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1.
Inorg Chem ; 58(19): 12609-12617, 2019 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-31503469

RESUMO

We have investigated two original hydrated cobalt arsenates based on Co2+ octahedral edge-sharing chains. Their different magnetocrystalline anisotropies induce different types of metamagnetic transitions: spin-flop versus spin-flip. In both compounds, a strong local anisotropy (Ising spins) is favored by the spin-orbit coupling present in the CoO6 octahedra, while ferromagnetic (FM) exchanges predominate in the chains. Co2(As2O7)·2H2O (1) orders antiferromagnetically below TN = 6.7 K. The magnetic structure is a noncollinear antiferromagnetic spin arrangement along the zigzag chains with DFT calculations implying frustrated chains and weakened anisotropy. A metamagnetic transition suggests a spin-flop process above µ0H = 3.2 T. In contrast, in BaCo2As2O8·2H2O (2) linear chains are arranged in disconnected layers, with only interchain ferromagnetic exchanges, therefore increasing its magnetocrystalline anisotropy. The magnetic structure is collinear with a magnetic easy axis that allows a spin-flop to a sharp spin-flip transition below TN = 15.1 K and above µ0H = 6.2 T.

2.
Inorg Chem ; 58(1): 81-92, 2019 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-30576119

RESUMO

Transmission electron microscopy, neutron diffraction, and synchrotron powder X-ray diffraction reveal a complex modulated structure on the doubly ordered perovskite NaLaCoWO6. Electron diffraction patterns as well as high-resolution transmission electron microscopy images clearly show a periodicity of 12 ap, where ap is the cell parameter of the generic perovskite, along either the [100]p or [010]p direction. Annular bright-field scanning transmission electron microscopy of slightly tilted samples shows that there is no chemical origin for the superstructure but that it is caused by geometric rearrangements. An atomic model of the superstructure is proposed on the basis of octahedral tilt twinning. At low temperature, NaLaCoWO6 undergoes a phase transition and the superstructure disappears. The compound takes on the more usual monoclinic P21 structure below the transition. Neutron powder diffraction reveals and electron diffraction confirms an unusually large temperature hysteresis, where the transition takes place at ∼180 K on cooling and at ∼320 K on heating. This hysteresis can be attributed to the necessity of rearranging the oxygen octahedra and the thus induced energy barrier for the transition.

3.
Chemistry ; 24(2): 388-399, 2018 Jan 09.
Artigo em Inglês | MEDLINE | ID: mdl-28858419

RESUMO

The characterization of the crystal structure, phase transitions, magnetic structure and dielectric properties has been carried out on [CH3 NH3 ][Co(COOH)3 ] (1) perovskite-like metal-organic compound through variable-temperature single-crystal and powder neutron and X-ray diffraction and relative permittivity measurements. The paraelectric to antiferroelectric-like phase transition observed at around 90 K is triggered by a structural phase transition; the structural studies show a change from Pnma space group at RT (1A) to P21 /n space group at low temperature (1B). This phase transition involves the occurrence of small distortions in the framework and counterions. Neutron diffraction studies have shown a magnetic order showing spontaneous magnetization below 15 K, due to the occurrence of a non-collinear antiferromagnetic structure with a weak ferromagnetic component, mainly due to the single-ion anisotropy of the CoII ions.

4.
Inorg Chem ; 56(14): 8478-8489, 2017 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-28678481

RESUMO

The compounds of the doubly ordered perovskite family NaLnCoWO6 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) were synthesized by solid-state reaction, nine of which (Ln = Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) are new phases prepared under high-temperature and high-pressure conditions. Their structural properties were investigated at room temperature by synchrotron X-ray powder diffraction and neutron powder diffraction. All of them crystallize in monoclinic structures, especially the nine new compounds have the polar space group P21 symmetry, as confirmed by second harmonic generation measurements. The P21 polar structures were decomposed and refined in terms of symmetry modes, demonstrating that the polar mode is induced by two nonpolar modes in a manner of Hybrid Improper Ferroelectricity. The amplitudes of these three major modes all increase with decreasing the Ln cation size. The spontaneous ferroelectric polarization is estimated from the neutron diffraction data of three samples (Ln = Y, Tb, and Ho) and can be as large as ∼20 µC/cm2.

5.
Inorg Chem ; 56(4): 2013-2021, 2017 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-28128934

RESUMO

Stabilizing new host structures through potassium extraction from K-based polyanionic materials has been proven to be an interesting approach to develop new Li+/Na+ insertion materials. Pursuing the same trend, we here report the feasibility of preparing langbeinite "Fe2(SO4)3" via electrochemical and chemical oxidation of K2Fe2(SO4)3. Additionally, we succeeded in stabilizing a new K2Cu2(SO4)3 phase via a solid-state synthesis approach. This novel compound crystallizes in a complex orthorhombic structure that differs from that of langbeinite as deduced from synchrotron X-ray and neutron powder diffraction. Electrochemically, the performance of this new phase is limited, which we explain in terms of sluggish diffusion kinetics. We further show that K2Cu2(SO4)3 decomposes into K2Cu3O(SO4)3 on heating, and we report for the first time the synthesis of fedotovite K2Cu3O(SO4)3. Finally, the fundamental attractiveness of these S = 1/2 systems for physicists is examined by neutron magnetic diffraction, which reveals the absence of a long-range ordering of Cu2+ magnetic moments down to 1.5 K.

