Magnetic Interactions in the Double Perovskites R₂NiMnO₆ (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R₂NiMnO₆ (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic <i>P</i>2₁/<i>n</i> space group, with an almost complete order between Ni²⁺ and Mn⁴⁺ cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La₂NiMnO₆, with a ferromagnetic structure involving Mn⁴⁺ and Ni²⁺ moments. This spin alignment can be rationalized taking into account the Goodenough–Kanamori rules. The magnetic ordering temperature (<i>T</i>_CM) decreases abruptly as the size of the rare earth decreases, since <i>T</i>_CM is mainly influenced by the superexchange interaction between Ni²⁺ and Mn⁴⁺ (Ni²⁺–O–Mn⁴⁺ angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below <i>T</i>_CM

Magnetic Structure, Single-Crystal to Single-Crystal Transition, and Thermal Expansion Study of the (Edimim)[FeCl₄] Halometalate Compound

This contribution addresses standing questions about the nature and consequences of the ion self-assembly and magnetic structures, as well as the molecular motion of the crystalline structure as a function of the temperature, in halometalate materials based on imidazolium cation. We present the magnetic structure and magnetostructural correlations of 1-ethyl-2,3-dimethylimidazolium tetrachloridoferrate, (Edimim)­[FeCl₄], resolved by neutron diffraction studies. Single-crystal, synchrotron powder X-ray diffraction and powder neutron diffraction techniques have been combined to follow the temperature evolution on its crystallographic structure from 2 K close to its melting point (340 K). In this sense, slightly above room temperature (307 K) (Edimim)­[FeCl₄] presents a single-crystal to single-crystal transition (SCSC), from phase <b>I</b> (space group <i>P</i>2₁/<i>n</i>) to phase <b>II</b> (<i>P</i>2₁/<i>m</i>), accompanied by a notable increase in the disorder of the imidazolium cation, as well as in the metal complex anion. The temperature evolution and solid-phase transitions of the presented compound were followed in detail by synchrotron X-ray powder diffraction (SXPD), which confirms the occurrence of another phase transition at 330 K, phase <b>III</b> (<i>P</i>2₁/<i>m</i>), the crystal structure of which was elucidated from the SXPD pattern. Moreover, this material presents an anisotropic thermal expansion with a switch from axial positive to negative thermal expansion coefficients as the temperature is raised above the first phase transition, which has been correlated with the molecular motion of the imidazolium-based molecules, producing not only a shortening of the counterion···counterion distances but also the occurrence of different quasi-isoenergetic crystal structures as a function of the temperature

Anion−π and Halide–Halide Nonbonding Interactions in a New Ionic Liquid Based on Imidazolium Cation with Three-Dimensional Magnetic Ordering in the Solid State

We present the first magnetic phase of an ionic liquid with anion−π interactions, which displays a three-dimensional (3D) magnetic ordering below the Néel temperature, <i>T</i>_N = 7.7 K. In this material, called Dimim­[FeBr₄], an exhaustive and systematic study involving structural and physical characterization (synchrotron X-ray, neutron powder diffraction, direct current and alternating current magnetic susceptibility, magnetization, heat capacity, Raman and Mössbauer measurements) as well as first-principles analysis (density functional theory (DFT) simulation) was performed. The crystal structure, solved by Patterson-function direct methods, reveals a monoclinic phase (<i>P</i>2₁ symmetry) at room temperature with <i>a</i> = 6.745(3) Å, <i>b</i> = 14.364(3) Å, <i>c</i> = 6.759(3) Å, and β = 90.80(2)°. Its framework, projected along the <i>b</i> direction, is characterized by layers of cations [Dimim]⁺ and anions [FeBr₄]⁻ that change the orientation from layer to layer, with Fe···Fe distances larger than 6.7 Å. Magnetization measurements show the presence of 3D antiferromagnetic ordering below <i>T</i>_N with the existence of a noticeable magneto–crystalline anisotropy. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order is originated by the antiparallel ordering of ferromagnetic layers of [FeBr₄]⁻ metal complex along the <i>b</i> direction. The magnetic unit cell is the same as the chemical one, and the magnetic moments are aligned along the <i>c</i> direction. The DFT calculations reflect the fact that the spin density of the iron ions spreads over the bromine atoms. In addition, the projected density of states (PDOS) of the imidazolium with the bromines of a [FeBr₄]⁻ metal complex confirms the existence of the anion−π interaction. Magneto–structural correlations give no evidence for direct iron–iron interactions, corroborating that the 3D magnetic ordering takes place via superexchange coupling, the Fe–Br···Br–Fe interplane interaction being defined as the main exchange pathway