Thomas Mazet

A Metal–Organic Framework as Attractive Cryogenic Magnetorefrigerant

Magnetocaloric effect: A Gd(III)-based metal-organic framework (MOF) has an unprecedented large magnetocaloric effect around 2 K. It was shown to be an interesting magnetorefrigerant for ultralow-temperature applications, because it combines the advantages of molecular materials and the robustness of a framework with strong 3D chemical connections (see figure).

Co4(OH)2(C10H16O4)3 metal-organic framework: slow magnetic relaxation in the ordered phase of magnetic chains.

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co(4)(OH)(2)(C(10)H(16)O(4))(3) metal-organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χT product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ(τ)/k(B) = 67(1) K for the [10-5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded C(P)(T) data are used for the calculation of the thermal variations of both the adiabatic temperature change ΔT(ad) and magnetic entropy change ΔS(m). The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length.

From hydrated Ni3(OH)2(C8H4O4)2(H2O)4 to anhydrous Ni2(OH)2(C8H4O4): impact of structural transformations on magnetic properties.

Dehydration of the hybrid compound [Ni3(OH)2(tp)2(H2O)4] (1) upon heating led to the sequential removal of coordinated water molecules to give [Ni3(OH)2(tp)2(H2O)2] (2) at T1 = 433 K and thereafter anhydrous [Ni2(OH)2(tp)] (3) at T2 = 483 K. These two successive structural transformations were thoroughly characterized by powder X-ray diffraction assisted by density functional theory calculations. The crystal structures of the two new compounds 2 and 3 were determined. It was shown that at T1 (433 K) the infinite nickel oxide chains built of the repeating structural unit [Ni3(μ3-OH)2](4+) in 1 collapse and lead to infinite porous layers, forming compound 2. The second transformation at T2 (483 K) gave the expected anhydrous compound 3, which is isostructural with Co2(OH)2(tp). These irreversible transitions directly affect the magnetic behavior of each phase. Hence, 1 was found to be antiferromagnetic at TN = 4.11 K, with metamagnetic behavior with a threshold field Hc of ca. 0.6 T. Compound 2 exhibits canted antiferromagnetism below TN = 3.19 K, and 3 is ferromagnetic below TC = 4.5 K.

Synthesis, ab initio X-ray powder diffraction crystal structure, and magnetic properties of Mn3(OH)2(C6H2O4S)2 metal-organic framework.

A new hydroxythiophenedicarboxylate metal-organic framework based on Mn(II) cations has been obtained by an aqueous two-step procedure including hydrothermal treatment. The structure of Mn(3)(OH)(2)(C(6)H(2)O(4)S)(2) has been determined ab initio from synchrotron X-ray powder diffraction data and consists of infinite inorganic ribbons which are interlinked by 2,5-thiophenedicarboxylate (tdc) molecules (monoclinic, space group P2(1)/c, a = 3.4473(1) Å, b = 19.1287(1) Å, c = 11.0069(1) Å, β = 97.48(1)°, V = 719.65(1) Å(3), and Z = 2). Each ribbon is built of three vertex-sharing chains of edge-sharing MnO(6) octahedrons. These ribbons are bridged together by the carboxylate functions of the tdc molecule to form a pseudo-2D inorganic subnetwork, while this molecule develops in the third dimension to pillar these pseudo-2D layers. An unprecedented hexadentate symmetric bridging mode is adopted by tdc which bridges two chains of a ribbon on one side and two ribbons of a pseudo-2D inorganic subnetwork on the other side. Magnetic measurements suggest that the titled compound is antiferromagnetic below T(N) = 17.7 K. Heat capacity measurements confirm the existence of a magnetic phase transition toward a 3D long-range ordered state. These C(P)(T) data have also been used for the calculation of the thermal variations of both the adiabatic temperature change ΔT(ad) and magnetic entropy change ΔS(m) of the material, namely its magnetocaloric effect.

