Synthesis and structure of (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>[UO<Subscript>2</Subscript>(CH<Subscript>3</Subscript>COO)<Subscript>3</Subscript>]<Subscript>2</Subscript>(NCS)

The compound (NH₄)₃[UO₂(CH₃COO)₃]₂(NCS) (I) was synthesized and examined by single crystal X-ray diffraction analysis. The compound crystallizes in the rhombic system with the unit cell parameters a = 11.5546(4), b = 18.5548(7), c = 6.7222(3) Å, V = 1441.19(10) ų, space group P2₁2₁2, Z = 2, R = 0.0345. The uranium-containing structural units of crystals of I are isolated mononuclear groups [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes. The specific features of packing of the uranium-containing complexes in the crystal structure are considered.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

High-temperature crystal chemistry of Na<Subscript>6</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>O(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>

Thermal deformations of Na₆(UO₂)₂O(MoO₄)₄ were studied by high-temperature powder X-ray diffraction. The compound crystallizes in the triclinic system, space group Р\(\bar 1\), a = 7.636(7), b = 8.163(6), c = 8.746(4) Å, α = 72.32(9)°, β = 79.36(4)°, γ = 65.79(5)°, V = 472.74(4) ų. It is stable in the temperature interval 20–700°С. The thermal expansion coefficients (TECs) are α₁₁ = 25.5 × 10^–6, α₂₂ = 7.8 × 10^–6, and α₃₃ = 1.1 × 10^–6 (°C)^–1. The orientation of the TEC pattern relative to the crystallographic axes is a₃₃^Z = 45°, a₃₃^X = 122°, a₂₂^Z = 59°, and a₂₂^X = 66°. The anisotropy of the thermal expansion is due to specific features of the crystal structure of the compound.     

Phase transitions and high-temperature crystal chemistry of polymorphous modifications of Cs<Subscript>2</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)

Phase transitions and thermal deformations of - and -Cs₂(UO₂)₂(MoO₄)₃ were studied by high-temperature X-ray diffraction analysis. In heating of -Cs₂(UO₂)₂(MoO₄)₃ to 625 ± 25°C, the reconstructive phase transition proceeds. -Cs₂(UO₂)₂(MoO₄)₃ is stable up to 700 ±25°C. The thermal expansion of both phases is sharply anisotropic: ₁₁ = 10 × 10^–6, ₂₂ = 33 × 10^–6, ₃₃ = 10 × 10^–6, _V = 53 × 10^–6 deg^–1 for -Cs(UO₂)₂(MoO₄)₃ and ₁₁ = 13 × 10^–6, ₃₃ = 3 × 10^–6, _V = 31 × 10^–6 deg^–1 for -Cs₂ (UO₂)₂ (MoO₄)₃. The anisotropy of thermal expansion is explained by features of the crystal structure of the compounds.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 405–407.Original Russian Text Copyright © 2004 by Nazarchuk, Krivovichev, Filatov.     

Dissolution of UO<Subscript>2</Subscript> in the N<Subscript>2</Subscript>O<Subscript>4</Subscript>-H<Subscript>2</Subscript>O system

The kinetics of the UO₂ dissolution in the N₂O₄-H₂O system was studied. At 25°C, the process is kinetically controlled, whereas at 55°C the process occurs initially under kinetic control (3 min) and then under diffusion-kinetic control. At 80°C, the process occurs exclusively under diffusion-kinetic control. The apparent activation energy was estimated at ∼39 kJ mol⁻¹.     

Liquid-phase epitaxy of NdAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and Yb-doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Epitaxial layers of NaAl₃(BO₃)₄ (NAB) and YAl₃(BO₃)₄〈Yb〉 (YAB〈Yb〉) containing up to 10 at % Yb have been grown by liquid-phase epitaxy on YAB substrates. Their growth kinetics have been studied at relative supersaturations of the high-temperature solution from 2 × 10^?2 to 16 × 10^?2. The ytterbium concentration in YAB〈Yb〉 has been shown to vary little during the epitaxial process. Near the edges of the substrate, the surface morphology of the layers is complicated by vicinals, which have a spiral form in the case of YAB〈Yb〉. On \(\{ 10\overline 1 1\} \) YAB substrates, homogeneous single-crystal NAB films have been grown.     

