Controlling Spin Transition in One-Dimensional Coordination Polymers through Polymorphism

A series of polymer <b>1</b> microcrystals with several different morphologies have been systematically synthesized by controlling experimental parameters, namely concentration of reactants, temperature, solvent nature, and the use of surfactants, and their valence tautomerism (VT) has been studied by combined electron microscopy, X-ray diffraction data, and magnetization. Our results indicate that all of them can be grouped exclusively into two different crystalline phases, or a mixture of them, that critically determine the VT process, independent of the morphology and/or dimensions of the crystals. Moreover, a difference in the critical temperature of both phases by more than 50 K allows us to regulate VT. These results head the use of valence tautomeric 1D polymers in devices where strict control and reproducibility of the switching behavior at different length scales and integration procedures is highly required

Influence of Li⁺ and H⁺ Distribution on the Crystal Structure of Li_7–<i>x</i>H_<i>x</i>La₃Zr₂O₁₂ (0 ≤ <i>x</i> ≤ 5) Garnets

With appropriate doping or processing, Li₇La₃Zr₂O₁₂ (LLZO) is an excellent candidate to be used in Li batteries either as a solid electrolyte or as a separator between the Li anode and a liquid electrolyte. For both uses, the reactivity with water either from the air or in aqueous media is a matter of interest. We address here the structural changes undergone by LLZO as a result of H⁺/Li⁺ exchange and relate them with the amount of H content and atomic distribution. Neutron diffraction is performed to elucidate Li and H location. Two different cubic phases derive from LLZO through H⁺/Li⁺ exchange: Deep hydration up to 150 °C yields a noncentrosymmetric <i>I</i>4̅3<i>d</i> phase in which octahedral Li ions are exchanged by H ions, tetrahedral Li ions split into two sites with very different occupancies, and H ions form O₄H₄ entities around the less occupied tetrahedral site. Annealing above 300 °C results in a centrosymmetric <i>Ia</i>3̅<i>d</i> phase with lower H content in which Li ions occupy the usual sites of the cubic garnets and H ions occupy a split pseudooctahedral site. The centrosymmetric or noncentrosymmetric character is determined by the temperature at which exchange is performed and the H content. Both factors are not independent: at low temperature, the high H content favors H ordering around the vacant tetrahedra, while low H content and higher mobility at 350 °C lead to a disordered configuration of Li and H ions. The deeply hydrated garnets are stable up to at least 300 °C and also upon aging at room temperature

Structural Variations in the Dithiadiazolyl Radicals <i>p</i>‑ROC₆F₄CNSSN (R = Me, Et, ^<i>n</i>Pr, ^<i>n</i>Bu): A Case Study of Reversible and Irreversible Phase Transitions in <i>p</i>‑EtOC₆F₄CNSSN

The 4′-alkoxy-tetrafluorophenyl dithiadiazolyls, ROC₆F₄CNSSN [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>n</i>Pr (<b>3</b>), ^<i>n</i>Bu­(<b>4</b>)] all adopt <i>cis-oid</i> dimers in the solid state. The methoxy derivative <b>1</b> adopts a π-stacked AA’AA’ motif, whereas propoxy (<b>3</b>) and butoxy (<b>4</b>) derivatives exhibit an AA’BB’ stacking. The ethoxy derivative (<b>2</b>) is polymorphic. The α-phase (<b>2α</b>) adopts an AA’BB’ motif comparable with <b>3</b> and <b>4</b>, whereas <b>2β</b> and <b>2γ</b> are reminiscent of <b>1</b> but combine a mixture of both monomers and dimers in the solid state. The structure of <b>2β</b> exhibits <i>Z</i>’ = 6 with two dimers and two monomers in the asymmetric unit but undergoes a thermally induced phase transition upon cooling below −25 °C to form <b>2γ</b> (<i>Z</i>’ = 14) with six dimers and two monomers in the asymmetric unit. The transition is associated with both rotation and translation of the dithiadiazolyl ring. Detailed differential scanning calorimetry and variable temperature powder X-ray diffraction studies coupled with SQUID magnetometry have been used to show that <b>2α</b> converts irreversibly to <b>2β</b> upon heating and that <b>2β</b> and <b>2γ</b> interconvert through a reversible phase transition with a small thermal hysteresis in its magnetic response

Neutron Diffraction Studies of the Molecular Compound [Co₂(bta)]_<i>n</i> (H₄bta =1,2,4,5-Benzenetetracarboxylic Acid): In the Quest of Canted Ferromagnetism

The exchange mechanism and magnetic structure of the organic–inorganic layered molecule-based magnet [Co₂(bta)]_<i>n</i> (<b>1</b>) (H₄bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800 G (metamagnetic behavior), and a slow magnetic relaxation from this ferromagnetic phase to the antiferromagnetic one is observed. The magnetic structure of <b>1</b> has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be of antiferromagnetic nature with the local cobalt­(II) spins (magnetic moments) being aligned ferromagnetically in the <i>ac</i> plane and antiferromagnetically coupled along the crystallographic <i>b</i> axis. No evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment

Neutron Diffraction Studies of the Molecular Compound [Co₂(bta)]_<i>n</i> (H₄bta =1,2,4,5-Benzenetetracarboxylic Acid): In the Quest of Canted Ferromagnetism

The exchange mechanism and magnetic structure of the organic–inorganic layered molecule-based magnet [Co₂(bta)]_<i>n</i> (<b>1</b>) (H₄bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800 G (metamagnetic behavior), and a slow magnetic relaxation from this ferromagnetic phase to the antiferromagnetic one is observed. The magnetic structure of <b>1</b> has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be of antiferromagnetic nature with the local cobalt­(II) spins (magnetic moments) being aligned ferromagnetically in the <i>ac</i> plane and antiferromagnetically coupled along the crystallographic <i>b</i> axis. No evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment