Thermal properties and structures of chlorocadmate, bromocadmate, and bromochlorocadmate glasses

Glass transition temperatures (T_g), crystallization temperatures (T_x), and ¹¹³Cd magic-angle spinning (MAS) NMR spectra of the chlorocadmate, bromocadmate, and bromochlorocadmate glasses and the CdCl₂, CdBrCl, and CdBr₂ crystals have been measured. The T_g and T_x, and thermal stabilities (T_x−T_g) decreased with increasing Br-content. No enhancement of (T_x−T_g) of bromochlorocadmate glasses by anion-mixing was observed. The CdCl₂ and CdBr₂ crystals gave the isotropic chemical shifts of 183 and −24 ppm, which are assigned to the CdCl₆ and CdBr₆ octahedra, respectively. The isotropic chemical shifts of CdBrCl crystal (186, 166, 135, 101, 63, 25 and −21 ppm) were assigned to CdBr_nCl_(6−n) (n=0-6) octahedra, respectively. It was found that the Br and Cl atoms in CdBrCl randomly occupy chlorine sites in the CdCl₂ structure. The ¹¹³Cd MAS NMR spectra revealed that the chlorocadmate, bromocadmate, and bromochlorocadmate glasses are composed of the CdCl₆, CdBr₆ and CdBr_nCl_(6−n) octahedra (n=0-6), respectively, and that the Cd²⁺ in all the bromochlorocadmate glasses was more preferentially bonded to Br⁻ than Cl⁻. The relationship between the thermal properties and the anion coordination environments around Cd²⁺ in these glasses is discussed.     

EPR parameters and local lattice structure study of V ions in CdCl2 and CsMgCl3 crystals

The local lattice structure and EPR parameters (D, g_‖, g_⊥) have been studied systematically on the basis of the complete energy matrix for a d³ configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V²⁺ ions in CdCl₂ and CsMgCl₃ crystals, respectively. The results show that the local lattice structure of CdCl₂:V²⁺ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl₃:V²⁺ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V²⁺ ion is smaller than that of Cd²⁺ ion or larger than that of Mg²⁺ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and g‐factors (g_‖, g_⊥) are discussed.     

P and Al NMR study of Al1−xGaxPO4

Mixed phosphates Al_1−xGa_xPO₄ (0.0≤x≤1.0), with orthorhombic structure, have been prepared by co-precipitation method followed by annealing at high temperature (T≈1350 °C) and investigated by using powder X-ray diffraction and NMR spectroscopy. A systematic decrease observed for the unit cell parameters with increasing Ga³⁺ contents, suggests the formation of a solid solution between AlPO₄ and GaPO₄. ³¹P MAS NMR spectra of both AlPO₄ and GaPO₄ show a single peak at δ=−24.6 and −8.5 ppm, respectively. For mixed phosphates, multi-component ³¹P NMR spectra characterized by varying values of chemical shifts, have been observed due to the existence of different types of ³¹P structural configurations formed by varying number of Al³⁺ and Ga³⁺ as its next nearest neighbours. The intensity analysis of these component peaks suggested a perfectly random distribution of Al³⁺ and Ga³⁺ cations in these mixed phosphates. Chemical shift anisotropy parameters for the different structural configurations of ³¹P have been evaluated from the intensity analysis of the spinning side bands corresponding to different isotropic peaks. ²⁷Al NMR spectra of mixed phosphate samples exhibited a slightly asymmetric broad peak at δ≈42.0 ppm, which is characteristic of the tetrahedral configuration of Al³⁺ ions with significant covalent bonding between the metal and oxygen ions.     

Finding confined water in the hexagonal phase of Bi0.05Eu0.05Y0.90PO4·xH2O and its impact for identifying the location of luminescence quencher

¹H MAS NMR spectra of Bi_0.05Eu_0.05Y_0.90PO₄·xH₂O show chemical shift from ?0.56 ppm at 300 K to ?3.8 ppm at 215 K and another one at 5–6 ppm, which are related to the confined or interstitial water in the hexagonal structure and water molecules on the surface of the particles, respectively. Negative value of the chemical shift indicates that H of H₂O is closer to metal ions (Y^3?+? or Eu^3?+?), which is a source of luminescence quencher. H coupling and decoupling ³¹P MAS NMR spectra at 300 and 250 K show the same chemical shift (?0.4 ppm) indicating that there is no direct bond between P and H. It is concluded that the confined water is not frozen even at 215 K because of the less number of H-bonding.     

