Nanoscale Hemispheres in Novel Mixed-Valent Uranyl Chromate(V,VI), (C₃NH₁₀)₁₀[(UO₂)₁₃(Cr₁₂⁵⁺O₄₂)(Cr⁶⁺O₄)₆(H₂O)₆](H₂O)₆

The structure of a novel mixed-valent chromium uranyl compound, (C₃NH₁₀)₁₀[(UO₂)₁₃(Cr₁₂⁵⁺O₄₂)­(Cr⁶⁺O₄)₆(H₂O)₆]­(H₂O)₆ (<b>1</b>), obtained by the combination of a hydrothermal method and evaporation from aqueous solutions with isopropylammonium, contains uranyl chromate hemispheres with lateral dimensions of 18.9 × 18.5 Ų and a height of about 8 Å. The hemispheres are centered by a UO₈ hexagonal bipyramid surrounded by six dimers of Cr⁵⁺O₅ square pyramids, UO₇ pentagonal bipyramids, and Cr⁶⁺O₄ tetrahedra. The hemispheres are linked into two-dimensional layers so that two adjacent hemispheres are oriented in opposite directions relative to the plane of the layer. From a topological point of view, the hemispheres have the formula U₂₁Cr₂₃ and can be considered as derivatives of nanospherical cluster U₂₆Cr₃₆ composed of three-, four-, and five-membered rings

Nanoscale Hemispheres in Novel Mixed-Valent Uranyl Chromate(V,VI), (C₃NH₁₀)₁₀[(UO₂)₁₃(Cr₁₂⁵⁺O₄₂)(Cr⁶⁺O₄)₆(H₂O)₆](H₂O)₆

The structure of a novel mixed-valent chromium uranyl compound, (C₃NH₁₀)₁₀[(UO₂)₁₃(Cr₁₂⁵⁺O₄₂)­(Cr⁶⁺O₄)₆(H₂O)₆]­(H₂O)₆ (<b>1</b>), obtained by the combination of a hydrothermal method and evaporation from aqueous solutions with isopropylammonium, contains uranyl chromate hemispheres with lateral dimensions of 18.9 × 18.5 Ų and a height of about 8 Å. The hemispheres are centered by a UO₈ hexagonal bipyramid surrounded by six dimers of Cr⁵⁺O₅ square pyramids, UO₇ pentagonal bipyramids, and Cr⁶⁺O₄ tetrahedra. The hemispheres are linked into two-dimensional layers so that two adjacent hemispheres are oriented in opposite directions relative to the plane of the layer. From a topological point of view, the hemispheres have the formula U₂₁Cr₂₃ and can be considered as derivatives of nanospherical cluster U₂₆Cr₃₆ composed of three-, four-, and five-membered rings

Cr(VI) Trioxide as a Starting Material for the Synthesis of Novel Zero‑, One‑, and Two-Dimensional Uranyl Dichromates and Chromate-Dichromates

Six different dichromate-based uranyl compounds were obtained. Their structures belong to four principally different but related structure types with different dimensionality of basic structural units. The units in Cs₂(UO₂)­(Cr₂O₇)­(NO₃)₂ (<b>1</b>) and (C₆H₁₁N₂)₂(UO₂)­(Cr₂O₇)₂(H₂O) (<b>2</b>) are unique, and these are the first “pure” uranyl-dichromates known to date. The compounds Rb₂(UO₂)­(CrO₄)­(Cr₂O₇) (<b>3</b>), (C₂NH₈)₂(UO₂)­(CrO₄)­(Cr₂O₇) (<b>4</b>), (C₂NH₈)₂(UO₂)­(CrO₄)₂(Cr₂O₇)­(H₂O)₂ (<b>5</b>), and (C₃NH₁₀)₂(UO₂)­(CrO₄)₂(Cr₂O₇)­(H₂O)₂ (<b>6</b>) are novel representatives of a rather small group of inorganic compounds containing both isolated CrO₄ tetrahedra and dichromate Cr₂O₇ groups. The structures of <b>5</b> and <b>6</b> contain compositionally identical but topologically different _∞²[(UO₂)­(CrO₄)₂(Cr₂O₇)]^2– sheets (thus corresponding to different geometrical isomers), which have not been reported previously in inorganic compounds. All novel phases have been prepared with an excess of CrO₃. “Pure” dichromates are formed at pH < 1.5 and with prior hydrothermal treatment of uranyl-chromate solution, whereas mixed chromate-dichromates are formed at higher pH > 2 values

pH Controlled Pathway and Systematic Hydrothermal Phase Diagram for Elaboration of Synthetic Lead Nickel Selenites

