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1.
IUCrJ ; 6(Pt 1): 105-115, 2019 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-30713708

RESUMO

The present article is devoted to the characterization of the structural phase transitions of the [CH3NH3][Co(COOH)3] (1) perovskite-like metal-organic compound through variable-temperature single-crystal neutron diffraction. At room temperature, compound 1 crystallizes in the orthorhombic space group Pnma (phase I). A decrease in temperature gives rise to a first phase transition from the space group Pnma to an incommensurate phase (phase II) at approximately 128 K. At about 96 K, this incommensurate phase evolves into a second phase with a sharp change in the modulation vector (phase III). At lower temperatures (ca 78 K), the crystal structure again becomes commensurate and can be described in the monoclinic space group P21/n (phase IV). Although phases I and IV have been reported previously [Boca et al. (2004). Acta Cryst. C60, m631-m633; Gómez-Aguirre et al. (2016). J. Am. Chem. Soc. 138, 1122-1125; Mazzuca et al. (2018). Chem. Eur. J. 24, 388-399], phases III and IV corresponding to the Pnma(00γ)0s0 space group have not yet been described. These phase transitions involve not only the occurrence of small distortions in the three-dimensional anionic [Co(HCOO)3]- framework, but also the reorganization of the [CH3NH3]+ counter-ions in the cavities of the structure, which gives rise to an alteration of the hydrogen-bonded network, modifying the electrical properties of compound 1.

2.
Sci Rep ; 8(1): 10665, 2018 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-30006614

RESUMO

(ND4)2[FeCl5(D2O)] represents a promising example of the hybrid molecular/inorganic approach to create materials with strong magneto-electric coupling. Neutron spherical polarimetry, which is directly sensitive to the absolute magnetic configuration and domain population, has been used in this work to unambiguously prove the multiferroicity of this material. We demonstrate that the application of an electric field upon cooling results in the stabilization of a single-cycloidal magnetic domain below 6.9 K, while poling in the opposite electric field direction produces the full population of the domain with opposite magnetic chirality. We prove the complete switchability of the magnetic domains at low temperature by the applied electric field, which constitutes a direct proof of the strong magnetoelectric coupling. Additionally, we refine the magnetic structure of the ordered ground state, deducing the underlying magnetic space group consistent with the direction of the ferroelectric polarization, and we provide evidence of a collinear amplitude-modulated state with magnetic moments along the a-axis in the temperature region between 6.9 and 7.2 K.

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.
Phys Chem Chem Phys ; 19(43): 29084-29091, 2017 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-29057417

RESUMO

A combined study involving DFT calculations, neutron scattering, heat capacity and magnetic measurements at very low temperatures demonstrates the long-range magnetic ordering of Ni(pyrazine)[Pt(CN)4] below 1.9 K, describing its antiferromagnetic spin arrangement. This compound belongs to the family of porous coordination polymers M(pyrazine)[Pt(CN)4] (M = divalent metal), renowned for showing interesting combinations of porosity and magnetic properties. The possibility of including long-range magnetic ordering, one of the most pursued functional properties, opens new perspectives for the multifunctionality of this class of compounds.

5.
Chem Sci ; 8(4): 3109-3120, 2017 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-28507686

RESUMO

Discrimination between different gases is an essential aspect for industrial and environmental applications involving sensing and separation. Several classes of porous materials have been used in this context, including zeolites and more recently MOFs. However, to reach high selectivities for the separation of gas mixtures is a challenging task that often requires the understanding of the specific interactions established between the porous framework and the gases. Here we propose an approach to obtain an enhanced selectivity based on the use of compartmentalized coordination polymers, named CCP-1 and CCP-2, which are crystalline materials comprising isolated discrete cavities. These compartmentalized materials are excellent candidates for the selective separation of CO2 from methane and nitrogen. A complete understanding of the sorption process is accomplished with the use of complementary experimental techniques including X-ray diffraction, adsorption studies, inelastic- and quasi-elastic neutron scattering, magnetic measurements and molecular dynamics calculations.