Structural and spectroscopic characterization of the quenched hexacelsian

The hexacelsian with disorder distribution of the Si⁴⁺ and Al³⁺ is synthesized by thermally induced transformation of a Ba-LTA zeolite. The initial Ba-LTA zeolite is annealed and quenched to room temperature. The crystal structure and microstructure i.e. long-range ordering is investigated from the polycrystalline material by the Rietveld refinement procedure while short-range ordering is investigated by the ²⁹Si and ²⁷Al MAS NMR, Raman and luminescence (Eu³⁺ doped hexacelsian) spectroscopy's. The crystal structure (space group P6₃/mcm) and microstructure (size-strain analysis) results indicate disorder distribution of the Si⁴⁺ and Al³⁺. Analysis of the spectra indicates disorder distribution of the Si⁴⁺ and Al³⁺ (²⁹Si and ²⁷Al MAS NMR spectroscopy), dynamical disorder in the structure and site symmetry lowering for the Ba²⁺ site at a low temperature (Raman and luminescence spectroscopy's).     

Spin-spin and spin-other-orbit effects of optical spectra and the spin-Hamiltonian parameters for Cr ions in MgO crystals

An extended complete diagonalization method/microscopic spin-Hamiltonian (CDM/MSH) program has been developed, which is applicable for d³ ions at sites of tetragonal symmetry type I (C_4v, D_2d, D₄, D_4h) and trigonal symmetry type I (C_3v, D₃, D_3d). The Hamiltonian includes the spin-spin (SS) and spin-other-orbit (SOO) magnetic interactions besides the spin-orbit (SO) magnetic interaction usually taken into account. Utilizing the extended CDM/MSH program, the optical spectra, the spin-Hamiltonian (SH) parameters of the ground state ⁴B₁, and the splitting δ(²E) of the first excited ²E state for Cr³⁺ (3d³) ions at C_4v symmetry sites in MgO crystals have been successfully investigated. It is found that although the SO magnetic interaction is the most important one, the contributions to the SH parameters and the optical spectra from the SS and SOO magnetic interactions for Cr³⁺:MgO crystals are appreciable and should not be omitted, especially reaching 27.8% for the zero field splitting parameter D.     

Characterization of the photolyase-like iron sulfur protein PhrB from Agrobacterium tumefaciens by Mössbauer spectroscopy

High field Mössbauer spectroscopy has been used to characterize the [4Fe-4S] ^{2 +}cluster of the protein PhrB from Agrobacterium tumefaciens which belongs to the cryptochrome/photolyase family (CPF) and which biological function has previously been shown to be DNA repair. Mössbauer spectra taken of the as prepared protein reveal δ = 0. 42 mms ^{? 1}, and Δ E _Q = 1. 26 mms ^{? 1}as well as an asymmetry parameter of η = 0. 8. These parameters are characteristic for a ferredoxin-type [4Fe-4S] ^{2 +}cluster. In order to investigate whether this cluster is involved in DNA-repair the protein has also been studied in its photoactivated state during DNA binding. The so obtained data sets exhibit essentially the same Mössbauer parameters as those of the non-activated PhrB. This indicates that during DNA repair the [4Fe-4S] ^{2 +}cluster of PhrB has no significant amounts of transition states which have conformational changes compared to the resting state of the protein and which have life times of several seconds or longer.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

A simple method for analyzing 51V solid-state NMR spectra of complex systems

Five vanadium complexes as models for biological systems were investigated using ⁵¹V-MAS–NMR spectroscopy. All spectra show an uncommon line shape, which can be attributed to a shorter relaxation time of the satellite transition in contrast to the central one. A method for the reliable analysis of such kind of spectra is presented for the first time and the most important NMR parameters of the investigated complexes (quadrupolar coupling constant C_Q, asymmetry of the EFG tensor η_Q, isotropic chemical shift δ_iso, chemical shift anisotropy δ_σ and asymmetry of the CSA tensor η_σ) are presented. These results are of particular importance with respect to the analysis of the ⁵¹V-MAS–NMR spectra of vanadium moieties in biological matrices such as vanadium chloroperoxidase, which show hitherto unexplained low intensity of the satellite sideband pattern.     

Local structure analysis of smoky and colorless topaz using single crystal Al NMR

The angular dependence of the ²⁷Al NMR spectrum was measured for single crystals of smoky and colorless topaz, Al₂SiO₄(F,OH)₂. Smoky topaz was obtained by irradiating high energy neutrons to colorless topaz. The quadrupole coupling constant e²Qq/h and the asymmetry parameter η were obtained from the analysis of the angular dependences of quadrupole splitting of the ²⁷Al NMR spectrum. The local structures around the aluminum atoms in smoky and colorless topaz were discussed from the magnitude and the direction of the electric field gradient. The directions of principal axes of the EFG tensor of ²⁷Al were close to the directions of Al-O and Al-F bonds. The difference in the bond lengths between Al(1)-F(1) and Al(1)-F(2) was found to affect the x and y components of the EFG tensor.     

