EPR Study of Structural Phase Transition in Manganese-Doped [(CH3

Article pubs.acs.org/JPCC

EPR Study of Structural Phase Transition in Manganese-Doped [(CH3)2NH2][Zn(HCOO)3] Metal−Organic Framework † Mantas Šimeṅ as,† Aneta Ciupa,‡ Mirosław Ma̧czka,‡ Andreas Pöppl,*,§ and Juras ̅ 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 § Faculty of Physics and Earth Sciences, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig, Germany ‡

S Supporting Information *

ABSTRACT: We present an electron paramagnetic resonance (EPR) study of [(CH3)2NH2][Zn(HCOO)3] metal−organic framework (MOF) powder doped with a small amount of paramagnetic Mn2+ ions. Our EPR measurements indicate a successful incorporation of local Mn2+ probes into the structure allowing us to detect and investigate an order− disorder structural phase transition in the studied MOF. The temperature-dependent continuous wave (CW) X- and Qband EPR measurements reveal a sudden change in the spectra at a phase transition temperature T0 = 163 K. Simulations were performed to determine the spin Hamiltonian parameters of the spectra which reflect the local symmetry of the Mn2+ probes in the disordered and ordered phases. The temperature dependence of the axial zero-field splitting parameter D demonstrates that the observed phase transition at T0 is discontinuous. Additionally, this dependence follows the prediction of the Landau theory. We also performed preliminary pulse EPR measurements which reveal a rather long phase memory time sufficient to detect a spin echo even in the high-temperature disordered phase. The modulation with the proton and nitrogen Larmor frequencies of the electron spin echo was observed as well.

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INTRODUCTION

room temperature, encouraging the search for coordination polymers with inherent ferroelectricity or related phenomena. Recently, Jain et al. reported an observation of such inherent phenomenon in the [(CH3)2NH2][M(HCOO)3] (dimethylammonium metal formate or DMMF, M = Zn2+, Mn2+, Fe2+, Co2+, and Ni2+) MOF family which is based on the perovskite AMX3 topology (A = (CH3)2NH2+, X = HCOO−).19,20 Heat capacity and dielectric measurements of DMZnF indicated a phase transition at approximately 160 K, which was assigned to the order−disorder antiferroelectric type. The structural X-ray diffraction (XRD) studies revealed that the compound crystallizes in the trigonal R3c̅ space group in the paraelectric phase. In this phase Zn2+ ions are surrounded by six oxygen atoms from HCOO− (formate) groups, forming slightly distorted octahedra which join together to a three-dimensional framework of ReO3-type cages. A (CH3)2NH2+ (DMA+, dimethylammonium) cation is trapped within such a cage, and it is disordered, meaning that the nitrogen from the amine group can occupy three locally equivalent positions by forming hydrogen bonds with oxygen atoms from the formate linkers (see Figure 1).19 Although the precise mechanism of the phase

Lately, a novel type of porous material called coordination polymers or metal−organic frameworks (MOFs) has emerged and immediately attracted the attention of the scientific community.1−3 These compounds consist of two main building units: metal centers acting as nodes and organic linker molecules connecting them together.4 Usually transition metal or lanthanide cations are chosen as metal centers which together with enormous diversity of organic linkers selfassemble into various MOF networks.5 The resulting porous topologies and different functionality of the underlying structural motifs are responsible for appealing physical and chemical properties.6−9 The main scientific interest regarding these hybrid compounds is focused on their pronounced ability to selectively adsorb,10 store,11,12 and separate13 gases, catalyze chemical reactions,14 or sense chemicals.15 Alongside these functionalities, some MOFs exhibit peculiar dielectric properties such as ferroelectricity which originates from polar organic molecules (e.g., ethanol) situated within the framework.16,17 Usually these molecules are introduced into the pores as guests and order below a certain order−disorder-type phase transition temperature creating a nonzero spontaneous polarization.18 However, the loosely bound guest nature of the polar entities makes such materials unstable on approaching © 2015 American Chemical Society

Received: September 5, 2015 Revised: October 5, 2015 Published: October 7, 2015 24522

DOI: 10.1021/acs.jpcc.5b08680 J. Phys. Chem. C 2015, 119, 24522−24528

Article

The Journal of Physical Chemistry C

(CW) and pulse EPR methods were successfully employed to study gas adsorption,40 local structural changes,41 and magnetic properties42,43 in nonferroelectric HKUST-1, MIL-53, and similar MOFs. Taking this into account EPR spectroscopy seems to be a natural choice for investigation of the structural phase transition in the DMMF MOF family. Note that the DMMnF MOF was already studied by CW EPR spectroscopy.20,28 However, due to the strong magnetic dipolar and exchange interactions between the Mn2+ centers the EPR spectrum of DMMnF consists of a single broad line which is barely sensitive to the ordering of the DMA+ cations. In this work we report the CW and pulse EPR study of DMZnF MOF powder doped with 0.05 Mn2+ mol %. The doping with such a small amount of Mn2+ ions is highly advantageous, since the lack of a strong dipolar interaction allowed us to record highly resolved CW and pulse EPR spectra at different temperatures. The incorporated Mn2+ probes proved to be highly sensitive to the structural phase transition permitting its further characterization.

