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Understanding Ferroelectricity in the Pb-Free PerovskiteLike Metal-Organic Framework [(CH3)2NH2]Zn(HCOO)3: Dielectric, 2D NMR, and Theoretical Studies Nandita Abhyankar, Jin Jung Kweon, Maylis Orio, Sylvain Bertaina, Minseong Lee, Eun Sang Choi, Riqiang Fu, and Naresh S. Dalal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00596 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017
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Understanding Ferroelectricity in the Pb-Free Perovskite-Like Metal-Organic Framework [(CH3)2NH2]Zn(HCOO)3: Dielectric, 2D NMR, and Theoretical Studies Nandita Abhyankar1, Jin Jung Kweon2, Maylis Orio3, Sylvain Bertaina4, Minseong Lee2, Eun Sang Choi2, Riqiang Fu2*, Naresh S. Dalal1, 2* 1 Department of Chemistry & Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee FL 32304 2 National High Magnetic Field Laboratory (NHMFL), 1800 E Paul Dirac Dr., Tallahassee, FL 32304 3 Aix Marseille Univ, CNRS, Cent Marseille, iSm2, Marseille, France 4 Aix-Marseille Univ, CNRS, IM2NP UMR 7334, Marseille, France
ABSTRACT. Dimethylammonium zinc formate ([(CH3)2NH2]Zn(HCOO)3 or DMZnF) is a model system for the study of hybrid perovskite-like dielectrics. It undergoes a phase transition from the paraelectric to ferroelectric phase at ~160 K. The mechanism of this phase transition has been shown to have contributions from ordering of the hydrogen bonds between DMA+ and
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the formate groups, as well as buckling of the metal-formate framework, but the transition dynamics and atomistic mechanism are not fully clear. This work presents dielectric constant measurements as evidence of cluster formation of the low-temperature phase and the relaxor-like behavior of this MOF above the phase transition.
13
C CP-MAS is used to track the evolution of
the chemical shift, T1, and T2 of the dimethylammonium cation and formate groups from room temperature down to 120 K. 2D
13
C-13C correlation measurements provide evidence of the
formation of pre-transitional clusters above the phase transition temperature. DFT calculations support the assignment of chemical shifts and the proposed model. The analysis of
13
C CP-MAS
spectra and DFT calculations is used to discuss the mechanism of the dielectric phase transition and the origin of relaxor-like behavior in DMZnF.
1. Introduction: Perovskite-like metal formates with the general formula ABX3 are a class of metal-organic frameworks (MOFs) that have received extensive research interest1-3 for their multiferroic properties, exhibiting simultaneous dielectric and magnetic ordering (ferroelectricity and ferromagnetism, respectively). One reason for this high interest is that they have the potential to become components of novel memory storage and manipulation devices: one could, in principle, encrypt a data set by using both electric and magnetic technologies, thus making it more effective and also more compact than current technologies, which use one or the other. Additionally, being metal-organic complexes, these ABX3 materials could serve as Pb-free alternatives to the PZT family of piezoelectric and ferroelectric materials that are currently used extensively in electro-optic and electric polarization-based devices.1-3 Dimethylammonium zinc formate, [(CH3)2NH2]Zn(HCOO)3 (DMZnF), is the forerunner of this family,1-7 and the first to
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exhibit ferroelectric behavior in this class.5-7 Before large-scale applications, however, characteristics such as operation temperature and magnetoelectric couplings must be optimized.13
It is thus important to understand the details of the mechanism of their ferroelectric phase
transition, in particular, whether the mechanism involves an order-disorder mechanism of the dimethylammonium (DMA+) cation, or a soft-mode behavior.1-3 This has not been possible despite focused effort.8--17 One of the possible reasons is the lack of high-resolution solid-state NMR measurements, even though NMR spectroscopy is known to be a sensitive probe of ferroelectric phase transitions.18-21 This query provided the main impetus for the present undertaking. The currently accepted model of structural and ferroelectric transitions in the [(CH3)2NH2]M(HCOO)3 family, where M is a d-metal cation (or a mixture thereof), is the orderdisorder model involving the dimethylammonium cation, henceforth abbreviated as DMA+. At temperatures above the phase transition temperature TC, DMA+ is found to be disordered over three sites, consistent with the space group R3-c, as determined by X-ray crystallography at T>TC. At T TC), the DMA+ resides at the center of the MOF cavity, and its nitrogen atom is disordered over three possible positions, through its hindered rotation. Figure 1 shows three adjacent cells of the low-temperature Cc phase, with DMA+ ions in one of the three possible orientations, based on the reported X-ray structure. When the MOF undergoes a structural phase transition to the monoclinic (Cc) phase, the DMA+ group is shifted off-center in the cavity (see Figure 1). In the Cc phase, the methyl as well as formate groups are structurally differentiated into two and three different types, respectively (see Figure 1). The structure determined from X-ray diffraction indicates that when the DMA+ group becomes off-center at T < TC, one of the methyl groups is pushed towards a corner of the cavity while the other is pulled away from the diagonally opposite corner. At the same time, the formate groups are structurally distinguished into three types.
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Figure 1. One of the three possible ordered arrangements of DMA+ ions inside adjacent metalformate cavities in the Cc phase below the dielectric transition in DMMnF, as proposed by Sanchez-Andujar et al.15 DMZnF is isostructural and therefore, is represented here by the same structure. The blue labels in the left-most cavity show the carbons corresponding to the peaks shown in Fig. 2. The three types of formate carbons are structurally different and are signified by the blue, red, and yellow colors of the attached oxygen atoms. Further explanation is provided in Section 3.1.1.
The NMR lineshapes are consistent with this structural differentiation, since the single methyl and formate signals seen at T>TC split into two methyl signals and three formate signals at T