Structural study of the Lu2Si2O7-Sc2Si2O7 system

Different compositions in the Lu₂Si₂O₇-Sc₂Si₂O₇ system have been synthesized following the ceramic method. All XRD patterns are compatible with the thortveitite structure (β-RE₂Si₂O₇ polymorph). Unit cell parameters change linearly with composition, which indicates a complete solid solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇. ²⁹Si MAS NMR spectra show a decrease of the ²⁹Si chemical shift with increasing Sc content. A correlation reported in the literature to predict ²⁹Si chemical shifts in silicates is applied here to obtain the theoretical variation in ²⁹Si chemical shift values in the system Lu₂Si₂O₇-Sc₂Si₂O₇ and the results compare favourably with the values obtained experimentally. The FWHM values of the ²⁹Si MAS NMR curves indicate a random distribution of Lu and Sc in the structure of the intermediate members. Finally, the IR study of the system confirms the solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇, showing the splitting of several modes in the intermediate members and a linear shift of the frequency on going from one end-member to the other.     

The use of photoinduced light scattering for the evaluation of photoelectric fields in Lithium Niobate Crystals

The photovoltaic and diffusion fields in nominally pure single crystals of stoichiometric composition (R = Li/Nb = 1) grown from the melt with 58.6 mol % of Li₂O (LiNbO₃ stoich), in the nominally pure single crystals of congruent composition (LiNbO₃), and in congruent single crystals doped with Cu²⁺, Zn²⁺, and Gd³⁺ are found from the parameters of the photoinduced light scattering indicatrix obtained with the use of a 60-mW He-Ne laser.     

Influence of porous morphology on optical dispersion properties of template free mesoporous titanium dioxide (TiO2) films

This paper focuses the influence of porous morphology on the microstructure and optical properties of TiO₂ films prepared by different sol concentration and calcination temperatures. Mesoporous TiO₂ thin films were prepared on the glass substrates by sol-gel dip coating technique using titanium (IV) isopropoxide. Porous morphology of the films can be regulated by chemical kinetics and is studied by scanning electron microscopy. The optical dispersion parameters such as refractive index (n), oscillator energy (E_d), and particle co-ordination number (N_c) of the mesoporous TiO₂ films were studied using Swanepoel and Wemple-DiDomenico single oscillator models. The higher precursor concentration (0.06 M), films exhibit high porosity and refractive index, which are modified under calcination treatment. Calcinated films of low metal precursor concentration (0.03 M) possess higher particle co-ordination number (N_c = 5.05) than that of 0.06 M films (N_c = 4.90) due to calcination at 400 °C. The lattice dielectric constant (E_∞) of mesoporous TiO₂ films was determined by using Spintzer model. Urbach energy of the mesoporous films has been estimated for both concentration and the analysis revealed the strong dependence of Urbach energy on porous morphology. The influence of porous morphology on the optical dispersion properties also has been explained briefly in this paper.     

Extended rigidity percolation and chemical thresholds in Ge-Te-Pb glasses as revealed by MDSC

Thermal analysis of Ge₂₀Te_80−xPb_x (2≤x≤8) glasses has been undertaken using modulated differential scanning calorimetry (MDSC). The compositional dependence of thermal parameters is investigated. The crystallization temperatures (T_c) estimated from the total heat flow show detectable changes at compositions x=4, 6.5 and 7.5. Further, the heat capacity change at the glass transition temperature, measured from the reversible heat flow curve (ΔC_p^R), is found to exhibit a maximum and an inflexion at compositions x=4 and 6.5 and minimum at x=7.5, respectively. Also, the relaxation enthalpy, estimated from the area under the non-reversing heat flow curve (ΔH^NR), exhibits similar features at the said compositions. From the observed MDSC results, it has been proposed that the compositions x=4 and x=6.5 denote to the onset and completion of rigidity percolation and x=7.5 corresponds to the chemical threshold of the system.     

Study on ethyl groups with two different orientations in [N(C2H5)4]2CuBr4

The crystal structure and phase transition temperature of [N(C₂H₅)₄]₂CuBr₄ are studied using X-ray diffraction and differential scanning calorimetry (DSC); measurements revealed a tetragonal structure and the two phase transition temperatures T_C of 204 K and 255.5 K. The structural geometry near T_C is discussed in terms of the chemical shifts for ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups are distinguishable by the ¹³C NMR spectrum. The molecular motions are discussed in terms of the spin–lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The T_1ρ results reveal that the ethyl groups undergo tumbling motion, and furthermore that the ethyl groups are highly mobile.     