The PbO–NiO–SeO₂ ternary system was fully studied using constant hydrothermal conditions at 473 K. It yields the establishment of the corresponding phase diagram using a systematic assignment of reaction products by both powder and single-crystal X-ray diffraction. It leads to the preparation of three novel lead nickel selenites, α-PbNi­(SeO₃)₂ (<b>I</b>), β-PbNi­(SeO₃)₂ (<b>II</b>), and PbNi₂(SeO₂OH)₂(SeO₃)₂ (<b>III</b>), and one novel lead cobalt selenite, α-PbCo­(SeO₃)₂ (<b>IV</b>), which have been structurally characterized. The crystal structures of the α-forms <b>I</b>, <b>IV</b>, and <b>III</b> are based on a 3D complex nickel selenite frameworks, whereas the β-PbNi­(SeO₃)₂ modification (<b>II</b>) consists of nickel selenite sheets stacked in a noncentrosymmetric structure, second-harmonic generation active. The pH value of the starting solution was shown to play an essential role in the reactive processes. Magnetic measurements of <b>I</b>, <b>III</b>, and <b>IV</b> are discussed

Bonding Scheme, Hydride Character, and Magnetic Paths of (HPO₃)^2– Versus (SeO₃)^2– Building Units in Solids

The abilities of the (HPO₃)^2– and (SeO₃)^2– anions as structure building units and as spin exchange paths between magnetic ions were investigated by preparing and analyzing the isostructural Fe₂(SeO₃)₃ and Fe₂(HPO₃)₃. In both compounds, the face-sharing Fe₂O₉ dimers are interconnected into chains by the (HPO₃)^2– and (SeO₃)^2– anions. The (HPO₃)^2– is the structural counterpart of the Se electron lone pair of (SeO₃)^2– due to the weak hydride character of the terminal hydrogen. However, they differ considerably as spin exchange paths between magnetic cations. Both compounds exhibit an effective magnetic dimer behavior, unexpectedly arising from the interdimer FeO···OFe exchange along the chain, but weaker in Fe₂(HPO₃)₃ by a factor of ∼3. It is consistent with the general tendencies of the phosphite anions to act as a weak magnetic mediator, which is caused by the through-bond effect of the P³⁺ ion in the FeO···P³⁺···OFe exchange path, much weaker than in the selenite phase in absence of P³⁺d contribution. Reasons for stronger exchanges through phosphates or sulfates are also discussed

Structural Evolution from 0D Units to 3D Frameworks in Pb Oxyhalides: Unexpected Strongly Corrugated Layers in Pb₇O₆Br₂

Novel Pb₇O₆Br₂ (<b>1</b>) lead oxybromide was prepared from Pb oxybromide melt by the “rapid quenching” route. Bonding scheme, thermal expansion, and structural properties were studied. The structural features of this unexpectedly complex phase are described on the basis of lone electron pair stereochemical activity and Pb–Br versus Pb–O bonding scheme. The structure of <b>1</b> contains a number of cavities, which can be assigned to the self-containments of the lone electron pairs on Pb²⁺ cations. “Empty” □Pb₄ chains are observed in between of the folding sides of the adjacent strongly corrugated oxocentered [Pb₇O₆]²⁺ layers. Highly isotropic thermal expansion of <b>1</b> appeared to be unexpected. The possible explanations of such a behavior in <b>1</b> are given. The structure of <b>1</b> is an interesting example of tetrahedral framework with mixed chemical bonding and is the densest known among Pb oxyhalides with the density of 18.4 tetrahedra/1000 ų. Current study shows that oxocentered layers derivatives from α-PbO can be very flexible and form rather dense three-dimensional structural topologies. The properties and structure are compared to other phases crystallizing in the anhydrous PbO–Pb<i>X</i>₂ (X = F, Cl, Br, I) systems, illustrate the complexity of lead oxyhalides, and reveal new and general pathways for the targeted synthesis of new phases with the Pb–O units of desired dimensionality. The indirect gap value of ∼2.04 eV obtained from generalized gradient approximation calculations demonstrates potentially good photocatalytic properties of <b>1</b>