On Mn²⁺ EPR Probing of the Ferroelectric Transition and Absence of Magnetoelectric Coupling in Dimethylammonium Manganese Formate (CH₃)₂NH₂Mn(HCOO)₃, a Metal–Organic Complex with the Pb-Free Perovskite Framework

We employ electron paramagnetic resonance (EPR) of Mn²⁺ as a spin probe to study the paraelectric–ferroelectric transition in dimethylammonium manganese formate, [(CH₃)₂NH₂]­Mn­(CHCO₂)₃, (DMMnF), which is considered a model metal–organic framework (MOF) with a Pb-free perovskite architecture. We study the variation of the Mn²⁺ EPR line shape and intensity at the X-band (∼9.5 GHz) over 80 to 300 K. The peaks are essentially Lorentzian, implying electron spin exchange at frequencies greater than 9.5 GHz. On cooling, an anomalous increase in the peak width is noted at 185 K but no anomalous change in the normalized, double-integrated EPR signal intensity around the <i>T</i>_C, indicating that DMMnF is transparent to microwave electric fields with a clear lack of magnetoelectric coupling, in contrast to an earlier report. Our analysis enables us to estimate change in lattice strain related to the ferroelectric transition, information that is difficult to obtain by other techniques

Increasing ¹³C CP-MAS NMR Resolution Using Single Crystals: Application to Model Octaethyl Porphyrins

Octaethyl porphyrin (OEP) and its Ni and Zn derivatives are considered as model compounds in biochemical, photophysical, and fossil fuel chemistry. They have thus been investigated by high-resolution solid-state ¹³C NMR using powders, but peak assignment has been difficult because of large line widths. Arguing that a significant cause of broadening might be the anisotropic bulk magnetic susceptibility, we utilized single crystals in our ¹³C cross-polarization magic angle spinning (CP-MAS) measurements and observed a nearly 2-fold line narrowing. This enhanced resolution enabled us to assign chemical shifts to each carbon for all the three compounds. The new assignments are now in agreement with X-ray structural data and allowed us to probe the motional dynamics of the methyl and methylene carbons of the OEP side chains. It is apparent that the use of single crystals in ¹³C CP-MAS measurements has a significantly wider impact than previously thought

Measuring Motional Dynamics of [(CH₃)₂NH₂]⁺ in the Perovskite-Like Metal–Organic Framework [(CH₃)₂NH₂][Zn(HCOO)₃]: The Value of Low-Frequency Electron Paramagnetic Resonance

Dimethylammonium (DMA) zinc formate is the precursor for a large family of multiferroics, materials which display co-existing magnetic and dielectric ordering. However, the mechanism underlying these orderings remains unclear. While it is generally believed that the dielectric transition is related to the freezing of the order–disorder dynamics of the DMA⁺ cation, no quantitative data on this motion are available. We surmise that this is due to the fact that the timescale of this cationic motion is on the borderline of the timescales of experimental techniques used in earlier reports. Using multifrequency electron paramagnetic resonance (EPR), we find that the timescale of this motion is ∼5 × 10 ^–9 s. Thus, S-band (4 GHz) EPR spectroscopy is presented as the technique of choice for studying these motional dynamics. This work highlights the value of the lower-frequency end of EPR spectroscopy. The data are interpreted using density functional theory calculations and provide direct evidence for the motional freezing model of the ferroelectric transition in these metal–organic frameworks with the ABX₃ perovskite-like architecture

Elucidating the Mechanism of Multiferroicity in (NH₄)₃Cr(O₂)₄ and Its Tailoring by Alkali Metal Substitution

