Abstract
Elastic properties are important mechanical properties which are dependent on the structure, and the coupling of ferroelasticity with ferroelectricity and ferromagnetism is vital for the development of multiferroic metal–organic frameworks (MOFs). The elastic properties and energy loss related to the disorder–order ferroelectric transition in [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] were investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The DSC curves of [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] exhibited anomalies near 256 K and 264 K, respectively. The DMA results illustrated the minimum in the storage modulus and normalized storage modulus, and the maximum in the loss modulus, normalized loss modulus and loss factor near the ferroelectric transition temperatures of 256 K and 264 K, respectively. Much narrower peaks of loss modulus, normalized loss modulus and loss factor were observed in [(CH3)2NH2][Mg(HCOO)3] with the peak temperature independent of frequency, and the peak height was smaller at a higher frequency, indicating the features of first-order transition. Elastic anomalies and energy loss in [NH4][Mg(HCOO)3] near 256 K are due to the second-order paraelectric to ferroelectric phase transition triggered by the disorder–order transition of the ammonium cations and their displacement within the framework channels, accompanied by the structural phase transition from the non-polar hexagonal P6322 to polar hexagonal P63. Elastic anomalies and energy loss in [(CH3)2NH2][Mg(HCOO)3] near 264 K are due to the first-order paraelectric to ferroelectric phase transitions triggered by the disorder–order transitions of alkylammonium cations located in the framework cavities, accompanied by the structural phase transition from rhombohedral Rc to monoclinic Cc. The elastic anomalies in [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] showed strong coupling of ferroelasticity with ferroelectricity.
Highlights
Multiferroic Metal–organic frameworks (MOFs) with at least two coexisting orders among the ferroelectricity, ferromagnetism and ferroelasticity are of particular interest, and their structure analysis, thermal properties, electric properties and magnetic properties have been widely studied using X-ray diffraction (XRD), neutron scattering, infrared spectroscopy, Raman spectroscopy, differential scanning calorimetry (DSC), heat capacity measurements, dielectric measurements, magnetic measurements and computer simulations [8,9,10,11,12,13,14]
In [NH4 ][M(HCOO)3 ] (M = Mg, Mn, Fe, Co, Ni, Zn), paraelectric to ferroelectric phase transitions triggered by the disorder–order transitions of the ammonium cations and their displacement within the framework channels occur between 191 K and 255 K (Co: 191 K, Zn: 192 K, Ni: 199 K, Fe: 212 K, Mn: 254 K, Mg: 255 K), accompanied by the structural phase transition from the non-polar hexagonal space group P63 22 to the polar hexagonal space group P63 [15,16,17,18,19,20,21,22,23,24,25,26,27]
[(CH3 )2 NH2 ][Mg(HCOO)3 ] single crystals, (d) normalized storage modulus E’T /E’298, (e) normalized loss modulus and (f) loss factor tanδ of [(CH3)2NH2][Mg(HCOO)3] pellet determined by dynamic mechanical analysis (DMA) during heating at the rate of 2 K/min
Summary
[Mg(HCOO)3 ] are around 255 K and 267 K, respectively, close to room temperature, leading to promising applications Their synthesis, structure analysis, thermal properties, dielectric properties, IR studies and Raman studies have been well studied [13,14,15,33,34,35,36,37,38,57,58]. Associated with ferroelectric transitions using dynamic mechanical analysis (DMA) at low frequencies of 0.1–10 Hz and at high stress and strain levels and investigated the relationship between the structure of MOFs and their elastic properties. The findings quantified the changes in elastic properties and energy loss related to the ferroelectric phase transition and contributed to the further understanding of the phase transition mechanism
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