6.
Dalton Trans ; 44(31): 14130-8, 2015 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-26174497

RESUMO

We have reinvestigated the crystal structure of the low-dimensional fluoride ß-FeF3(H2O)2·H2O using high resolution neutron and X-ray diffraction data. Moreover we have studied the magnetic behavior of this material combining medium resolution and high flux neutron powder diffraction together with magnetic susceptibility measurements. This fluoride compound exhibits vertex-shared 1D Fe(3+) octahedral chains, which are extended along the c-axis. The magnetic interactions between adjacent chains involve super-superexchange interactions via an extensive network of hydrogen bonds. This interchain hydrogen bonding scheme is sufficiently strong to induce a long range magnetic order appearing below T = 20(1) K. The magnetic order is characterized by the propagation vector k = (0, 0, 1/2), giving rise to a strictly antiferromagnetic structure where the Fe(3+) spins are lying within the ab-plane. Magnetic exchange couplings extracted from magnetization measurements are found to be J∥/kb = -18 K and J⊥/kb = -3 K. These values are in good agreement with the neutron diffraction data, which show that the system became antiferromagnetically ordered at ca. TN = 20(1) K.

7.
Inorg Chem ; 52(2): 753-60, 2013 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-23273254

RESUMO

The crystal and magnetic structures of the organic-inorganic hybrid compound Cr(II) ammoniumethylphosphonate chloride monohydrate, Cr[D(3)N-(CH(2))(2)-PO(3))(Cl)(D(2)O)] (1), have been studied by temperature-dependent neutron powder diffraction and superconducting quantum interference device (SQUID) magnetometry. The compound represents a rare example of a magnetoelectric polar organic-inorganic hybrid solid, containing high spin Cr(2+) ions (S = 2) and is a canted antiferromagnet (weak ferromagnet) below T(N) = 5.5 K. The neutron powder diffraction pattern recorded at T = 10 K, shows that the partially deuterated compound crystallizes in the same non centrosymmetric monoclinic space group P2(1) (No. 4) with the following unit-cell parameters: a = 5.24041(4) Å, b =13.93113(8) Å, c = 5.26081(4) Å, and ß = 105.4347(5)°. Powder neutron diffraction of a partially deuterated sample has enabled us, for the first time, to locate the water molecule. At low temperature, the compound presents a canted antiferromagnetic state characterized by k = 0 resulting in the magnetic symmetry P2(1)'. This symmetry is in agreement with the previously reported large magnetodielectric effect. The crystal structure of (1) can be described as being built up of triangular lattice planes made up of [Cr(II)O(4)Cl] square pyramids which are separated by ammonium ethyl groups along the b axis. The transition from paramagnetic to weakly ferromagnetic state results from super-superexchanges only. Surprisingly, while the overall magnetic behavior is antiferromagnetic, the Cr(II)O(4)Cl planes are ferromagnetic, and the strongest antiferromagnetic coupling is via the ammonium ethyl groups. Our density functional calculations confirm these aspects of the spin exchange interactions of (1) and that the spin exchange interactions between Cr(II) ions are considerably weak compared with the single-ion anisotropy of Cr(II).

8.
J Am Chem Soc ; 134(48): 19772-81, 2012 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-23130914

RESUMO

Neutron diffraction studies have been carried out to shed light on the unprecedented order-disorder phase transition (ca. 155 K) observed in the mixed-valence iron(II)-iron(III) formate framework compound [NH(2)(CH(3))(2)](n)[Fe(III)Fe(II)(HCOO)(6)](n). The crystal structure at 220 K was first determined from Laue diffraction data, then a second refinement at 175 K and the crystal structure determination in the low temperature phase at 45 K were done with data from the monochromatic high resolution single crystal diffractometer D19. The 45 K nuclear structure reveals that the phase transition is associated with the order-disorder of the dimethylammonium counterion that is weakly anchored in the cavities of the [Fe(III)Fe(II)(HCOO)(6)](n) framework. In the low-temperature phase, a change in space group from P31c to R3c occurs, involving a tripling of the c-axis due to the ordering of the dimethylammonium counterion. The occurrence of this nuclear phase transition is associated with an electric transition, from paraelectric to antiferroelectric. A combination of powder and single crystal neutron diffraction measurements below the magnetic order transition (ca. 37 K) has been used to determine unequivocally the magnetic structure of this Néel N-Type ferrimagnet, proving that the ferrimagnetic behavior is due to a noncompensation of the different Fe(II) and Fe(III) magnetic moments.

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