Figure 1. Room-temperature crystal structure of DMZnF MOF based on the perovskite AMX3 topology. The dimethylammonium cation is shown as a superposition of three equivalent positions it can occupy (each nitrogen atom has an occupational probability of 1/3, two hydrogen atoms bonded to nitrogen are not shown). Structure taken from ref 19.

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EXPERIMENTAL DETAILS Sample Preparation. ZnCl2 (99%, Fluka), MnCl2 (99%, Sigma-Aldrich), 2.0 M solution of (CH3)2NH in methanol (Sigma-Aldrich), methanol (99.8%, Sigma-Aldrich), and formic acid (98%, Fluka) were commercially available and used without further purification. [(CH3)2NH2][Zn(HCOO)3]: 0.05 Mn2+ mol % (1) was obtained by a slow diffusion method. In a typical experiment, 16 mL of methanol solution containing 12.8 mmol of (CH3)2NH and 12.8 mmol of formic acid was placed at the bottom of a glass tube (9 mm inner diameter). On this solution, 16 mL of methanol solution containing 1.5992 mmol of ZnCl2 and 0.0008 mmol of MnCl2 was gently added. The tube was sealed and kept undisturbed. Colorless crystals were harvested after 5 days. Powder X-ray diffraction pattern (PXRD) of 1 was measured to check the structure of the sample, and we found that it indeed corresponds to DMZnF (see the Supporting Information, SI). No additional impurity phases were detected. EPR Spectroscopy. The CW and pulse EPR measurements at X-band microwave (mw) frequency (∼9.5 GHz) were performed using a conventional Bruker ELEXYS E580 EPR spectrometer. The Q-band (∼34 GHz) CW EPR spectra were recorded with a Bruker EMX 10-40 spectrometer. For most of the CW measurements 2 mW mw power was used. To avoid saturation effects, the measurements at low temperature were performed at 0.02 mW mw power. The strength and frequency of the modulation field were 4 G and 100 kHz, respectively. Pulse EPR spectra were recorded using nonselective pulses (typical π/2 pulse length was 16 ns). Samples were cooled with liquid helium in an Oxford flow cryostat. A 100 Ω platinum resistor connected to a Keithley Integra 2700 multimeter was used for precise temperature measurements. The temperature determination error was minimized (±0.3 K) by placing the sensor near the sample within the sample tube. Spectral simulations were performed using EasySpin 5.0.2 simulation software.44

transition and the structure of DMZnF below the phase transition temperature is still unsolved, it is believed that the DMA+ cations order cooperatively and during this process the symmetry of the structure is reduced from trigonal to monoclinic.20 In addition to the heat capacity, dielectric, and XRD measurements,19 the DMZnF MOF was intensively investigated using other experimental techniques such as nuclear magnetic resonance,21,22 infrared, and Raman spectroscopies23 as well as mechanical study.24 However, despite this huge effort the precise nature of the phase transition in DMZnF is still obscured. It should be noted that Sánchez-Andújar et al. reported an XRD study in which they claim to have solved the structure of DMMnF in the low-temperature phase.25 It was assigned to the monoclinic Cc space group, which is indeed noncentrosymmetric as required for ferroelectricity-related phenomena to occur. In this phase the dimethylammonium cations occupy a single crystallographic position, while the Mn2+ cations are in a strongly distorted octahedral environment with six different Mn−O distances. It is also believed that the phase transitions in DMZnF and DMMnF should have the same mechanism, since both compounds are isostructural and have similar phase transition temperatures (approximately 185 K for DMMnF).20 Moreover, it was found that compounds with Mn2+, Fe2+, Co2+, and Ni2+ exhibit the magnetic ordering in the lowtemperature region ( 163 K the outer fs transitions are not resolved, indicating a broad distribution of the D parameter. A Gaussian distribution of D with the fwhm of ΔD ≈ 150 MHz 24525