Stability limits of graphite intercalation compounds in the systems graphite-HNO<Subscript>3</Subscript>(H<Subscript>2</Subscript>SO<Subscript>4</Subscript>)-H<Subscript>2</Subscript>O-KMnO<Subscript>4</Subscript>

The formation of binary graphite intercalation compounds (GICs) with nitric and sulfuric acids in the presence of a strong oxidant has been studied by x-ray diffraction and potentiometry in a wide range of acid concentrations. The redox potential of the oxidizing solution and the intercalation ability of the acid are shown to influence the stage number (the number of graphite layers between two successive intercalate layers) of the forming GIC and the concentration ranges of GIC formation. The (\(E_{H_2 } \))-H ₀ (redox potential of the oxidizing solution-Hammett function of the acid) stability fields of graphite nitrate and graphite bisulfate are presented. Our results are the first to demonstrate that KMnO₄ extends the concentration ranges of GIC formation and reduces the threshold acid concentration for the synthesis of binary GICs (to 40%).     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Thermodynamic properties of NaZr<Subscript>2</Subscript>(AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The heat capacity C _p ⁰ of crystalline NaZr₂(AsO₄)₃ has been measured in the range 7–650 K using precision adiabatic calorimetry and differential scanning calorimetry. The experimental data have been used to calculate the standard thermodynamic functions of the arsenate: C _p ⁰, enthalpy H ⁰(T) − H ⁰(0), entropy S ⁰(T), and Gibbs function G ⁰(T) − H ⁰(0) from T → 0 to 650 K. The standard entropy of its formation from elements is Δ_f S ⁰(NaZr₂(AsO₄)₃, cr, 298.15 K) = −1087 ± 1 J/(mol K).     

Erbium zirconium phosphate Er<Subscript>0.33</Subscript>Zr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> of the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> family. Structure contraction on heating

Erbium zirconium phosphate Er_0.33Zr₂(PO₄)₃, a member of the family of structural analogs of NaZr₂(PO₄)₃ (NZP), was prepared by the sol-gel process and studied by X-ray phase analysis, IR spectroscopy, and differential scanning calorimetry. The behavior of erbium zirconium phosphate on heating in the temperature interval from 25 to 625°C was studied by high-temperature X-ray diffraction. Expansion and contraction along different crystallographic directions and contraction of the structure as a whole were found. The overall contraction is due to higher contribution of the negative axial thermal expansion coefficients α _a and α _b to α_av and hence to the volume expansion of the phosphate. On heating to 900°C, the NZP structure is preserved.     

Effect of complexation on crystallization of plutonium(IV) oxalate in the system Pu(NO<Subscript>3</Subscript>)<Subscript>4</Subscript>-HNO<Subscript>3</Subscript>-H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The published data on complexation in the system Pu(NO₃)₄-HNO₃-H₂C₂O₄ were treated on the basis of a unified approach to determination of the oxalate ion concentration. Because of discrepancies between results published by different researchers, additional experiments on crystallization of Pu(IV) oxalate were carried out at widely varied excess and deficiency of oxalic acid. These experiments confirmed high stability of the complex cations PuC₂O ₄ ²⁺ . The upper boundary of the field of metastable supersaturated solutions of Pu oxalate at the initial Pu concentration of 15–50 g l^?1 was determined.     

Wet chemical etching of ZnO films using NH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-based (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>CO<Subscript>3</Subscript> and NH<Subscript>4</Subscript>OH alkaline solution

In this paper, we deposited ZnO thin films by RF magnetron sputtering at room temperature from un-doped targets. Wet chemical etching of ZnO films in (NH₄)₂CO₃ and NH₄OH solutions were examined. For comparison, hydrochloric acid was also used as an etchant. The NH _x -based alkaline solutions provide well-controlled etching rate, and smooth surface and sidewall profiles. Although NH _x -based alkaline solution etch rates for ZnO were relatively low, they were enhanced with the use of a H₃O stabilizer. In this case, the NH₄OH solution went from reaction-dominant mode to diffusion-dominant mode, which is beneficial for smooth surface morphology.     

