Low-temperature X-ray Research Articles

Synthesis and Characterization of SF4 Adducts with Polycyclic Amines.

Chalcogen-bonding interactions of SF4 with the polycyclic amines DABCO (C6H12N2) and HMTA (C6H12N4) were studied by low-temperature Raman spectroscopy and X-ray crystallography, revealing the 2:1 adducts C6H12N2·2SF4 and C6H12N4·2SF4 obtained from SF4 solvent. In C6H12N2·2SF4, the sulfur in each SF4 molecule is pentacoordinate with each SF4 coordinated by a single amine group, whereas C6H12N4·2SF4 forms a one-dimensional coordination polymer with three of the four nitrogen atoms in HMTA exhibiting N---S chalcogen bonds: one terminal N---SF4 and one experimentally unprecedented bridging N---(SF4)---N moiety. Solvolysis of C6H12N2·2SF4 by HF yielded crystals of [C6H12N2H]+2F-[SF5]-·6SF4, in which SF4 acts as a chalcogen-bond donor toward N as well as F. Solvolysis of C6H12N4·2SF4 resulted in the formation of the monoprotonated HMTA salt [C6H12N4H]+[HF2]-·SF4. Excess HF also led to isolation of monoprotonated HTMA, as seen in the crystal structure of the [C6H12N4H]+[H2F3]-·HF salt. The reaction of bicyclic, monobasic quinuclidine with SF4 and HF gave [C7H13NH]+F-·3.5SF4, which contains N-H---F----SF4 interactions, as well as an interstitial, disordered SF4 molecule.

Elucidating the Defect-Induced Changes in the Photocatalytic Activity of TiO2

The adsorption and subsequent photo-oxidation of carbon monoxide on the anatase TiO2(101) and rutile TiO2(110) single crystal surfaces was investigated using low temperature X-ray photoelectron spectroscopy. Anatase was shown to significantly outperform rutile in terms of the rate of carbon dioxide yield on the stoichiometric surface, and further to this, the presence of defects was shown to heavily influence the photocatalytic efficiency. The oxidation rate was reduced on anatase but increased on rutile. This change is attributed to the location of defects within the crystal structure and is further discussed in this work. These findings are of significant importance and demonstrate the possibilities of defect engineering in photocatalysis.

Synthesis, spectroscopic characterization, crystal structure, DFT studies and biological activities of new hydrazone derivative: 1-(2,5-bis((E)-4-isopropylbenzylidene)cyclopentylidene)-2-(2,4-dinitrophenyl) hydrazine

An aromatic α, β-unsaturated Ketone (2E,5E)-2,5 bis(4-isopropyl benzylidene) cyclopentanone (A) have been achieved by a Claisen-Schmidt reaction. A new hydrazone derivative 1-(2,5-bis((E)-4-isopropyl benzylidene) cyclopentylidene)-2-(2,4-dinitrophenyl) hydrazine(B) was synthesized by reacting under reflux chalcone (A) with 2,4-dinitrophenylhydrazine and evaluated for its biological activities. The structure of the title compound (B) was studied using different spectroscopic techniques such as 1H and 13C NMR, FT-IR, UV–visible and confirmed by low-temperature single-crystal X-ray diffraction analysis. Complementary computation studies using DFT and TD-DFT at B3LYP reproduced well the experimental geometrical parameters and the spectroscopic properties. The obtained biological results revealed that the synthesized compound displayed higher antioxidant activity (IC50 = 6.95 ± 0.03 μM) in comparison to BHA and BHT standards by superoxide anion radical assay, and an interesting anti-tyrosinase activity (IC50 = 15.84 ± 1.10 μM), approximately 2-fold more than that of Kojic acid which was used as a reference.

Low-Temperature Crystal Chemistry of Hingganite-(Y), from the Wanni Glacier, Switzerland

Hingganite from the Wanni glacier (Switzerland) was studied by means of energy dispersive and wavelength-dispersive spectroscopy, Raman spectroscopy, and low-temperature single-crystal X-ray diffraction. According to its chemical composition, the investigated mineral should be considered as hingganite-(Y). It showed a relatively high content of Gd, Dy, and Er and had limited content of lighter rare-earth element (REE), which is typical for Alpine gadolinite group minerals. The most intense Raman bands were 116, 186, 268, 328, 423, 541, 584, 725, 923, 983, 3383, and 3541 cm−1. Based on data of low-temperature [(−173)–(+7) °C] in situ single-crystal X-ray diffraction, it was shown that the hingganite-(Y) crystal structure was stable in the studied temperature range and no phase transitions occurred. Hingganite-(Y) demonstrated low volumetric thermal expansion (αV = 9(2) × 10−6 °C−1) and had a high thermal expansion anisotropy up to compression along one of the directions in the layer plane. Such behavior is caused by the shear deformations of its monoclinic unit cell.

