Synthesis and characterization of the rare-earth dicyanamides Ln[N(CN)2]3 with Ln=La, Ce, Pr, Nd, Sm, and Eu

The rare-earth dicyanamides Ln[N(CN)₂]₃ (Ln=La, Ce, Pr, Nd, Sm, Eu) were obtained via ion exchange in aqueous medium and subsequent drying: The crystal structures were solved and refined based on X-ray powder diffraction data and they were found to be isotypic: Ln[N(CN)₂]₃; Cmcm (no. 63), Z=4, Ln=La: , , ; Ce: , , ; Pr: , , ; Nd: , , ; Sm: , , ; Eu: , , ). The compounds represent the first dicyanamides with trivalent cations. The Ln³⁺ ions are coordinated by three bridging N atoms and six terminal N atoms of the dicyanamide ions forming a three capped trigonal prism. The structure type is related to that of PuBr₃. The novel compounds Ln[N(CN)₂]₃ have been characterized by IR and Raman spectroscopy (Ln=La) and the thermal behavior has been monitored by differential scanning calorimetry (Ln=Ce, Nd, Eu).     

Crystal growth, characterization and physical properties of PrNiSb3, NdNiSb3 and SmNiSb3

The crystal structures of three new intermetallic ternary compounds in the LnNiSb₃ (Ln=Pr, Nd and Sm) family have been characterized by single crystal X-ray diffraction. PrNiSb₃, NdNiSb₃ and SmNiSb₃ all crystallize in an orthorhombic space group, Pbcm (No. 57), Z=12, with , , , and ; , , , and ; and , , , and , for Ln=Pr, Nd and Sm, respectively. These compounds consist of rare-earth atoms located above and below layers of nearly square, buckled Sb nets, along with layers of highly distorted edge- and face-sharing NiSb₆ octahedra. Resistivity data indicate metallic behavior for all three compounds. Magnetization measurements show antiferromagnetic behavior with (PrNiSb₃), 4.6 K (NdNiSb₃), and 2.9 K (SmNiSb₃). Effective moments of 3.62 μ_B, 3.90 μ_B and 0.80 μ_B are found for PrNiSb₃, NdNiSb₃ and SmNiSb₃, respectively, and are consistent with Pr³⁺ (f ²), Nd³⁺ (f ³), and Sm³⁺ (f ⁴).     

Effect of initial monomer concentration on the equilibrium swelling and elasticity of hydrogels

The linear swelling ratio α and the effective network chain length N of a series of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration . PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that α is not a monotonic function of . As is increased, α first decreases up to about and remains constant in a narrow range of , but then it increases continuously. The -dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of below 0.1, N follows the scaling relationship , while at higher concentrations, N varies only slightly with . The increase of α with N obeys the relation , as predicted by the Flory-Rehner theory.     

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3-MO-Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.     

Monitoring structural transformations in crystals. Part 4. Monitoring structural changes in crystals of pyridine analogs of chalcone during [2+2]-photodimerization and possibilities of the reaction in hydroxy derivatives

We studied the [2+2]-photodimerization in crystals of pyridine analogs and hydroxy derivatives of chalcone using the X-ray structure analysis. The mutual orientation of adjacent molecules in the crystals was analyzed in a quantitative way and the results were compared with data for known photoactive crystals undergoing the [2+2]-photodimerization. In the case of one pyridine analog, we processed the single-crystal-to-single-crystal photodimerization and determined the structure for the mixed crystal containing both the substrate and the product. We also explained a role of hydrogen bonds in the [2+2]-photodimerization in the case of the hydroxy derivatives of chalcone. C₅H₄N-CO-CHCH-C₆H₅: , , , β=91.318(10)°, monoclinic, . The irradiated crystal of the above analog: , , , β=91.870(10)°, monoclinic, . C₆H₅-CO-CHCH-C₅H₄N: , , , β=110.01(3)°, monoclinic, C2/c,Z=8. C₆H₅-CO-CHCH-C₆H₄(o-OH): , , , β=109.73(5)°, monoclinic, . C₆H₅-CO-CHCH-C₆H₄(p-OH): , , , orthorhombic, .     

Tetragonal low-temperature structure of LiAl

The cubic face-centered structure of LiAl (, at ) transforms into a tetragonal body-centered structure (I4₁/amd, , at ). This first-order phase transition at about during heating is probably the reason for the so-called “ anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.     

Synthesis, structure, and magnetism of a new heavy-fermion antiferromagnet, CePdGa6

A new compound, CePdGa₆, and its isostructural analog, LaPdGa₆ have been synthesized by flux growth and characterized by single-crystal X-ray diffraction. The compounds adopt a tetragonal structure with P4/mmm space group, Z=1. The lattice parameters for CePdGa₆ are and and and for LaPdGa₆. Magnetic and thermal measurement have revealed that CePdGa₆ is a heavy-fermion with the specific heat coefficient and Ce f moments order antiferromagnetically along c-axis at . Reconfiguration of spin occurs at to induce a ferromagnetic component only in the a-b plane. This strong anisotropy in the magnetism might be related to its unique layered structure.     

Interactions of Proline in Non-aqueous Anionic, Cationic and Nonionic Surfactants at Different Temperatures

Density and viscosity data of proline (Pro) in sodium dodecyl sulfate/cetyltrimethylammonium bromide/poly (oxyethylene) isooctyl phenyl ether in formamide were measured at 298.15, 303.15, 308.15, and 313.15 K and 0.1 MPa. The density data were utilized to evaluate standard partial molar volumes ( ) and partial molar isobaric expansibility ( ). The viscosity data were used to evaluate A- and B-coefficients, free energy of activation of viscous flow ( ) and ( ), per mole of solvent and solute respectively, enthalpy (ΔH^*) and entropy (ΔS^*) of activation of viscous flow. The results obtained were utilized in the qualitative elucidation of the Pro-surfactant/formamide and Pro-Pro interactions in the present systems.     