Beta-LaSc<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>: correction of the crystal structure

The published structure data of trigonal beta-LaSc₃(BO₃)₄ are incorrect because they are not compatible with the formula of the compound. After correcting the positional atom co-ordinates of one O atom the structure is found to be isotypic with CeSc₃(BO₃)₄ which crystallizes with the huntite CaMg₃(CO₃)₄ structure type.Response to paper, titled "Structure of medium temperature phase -LaSc₃(BO₃)₄ crystal," by He MY et al., published in MRI, vol. 2, issue 6, pp. 345–348, DOI     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Low-loss,high-purity (TeO<Subscript>2</Subscript>)<Subscript>0.75</Subscript>(WO<Subscript>3</Subscript>)<Subscript>0.25</Subscript> glass

By melting a mixture of high-purity oxides in a platinum crucible under flowing purified oxygen, we have prepared (TeO₂)_0.75(WO₃)_0.25 glass with a total content of 3d transition metals (Fe, Ni, Co, Cu, Mn, Cr, and V) within 0.4 ppm by weight, a concentration of scattering centers larger than 300 nm in size below 10² cm⁻³, and an absorption coefficient for OH groups (λ ∼ 3 μm) of 0.008 cm⁻¹. The absorption loss in the glass has been determined to be 115 dB/km at λ = 1.06 μm, 86 dB/km at λ = 1.56 μm, and 100 dB/km at λ = 1.97 μm. From reported specific absorptions of impurities in fluorozirconate glasses and the impurity composition of the glass studied here, the absorption loss at λ ∼ 2 μm has been estimated at ≤100 dB/km. The glass has been drawn into a glass-polymer fiber, and the optical loss spectrum of the fiber has been measured.     

Synthesis and crystal structure CoNi<Subscript>2</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

A new metal orthoborate compound, cobalt dinickel orthoborate, CoNi₂(BO₃)₂ has been successfully synthesized for the first time. The title compound was synthesized by thermally-induced solid-state chemical reaction at 900°C between the initial reagents of Co(NO₃)₂ · 6H₂O, Ni(NO₃)₂ · 6H₂O and H₃BO₃ which were mixed with the mol ratio of 1: 2: 2 respectively. The obtained product was structurally characterized by X-ray powder diffraction technique. It has been found that the CoNi₂(BO₃)₂ crystallizes in the kotoite type and isostructural with the compounds having the chemical formula M₃(BO₃)₂ where M—Mg, Co and Ni. The synthesized compound belongs to the orthorhombic crystal system with the refined unit cell parameters of a = 5.419(9) Å, b = 8.352(0) Å, c = 4.478(8) Å and Z = 2. The space group was determined as Pnmn. Further characterizations by FTIR, elemental analysis and thermal analysis were also performed.     

Phonons in mixed superionic fluorites (BaF<Subscript>2</Subscript>)<Subscript>1-x</Subscript>(LaF<Subscript>3</Subscript>)<Subscript>x</Subscript>

In recent years, the fluorite-structured solid solutions with the general formula, (MF₂)_1-x(RF₃)_x (M = Ca, Sr, Ba, Pb and R is a rare-earth element or Y), have been the subject of numerous experimental studies focussed on their superionic properties. The overall cubic crystal symmetry (space group Fm3m) is conserved up to x ≶ x_max, where x_max ⊁ 0.4-0.5 depending on M and R. The zone centre phonons and phonon dispersion along three symmetry directions of the mixed superionic compound (BaF₂)_1-x(LaF₃)_x have been investigated by applying de Launey angular force model for x ≶ x_max. The calculated results are compared and explained with available experimental results.     

Methylammonium cation deficient surface for enhanced binding stability at TiO<Subscript>2</Subscript>/CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript> interface

Heterojunction interfaces in perovskite solar cells play an important role in enhancing their photoelectric properties and stability.Till date,the precise lattice arrangement at TiO2/CH3NH3PbI3 heterojunction interfaces has not been investigated clearly.Here,we examined a TiO2/CH3NH3PbI3 interface and found that a heavy atomic layer exists in such interfaces,which is attributed to the vacancies of methylammonium (MA) cation groups.Further,first-principles calculation results suggested that an MA cation-deficient surface structure is beneficial for a strong heterogeneous binding between TiO2 and CH3NH3PbI3 to enhance the interface stability.Our research is helpful for further understanding the detailed interface atom arrangements and provides references for interfacial modification in perovskite solar cells.     

Phase transitions of the NASICON-type mixed phosphates LiM<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> (M = Ti,Zr) and LiInNb(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The structural phase transitions of LiTi₂(PO₄)₃, LiInNb(PO₄)₃, and LiZr₂(PO₄)₃ have been studied by X-ray diffraction, impedance spectroscopy, ⁷Li NMR spectroscopy, and calorimetry. The results indicate that, as the temperature is raised, the lithium ions in the structure of LiTi₂(PO₄)₃ and LiInNb(PO₄)₃ redistribute between the M1 and M2 sites. The thermal expansion coefficients along the crystallographic axes of LiTi₂(PO₄)₃ and LiInNb(PO₄)₃ are estimated.