Effect of Precursors on the Preparation and Texture of Mesoporous γ-Al2O3 Powders

We have studied processes underlying the formation of γ-Al2O3 from different precursors (aluminum hydroxide, aluminum isopropoxide, and aluminum nitrate) and synthesized mesoporous γ-Al2O3 powders at temperatures of 500 and 600°C. Texture parameters of the powders (specific surface area, average crystallite size, pore volume, pore shape, and pore size distribution) have been studied using low-temperature nitrogen adsorption measurements and X-ray diffraction, and the effect of the precursors on the texture parameters of γ-Al2O3 has been examined. We have demonstrated the possibility of obtaining γ-Al2O3 with a unimodal pore size distribution (2.5–4.5 and 4.5–8.0 nm, depending on the precursor) and a pore volume of ~0.550 cm3/g. The experimental results obtained here show that the texture characteristics of the synthesized γ-Al2O3 powders as catalyst supports compare well to those of their foreign analogues.

Hydrothermal SiO2 Nanopowders: Obtaining Them and Their Characteristics.

The technological mode of obtaining amorphous SiO2 nanopowders based on hydrothermal solutions is proposed in this study. Polycondensation of orthosilicic acid as well as ultrafiltration membrane separation, and cryochemical vacuum sublimation were used. The characteristics of nanopowders were determined by tunneling electron microscopy, low-temperature nitrogen adsorption, X-ray diffraction, and small-angle X-ray scattering. The scheme allows to adjust density, particle diameters of nanopowders, specific surface area, as well as diameters, area and volume of the pore. Thus, the structure of nanopowders is regulated—the volume fraction of the packing of spherical particles in aggregates and agglomerates, the size of agglomerates, and the number of particles in agglomerates. The pour densities of the nanopowders depend on the SiO2 content in sols, which were 0.02 to 0.3 g/cm3. Nanoparticles specific surface area was brought to 500 m2/g by low temperature polycondensation. Nanoparticle aggregates specific pore volume (0.2–0.3 g/cm3) weakly depend on powders density. The volume fraction of the packing of SiO2 nanoparticles in aggregates was 0.6–0.7. Solid samples of compacted nanopowders had a compressive strength of up to 337 MPa. Possible applications of hydrothermal SiO2 nanopowders are considered.

Jahn-Teller and geometric frustration effects on the structural and magnetic ground states of substituted spinels (Ni,A)Cr2O4 (A = Mn/Cu)

Comparative study of the effects of substitution (25%) of Jahn-Teller (JT) active Cu2+ and non JT active Mn2+ cations, at the Ni2+ site, on the geometric frustration (GF) and cooperative JT distortion (CJTD) in multiferroic NiCr2O4 has been carried out with the help of low-temperature magnetization, x-ray diffraction and time of flight (TOF) neutron powder diffraction (NPD) measurements. On Mn2+ substitution the CJTD, which causes elongation of the NiO4 tetrahedra in I41/amd phase of NiCr2O4, gets suppressed at room-temperature (RT) and stabilizes the Fd-3m phase in (Ni0.75Mn0.25)Cr2O4. On the contrary the Cu2+ substitution inverts the effect of CJTD and the tetrahedra get flattened at RT, which in turn oppositely distorts the I41/amd lattice in Ni0.75Cu0.25Cr2O4. The frustration index decreases on substitution. Intriguingly, the tetragonal to orthorhombic phase transition and the antiferromagnetic ordering, which are clearly evident in the pristine NiCr2O4 and the Cu2+ substituted sample respectively at 30 K and 70 K, have not been observed for the Mn2+ substituted sample. This has been attributed to the change in Cr3+- Cr3+ direct magnetic-interaction on structural transition. However, the long-range ferrimagnetic order remains intact for all the three compositions i.e. NiCr2O4, Ni0.75Mn0.25Cr2O4 and Ni0.75Cu0.25Cr2O4 occurring at 75 K, 70 K and 95 K, respectively. The Mn2+ substituted compound shows short-range magnetic correlation at low-temperatures, as evident from the diffuse and broad hump in the intensity-vs-d spacing plot of TOF NPD data. This short-range magnetic order has been attributed to the known spiral magnetic order.