Synthesis, structure, band gap, and electronic structure of CsAgSb4S7

The compound CsAgSb₄S₇ has been synthesized by the reaction of the elements in a Cs₂S₃ flux at 773 K. The compound crystallizes in a new structure type with eight formula units in space group C2/c of the monoclinic system in a cell at 153 K of dimensions , , , β=97.650(1)°, and . The structure contains two-dimensional layers separated by Cs atoms. Each layer is built from edge-sharing one-dimensional and chains. Each Ag atom is tetrahedrally coordinated to four S atoms. Each Sb³⁺ center is pyramidally coordinated to three S atoms to form an SbS₃ group. CsAgSb₄S₇ is insulating with an optical band gap of 2.04 eV. Extended Hückel calculations indicate that the band gap in CsAgSb₄S₇ is dominated by the Sb 5s and S 3p states above and below the Fermi level.     

Temperature-dependent diffraction studies on the phase evolution of tetraindium heptabromide

The mixed-valent compound In₄Br₇ undergoes a higher-order phase transition below which leads to a decrease in symmetry from the trigonal to the monoclinic (C2/c) system via . The phase transition has been monitored by X-ray powder diffraction using a linear position-sensitive detector between 15 and , and the crystal structures at room temperature and at 90 K have been refined by means of time-of-flight neutron powder-diffraction data; at , the lattice parameters are , , , and β=98.20(1)°; the new unit cell contains 88 atoms (Z=8) of which 12 are symmetry-independent. Due to their electronic instability because of a second-order Jahn-Teller effect, two of the three crystallographically independent monovalent indium cations are severely affected by the phase transition with respect to their coordination spheres; bond-valence calculations reveal significant strengthening of In⁺-Br⁻ bonding upon symmetry reduction. Structural changes and group-subgroup relationships as well as possible intermediate phases are discussed.     

Synthesis, crystal structure, and electronic structure of RbVSe2

RbVSe₂ has been synthesized at 773 K through the reaction of V and Se with a Rb₂Se₃ reactive flux. The compound crystallizes in the orthorhombic space group D_2h²⁴-Fddd with 16 formula units in a cell of dimensions , , and at . The structure possesses infinite one-dimensional chains of edge-sharing VSe₄ tetrahedra separated from the Rb⁺ ions. These chains distort slightly to chains. The V-V distance within these chains is 2.8362(4) Å. First-principles total energy calculations indicate that a non-magnetic configuration for the V³⁺ cations is the most stable.     

Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

Heat capacities, order-disorder transitions, and thermodynamic properties of rare-earth orthoferrites and rare-earth iron garnets

Rare-earth orthoferrites, RFeO₃, and rare-earth iron garnets (RIGs) R₃Fe₅O₁₂ (R=rare-earth elements) were prepared by citrate-nitrate gel combustion method and characterized by X-ray diffraction method. Isobaric molar heat capacities of these oxides were determined by using differential scanning calorimetry from 130 to 860 K. Order-disorder transition temperatures were determined from the heat capacity measurements. The Néel temperatures (T_N) due to antiferromagentic to paramagnetic transitions in orthoferrites and the Curie temperatures (T_C) due to ferrimagnetic to paramagnetic transitions in garnets were determined from the heat capacity data. Both T_N and T_C systematically decrease with increasing atomic number of R across the series. Lattice, electronic and magnetic contributions to the total heat capacity were calculated. Debye temperatures as a function of absolute temperature were calculated for these compounds. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

The thermodynamic dissociation constants of the anticancer drugs camptothecine, 7-ethyl-10-hydroxycamptothecine, 10-hydroxycamptothecine and 7-ethylcamptothecine by the least-squares nonlinear regression of multiwavelength spectrophotometric pH-titration data

The mixed dissociation constants of four anticancer drugs - camptothecine, 7-ethyl-10-hydroxycamptothecine, 10-hydroxycamptothecine and 7-ethylcamptothecine, including diprotic and triprotic molecules at various ionic strengths I of range 0.01 and 0.4, and at temperatures of 25 and 37 °C - were determined with the use of two different multiwavelength and multivariate treatments of spectral data, SPECFIT32 and SQUAD(84) nonlinear regression analyses and INDICES factor analysis. A proposed strategy for dissociation constants determination is presented on the acid-base equilibria of camptothecine. Indices of precise modifications of the factor analysis in the program INDICES predict the correct number of components, and even the presence of minor ones, when the data quality is high and the instrumental error is known. The thermodynamic dissociation constant was estimated by nonlinear regression of {pK_a, I} data at 25 and 37 °C: for camptothecine and 3.02(8), and 10.23(8); for 7-ethyl-10-hydroxycamptothecine, and 2.46(6), and 8.74(3), and 9.47(8); for 10-hydroxycamptothecine and 2.84(5), and 8.92(2), and 9.98(4); and for 7-ethylcamptothecine and 3.30(16), and 10.98(18). Goodness-of-fit tests for various regression diagnostics enabled the reliability of the parameter estimates found to be proven. Pallas and Marvin predict pK_a being based on the structural formulae of drug compounds in agreement with the experimental value.