Spin-density studies of the multiferroic metal-organic compound [NH2(CH3)2][FeIIIFeII(HCOO)6]

Polarized neutron diffraction is used to study in depth the magnetic properties of the heterometallic compound [NH₂(CH₃)₂][Fe^IIIFe^II(HCOO)₆] and give insight into its magnetic behaviour, addressing open questions that will contribute to a better understanding of this attention-grabbing material and other related ones. Previous results revealed that upon cooling, the magnetic moments of the Fe^II and Fe^III sites do not order simultaneously: the magnetization of the Fe^II site increases faster than that of the Fe^III sites. Unpolarized neutron diffraction measurements at 2 K with no external field revealed some discrepancies in the saturation value of the magnetic signal on the Fe^III sites and in the ferromagnetic moment along the c axis. These discrepancies could be related to the actual distribution of magnetic moment, since unpolarized neutron diffraction gives information on the magnetic moment localized only on the magnetic ions. Polarized neutron diffraction allows an analysis of the magnitude of the spin density over magnetic and non-magnetic ions (the organic ligand and the counterion), which can give a clue to explain the low saturation on the Fe^III sites and the correlation with the physical measurements. The present study also contributes to the understanding of the magneto-electric behaviour of this compound, giving insight into the role of metal disorder in the origin of the structural phase transition, which is responsible for its antiferrolelectric order, and into the influence of spin-density delocalization on its magneto-electric properties, allowing a discussion of the alternative explanations given so far for its electric properties at low temperature.

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.

Tunable Ferromagnetic Strength in Niccolite Structural Heterometallic Formate Framework Based on Orthogonal Magnetic Orbital Interactions

A series of heterometallic formate framework templated by amines were solvothermally prepared. They feature the formula of [A^I][CrM^II(HCO₂)₆] (A^I = NH₄H₂O^I and M = Mn for 1, A^I = CH₃NH₃^I and M = Fe for 2, A^I = CH₃NH₂CH₃^I and M = Co for 3, A^I = CH₃NH₃^I and M = Ni for 4). The title compounds exhibit isostructural niccolite architectures with differences only in the host metal ions and guest amines. Tunable ferromagnetic (FO) strength was realized in the resulting framework under the guidance of orthogonal magnetic orbital analysis of Cr^III (t_2g³e_g) and M^II (t_2g³e_g² for Mn^II, t_2g⁴e_g² for Fe^II, t_2g⁵e_g² for Co^II, t_2g⁶e_g² for Ni^II) ions. The magnetic ordering temperatures derived from the experimental magnetic measurements for 1-4 are lower than 2, 10.3, 7.6, and 22.0 K, respectively. Notably, thanks to the weak FO coupling between Cr^III and Mn^II ions, compound 1 displays a large magnetocaloric effect bearing the maximum of magnetic entropy change (-Δ S_m^max) up to 43.9 J kg^-1 K^-1 with Δ H = 7 T and T = 3.5 K, larger than most reported transition metal-based complexes and commercial gadolinium gallium garnet (Gd₃Ga₅O₁₂) (-Δ S_m^max = 38.4 J kg^-1 K^-1 with Δ H = 7 T). From 1, 2/3, to 4, an enhancement of the magnetic ordering temperature is observable due to the increasing strength of FO interactions between Cr^III and M^II ions. Our work provides a successful instance to modulate the strength of FO exchange via analyzing the orthogonal magnetic orbitals of heterometallic ions.

Ferroelastic Phase Transition and Switchable Dielectric Constant in Heterometallic Niccolite Formate Frameworks

Four heterometallic formate frameworks templated by various alkylamine cations with the general formula [cat][Ga^IIIMn^II(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.

Giant Room-Temperature Magnetodielectric Response in a MOF at 0.1 Tesla

A giant room-temperature magnetodielectric (MD) response upon the application of a small magnetic field is of fundamental importance for the practical application of a new generation of devices. Here, the giant room-temperature magnetodielectric response is demonstrated in the metal-organic framework (MOF) of [NH₂ (CH₃ )₂ ]_n [Fe^III Fe^II_(1-x) Ni^II_x (HCOO)₆ ]_n (x ≈ 0.63-0.69) (1) with its MD coefficient remaining between -20% and -24% in the 300-410 K temperature range, even at 0.1 T. Because a room-temperature magnetodielectric response has never been observed in MOFs, the present work not only provides a new type of magnetodielectric material but also takes a solid step toward the practical application of MOFs in a new generation of devices.