The antiferromagnetic Cr­(V) peroxychromates, M₃Cr­(O₂)₄, M = K, Rb, and Cs, become ferroelectric when mixed with NH₄⁺, but the underlying mechanism is not understood. Our dielectric relaxation, Raman scattering, and high-frequency EPR measurements on the M_3–<i>x</i>(NH₄)_<i>x</i>Cr­(O₂)₄ family clarify this mechanism. At 295 K, (NH₄)₃Cr­(O₂)₄ is tetragonal (<i>I</i>4̅2<i>m</i>), with the NH₄⁺ ions occupying two distinctly different sites, N1 and N2. A ferroelectric transition at <i>T</i>_c1 = 250 K is revealed by λ-type anomalies in <i>C</i>_p and dielectric constant, and lowering of symmetry to C<i>mc</i>2­(1). Below <i>T</i>_c1, the N1 sites lose their tetrahedral symmetry and thus polarization develops. Raman detection of translational modes involving the NH₄⁺ ions around 193 cm^–1 supports this model. EPR around <i>T</i>_c1 revealed that the [Cr­(O₂)₄]^<b>3–</b> ions reorient by about 10°. A minor peak at <i>T</i>_c2 ≈ 207 K is attributed to a short-range ordering that culminates in a long-range, structural order at <i>T</i>_c3 ≈ 137 K. At <i>T</i>_c3, the symmetry is lowered to <i>P</i>1 with significant changes in the cell parameters. Rb⁺ and Cs⁺ substitutions that block the N1 and N2 sites selectively show that T_c1 is related to the torsional motion of the N1 site, while <i>T</i>_c2 and <i>T</i>_c3 are governed by the motional slowing down of the N2 site. These data show that the multiferroic behavior of this family is governed by the rotational and translational dynamics of the NH₄⁺ ions and is tunable by their controlled substitutions. Relevance to other classes of possible multiferroics is pointed out

Di‑, Tri‑, and Tetranuclear Nickel(II) Complexes with Oximato Bridges: Magnetism and Catecholase-like Activity of Two Tetranuclear Complexes Possessing Rhombic Topology

Oxime-based tridentate Schiff base ligands 3-[2-(diethylamino)­ethylimino]­butan-2-one oxime (HL¹) and 3-[3-(dimethylamino)­propylimino]­butan-2-one oxime (HL²) produced the dinuclear complex [Ni₂L¹₂]­(ClO₄)₂ (<b>1</b>) and trinuclear complex [Ni₃(HL²)₃(μ₃-O)]­(ClO₄)₄·CH₃CN (<b>2</b>), respectively, upon reaction with Ni­(ClO₄)₂·6H₂O. However, in a slightly alkaline medium, both of the ligands underwent hydrolysis and resulted in tetranuclear complexes [{Ni­(deen)­(H₂O)}₂(μ₃-OH)₂{Ni₂(moda)₄}]­(ClO₄)₂·2CH₃CN (<b>3</b>) and [{Ni­(dmpn)­(CH₃CN)₂}₂(μ₃-OH)₂{Ni₂(moda)₄}]­(ClO₄)₂·CH₃CN (<b>4</b>), where deen = 2-(diethylamino)­ethylamine, dmpn = 3-(dimethylamino)-1-propylamine, and modaH = diacetyl monoxime. All four complexes have been structurally characterized. Complex <b>1</b> is a centrosymmetric dimer where the square planar nickel­(II) atoms are joined solely by the oximato bridges. In complex <b>2</b>, three square planar nickel atoms form a triangular core through a central oxido (μ₃-O) and peripheral oximato bridges. Tetranuclear complexes <b>3</b> and <b>4</b> consist of four distorted octahedral nickel­(II) ions held together in a rhombic chair arrangement by two central μ₃-OH and four peripheral oximato bridges. Magnetic susceptibility measurements indicated that dinuclear <b>1</b> and trinuclear <b>2</b> exhibited diamagnetic behavior, while tetranuclear complexes <b>3</b> and <b>4</b> were found to have dominant antiferromagnetic intramolecular coupling with concomitant ferromagnetic interactions. Despite its singlet ground state, both <b>3</b> and <b>4</b> serve as useful examples of Kahn’s model for competing spin interactions. High-frequency EPR studies were also attempted, but no signal was detected, likely due to the large energy gap between the ground and first excited state. Complexes <b>3</b> and <b>4</b> exhibited excellent catecholase-like activity in the aerial oxidation of 3,5-di-<i>tert</i>-butylcatechol to the corresponding <i>o</i>-quinone, whereas <b>1</b> and <b>2</b> did not show such catalytic activity. Kinetic data analyses of this oxidation reaction in acetonitrile revealed that the catalytic activity of <b>3</b> (<i>k</i>_cat = 278.4 h^–1) was slightly lower than that of <b>4</b> (<i>k</i>_cat = 300.0 h^–1). X-band EPR spectroscopy indicated that the reaction proceeded through the formation of iminoxyl-type radicals