Article


We decided to check whether the obtained D vs T dependence at T < T0 can be described using the Landau theory of phase transitions.52 First, the axial zero-field splitting parameter can be expanded in series of the order parameter η53

D=

∑ Cmηm m

(3)

where Cm are expansion coefficients. Usually the first expansion term containing η is sufficient to describe the experimental data. We assume that the local symmetry at the Mn2+ ion has an inversion center, and thus, the coefficient C1 must vanish. Then it follows that D ≈ C0 + C2η2

(4)

Here C0 corresponds to the value of D in the high-temperature phase (T > T0). It is known from Landau theory that the square of the order parameter for the discontinuous phase transition behaves as (T0−T)1/2 at T < T0.52,54 Thus, according to eq 4, D should follow the (T0 − T)1/2 law. We indeed found that such a temperature dependence describes the experimental data very well (see inset in Figure 8), indicating that Landau theory can be applied to characterize the phase transition in 1.

Figure 7. Simulation of the Q-band EPR spectrum of 1 recorded at 80 K. The central and outer transitions are indicated in a and b, respectively.

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CONCLUSIONS We employed CW and pulse EPR spectroscopy to study the structural order−disorder phase transition in [(CH3)2NH2][Zn(HCOO)3] MOF powder doped with 0.05 Mn2+ mol %. The crystallites of the investigated compound were obtained using the slow diffusion method. The observed narrow lines in the EPR spectra indicate that paramagnetic Mn2+ ions are well isolated from each other and evenly distributed within the sample volume. The spin Hamiltonian parameters were obtained by simultaneously simulating the CW X- and Qband spectra. The determined value of the isotropic hyperfine coupling constant Aiso indicates Mn−O coordination, implying that Mn2+ probes substitute Zn2+ ions and form MnO6 octahedra. An abrupt change in the temperature-dependent EPR spectra at T0 = 163 K demonstrates that the Mn2+ centers are indeed sensitive to the structural transformation occurring in the MOF. In the high-temperature disordered phase we observed a slightly noncubic environment of Mn2+ ions and a broad distribution of the axial zfs parameter D. In contrast, the spin Hamiltonian parameters of the ordered phase indicate a much more pronounced distortion of the MnO6 octahedra and a narrow distribution of D. The temperature dependence of the spin Hamiltonian parameters reveals that a phase transition at T0 is discontinuous. Besides, the determined temperature dependence of D can be well described within the framework of Landau theory. We also performed pulse EPR measurements to explore whether such experiments can provide additional information. Surprisingly, we detected a spin echo up to 190 K which allowed us to record the two-pulse echo-detected FS EPR spectra of both structural phases. The observation of the spin echo at such a high temperature is unusual for the Mn2+ center. This again proves that paramagnetic probes are well isolated. We also observed modulation of the spin echo with the proton and nitrogen Larmor frequencies. These preliminary results imply that pulse EPR as well as pulse ENDOR spectroscopy may be applied to thoroughly study the phase transition in DMZnF MOF.

The observed much higher value of D as well as nonnegligible E demonstrate that in the low-temperature phase the symmetry of the MnO6 octahedron deviates significantly from cubic. Note that a highly noncubic environment of Mn2+ (six different Mn−O distances) was found in DMMnF MOF using XRD.25 In addition, a much smaller distribution width ΔD may occur, because of the long-range ordering of the DMA+ cations. In such a situation most MnO6 octahedra are deformed in the same manner, resulting in a narrow distribution of the fs parameters. The temperature dependence of the spin Hamiltonian parameters was determined from the X- and Q-band EPR spectra measured at different temperatures (Figures 2 and 3). We did not observe any change of the g factor or Aiso. However, the zfs parameter D increased substantially in the lowtemperature phase. The obtained temperature dependence of D (Figure 8) clearly indicates a discontinuous phase transition at T0 = 163 K. Note that this curve is reproduced on heating (not shown here) with a small temperature hysteresis (ΔT ≈ 3 K).

Figure 8. Temperature dependence of the axial zfs parameter D. The dependence just below T0 is presented in the inset. The red solid curve indicates the best fit to the Landau theory. 24526


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08680. XRD powder pattern and additional EPR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 341 9732608. Fax: +49 341 9732649. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the STSM Grant from the COST Action MP1308-TO-BE and the Deutsche Forschungsgemeinschaft (DFG) within the priority programs SPP 1601 and SPP 1362. The authors thank R. Juškėnas for performing PXRD measurements and R. Böttcher for extensive discussion.

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EPR Study of Structural Phase Transition in Manganese-Doped [(CH3

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