A Series of Bimetallic Ammonium AlNa Formates

A series of AlNa bimetallic ammonium metal formate frameworks (AlNa AMFFs) have been prepared by employing various ammoniums from NH₄⁺ to large linear polyammoniums. The series consists of six perovskites of (4¹² ⋅6³ ) topology for mono-ammoniums, two chiral (4⁹ ⋅6⁶ ) frameworks incorporating polyethylene ammoniums, two niccolites with (4¹² ⋅6³ )(4⁹ ⋅6⁶ ) topology containing diammoniums, and two layered compounds made of 2D (4,4) AlNa formate sheets intercalated by small diammoniums. The first ten compounds present the structural hierarchy of (4¹² ⋅6³ )_m (4⁹ ⋅6⁶ )_n framework topologies for (m, n)=(1, 0), (0, 1), and (1, 1), respectively, in parallel to the homometallic AMFFs for divalent metals. The symmetry lowering, asymmetric formate bridges, and different hydrogen-bonding strengths appeared in the bimetallic structures owing to the different charge and size of Al³⁺ and Na⁺ seemingly inhibits the occurrence of phase transitions for more than half the AlNa AMFFs within the series, and the bimetallic members undergoing phase transitions show different transition behaviors and dielectric properties compared with the homometallic analogs. Anisotropic/negative/zero thermal expansions of the materials could be rationally attributed to the librational motion, or flip movement between different sites, of the ammonium cations, and the coupled change of AlNa formate frameworks. The thermal and IR spectroscopic properties have also been investigated.

Dielectric relaxation and anhydrous proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal–organic frameworks

First experimental direct evidence of anhydrous conductivity in the perovskite-like metal-formate framework in its ferroelectric phase.

Polar metal–formate frameworks templated with 1,2-diaminoethane–water assemblies showing ferromagnetic and ferroelectric properties

A set of formate frameworks templated with 1,2-diaminoethane-water assemblies of the formula: [NH₃(CH₂)₂NH₃]M₂(HCOO)₆·H₂O (M = Mg, Mn, Co, Ni, Zn) has been synthesized. They show room-temperature ferroelectricity and ferromagnetic ordering below 35 K.

Synthesis and temperature-dependent studies of a perovskite-like manganese formate framework templated with protonated acetamidine

Acetamidinium manganese formate undergoes a phase transition from the Imma to the P2₁/n structure at 304 K and magnetic order below 9 K.

Raman scattering studies of pressure-induced phase transitions in perovskite formates [(CH3)2NH2][Mg(HCOO)3] and [(CH3)2NH2][Cd(HCOO)3]

Pressure-dependent Raman studies were preformed on two dimethylammonium metal formates, [(CH3)2NH2][Mg(HCOO)3] (DMMg) and [(CH3)2NH2][Cd(HCOO)3] (DMCd). They revealed three pressure-induced transitions in the DMMg near 2.2, 4.0 and 5.6 GPa. These transitions are associated with significant distortion of the anionic framework and the phase transition at 5.6 GPa has also great impact on the DMA+ cation. The DMCd undergoes two pressure-induced phase transitions. The first transition occurred between 1.2 and 2.0 GPa and the second one near 3.6 GPa. The first transition leads to subtle structural changes associated with distortion of anionic framework and the later leads to significant distortion of the framework. In contrast to the DMMg, the third transition associated with distortion of DMA+ cation is not observed for the DMCd up to 7.8 GPa. This difference can be most likely associated with larger volume of the cavity occupied by DMA+ cation in the DMCd and thus weaker interactions between anionic framework and DMA+ cations.

Structural, electronic and magnetic properties of metal–organic-framework perovskites [AmH][Mn(HCOO)3]: a first-principles study

Novel multiferroic Metal–Organic-Frameworks (MOFs) [AmH][M(HCOO)₃] are investigated in structural, electronic and magnetic properties using density functional theory.

Temperature- and pressure-dependent studies of niccolite-type formate frameworks of [NH3(CH2)4NH3][M2(HCOO)6] (M = Zn, Co, Fe)

Structural changes occurring in [H₃N(CH₂)₄NH₃][M₂(HCOO)₆] (M = Zn, Co, Fe) niccolites due to temperature- and pressure-induced transitions were monitored by dielectric, IR and Raman spectroscopy.

Dielectric relaxation behavior in antiferroelectric metal organic framework [(CH3)2NH2][FeIIIFeII(HCOO)6] single crystals

The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal–organic framework, multiferroic [(CH₃)₂NH₂][Fe³⁺Fe²⁺(HCOO)₆] single crystals have been reported, covering eight decades in frequency (10⁻² ≤ f ≤ 10⁶) in the temperature range 120 K ≤ T ≤ 250 K.

Phase transitions and chromium(iii) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5−x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C₂H₅NH₃][Na_0.5Cr_xAl_0.5−x(HCOO)₃] (x = 0, 0.025, 0.5).

Experimental and theoretical studies of structural phase transition in a novel polar perovskite-like [C2H5NH3][Na0.5Fe0.5(HCOO)3] formate

We report the synthesis and studies of a novel heterometallic formate [C₂H₅NH₃][Na_0.5Fe_0.5(HCOO)₃].