6.
Inorg Chem ; 56(1): 197-207, 2017 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-27935298

RESUMO

A study of the magnetic structure of the [NH2(CH3)2]n[FeIIIMII(HCOO)6]n niccolite-like compounds, with MII = CoII (2) and MnII (3) ions, has been carried out using neutron diffraction and compared with the previously reported FeII-containing compound (1). The inclusion of two different metallic atoms into the niccolite-like structure framework leads to the formation of isostructural compounds with very different magnetic behaviors due to the compensation or not of the different spins involved in each lattice. Below TN, the magnetic order in these compounds varies from ferrimagnetic behavior for 1 and 2 to an antiferromagnetic behavior with a weak spin canting for 3. Structure refinements of 2 and 3 at low temperature (45 K) have been carried out combining synchrotron X-ray and high-resolution neutron diffraction in a multipattern approach. The magnetic structures have been determined from the difference patterns between the neutron data in the paramagnetic and the magnetically ordered regions. These difference patterns have been analyzed using a simulated annealing protocol and symmetry analysis techniques. The obtained magnetic structures have been further rationalized by means of ab initio DFT calculations. The direction of the magnetic moment of each compound has been determined. The easy axis of the MII for compound 1 (FeII) is along the c axis; for compound 2 (CoII), the moments are mainly within the ab plane; finally, for compound 3 (MnII), the calculations show that the moments have components both in the ab plane and along the c axis.

7.
Chemistry ; 20(26): 7956-61, 2014 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-24807735

RESUMO

Fe(II)(Metz)6](Fe(III)Br4)2 (Metz = 1-methyltetrazole) is one of the rare systems combining spin-crossover and long-range magnetic ordering. A joint neutron and X-ray diffraction and magnetometry study allows determining its collinear antiferromagnetic structure, and shows an increase of the Néel temperature from 2.4 K at ambient pressure, to 3.9 K at 0.95 GPa. Applied pressure also enables a full high-spin to low-spin switch at ambient temperature.

8.
Angew Chem Int Ed Engl ; 52(40): 10458-62, 2013 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-23913399

RESUMO

The first pressure-induced solid-phase synthesis of a zeolite has been found through compression of a common zeolite, ITQ-29 (see scheme, Si yellow, O red). The new microporous structure, ITQ-50, has a unique structure and improved performance for propene/propane separation with respect the parent material ITQ-29.

9.
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.

10.
Inorg Chem ; 51(20): 11126-32, 2012 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-23009692

RESUMO

Here we describe the synthesis, structure, and magnetic properties of two related coordination polymers made up of self-assembling Fe(II) ions, pyrazine (pz), and the tetrathiocyanopalladate anion. Compound {Fe(MeOH)(2)[Pd(SCN)(4)]}·pz (1a) is a two-dimensional coordination polymer where the Fe(II) ions are equatorially coordinated by the nitrogen atoms of four [Pd(SCN)(4)](2-) anions, each of which connects four Fe(II) ions, forming corrugated layers {Fe[Pd(SCN)(4)]}(∞). The coordination sphere of Fe(II) is completed by the oxygen atoms of two CH(3)OH molecules. The layers stack one on top of each other in such a way that the included pz molecule establishes strong hydrogen bonds with the coordinated methanol molecules of adjacent layers. Compound {Fe(pz)[Pd(SCN)(4)]} (2) is a three-dimensional porous coordination polymer formed by flat {Fe[Pd(SCN)(4)]}(∞) layers pillared by the pz ligand. Thermal analysis of 1a shows a clear desorption of the two coordinated CH(3)OH molecules giving a rather stable phase (1b), which presumably is a polymorphic form of 2. The magnetic properties of the three derivatives are typical of the high-spin Fe(II) compounds. However, compounds 1b and 2, with coordination sphere [FeN(6)], show thermal spin crossover behavior at pressures higher than ambient pressure (10(5) MPa).

