New Ln[B6O9(OH)3] borates (Ln = Sm-Lu): Structure, properties, and structural relation to cationic Li4[B7O12]Cl conductors (Li-Boracites)

Crystals of new rare earth borates of the composition Ln[B₆O₉ (OH)₃] (Ln = Sm-Lu), sp. gr. R3c are synthesized under hydrothermal conditions. Their crystal structures are determined on single crystals with Ln = Ho, Gd without preliminary determination of their chemical formulas. The polar anionic framework of the crystals consists of BO₃ triangles and BO₄ tetrahedra and has wide channels along the threefold axis of the structure, which are similar to the channels along the a, b, and c axes in cubic Li₄[B₇O₁₂]Cl boracite with Li conductivity. Rare earth atoms are arranged in the structure over the cubic F pseudolattice, whereas the analogous positions in Li boracites are filled with Cl anions. The squared optical nonlinearity of the new crystals is comparable with the nonlinearity of quartz, whereas the electrical conductivity in borates at 300°C exceeds 10⁻⁶ S/cm. __________ Translated from Kristallografiya, Vol. 49, No. 4, 2004, pp. 681–691. Original Russian Text Copyright ? 2004 by Belokoneva, Ivanova, Stefanovich, Dimitrova, Kurazhkovskaya.     

Critical Points in a Crystal and Procrystal

The critical points in the model electron density distributions of LiF, NaF, NaCl, and MgO crystals, constructed from accurate X-ray diffraction data, are determined. For LiF and MgO they are compared with those obtained from a Hartree–Fock electron density calculation. Both experiment and theory show the same type of critical points on the bond lines. The topological features in areas between structural units, where the electron density is low and near-uniform, turn out to be model dependent and cannot be established well with the data available. Topological analysis of procrystals (hypothetical systems consisting of spherical atoms or ions placed on the same sites as atoms in real crystal) show that (3, –1) critical points, usually connected with bonding interaction, are observed on interatomic lines in these nonbonded systems as well.     

Synthesis and Crystal Structure of Fluoride K2(Ta0.9I0.1)F7 with the Micro-Isomorphic Substitution in the Ta Cationic Position

Crystallography Reports - Fluoride crystals K2(Ta0.9I0.1)F7 (sp. gr. Р21/c) complementary to the K2TaF7 and K2NbF7 fluoride family have been obtained by hydrothermal synthesis. Their...     

Synthesis and Crystal Structure of New Indium Iodate (K<Subscript>0.6</Subscript>Na<Subscript>0.4</Subscript>Ba)In[IO<Subscript>3</Subscript>]<Subscript>6</Subscript>

Crystals of new indium iodate (K_0.6Na_0.4Ba)In[IO₃]₆ were prepared by the hydrothermal synthesis. The unit-cell parameters are a = 11.3984(3) Å, с = 11.3817(3) Å, sp. gr. R3?. The chemical formula of the compound was derived from the structure determination and refinement with anisotropic displacement parameters to R = 0.0284. In the structure the InO₆ octahedra share vertices with six umbrella-like [IO₃]–groups typical of iodates and form isolated 3?-symmetric charged \(\rm{In[IO_{3}]_6^{3-}}\) clusters. Large Ba, K, and Na cations occupy a common site on a threefold axis due to the isomorphous substitution and compensate the charge of the clusters. The new structure extends the family of the recently discovered alkali-metal and barium iodates containing Ti and Zr atoms in octahedral sites. The iodate K₂Ge[IO₃]₆ containing Ge atoms in the centers of octahedra is the parent compound of this structural family.     