Freeze-drying process for the design of porous formulations based on bismuth-potassium-ammonium citrate

The objective of this study was to find optimal freeze-drying process parameters to design porous and friable freeze-dried bismuth-potassium-ammonium citrate (FD BPAC) with improved dissolution behavior. The nucleation and melting temperatures of frozen BPAC solution in water and tert-butanol/water co-solvent system were studied with use of thermal analysis. The phases that resulted on cooling were studied by low-temperature powder x-ray diffraction. BPAC formed an amorphous freeze concentrate on cooling and remained amorphous during freeze-drying and subsequent storage. The nucleation of colloidal bismuth subcitrate (CBS) in the FD BPAC and starting BPAC systems in 0.1 M and 0.01 M HCl was studied by dynamic light scattering (DLS) method. The proposed preparation method of FD BPAC can be the basis for design of formulations with enhanced dissolution rate for bismuth-based triple/quadruple therapy, where FD BPAC simultaneously act as a carrier and an active ingredient.

Evidence for the coexistence of spin-glass and ferrimagnetic phases in BaFe12O19 due to basal plane freezing.

We present here the results of low-temperature magnetization and X-ray magnetic circular dichroism studies on the single crystals of BaFe12O19 which reveal for the first time the emergence of a spin glass phase, in coexistence with a long-range ordered ferrimagnetic phase, due to the freezing of the basal plane spin component.

Preparation of Activated Carbon from Cashew Nut Shells for Water Purification

The application of the agricultural waste of cashew nut shells (CNSs) from the Ivory Coast is proposed for the production of activated carbon (AC) used in water treatment by the physical activation. Washed and crushed CNSs are carbonized at 800°C. The crushed CNS carbonizate is activated by the physical method in a temperature range from 400 to 700°C. The specific surface (SBET) and porous structure of the AC samples are investigated by low-temperature nitrogen desorption and X-ray structural (X-ray phase) analysis. The results show that an increase in the activation temperature at a fixed time leads to an increase in the material specific surface, the development of a microporous structure, and an increase in the summary volume of mesopores and micropores of AC. The X-ray phase analysis data shows that the degree of graphitization, interplanar spacing, and crystallite sizes vary insignificantly. The possibility of using CNSs to fabricate AC no worse in sorption properties than its analogs currently in use to purify water is proven.

Crystal structure and thermodynamic properties of dinickel diphosphate dihydrate Ni2(H2O)2[P2O7

Single crystals of dinickel diphosphate dihydrate, Ni2(H2O)2[P2O7], have been synthesized by a hydrothermal method. Its structure was refined in the monoclinic P21/n space group (unit cell parameters a = 6.2517(1) Å, b = 13.7892(3) Å, c = 7.2894(2) Å, β = 94.507(2)°, V = 626.45(2) Å3, and Z = 4) based on low-temperature X-ray diffraction data until R- 0.016. Corrugated chains of NiO5(H2O) octahedra sharing edges are aligned in the [101[combining macron]] direction. They are linked into a three-dimensional framework through diphosphate groups and hydrogen bonds. A detailed crystal chemical analysis of the family Me2(H2O)2[X2O7] revealed correlations between the unit-cell parameters of the isotypic transition metal phosphates and arsenates, their structural features and the sizes of structure forming cations. Despite the isolation of the cis and trans edge-sharing infinite zigzag chains of Ni-centered octahedra from each other no pronounced low dimensionality is seen in the magnetic response of Ni2(H2O)2[P2O7]. The magnetic susceptibility χ evidences a short range correlation maximum at Tmax = 11.9 K accompanied by the onset of long-range magnetic order at TN = 9.4 K. Below TN, the title compound exhibits the features of an archetype three-dimensional easy-axis antiferromagnet which experiences a sequence of spin-flop and spin-flip phase transitions. Basing on specific heat Cp and magnetization M studies, the magnetic phase diagram of Ni2(H2O)2[P2O7] has been established.

