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

The ordering of polar hydrogen bonds may break space inversion symmetry and induce ferroelectricity or antiferroelectricity. This process is usually immune to external magnetic fields so that magnetic control of hydrogen bonds is very challenging. Here we demonstrate that the ordering of hydrogen bonds in the metal-organic frameworks [(CH_{3})_{2}NH_{2}]M(HCOO)_{3} (M=Fe, Co) can be manipulated by applying magnetic fields. After cooling in a high magnetic field, the order-disorder transition of hydrogen bonds shifts to a lower or higher temperature, depending on antiferroelectricity or ferroelectricity induced by hydrogen bond ordering. Besides, the order-disorder transition leads to a giant thermal expansion, exceeding ∼3.5×10^{4} and ∼2×10^{4}  ppm for M=Fe and Co, respectively, which is much higher than that of inorganic ferroelectrics. The influence of magnetic field on hydrogen bond ordering is discussed in terms of the magnetoelastic coupling.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni)

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH₃)₂NH₂] M(HCOO)₃ ( M = Mn²⁺, Co²⁺, Ni²⁺) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH₃)₂NH₂]Mn(HCOO)₃ takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across T_C reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH₃)₂NH₂]Co(HCOO)₃ reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co²⁺ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Transformation of the surface compositions of titanium during alkali and heat treatment at different vacuum degrees

The AH method at different vacuum degrees had a significant influence on the surface composition, structure, wettability, bioactivity and other properties of titanium.

Insights into the nitridation of zero-valent iron nanoparticles for the facile synthesis of iron nitride nanoparticles

The process of nitridation of zero-valent iron nanoparticles (ZVINPs) is investigated by employing two different synthesis strategies such as solvothermal method and gas diffusion using N₂ and NH₃.