Ferroelastic Phase Transition and Switchable Dielectric Constant in

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Ferroelastic Phase Transition and Switchable Dielectric Constant in Heterometallic Niccolite Formate Frameworks Kai Wang, Jian-Bo Xiong, Bin Xia, Qing-Lun Wang,* Yu-Zhang Tong, Yue Ma, and Xian-He Bu College of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, P. R. China S Supporting Information *

DMAFeFe and DEtAFeFe. The unique feature of both complexes is the presence of an intervalence charge-transfer transition,9,15 which indicates the presence of electron transfer from divalent-to-trivalent Fe ions. Here, we report the synthesis and study of four heterometallic formates, [MA][GaIIIMnII(HCOO)6] (1; MA = CH3NH3+), [DMA][GaIIIMnII(HCOO)6] [2; DMA = (CH3)2NH2+], [EtA][GaIIIMnII(HCOO)6] (3; EtA = CH3CH2NH3+), and [DEtA][GaIIIMnII(HCOO)6] (4; DEtA = (CH3CH2)2NH2+). Crystal data and details of structural refinements of complexes 1−4 are summarized in Table S1. Selected bond lengths and angles are given in Table S2. Complex 4 exhibits a reversible ferroelastic phase transition (P31̅ c ↔ P1̅) triggered by disorder−order DEtA cations around 220 K, as in DEtAFeFe reported by Mac̨ zka and Bu et al.13,14 The colorless crystals of complex 1 were synthesized by solvothermal reaction of GaCl3 and MnCl2·4H2O in a mixed solution containing formic acid and N-methylformamide (Supporting Information). Complexes 2−4 were prepared by the same procedure using N,N-dimethylformamide (DMF; for 2), N-ethylformamide (for 3), and N,N-diethylformamide (for 4) instead of N-methylformamide. It has long been known that DMF can be hydrolyzed to formate and DMA, and the rate increases especially at high temperature.18 Under our experimental conditions, high temperature and pressure favor the hydrolyzation of amide. The corresponding amide played an important role in the synthesis of the compounds. Besides behaving as the source of formate and the alkyl ammonium cations, amide also serves as the solvent for the crystals to be separated out because the products are not very soluble in these solvents. Complexes 1−4 are isostructural with the previously reported metal formates [DMA][MIIFeIII(HCOO)6] with MII = Fe, Co, and Mn,10 which crystallize in the trigonal space group P31̅ c [except for the low-temperature phase (LTP) of 4 in the triclinic space group P1̅]. They possess the same [GaMn(HCOO)6]− anionic framework with binodal (412·63)(49·66) topology, and only the counterions in the cavity are different (Figure 1). The GaIII ions are located in the (412·63) nodes, and the MnII ions are in the (49·66) nodes. They are both 6-connected. Each GaIII ion connects with six adjacent MnII ions through six anti−anti formate ligands to give octahedral geometry. Each MnII ion connects with six adjacent GaIII ions through six anti−anti formate ligands to form a trigonal prism. Two kinds of metal

ABSTRACT: Four heterometallic formate frameworks templated by various alkylamine cations with the general formula [cat][Ga I I I Mn I I (HCOO) 6 ] {cat is MA (CH3NH3+) for 1, DMA [(CH3)2NH2+] for 2, EtA (CH3CH2NH3+) for 3, and DEtA [(CH3CH2)2NH2+] for 4} have been prepared and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric studies. All of the complexes have niccolite-like structures, which possess the same [GaMn(HCOO)6]− anionic framework with binodal (412·63)(49·66) topology; only the counterions in the cavity are different. Complex 4 undergoes a reversible ferroelastic phase transition around 220 K accompanied by a thermally switchable dielectric constant transition triggered by the freezing of the order− disorder DEtA cations.