Investigation of new magnetoelastic and magnetic transitions accompanied with magnetoelectric coupling in multiferroic

A new multiferroic solid solution has been developed and characterized for structure, phase transition, magnetoelectric and magnetoelastic coupling. Temperature dependent measurement of dc-magnetization on ceramic shows two magnetic transitions one around 42 K and the second at 130 K. The real part of dielectric permittivity exhibits step like change at the magnetic anomaly temperature (130 K) which indicates the presence of magnetoelectric coupling. The change in the value of dielectric permittivity on the application of magnetic field confirms the presence of magnetoelectric coupling in ceramic. The room temperature polarization (P)-electric field (E) hysteresis loop measurement shows week ferroelectric nature of sample while the magnetization (M) versus magnetic field (H) measurement suggest weakly ferromagnetic character. The ferroelectric nature of sample was further confirmed by calculating remanent polarization using PUND measurement. The Rietveld structural analysis of low temperature x-ray powder diffraction data does not reveal any crystallographic phase transition in terms of peak splitting or new reflections. However, temperature dependence of lattice parameters, tetragonality, unit cell volume, octahedral tilt angle (), bond length and bond angles reveal discontinuous changes at both the magnetic transitions observed in temperature dependence of magnetization. This confirms that both the magnetic anomalies (around 42 K and 130 K) exhibit magnetoelastic coupling accompanied with isostructural transitions.

Evidence for Spin Glass Transition in Hexagonal DyMnO3 without Substitutional Disorder

The magnetic interactions in the multiferroic hexagonal (h-) REMnO3 with magnetic RE3+ ions are geometrically frustrated, but only long-range ordering transitions have been reported so far. Using dc and ac susceptibility measurements on well-characterized single crystals of h-DyMnO3, we present here evidence for a spin glass (SG) transition around the spin-reordering transition at TDy3+ ≈ 7 K. This transition is shown to exhibit history-dependent irreversibility, de Almeida–Thouless line in the T–H plane, and critical slowing down of spin dynamics as per Vogel–Fulcher and power law. We have also confirmed that the SG phase exhibits nonexponential decay of thermoremanent magnetization, as well as memory and rejuvenation effects. The SG phase of h-DyMnO3 is unconventional as it occurs without any apparent disorder, like in other geometrically frustrated systems, but bears perfect phenomenological similarities with the SG phases in disordered systems. Using low-temperature X-ray diffraction studies, we also ...

Iron(II) Corrole Anions.

Reduction of [Fe(TPC)(THF)] (TPC = trianion of 5,10,15-triphenylcorrole) with KC8 generates the iron(II) corrole anion, K(THF)2[FeII(TPC)] (3a). Compound 3a represents the first example of an isolated and crystallographically characterized corrole complex of divalent iron. The compound adopts an intermediate-spin state (S = 1), displaying square-planar geometry about the iron atom. All-electron density functional theory (OLYP and B3LYP) calculations with STO-TZP basis sets indicate two essentially equienergetic d electron configurations, dxy2dz22dxz1dyz1 (occupation 1) and dxy2dz21dxz1dyz2 (occupation 2), as likely contenders for the ground state of [FeII(TPC)]-, with the optimized geometry of the former in slightly better agreement with the low-temperature X-ray structure. Solutions of 3a react with carbon monoxide to afford the low-spin (S = 0) complex, [Fe(TPC)(CO)]-, whereas introduction of oxygen at -78 °C leads to a putative O2 adduct, [Fe(TPC)(O2)]-, which decays rapidly even at low temperatures. Treatment of 3a with organic electrophiles results in formal oxidative addition to give both iron(III) and iron(IV) corrole species. With iodomethane, [Fe(TPC)Me] is produced, illustrating the first instance of alkyl ligand coordination in an iron corrole complex.

Germination of Pulsatilla Seeds as Influenced by Seed Morphology, Moist 5 °C and Gibberellin (GA3) Treatment, and Detection of Nickel in Seeds