11.
J Am Chem Soc ; 134(11): 5083-9, 2012 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-22364147

RESUMO

A quasielastic neutron scattering and solid-state (2)H NMR spectroscopy study of the polymeric spin-crossover compound {Fe(pyrazine)[Pt(CN)(4)]} shows that the switching of the rotation of a molecular fragment--the pyrazine ligand--occurs in association with the change of spin state. The rotation switching was examined on a wide time scale (10(-13)-10(-3) s) by both techniques, which clearly demonstrated the combination between molecular rotation and spin-crossover transition under external stimuli (temperature and chemical). The pyrazine rings are seen to perform a 4-fold jump motion about the coordinating nitrogen axis in the high-spin state. In the low-spin state, however, the motion is suppressed, while when the system incorporates benzene guest molecules, the movements of the system are even more restricted.

12.
Inorg Chem ; 50(15): 7129-35, 2011 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-21736295

RESUMO

Neutron diffraction techniques have been used to determine the low temperature crystal structure and to shed light on the magnetic behavior of the [Mn(3)(suc)(2)(ina)(2)](n) (suc = succinate and ina = isonicotinate) complex. The ferromagnetic signal observed below T(c) ≈ 5 K in this compound is due to a noncompensation of homometallic spins in the 3D framework. The Mn(II) magnetic moments obtained from neutron diffraction refinements are slightly lower than those observed for isolated Mn(II) ions; this can be due to covalent spin delocalization or geometrical magnetic fluctuations. A small discrepancy between the value of the magnetic moments of each Mn(II) site is also observed [Mn(1) 4.1(2) µ(B) and the Mn(2) 3.9(1) µ(B)]. These differences between the theoretical and observed manganese magnetic moments are not unexpected in this large spin metal complex, and qualitatively reasonable given the synergistic interaction between the metal ions through oxo-bridge. The competition among different interactions, principally those covalent through organic ligands and dipolar interaction, drive to a final 3D ferrimagnetic order.

13.
Chemistry ; 16(29): 8785-96, 2010 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-20583049

RESUMO

In the series of polymeric spin-crossover compounds Fe(X-py)(2)[Ag(CN)(2))](2) (py=pyridine, X=H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five members of the family (X=H, 4-methyl, and X=3,4-dimethyl). The two other compounds (X=3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin-crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3-methyl undergo light-induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different temperatures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two-step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.

14.
J Phys Chem B ; 111(6): 1256-61, 2007 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-17253737

RESUMO

The two-dimensional (2D) polymeric spin crossover (SCO) compound Fe(py)2[Ag(CN)2]2 has been synthesized. The compound shows a two-step spin transition detected by magnetic, heat capacity, and X-ray diffraction measurements. The magnetic moment shows a high-temperature step (step 1) occurring at 146.3 K without hysteresis, while the low-temperature step (step 2) happens at 84 K on cooling and 98.2 K on heating. These measurements reveal a large amount of residual high spin (HS) species (23%) and that HS state trapping occurs at cooling rates of around 1 K min(-1) or higher. The two-step behavior has been confirmed by heat capacity, which gives, for steps 1 and 2, respectively, DeltaH1 = 3.33 kJ mol(-1), DeltaS1 = 22.6 J mol(-1) K(-1), and DeltaH2 = 1.51 kJ mol(-1), DeltaS2 = 15.7 J mol(-1) K(-1). For step 2 a hysteresis of 10 K has been determined with dynamic measurements. Powder X-ray diffraction at room temperature shows that the compound is isostructural to Cd(py)2[Ag(CN)2]2 previously reported. Powder X-ray diffraction indicates that there is only one crystallographic site for iron(II) in the whole temperature range, confirmed by Mössbauer spectroscopy. The X-ray diffraction study at different temperatures do not show any superstructure in the region between the transitions, discarding a crystallographic phase transition as the origin of the two-step behavior. However, an unexpected increase of the thermal factor is detected on lowering the temperature and considered as a manifestation of a disordered state between the two steps, consisting of a mixing of HS and LS species without long-range order.

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