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mmonium metal formate frameworks are becoming research hotspots in recent years because of their structure, phase transitions, and tunable ferroelectric, magnetic, and luminescent properties.1−3 Mixed-valence niccolite formate frameworks [cat][MIIINII(HCOO)6] (type A) get relatively fewer studies compared with the widely studied [cat][NII(HCOO)3] (type B; N = Mn, Fe, Co, Ni, Cu) frameworks.4−8 The initial studies of mixed-valence niccolite formate frameworks were reported by Hagen et al. in 2009.9 The metal formate framework consists of mixed-valence iron(II)−iron(III) and (CH3)2NH2+ cations, DMAFeFe. It displays Néel N-type ferrimagnet behavior accompanied by an order−disorder reversible phase transition (P3̅1c ↔ R3̅c) at 155 K.10−12 DMAFeFe is the first one of the only two known mixed-valence formates that display phase transitions. The other is DEtAFeFe, reported by Mac̨ zka and Bu et al., which possesses four different types of bistability in response to magnetic field and temperature changes.13,14 The extensively researched compounds with the general formulas [(CH3)2NH2][FeIIIMII(HCOO)6] (MII = Mn, Co, Ni, Cu, Zn, Mg)10−16 and [(CH3)2NH2][CrIIIMII(HCOO)6] (MII = Zn, Ni, Cu),17 crystallize in the trigonal space group P3̅1c (except for the C2/c space group of the Cu analogue) and do not exhibit any structural phase transition. In fact, most of such niccolite structures are stable and do not experience structural phase transitions.15−17 The different charge and size of M3+ and N2+ in type A results in symmetry lowering, asymmetric formate bridges, and different hydrogen-bonding strengths. Such structural features can reduce the chance of phase transitions occurring. This explanation is, however, not valid for © XXXX American Chemical Society

Received: September 18, 2017

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DOI: 10.1021/acs.inorgchem.7b02386 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

P31̅ c with GaIII−O = 1.9705(15) Å and MnII−O = 2.1751(16) Å. The DEtA cations are 3-fold and 2-fold parallel and perpendicular to the c axis, respectively. This differs from the pseudo-D3h symmetric of DMA cations in previous reports,13,19 DEtA cations have pseudo-D3 symmetry. When the dynamic rotations of the guest DEtA cations are frozen, the pseudosymmetry is broken, leading to a phase transition from HTP to LTP.14 The LTP (110 K) of complex 4 belongs to the triclinic space group P1̅, in which the trivalent GaIII metal sites can be classified as two crystallographically independent ions (denoted as Ga1 and Ga2 hereafter), with bond lengths of Ga1− O = 1.955(5)−1.971 (5) Å and Ga2−O = 1.966(5)−1.984(5) Å. Divalent MnII ions of each asymmetric unit (denoted as Mn1 hereafter) exhibit a slight distortion with Mn1−O = 2.153(5)− 2.195(5) Å. The DEtA cations in the LTP display complete ordering. A 3-fold order−disorder change of the DEtA cations is key to the occurrence of the reversible phase transition. The differential scanning calorimetry (DSC) measurements indicate that there is no obvious peak for complexes 1−3. However, for 4, the DSC data in the cooling and heating processes show two reversible broad peaks around 220 and 240 K (Figure 3). The overall calculated changes in the enthalpy (ΔH)

Figure 1. (a) Coordination environment of the (49·66) and (412·63) nodes in complexes 1−4. The pink trigonal prism is the (49·66) node of MnII. The yellow octahedron is the (412·63) node of GaIII. (b) (49· 66)(412·63) topological view of 1−4. MnII are in pink and GaIII in yellow. (c) View of the free cations in 1−4 filled in the cavities. MnII are in pink, GaIII in yellow, O in red, and C in gray. Blue circles represent countercations of alkylamine cations.

nodes are alternately connected by an anti−anti coordination pattern of formate ligands to construct a three-dimensional niccolite skeleton. Protonated amine cations are present as counterions in the framework cavity. Variable-temperature single-crystal X-ray diffraction studies showed that, except for the LTP (113 K) of complex 4, which has a complete ordering of DEtA cations, the other organic−amine cations are accommodated in anionic cages, and all display 3-fold disordering (Figure 2) in complexes 1−3. The high-temperature phase (HTP; 300 K) of complex 4 belongs to the trigonal space group

Figure 3. DSC cycles for complexes 1−4.

