Magnetic-Field Tuning of Hydrogen Bond Order-Disorder Transition in Metal-Organic Frameworks

A Paramagnetic Compass Based on Lanthanide Metal-Organic Framework

Macroscopic compass-like magnetic alignment at low magnetic fields is natural for ferromagnetic materials but is seldomly observed in paramagnetic materials. Herein, we report a "paramagnetic compass" that magnetically aligns under ∼mT fields based on the single-crystalline framework constructed by lanthanide ions and organic ligands (Ln-MOF). The magnetic alignment is attributed to the Ln-MOF's strong macroscopic anisotropy, where the highly-ordered structure allows the Ln-ions' molecular anisotropy to be summed according to the crystal symmetry. In tetragonal Ln-MOFs, the alignment is either parallel or perpendicular to the field depending on the easiest axis of the molecular anisotropy. Reversible switching between the two alignments is realized upon the removal and re-adsorption of solvent molecules filled in the framework. When the crystal symmetry is lowered in monoclinic Ln-MOFs, the alignments become even inclined (47°-66°) to the field. These fascinating properties of Ln-MOFs would encourage further explorations of framework materials containing paramagnetic centers.

Magnetoelectric and Magnetostrictive Effects in Scheelite-Type HoCrO4

Scheelite-type HoCrO₄ was prepared by treating the ambient-pressure zircon-type precursor phase under 8 GPa and 700 K. A long-range antiferromagnetic phase transition is found to occur at T_N ≈ 23 K due to the spin order of Ho³⁺ and Cr⁵⁺ magnetic ions. However, the antiferromagnetic ground state is sensitive to an external magnetic field and a moderate field of about 1.1 T can induce a metamagnetic transition, giving rise to the presence of a large magnetization up to 8.5 μ_B/f.u. at 2 K and 7 T. Considerable linear magnetoelectric effect is observed in the antiferromagnetic state, while the induced electric polarization experiences a sharp increase near the critical field of the metamagnetic transition. Ferromagnetism and ferroelectricity thus rarely coexist under higher magnetic fields in scheelite-type HoCrO₄. Moreover, a magnetic field also plays an important role in the longitudinal constriction of HoCrO₄, and a significant magnetostrictive effect with a value of up to 300 ppm is observed at 2 K and 9 T, which can be attributed to the strong anisotropy of the rare-earth Ho³⁺ ion. Possible coupling between magnetoelectric and magnetoelastic effects is discussed.

Pressure Control of the Structure and Multiferroicity in a Hydrogen-Bonded Metal-Organic Framework

Multiferroic materials with the cross-coupling of magnetic and ferroelectric orders provide a new platform for physics study and designing novel electronic devices. However, the weak coupling strength of ferroelectricity and magnetism is the main obstacle for potential applications. The recent research focuses on enhancing the coupling effect via synthesizing novel materials in a chemical route or tuning the multiferroicity in the physical way. Among them, pressure is an effective method to modify multiferroic materials, especially when the chemical doping has reached its tuning limit. In this work, we systemically studied the multiferroic properties in a hydrogen-bonded metal-organic framework (MOF) [(CH₃)₂NH₂]Ni(HCOO)₃ under high pressure. X-ray diffraction and Raman scattering reveal that a structural phase transition occurs in a pressure region of 6-9 GPa, and the crystal structure is greatly modified by pressure. With the ac magnetic susceptibility, pyroelectric current, and dielectric constant measurements, we obtain the multiferroic property evolution under high pressure and create a temperature-pressure phase diagram. Our study demonstrates that the pressure can modify the magnetic superexchange interaction and hydrogen bonding simultaneously in these perovskite-like MOFs. The multiferroic phase region has been expanded to higher temperature due to the pressure-enhanced spin-phonon coupling effect.

Negative Linear Compressibility in Organic-Inorganic Hybrid Perovskite [NH2NH3]X(HCOO)3 (X = Mn, Fe, Co)

Hybrid organic-inorganic perovskites [NH₂NH₃][X(HCOO)₃] (X = Mn, Fe, Co) have a so-called "wine-rack" type of geometry that could give origin to the rare property of negative linear compressibility, which is an exotic and highly desirable material response. We use first-principles density functional theory computations to probe the response of these materials to hydrostatic pressure and predict that, indeed, all three of them exhibit negative linear compressibility above a critical pressure of 1 GPa. Calculations reveal that, under pressure, XO₆ octahedra and -HCOO ligands remain relatively rigid while XO₆ octahedra tilt significantly, which leads to highly anisotropic mechanical properties and expansion along certain directions. These trends are common for the three materials considered.