Synthesis and crystal structure of BaFe[PO<Subscript>4</Subscript>](OH), a new orthophosphate with a mixed framework related to alumosilicates of the nepheline group

A new orthophosphate BaFe[PO₄](OH) is obtained by hydrothermal synthesis. The unit cell parameters are as follows: a = 9.711(6), b = 8.991(5), and c = 4.912(2) Å; space group P2₁2₁2₁. The structure has no features in common with the structures of three Ba-Fe phosphates studied. At the same time, the mixed framework, which is composed of five-vertex polyhedra of Fe and P-tetrahedra, exhibits a similarity to the frameworks of a number of alumosilicates belonging to the group of nepheline (nepheline NaAlSiO₄, kalsilite KAlSiO₄, and RbAlSiO₄), as well as the beryllonite NaBePO₄ beryllophosphate and H₃O · ZnPO₄ zin-cophosphate. All the frameworks contain six-membered rings which consist of alternating tetrahedra of different kinds with compensating cations at the ring centers. The topology of the new framework is most closely related to that of nepheline. Structures of this type are of interest as potential ion exchangers and ion-conducting materials.     

Electron density in lead bromine and chlorine hilgardites on the basis of precise x‐ray diffraction data and ab‐initio calculations and correlation with the properties

By a precise X‐ray diffraction experiment carried out on grown crystals of lead bromine and chlorine hilgardites (Pb₂B₅O₉Br and Pb₂B₅O₉Cl) we solved the structure of Pb₂B₅O₉Cl and refined the structure of Pb₂B₅O₉Br. The electron density distribution in the Pb₂B₅O₉Br was analyzed and its spatial distribution was linked to the unusually high optical nonlinearity of the Pb₂B₅O₉Br. Ab‐initio calculations confirmed the role of the lead atoms as the major contributors to the optical nonlinearity. By the Bader critical point approach, we analyzed the coordination polyhedrons of lead and halogen atoms, we showed that despite a long distance the bonding between lead and halogen atoms exists indeed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure of a new chain diborate TmH[B2O5] and its comparison with related compound GdH[B2O5] based on the pseudosymmetry and the group-subgroup relation

Crystals of a new rare-earth chain diborate TmH[B₂O₅], space group C2/c, are obtained under hydrothermal conditions. The structure is determined without preliminary knowledge of the chemical formula. It is close to that of the GdH[B₂O₅] diborate studied earlier, but has a different orientation of the monoclinicity axis. In both structures the arrangement of atoms follows the pattern of the orthorhombic pseudosupergroup Cmca, which is most distinctly violated by the displacements of the rare-earth atoms. Their common anionic chain radical consists of diborate groups, namely, BO₃ triangles and BO₄ tetrahedra, 2[1T + 1Δ]. It is shown that the existence of two varieties that are based on the hypothetical orthorhombic prototype is determined by the pseudosymmetry and the difference between the ionic radii of the elements located at the middle and the end of the rare-earth series. The twinning and poor quality of the crystals are related to the effect of the m pseudoplane of the supergroup and cleavage. The structures of natural megaborates that contain diborate chains in complex boron-oxygen radicals are discussed.     

New isoformula iodates In(IO<Subscript>3</Subscript>)<Subscript>3</Subscript> and Sm(IO<Subscript>3</Subscript>)<Subscript>3</Subscript> with different crystal structures: Specific structural features of I<Superscript>5+</Superscript> compounds

New iodates, namely, In(IO₃)₃ (space group R \(\bar 3\)) and Sm(IO₃)₃ (space group P2₁/a), are synthesized under hydrothermal conditions. The original crystal structure of the In(IO₃)₃ compound is determined without prior knowledge of the chemical formula. The Sm(IO₃)₃ compound is isostructural to the Gd(IO₃)₃ compound. The [IO₄]^3- tetrahedra with three short I-O bonds have an umbrella coordination, which is characteristic of pentavalent iodine, and form anionic radicals, such as a ring radical in the In(IO₃)₃ iodate, a triple helix in the isoformula compound Fe(IO₃)₃, a complex chain in the Sm(IO₃)₃ iodate, and a linear triortho group in the Sm(IO₃)₃·H₂O compound. All radicals contain triortho groups. The structural differences are determined by different ionic radii and shapes of the coordination polyhedra of the cations (indium and iron octahedra and an eight-vertex samarium polyhedron).