and entropy (ΔS) were estimated to be 2376 J mol−1 and 10.8 J mol−1 K−1, respectively. According to the X-ray diffraction data, the DEtA cation has trigonal disorder in the HTP and the transition at the LTP is associated with a loss of the trigonal disorder of DEtA cations. For an order−disorder transition, ΔS = R ln(N),20 where N is the number of sites for the disordered system. Therefore, for a simple 3-fold order−disorder model, the change in the entropy at this transition is expected to be R ln(3) = 9.1 J mol−1 K−1. The experimental value of ΔS (10.8 J mol−1 K−1) suggests that the phase transition at the LTP is related to the complete ordering of DEtA cations, indicating a 3-fold order− disorder character of the phase transition. In comparison, the change of the entropy associated with complete ordering of these cations in [DEtA][FeIIIFeII(HCOO)6] was found to be 7.2 J mol−1 K−1.13 Variable-temperature dielectric spectra were measured in the temperature range 120−260 K. The results for complex 4 clearly reveal that the high dielectric state exists in the paraelastic phase (P3̅1c), while the low dielectric state is in the ferroelastic phase (P1̅; Figure 4), which should be attributed to the motional transitions of the polar DEtA cations, suggesting the occurrence of an order−disorder phase transition between LTP and HTP. The dielectric response ε′ of 4 exhibits little frequency dependence in the measuring frequency range of 5 kHz to 1 MHz, indicating that the polar motions are much faster than 1

Figure 2. (a) Disordered MA cations in the cavities of 1. (b) Disordered DMA cations in the cavities of 2. (c) Disordered EtA cations in the cavities of 3. (d) Disordered DEtA cations in the cavities of 4 (HTP, 293 K). (e) Ordered DEtA cations in the cavities of 4 (LTP, 113 K). H atoms have been omitted. B

DOI: 10.1021/acs.inorgchem.7b02386 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

103 K (0.33 eV; 3), respectively, and the preexponential factors τ0 = 1.74 × 10−15 s (1), 2.03 × 10−14 s (2), and 2.24 × 10−15 s (3). These values of the dielectric relaxation parameters are appropriate for dielectrics23 and comparable to other amine metal formate frameworks previously reported.24 The MA, DMA, and EtA cations in complexes 1−3 remain disordered even at low temperatures; i.e., these cations rotate dynamically at room temperature, but the reorientational motion slows down upon cooling and freezes at low temperatures, leading to static disorder at low temperatures. In summary, we have synthesized four heterometallic formate frameworks crystallizing in a niccolite-like architecture. Although dielectric studies of complexes 1−4 led to a dielectric anomaly in the test range of 120−260 K, variable-temperature single-crystal X-ray diffraction and DSC studies have shown that only complex 4 undergoes a first-order ferroelastic phase transition at about 220 K, while complexes 1−3 are stable above 110 K. It is noteworthy that complexes that have the [GaMn(HCOO)6]− anion niccolite framework have not been reported before. Complex 4 with DEtA cations is the first heterometallic niccolite formate of type A exhibiting an order−disorder phase transition. Cooperative freezing of the reorientational motions of DEA cations plays an important role in the phase-transition mechanism in complex 4.

Figure 4. Temperature dependence of the real part of the dielectric constant of 4 between 5 kHz and 1 MHz.

MHz.14 Plots of ε′ versus T reveal an obvious broad hysteresis between cooling and heating at about 200−240 K. The drastic change of ε′ is consistent with the DSC measurement. Inconsistent with complex 4, complexes 1−3 display relaxation and dielectric dispersion. The tan δ versus T traces displayed a strong frequency ( f) dispersion, and the tan δ peaks correspond to the fall in the ε′ traces due to the Kramers−Krönig relationship (Figure 5 and Figure S1).21 The tan δ peaks went smoothly from HT−HF (high frequencies) to LT−LF (low frequencies). The f versus TP (the peak temperature in tan δ) data of 1−3 could be fitted by the Arrhenius law of τ = τ0 exp(Ea/ kBT)[τ = (2πf)−1],22 resulting in activation energy Ea/kB values of 3.50 × 103 K (0.30 eV; 1), 2.60 × 103 K (0.22 eV; 2), and 3.80 ×

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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.inorgchem.7b02386. Experimental details, Figure S1 and Tables S1 and S2 (PDF) Accession Codes

CCDC 1570105−1570112 and 1583085 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qing-Lun Wang: 0000-0002-0983-6280 Xian-He Bu: 0000-0002-2646-7974 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21371104, 21771111, 21471084, and 21601092) and MOE Innovation Team (IRT13022) of China.

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REFERENCES

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Figure 5. (a) Temperature-dependent traces of the dielectric permittivities for 2. (b) Arrhenius plots for the dielectric relaxations for 1−3. C

DOI: 10.1021/acs.inorgchem.7b02386 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.7b02386 Inorg. Chem. XXXX, XXX, XXX−XXX