Structure–Property Correlations in the Heterobimetallic 4f/3d Materials Ln₂M(TeO₃)₂(SO₄) (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; M = Co or Zn)

Eighteen new lanthanide transition metal heterobimetallic compounds, Ln₂Co­(TeO₃)₂(SO₄)₂ (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) and Ln₂Zn­(TeO₃)₂(SO₄)₂ (Ln = Sm, Gd, Dy, Ho, Er, or Yb), have been prepared. They crystallize in triclinic space group <i>P</i>1̅ with two different structural topologies occurring because of a reduction in the Ln³⁺ coordination number from eight to seven with the smallest lanthanides, Yb³⁺ and Lu³⁺. Magnetic susceptibility studies of compounds with diamagnetic lanthanides and lanthanide-like ions suggest that antiferromagnetic interactions occur between the Co²⁺ ions. Similarly, the replacement of Co²⁺ with Zn²⁺ yields Ln₂Zn­(TeO₃)₂(SO₄)₂ (Ln = Gd, Dy, Ho, or Er), and these materials allow for the resolution of the nature of the interactions between lanthanide ions. The data suggest that the short-range Ln³⁺···Ln³⁺ interactions are ferromagnetic. However, a wide range of ferro- and antiferromagnetic interactions occur between the Ln³⁺ and the Co²⁺ cations, with several compounds exhibiting short-range magnetic correlations below 25 K. The results are discussed and contrasted with those recently reported for the related Ln₂Cu­(TeO₃)₂(SO₄)₂ family

Di‑, Tri‑, and Tetranuclear Nickel(II) Complexes with Oximato Bridges: Magnetism and Catecholase-like Activity of Two Tetranuclear Complexes Possessing Rhombic Topology

Oxime-based tridentate Schiff base ligands 3-[2-(diethylamino)­ethylimino]­butan-2-one oxime (HL¹) and 3-[3-(dimethylamino)­propylimino]­butan-2-one oxime (HL²) produced the dinuclear complex [Ni₂L¹₂]­(ClO₄)₂ (<b>1</b>) and trinuclear complex [Ni₃(HL²)₃(μ₃-O)]­(ClO₄)₄·CH₃CN (<b>2</b>), respectively, upon reaction with Ni­(ClO₄)₂·6H₂O. However, in a slightly alkaline medium, both of the ligands underwent hydrolysis and resulted in tetranuclear complexes [{Ni­(deen)­(H₂O)}₂(μ₃-OH)₂{Ni₂(moda)₄}]­(ClO₄)₂·2CH₃CN (<b>3</b>) and [{Ni­(dmpn)­(CH₃CN)₂}₂(μ₃-OH)₂{Ni₂(moda)₄}]­(ClO₄)₂·CH₃CN (<b>4</b>), where deen = 2-(diethylamino)­ethylamine, dmpn = 3-(dimethylamino)-1-propylamine, and modaH = diacetyl monoxime. All four complexes have been structurally characterized. Complex <b>1</b> is a centrosymmetric dimer where the square planar nickel­(II) atoms are joined solely by the oximato bridges. In complex <b>2</b>, three square planar nickel atoms form a triangular core through a central oxido (μ₃-O) and peripheral oximato bridges. Tetranuclear complexes <b>3</b> and <b>4</b> consist of four distorted octahedral nickel­(II) ions held together in a rhombic chair arrangement by two central μ₃-OH and four peripheral oximato bridges. Magnetic susceptibility measurements indicated that dinuclear <b>1</b> and trinuclear <b>2</b> exhibited diamagnetic behavior, while tetranuclear complexes <b>3</b> and <b>4</b> were found to have dominant antiferromagnetic intramolecular coupling with concomitant ferromagnetic interactions. Despite its singlet ground state, both <b>3</b> and <b>4</b> serve as useful examples of Kahn’s model for competing spin interactions. High-frequency EPR studies were also attempted, but no signal was detected, likely due to the large energy gap between the ground and first excited state. Complexes <b>3</b> and <b>4</b> exhibited excellent catecholase-like activity in the aerial oxidation of 3,5-di-<i>tert</i>-butylcatechol to the corresponding <i>o</i>-quinone, whereas <b>1</b> and <b>2</b> did not show such catalytic activity. Kinetic data analyses of this oxidation reaction in acetonitrile revealed that the catalytic activity of <b>3</b> (<i>k</i>_cat = 278.4 h^–1) was slightly lower than that of <b>4</b> (<i>k</i>_cat = 300.0 h^–1). X-band EPR spectroscopy indicated that the reaction proceeded through the formation of iminoxyl-type radicals