Magnetism in the (Co1−xFex)2(OH)2(C8H4O4) solid solutions: a combined neutron diffraction and magnetic measurements study

The magnetic properties of the solid solution (Co1−xFex)2(OH)2(C8H4O4) x = 0.25 (1), x = 0.5 (2), x = 0.75 (3), x = 0.88 (4) and of the pure iron compound x = 1.0 (5) (C2/m) were investigated by magnetic susceptibility measurements and neutron diffraction experiments. Magnetic measurements show that the pure iron compound is antiferromagnetic below TN = 65 K. The AC susceptibility vs. temperature curves for the iron-cobalt solid solutions reveal a complicated magnetic behavior. This behavior is discussed with that of the pure cobalt compound which is known for stabilizing several magnetic states as a function of the temperature and of the applied field. These bimetallic compounds were also studied by magnetization vs. applied magnetic field (up to 9 T) and they appear to be a simple way to tune the giant hysteretic effect known for Co2(OH)2(C8H4O4) at low temperature. The neutron diffraction study at 2 K reveals that in the pure iron compound the magnetic moments are aligned along the b axis. Several compositions of the hybrid solid solution were also investigated, the analysis of the data display a reorientation of the magnetic moment directions along the c axis for x < 0.50, as observed in the pure cobalt compound.

Investigation of compounds for magnetocaloric applications: YFe2H4.2, YFe2D4.2, and Y0.5Tb0.5Fe2D4.2

The magnetocaloric properties of powder samples of the monoclinic YFe2H4.2, YFe2D4.2, and Y0.5Tb0.5Fe2D4.2 compounds have been studied at their itinerant electron metamagnetic transition (TM0=131, 84, and 127 K, respectively). Large reversible entropy changes are observed, up to −ΔSM=10.83 J K−1 kg−1 for a field variation of 5 T in YFe2D4.2, making these alloys candidates for magnetic refrigeration applications. The results are compared with previously published data on other potential magnetic refrigerants with itinerant electron metamagnetic transitions.

Magnetic structure and dynamics of a strongly one-dimensional cobalt II metal-organic framework

We investigate the magnetism of the CoII4(OH)2(C10H16O4)3 metal-organic framework, which displays complex inorganic chains separated from each other by distances of 1 to 2 nm and orders at 5.4 K. The zero-field magnetic structure is determined using neutron powder diffraction: it is mainly antiferromagnetic but possesses a ferromagnetic component along the c axis. This magnetic structure persists in presence of a magnetic field. Alternating current susceptibility measurements confirm the existence of a single thermally activated regime over seven decades in frequency (E/kB≈64 K), whereas time-dependent relaxation of the magnetization after saturation in an external field leads to a two times smaller energy barrier. These experiments probe the slow dynamics of domain walls within the chains: we propose that the ac measurements are sensitive to the motion of existing domain walls within the chains, while the magnetization measurements are governed by the creation of domain walls.

Magnetocaloric effect in gadolinium-oxalate framework Gd2(C2O4)3(H2O)6⋅(0⋅6H2O)

Magnetic refrigerants incorporating Gd3+ ions and light organic ligands offer a good balance between isolation of the magnetic centers and their density. We synthesized the framework material Gd2(C2O4)3(H2O)6⋅0.6H2O by a hydrothermal route and characterized its structure. The honeycomb lattice of Gd3+ ions interlinked by oxalate ligands in the (a,c) plane ensures their decoupling in terms of magnetic exchange interactions. This is corroborated by magnetic measurements indicating negligible interactions between the Gd3+ ions in this material. The magnetocaloric effect was evaluated from isothermal magnetization measurements. The maximum entropy change −ΔSMmax reaches 75.9 mJ cm−3 K−1 (around 2 K) for a moderate field change (2 T).

Location of metallic elements in (Co1−xFex)2(OH)2(C8H4O4): use of MAD, neutron diffraction and 57Fe Mössbauer spectroscopy

Four samples of the solid solution (Co1−xFex)2(OH)2(C8H4O4) x = 0.25 (1), x = 0.5 (2), x = 0.75 (3); x = 0.88 (4) (C2/m) were investigated by synchrotron radiation using Multi-wavelength Anomalous Diffraction (MAD) and multipattern refinements in order to locate the two neighbour metallic elements Fe and Co. Although there is no cation ordering, the two metallic sites M1 (0, 0, 0) and M2 (0, 0.5, 0.5) are preferentially occupied by Co and Fe respectively. In this article we demonstrate that this distribution is in agreement with neutron data and 57Fe Mossbauer spectra at 100 K. Consequently, the MAD method can be used to locate close d-metallic elements in metal–organic frameworks.

Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study

We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder diraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni3Sn2 type of structure (Pnma) and comprise two inequivalent sites for the transition metal atoms (4c and 8d) and two Sn sites (4c and 4c). The neutron data show that the substituting Fe atoms predominantly occupy the 4c transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the b or c axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the b or c axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and non-collinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diraction experiments on Mn3Sn2 reveal a weak monoclinic distortion at low temperature (90.08 at 175 K). This lowering of symmetry could explain the stabilization of the c-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.