Iodide-Iodates M₃[IO₃]₁₂·Ag₄I, M = Bi, Tb, with a Framework Structure and High Second-Harmonic Generation Optical Response

Single crystals of two new iodide-iodates, Bi₃[IO₃]₁₂·Ag₄I and Tb₃[IO₃]₁₂·Ag₄I, are synthesized in hydrothermal systems. The anionic parts in both iodide-iodates are characterized as a complex charged framework of isolated IO₃ umbrella-like groups and large Bi­(Tb)–O polyhedra similar to those previously found in La₃[IO₃]₁₂­[IO₃]­(Pb₃O). Broad channels along the <i>c-</i>axis contain compensators: (Ag₃I)²⁺ umbrella-like groups and additional Ag⁺ ions which form Ag₄⁴⁺ tetrahedral clusters augmented with I^– halogen. New iodates possess significantly higher second-order nonlinear optical characteristics compared to the previously known lead-containing compounds REE₃[IO₃]₁₂­[IO₃]­(Pb₃O), REE = La, Pr, Nd. The difference is related to the polar ordering of umbrella-like (Ag₃I)²⁺ groups in the channels in the new iodide-iodate. Additionally, planar-coordinated Ag atoms add three Ag atoms in umbrellas forming [Ag₄I]³⁺ polar clusters in the channels

Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Structural properties of a quadruple perovskite BiMn₇O₁₂ were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above <i>T</i>₁ = 608 K, BiMn₇O₁₂ crystallizes in a parent cubic structure with space group <i>Im</i>3̅. Between 460 and 608 K, BiMn₇O₁₂ adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as <i>I</i>2/<i>m</i>(o)), and orbital order appears below <i>T</i>₁. Below <i>T</i>₂ = 460 K, BiMn₇O₁₂ is likely to exhibit a transition to space group <i>Im</i>. Finally, below about <i>T</i>₃ = 290 K, a triclinic distortion takes place to space group <i>P</i>1. Structural analyses of BiMn₇O₁₂ are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the <i>Im</i> and <i>P</i>1 models is very small, and this fact can explain why the <i>Im</i> to <i>P</i>1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMn₇O₁₂ is in striking contrast with that of LaMn₇O₁₂ and could be caused by effects of the Bi³⁺ lone electron pair

Syntheses, Crystal Structures, and Nonlinear Optical Activity of Cs₂Ba[AnO₂(C₂H₅COO)₃]₄ (An = U, Np, Pu) and Unprecedented Octanuclear Complex Units in KR₂(H₂O)₈[UO₂(C₂H₅COO)₃]₅ (R = Sr, Ba)

X-ray diffraction was applied to the elucidation of crystal structures of single crystals of Cs₂Ba­[AnO₂(C₂H₅COO)₃]₄, where An = U­(<b>I</b>), Np­(<b>II</b>), Pu­(<b>III</b>), and KR₂(H₂O)₈[UO₂(C₂H₅COO)₃]₅, where R = Sr­(<b>IV</b>), Ba (polymorphs <b>V-a</b> and <b>V</b><b>-</b><b>b</b>). FTIR spectra were analyzed for the uranium-containing crystals <b>I</b>, <b>I</b><b>V</b>, and <b>V</b><b>-</b><b>b</b>. Isostructural cubic crystals <b>I</b>–<b>I</b><b>I</b><b>I</b> are constructed of typical mononuclear anionic complex units [AnO₂(C₂H₅COO)₃]⁻ and charge-balancing Cs and Ba cations. Features of actinide contraction in the six U–Np–Pu isostructural series known to date are analyzed. In crystal structures of <b>IV</b> and <b>V</b> two typical complexes [UO₂(C₂H₅COO)₃]⁻ bind with a hydrated Sr or Ba cation to form the rare trinuclear neutral complex unit {<i>R</i>(H₂O)₄[UO₂(C₂H₅COO)₃]₂}, where R = Sr, Ba. Two such trinuclear units and one typical mononuclear unit further bind with a K cation to form the unprecedented octanuclear neutral complex unit K­[UO₂(C₂H₅COO)₃]­{R­(H₂O)₄[UO₂(C₂H₅COO)₃]₂}₂. As the derived polynuclear complexes of uranyl ion with carboxylate ligands in the crystal structures of <b>IV</b> and <b>V</b> are not the first but are rare examples, the equilibrium between mono and polynuclear complex units in aqueous solutions is discussed. The two polymorphic modifications <b>V-a</b> and <b>V-b</b> were studied at 100 K and at room temperature, respectively. Peculiarities of noncovalent interactions in crystal structures of the two polymorphs are revealed using Voronoi–Dirichlet tessellation. The nonlinear optical activity of noncentrosymmetric crystals <b>I</b> was estimated by its ability for second harmonic generation

New Solid Electrolyte Na₉Al(MoO₄)₆: Structure and Na⁺ Ion Conductivity

Solid electrolytes are important materials with a wide range of technological applications. This work reports the crystal structure and electrical properties of a new solid electrolyte Na₉Al­(MoO₄)₆. The monoclinic Na₉Al­(MoO₄)₆ consists of isolated polyhedral [Al­(MoO₄)₆]^9– clusters composed of a central AlO₆ octahedron sharing vertices with six MoO₄ tetrahedra to form a three-dimensional framework. The AlO₆ octahedron also shares edges with one Na1O₆ octahedron and two Na2O₆ octahedra. Na3–Na5 atoms are located in the framework cavities. The structure is related to that of sodium ion conductor II-Na₃Fe₂(AsO₄)₃. High-temperature conductivity measurements revealed that the conductivity (σ) of Na₉Al­(MoO₄)₆ at 803 K equals 1.63 × 10^–2 S cm^–1. The temperature behavior of the ²³Na and ²⁷Al nuclear magnetic resonance spectra and the spin-lattice relaxation rates of the ²³Na nuclei indicate the presence of fast Na⁺ ion diffusion in the studied compound. At <i>T</i><490 K, diffusion occurs by means of Na⁺ ion jumps exclusively through the sublattice of Na3–Na5 positions, whereas Na1 and Na2 become involved in the diffusion processes (through chemical exchange with the Na3–Na5 sublattice) only at higher temperatures