Expansion of the Rich Structures and Magnetic Properties of Neptunium Selenites: Soft Ferromagnetism in Np(SeO₃)₂

Two new neptunium selenites with different oxidation states of the metal centers, Np^IV(SeO₃)₂ and Np^VIO₂(SeO₃), have been synthesized under mild hydrothermal conditions at 200 °C from the reactions of NpO₂ and SeO₂. Np­(SeO₃)₂ crystallizes as brown prisms (space group <i>P</i>2₁/<i>n</i>, <i>a</i> = 7.0089(5) Å, <i>b</i> = 10.5827(8) Å, <i>c</i> = 7.3316(5) Å, β = 106.953(1)°); whereas NpO₂(SeO₃) crystals are garnet-colored with an acicular habit (space group <i>P</i>2₁/<i>m</i>, <i>a</i> = 4.2501(3) Å, <i>b</i> = 9.2223(7) Å, <i>c</i> = 5.3840(4) Å, β = 90.043(2)°). Single-crystal X-ray diffraction studies reveal that the structure of Np­(SeO₃)₂ features a three-dimensional (3D) framework consisting of edge-sharing NpO₈ units that form chains that are linked via SeO₃ units to create a 3D framework. NpO₂(SeO₃) possesses a lamellar structure in which each layer is composed of NpO₈ hexagonal bipyramids bridged via SeO₃^2– anions. Bond-valence sum calculations and UV-vis-NIR absorption spectra support the assignment of tetravalent and hexavalent states of neptunium in Np­(SeO₃)₂ and NpO₂(SeO₃), respectively. Magnetic susceptibility data for Np­(SeO₃)₂ deviates substantially from typical Curie–Weiss behavior, which can be explained by large temperature-independent paramagnetic (TIP) effects. The Np^IV selenite shows weak ferromagnetic ordering at 3.1(1) K with no detectable hysteresis, suggesting soft ferromagnetic behavior

Quantum Phase Transition from Superparamagnetic to Quantum Superparamagnetic State in Ultrasmall Cd_1–<i>x</i>Cr(II)_<i>x</i>Se Quantum Dots?

Despite a long history of success in formation of transition-metal-doped quantum dots (QDs), the origin of magnetism in diluted magnetic semiconductors (DMSs) is yet a controversial issue. Cr(II)-doped II–VI DMSs are half-metallic, resulting in high-temperature ferromagnetism. The magnetic properties reflect a strong p–d exchange interaction between the spin-up Cr(II) t_2g level and the Se 4p. In this study, ultrasmall (∼3.1 nm) Cr(II)-doped CdSe DMSQDs are shown to exhibit room-temperature ferromagnetism, as expected from theoretical arguments. Surprisingly, a low-temperature phase transition is observed at 20 K that is believed to reflect the onset of long-range ordering of the single-domain DMSQD

Evidence from 900 MHz ¹H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH₄H₂AsO₄

NH₄H₂AsO₄ (ADA) is a model compound for understanding the mechanism of phase transitions in the KH₂PO₄ (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at <i>T</i>_N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (¹H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, <i>T</i>₁, of the O–H···O and NH₄⁺ protons through the <i>T</i>_N. As <i>T</i> → <i>T</i>_N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its <i>T</i>_N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at <i>T</i>_N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH₄⁺ protons. Both sets of protons exhibited order–disorder characteristics in their <i>T</i>₁ data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into <i>order–disorder</i> and <i>displacive</i> ones, should rather be “<i>order–disorder cum displacive</i>” type