Elastic Properties and Energy Loss Related to the Disorder-Order Ferroelectric Transitions in Multiferroic Metal-Organic Frameworks [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3]

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 R3¯c to monoclinic Cc. The elastic anomalies in [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] showed strong coupling of ferroelasticity with ferroelectricity.

Elastic Properties and Energy Dissipation Related to the Disorder-Order Ferroelectric Transition in a Multiferroic Metal-Organic Framework [(CH3)2NH2][Fe(HCOO)3] with a Perovskite-Like Structure

The elastic properties and the coupling of ferroelasticity with ferromagnetism and ferroelectricy are crucial for the development of multiferroic metal-organic frameworks (MOFs) with strong magnetoelectric coupling. Elastic properties and energy dissipation related to the disorder-order ferroelectric transition in [(CH₃)₂NH₂][Fe(HCOO)₃] were studied by differential scanning calorimetry (DSC), low temperature X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). DSC result indicated the transition near 164 K. XRD showed the first-order structural transition from rhombohedral R3-c to monoclinic Cc at ~145 K, accompanied by the disorder-order transition of proton ordering in the N-H…O hydrogen bonds in [(CH₃)₂NH₂]⁺ as well as the distortion of the framework. For single crystals, the storage modulus was ~1.1 GPa and the loss modulus was ~0.02 GPa at 298 K. DMA of single crystals showed quick drop of storage modulus and peaks of loss modulus and loss factor near the ferroelectric transition temperature ~164 K. DMA of pellets showed the minimum of the normalized storage modulus and the peaks of loss factor at ~164 K with weak frequency dependences. The normalized loss modulus reached the maximum near 145 K, with higher peak temperature at higher frequency. The elastic anomalies and energy dissipation near the ferroelectric transition temperature are caused by the coupling of the movements of dimethylammonium cations and twin walls.

Unique cation-template three-dimensional hybrid material demonstrates dielectric switchable response

The strict tolerance space of three-dimensional (3D) crystalline structures is still a significant challenge in the area of switching dielectrics in comparison with lower-dimensional structures. Generally, the function of crystalline materials can be given or adjusted by controlling the environment in which synthesis takes place or the packing rearrangement. Using this method, special functional enhancements or changes in the dielectrics can be realized by improving the synthetic strategies. Here, a 3D switchable dielectric compound [MeHdabco]K(BF4)3 was achieved by employing the temperature selective effect. In particular, its structure is completely different from the usual 3D perovskite structure, which is constructed using two different cation-template frameworks. Moreover, the 3D [MeHdabco]K(BF4)3 shows a structural phase transition at 358 K. The thermal analysis (differential scanning calorimetry (DSC)) and X-ray diffractometry results provided evidence of these phase changes. This work provides a feasible strategy that can be used to achieve the different structures of an 'isomer', and enrich the method used for designing diverse functional materials.

Pressure Effect on Order-Disorder Ferroelectric Transition in a Hydrogen-Bonded Metal-Organic Framework

Perovskite-like ABX₃ metal-organic frameworks (MOFs) have gathered great interest due to their intriguing chemical and physical properties, including their magnetism, ferroelectricity, and multiferroicity. Pressure is an effective thermal parameter in tuning related properties in MOFs due to the adjustable organic framework. Though spectrum experiments have been made on the structural evolution during decompression, there is a lack of electrical studies on the order-disorder ferroelectric transition in the metal-organic frameworks under pressure. In this work, we use a static pyroelectric current measurement, a dynamic dielectric method combined with a Raman scattering technique with applying in situ pressure, to explore the order-disorder ferroelectric transition in [(CH₃)₂NH₂]Co(HCOO)₃. The ferroelectric transition vanishes around the external pressure of 1.6 GPa, emerging with a new paraelectric phase. Another phase transition was observed at 6.32 GPa, mainly associated with the distortive transition of DMA⁺ cations. A phenomenological theory of ferroelectricity vanishing at 1.6 GPa for [(CH₃)₂NH₂]Co(HCOO)₃ is also discussed. Our study gives a comprehensive understanding in the pressure tuning of ferroelectric properties in hybrid inorganic-organic materials.

Static disorder in a perovskite mixed-valence metal–organic framework

Effects of A-site and M-site substitutions on the structural properties of perovskite dimethylammonium iron formate.

On the impact of metal ion proportion on the physical properties of heterometallic metal–organic frameworks

In the studied heterometallic structures, the interactions between the built-in EtA⁺ cation and the environment are modified by the K/Na ratio.