. Anomaly of the Proton SpinLattice Relaxation Time near the Critical Temperature of Dicalcium Lead Propionate Ca2 Pb(C2 H5 COO)6 . Journal of the Physical Society of Japan 1972, 33, 438–445. (57) Brosowski, G.; Buchheit, W.; Müller, D.; Petersson, J. Proton Spin-Lattice Relaxation in the Hydrogen-Bonded Ferroelectrics Colemanite, KFCT, and TGS. Physica Status Solidi (b) 1974, 62, 93–102. (58) Rigamonti, A. NMR-NQR studies of structural phase transitions. Advances in Physics 1984, 33, 115–191. (59) Šim˙enas, M.; Balči¯ unas, S.; Mączka, M.; Banys, J.; Tornau, E. Structural Phase Tran-

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sition in Perovskite Metal-Formate Frameworks: a Potts-Type Model with Dipolar Interactions. Physical Chemistry Chemical Physics 2016, (60) Pöppl, A.; Baglioni, P.; Kevan, L. Electron Spin Resonance and Electron Spin Echo Modulation Studies of the Incorporation of Macrocyclic-Complexed Cupric Ions into Siliceous MCM-41. The Journal of Physical Chemistry 1995, 99, 14156–14160. (61) Rubins, R. S.; Drumheller, J. E. The Temperature Dependence of the EPR Spectrum of Cu2+ in ZnTiF6 · 6 H2 O between 4 and 160 K. The Journal of Chemical Physics 1987, 86, 6660–6664. (62) Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Oxford University Press, 2001.

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Graphical TOC Entry

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EPR of Structural Phase Transition in Manganese- and Copper-Doped

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