²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance, Fourier-Transform Infrared, and Monte Carlo Study of Synthetic Tetrasilicic Magnesium Mica Solid Solutions

The parallel 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) and Fourier-transform infrared study of synthetic micas made it possible to compare structural features of the tetrasilicic magnesium mica K(Mg2.5□0.5) Si4O10(OH)2 (TMM) and their K(Mg3)(Si3.5Mg0.5)O10(OH)2 (TMMA) and K(Mg3)(Si3.5Be0.5)O10(OH)2 (TMMB) derivatives. In the TMM mica, SiO4 tetrahedra are elongated in the plane ab and shortened along the c* direction with respect to those of the phlogopite (Phl) K(Mg3)(Si3Al)O10(OH)2. The substitution of Si4+ by R2+ (Mg2+ or Be2+) produces, besides the 29Si MAS NMR signal of Si (3Si) at −91.2 ppm, new components at −84.4 or −87.5 ppm that correspond to Si (2Si1Mg) or Si(2Si1Be) environments. Tetrahedral cation distributions in TMM/TMMA, TMM/TMMB solid solutions are investigated with respect to the TMM/Phl series by means of NMR and Monte Carlo simulations, concluding that divalent Mg2+ and Be2+ are further dispersed than trivalent Al3+ cations in tetrahedral sheets of micas. In three analyzed series, cation distributions display features between those of the homogeneous dispersion of charges of phlogopites and the maximum dispersion of charges of TMM derivatives. In three series, the location of charge deficits that compensate K+ cations changes from octahedral in TMM to tetrahedral sheets in phlogopite and TMMA and TMMB derivatives

Influence of the Preparation Temperature on the Photocatalytic Activity of 3D-Ordered Macroporous Anatase Formed with an Opal Polymer Template

Even if many aspects of three-dimensionally ordered macroporous (3DOM) titania have been extensively studied, the interplay between the amorphous and crystalline components of their skeleton wall has not attracted too much attention, although it should strongly influence the properties of these materials. In order to gain new insight on this interplay, we have studied in detail the wall structure of three 3DOM titania samples prepared by heating the 3DOM titania precursor at 673, 773, and 873 K by X-ray diffraction, thermogravimetric analysis, differential thermal analysis, N₂ adsorption/desorption, pore-size distribution, high-resolution transmission electron microscopy, and ¹H magic-angle-spinning NMR. In addition, their photoreactivity toward the partial oxidation and mineralization of 4-methoxybenzyl alcohol was determined and compared with that of commercial anatase and P25 samples. The results show that when the 3DOM sample heated at 673 K, whose skeleton wall is formed by very small anatase nanoparticles covered by relatively thick layers of amorphous titania, is heated at 773 K, most amorphous titania is eliminated, leaving a very thin amorphous titania layer covering the slightly grown anatase nanoparticles. The photoreactivity results show that the sample heated at 773 K has the highest overall photoreactivity, which was lower than those of commercial anatase and P25 samples; however, its reactivity toward the alcohol partial oxidation was the highest compared with those obtained with all of the other samples. These results indicate that amorphous titania negatively affects the overall photocatalytic activity; however, its particular distribution in the 3DOM samples enhances the partial oxidation reaction

Synthesis and Characterization of NaNiF₃·3H₂O: An Unusual Ordered Variant of the ReO₃ Type

A new hydrated sodium nickel fluoride with nominal composition NaNiF₃·3H₂O was synthesized using an aqueous solution route. Its structure was solved by means of ab initio methods from powder X-ray diffraction and neutron diffraction data. NaNiF₃·3H₂O crystallizes in the cubic crystal system, space group <i>Pn</i>3̅ with <i>a</i> = 7.91968(4) Å. The framework, derived from the ReO₃ structure type, is built from NaX₆ and NiX₆ (X = O, F) corner-shared octahedra, in which F and O atoms are randomly distributed on a single anion site. The 2<i>a</i> × 2<i>a</i> × 2<i>a</i> superstructure arises from the strict alternate three-dimensional linking of NaX₆ and NiX₆ octahedra together with the simultaneous tilts of the octahedra from the cube axis (φ = 31.1°), with a significant participation of hydrogen bonding. NaNiF₃·3H₂O corresponds to a fully cation-ordered variant of the In­(OH)₃ structure, easily recognizable when formulated as NaNi­(XH)₆ (X = O, F). It constitutes one of the rare examples for the <i>a</i>⁺<i>a</i>⁺<i>a</i>⁺ tilting scheme with 1:1 cation ordering in perovskite-related compounds. The Curie-like magnetic behavior well-reflects the isolated paramagnetic Ni²⁺ centers without worth mentioning interactions. While X-ray and neutron diffraction data evidence Na/Ni order in combination with O/F disorder as a main feature of this fluoride, results from Raman and magic-angle spinning NMR spectroscopies support the existence of specific anion arrangements in isolated square windows identified in structural refinements. In particular, formation of water molecules derives from unfavorable FH bond formation

Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphous-to-Crystalline Transformations

The chemistry of metal phosphonates has been progressing fast with the addition of new materials that possess novel structural features and new properties, occasionally in a cooperative manner. In this paper, we report a new family of functional lanthanide-carboxyphosphonate materials. Specifically, the lanthanide is La, Ce, Pr, Sm, Eu, Gd, Tb, or Dy and the carboxyphosphonate ligand is 2-hydroxyphosphonoacetic acid (H₃HPA). All reported LnHPA compounds, Ln₃(H_0.75O₃PCHOHCOO)₄·<i>x</i>H₂O (<i>x</i> = 15–16), crystallize in the orthorhombic system. Two types of structures were isolated: series I and II polymorphs. For both series, the three-dimensional (3D) open frameworks result from the linkage of similar organo-inorganic layers, in the <i>ac</i>-plane, by central lanthanide cations, which yield trimeric units also found in other metal-HPA hybrids. Large oval-shaped 1D channels are formed by the spatial separation of the layers along the <i>b</i>-axis and filled with lattice water molecules. LnHPA materials undergo remarkable crystalline-to-amorphous-to crystalline transformations upon dehydration and rehydration cycles, as confirmed by thermodiffraction and NMR spectroscopy. The highest proton conductivity was observed for GdHPA (series II), 3.2 × 10^–4 S cm^–1 at 98% RH and <i>T</i> = 21 °C. The dehydration–rehydration chemistry was also followed by photoluminescence spectroscopy. It was shown that loss and reuptake of water molecules are accompanied by clear changes in the photoluminescence spectra and lifetimes of the Eu analog (series II). Our present results reveal a wide family of well-characterized, multifunctional lanthanide-based phosphonate 3D-structured metal–organic frameworks (MOFs) that show reversible crystalline-to-amorphous-to-crystalline transformations and, at the same time, exhibit high proton conductivity

Multifunctional Luminescent and Proton-Conducting Lanthanide Carboxyphosphonate Open-Framework Hybrids Exhibiting Crystalline-to-Amorphous-to-Crystalline Transformations

The chemistry of metal phosphonates has been progressing fast with the addition of new materials that possess novel structural features and new properties, occasionally in a cooperative manner. In this paper, we report a new family of functional lanthanide-carboxyphosphonate materials. Specifically, the lanthanide is La, Ce, Pr, Sm, Eu, Gd, Tb, or Dy and the carboxyphosphonate ligand is 2-hydroxyphosphonoacetic acid (H₃HPA). All reported LnHPA compounds, Ln₃(H_0.75O₃PCHOHCOO)₄·<i>x</i>H₂O (<i>x</i> = 15–16), crystallize in the orthorhombic system. Two types of structures were isolated: series I and II polymorphs. For both series, the three-dimensional (3D) open frameworks result from the linkage of similar organo-inorganic layers, in the <i>ac</i>-plane, by central lanthanide cations, which yield trimeric units also found in other metal-HPA hybrids. Large oval-shaped 1D channels are formed by the spatial separation of the layers along the <i>b</i>-axis and filled with lattice water molecules. LnHPA materials undergo remarkable crystalline-to-amorphous-to crystalline transformations upon dehydration and rehydration cycles, as confirmed by thermodiffraction and NMR spectroscopy. The highest proton conductivity was observed for GdHPA (series II), 3.2 × 10^–4 S cm^–1 at 98% RH and <i>T</i> = 21 °C. The dehydration–rehydration chemistry was also followed by photoluminescence spectroscopy. It was shown that loss and reuptake of water molecules are accompanied by clear changes in the photoluminescence spectra and lifetimes of the Eu analog (series II). Our present results reveal a wide family of well-characterized, multifunctional lanthanide-based phosphonate 3D-structured metal–organic frameworks (MOFs) that show reversible crystalline-to-amorphous-to-crystalline transformations and, at the same time, exhibit high proton conductivity