The phase shifts leading to the broadening and shift of spectral lines

The classical theory of collisional broadening and shift parameters (β, δ) of an isolated spectral line was used to obtain simple analytical formulas for calculating both β and δ. These formulas were obtained on the assumption that the short range interaction is effective only in the broadening while the long range is effective in the shift of the spectral line. These parameters β and δ depend on the limiting phase shifts responsible for broadening η_b and shift ηδ. It was found that the values of η_b and η_δ are not equal to each other as was proposed by Weisskopf η_b=η_δ=1. The maximum and average values of η_b (η_b max, η_b av) and η_δ (η_δ max, η_δ av) were obtained by numerical evaluation, using different inverse power potentials. By introducing these parameters into the approximated formulas for β and δ using Van der Waals and Lennard-Jones potential, it was found that the results of calculations for (β and δ) with different atomic transitions perturbed by different inert gases are in close agreement with earlier results. Those results, obtained earlier, were based on the Lindholm-Foley theory especially with the average values of η_b [η_b av=0.6057] and the maximum values of η_δ [η_δ max=1.57625]. The impact parameters ρ_b and ρ_δ leading to the broadening and shift of the spectral line were also obtained for different interactions. It was found that the end parameter for the broadening ρ_b is not equal to the starting parameter for the shift ρ_δ.     

Study of potassium-zinc borophosphate glasses

Potassium-zinc borophosphate glasses were prepared and studied in two compositional series xK₂O-(50−x)ZnO-10B₂O₃-40P₂O₅ and xK₂O-(50−x)ZnO-20B₂O₃-30P₂O₅ with x=0, 10, 20, 30, 40 and 50 mol% K₂O. The replacement of zinc by potassium decreases the density and increases the molar volume of these glasses, whereas glass transition temperature and chemical durability decrease with increasing potassium content. Structural changes were studied by ¹¹B and ³¹P MAS NMR and Raman spectroscopy. The observed changes in the spectra and the properties of the studied glasses can be ascribed to several reasons and namely to the differences in the space occupied by cations Zn²⁺ and 2K⁺, the differences in the electronegativity of zinc and potassium and a large difference in the field strength of Zn²⁺ and K⁺ cations and thus higher ionicity of K-O bonds in comparison with Zn-O bonds.     

High-resolution photoemission study of FeSr2YCu2O7+δ

We have performed high-resolution photoemission spectroscopy (PES) on FeSr₂YCu₂O_7+δ, of which superconductivity of T_c=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δ_max) to be 10 meV at 15 K. This gap value gives 2Δ_max/k_BT_c∼5, suggesting a strong-coupling nature of superconductivity in FeSr₂YCu₂O_7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.     

RETRACTED ARTICLE: Density,viscosity and speed of sound of benzaldehyde with benzene at 303.15, 308.15, and 313.15 K

The values of density, viscosity and speed of sound for the binary liquid mixture of Benzaldehyde with Benzene were measured over the entire range of composition at 303.15, 308.15, and 313.15 K. These values are used to calculate the excess molar volume (V ^E), deviation in viscosity (Δη), deviation in speed of sound (ΔU), deviation in isentropic compressibility (Δβ _s ), excess internal pressure (Δπ), excess intermolecular free length (ΔL _f ), excess free volume (V _E ^f ) and excess acoustic impedance (ΔZ). McAllister’s three-body interaction model is used for correlating Kinematic Viscosity of binary mixtures. The excess values were correlated using the Redlich–Kister polynomial equation to obtain their coefficients and standard deviations. The thermophysical properties (density, viscosity, and speed of sound) under the study were fit to the Jouyban–Acree model.     

Structural Nature of 7Li and 11B Sites by Static NMR and MAS NMR in Nonlinear Optical Material LiCsB6O10

The relationship between structure and nonlinear optical properties in LiCsB₆O₁₀ is characterized using single-crystal nuclear magnetic resonance (NMR) and magic-angle spinning (MAS) NMR. Although the quadrupole parameters for B(1) and B(2) sites were obtained using single-crystal NMR, the T ₁ values for these atomic sites could not be distinguished in this way. Thus, the structural nature of lithium and boron sites in LiCsB₆O₁₀ was investigated using MAS NMR. B(1) and B(2) sites could be distinguished based on the spectrum and T _1ρ obtained from ¹¹B MAS NMR. In addition, the T ₁ and T _1ρ values and activation energies for ⁷Li and ¹¹B are compared. No significant changes were seen in the T _1ρ at the lithium and boron nuclei in LiCsB₆O₁₀.