Seeds of Pulsatilla turczaninovii were categorized to full and empty seeds based on observations under a light microscope and on X-ray images. A germination test for full or empty seeds was evaluated as affected by the duration of gibberellin GA3 and the moist 5 °C cold stratification (CS) treatment. The morphological and elemental components of P. turczaninovii and P. cernua var. koreana seeds were analyzed using low-temperature scanning electron microscopy (LT-SEM) and energy dispersive X-ray diffraction analysis. The results showed that 64% of full and semifull P. turczaninovii seeds 10 weeks after harvesting germinated in 17 to 19 days; however, the germination rate, including empty and semiempty seeds, was lower (52.6%). Full seeds with damaged or dried vegetative organs (embryo or endosperm) and semifull seeds with severely damaged vegetative organs were observed, and this result could be related to low germination rates. Germination patterns of seeds stored dry at 5 °C for 44 weeks that showed a sigmoid pattern were increased by immersing seeds in a GA3 solution for 8 hours and treating seeds with 16 or 32 days of CS. More seeds germinated between 12 and 17 days (as compared with 17 to 29 days), especially when they were treated with GA3 and received 32 days of CS. Comparison of germination rates of visually full seeds upon harvest (52.6%) with those that had been stored dry for 44 weeks (26.3% to 29.7%) suggested that the viability of seeds may have decreased. Dormancy could be a factor that decreased germination and can be removed by low temperature and GA3 treatment. LT-SEM revealed a valley-like, sunken streak in empty seeds of P. cernua var. koreana. The nickel content in the trichome and seedcoat of full and empty seeds of both taxa ranged from 2.98 to 4.62 (weight %), as determined on X-ray images. Our study suggested that the low germination rate was due to either the presence of dormancy, damage to either embryo or endosperm, a loss in viability, or the presence of nickel in the seeds.

Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy

Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM-based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times on the order of 0.1–1 s.

Reversible Oxidation of Blue Phosphorus Monolayer on Au(111)

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.

MgO Modified with MgF2 as an Electrolyte Immobilizing Agent for the High-Temperature Cells

Magnesium oxide, generally applied as a filler in high-temperature cells (with an electrolyte melting point above 250 °C), was modified with magnesium fluoride to improve its mechanical and electrical properties. Samples containing 10 and 25 mol.% MgF2 were prepared and calcined at 500, 600, and 700 °C. They were characterized by low-temperature nitrogen adsorption and X-ray diffractometry (XRD). Moreover, the electrolyte absorption, mechanical strength of pellets made of filler and electrolyte, and volume of unfilled spaces were determined. It was shown that the introduction of MgF2 in the amount of 10 and 25 mol.% results in a considerable decrease in the surface area of the initial MgO, which testifies to the covering of MgO by the formed fluoride. However, no new crystalline phases were formed as concluded from the XRD analysis. The pellets consisting of electrolyte and MgF2/MgO filler (the electrolyte + 40 wt.% of the filler) had a higher mechanical strength compared to bare MgO filler. In particular, they outperformed MgO in the ionic conductivity of molten electrolyte. The latter was almost three times as high as that of MgO filler, when the filler containing 25 mol.% MgF2 was employed. The aforementioned properties of MgF2/MgO materials predispose them for use as fillers in high-temperature cells.

Low temperature magnetic and x-ray diffraction studies of YBa1-xSrxCuFeO5 (x = 0, 0.5)

We report low temperature magnetic and structural properties of Sr doped layered perovskite material YBaCuFeO5. YBaCuFeO5 is the antiferromagnetic compound with commensurate to incommensurate transition temperature at 200 K. Low temperature x-ray diffraction shows the lattice contraction effect with decreasing temperature. Below 15 K a strong bifurcation between the ZFC and FC curves has been observed in YBa0.5Sr0.5CuFeO5, which may arise due to lattice contraction effect which changes the interaction between the spin and the lattice.

LOW TEMPERATURE CRYSTAL STRUCTURE AND MAGNETIC BEHAVIOR OF BIS(N-METHYL-2-PHENYLETHYLAMININIUM) TETRABROMIDOCUPRATE

The low-temperature X-ray structure, and magnetic properties of (MePhenetNH2)2[CuBr4] (MePhenetNH2=N-methyl-2-phenylethylammonium) (1) are reported. The compound was characterized using IR, combustion analysis, X-ray powder diffraction, single crystal X-ray diffraction, and temperature-dependent magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group C2/c. In the structure, the tetrabromidocuprate ions are packed into zig-zag chains parallel to the c-axis which are stabilized by hydrogen bonds between the ammonium cations and bromide anions. Magnetic measurements (1.8-310 K) indicate weak antiferromagnetic interactions propagated via the two-halide exchange pathway through short Br…Br contacts. Fitting the data with the S = ½ uniform chain model gave J= -7.7(1) K with C = 0.403(1).