The band electronic structures of (BEDTTTF)2X,X− = BrO4− and ReO4−

The strictly isostructural organic metals, (BEDTTTF)₂ReO₄ and (BEDTTTF)₂BrO₄ consist of sheets of BEDT-TTF (abbreviated ET) donor molecules which form a novel two-dimensional network. Despite the similar structures these salts possess different physical properties. (ET)₂ReO₄ is a highly anisotropic metal down to 80 K but becomes insulating below this temperature. An applied pressure greater than 4.5 kbar reportedly suppresses the MI transition and gives superconductivity at 2 K. However, no superconductivity has yet been observed for (ET)₂BrO₄. Band electronic structure calculations derived from their ET molecule network geometries at 298 and 120 K, and intrastack and interstack energies between nearest neighbor ET molecules, show that (ET)₂ReO₄ and (ET)₂BrO₄ are highly anisotropic metals, and both possess very similar band structures.     

Stability,metastability and spectroscopic constants of electronic states of NH− and CH−

Highly correlated ab initio methods were used in order to calculate potential energy curves (PECs) of the bound electronic states of CH^? and NH^? anions and the long-range parts of their excited states. The spin–orbit interaction between electronic states has been calculated for the cases in which the couplings were assumed to be responsible for perturbations. The spectroscopic constants of bound states were calculated from the PECs and compared with previous theoretical and/or available experimental values. Also, from the ground states of CH/CH^? and NH/NH^?, the adiabatic electron affinities were given and discussed.     

Structural,electronic, optical and thermodynamic properties of NaxRb1−xH and NaxK1−xH alloys

A theoretical study of the structural, electronic, optical and thermodynamic properties of Na_xRb_1?xH and Na_xK_1?xH ternary alloys in NaCl phase has been carried out using the first-principles method. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependences on the composition of the lattice constant, band gap, dielectric constant, refractive index, Debye temperature, mixing entropy and heat capacities were analyzed for x=0, 0.25, 0.50, 0.75 and 1. The lattice constants of Na_xRb_1?xH and Na_xK_1?xH exhibit a marginal deviation from Vegard's law. A strong deviation of the bulk modulus from linear concentration dependence was observed for both alloys. We found that the composition dependence of the energy band gap is highly non linear and the large bowing coefficient for Na_xRb_1?xH is sensitive to the composition. Using the approach of Zunger and co-workers, the microscopic origins of the gap bowing were detailed and explained. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds, which is a support for those of the ternary alloys that we report for the first time.     

Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Structure and relaxor behavior of BaBi4Ti4−xZrxO15 ceramics

BaBi₄Ti_4−xZr_xO₁₅ with x = 0.1, 0.2, 0.3 and 0.5, has been synthesized via modified solid state reaction route. X-ray diffraction studies confirmed the formation of single phase Zr⁴⁺ substituted BaBi₄Ti₄O₁₅ up to x = 0.2. ZrO₂ and Bi₂O₃ based impurity phases were found at x = 0.3 and 0.5 substitutions. However, Rietveld refinement showed the increase in lattice parameters of BaBi₄Ti₄O₁₅ up to x = 0.5 substitutions. A broad dielectric peak associated with frequency dependence dielectric maximum temperature was observed at low substitutions. Relaxor behavior was suppressed at x = 0.5 substitution. A broadening and shifting of permittivity-temperature peak was found for the substitution. The high temperature slopes of dielectric peaks were analyzed by quadratic law for relaxors. The degree of relaxation and phase transformation diffusiveness were investigated at different substitutions.     

Synthesis,microstructure and dielectric properties of (1−x)PSN−xPLuN

Ceramic lead niobates and their solid solutions PSN–PLuN (pure lutecium niobate) were synthesized by solid state reactions. The sequence of phases formed at PSN–PLuN synthesis has been studied by X-ray analysis. Their symmetry changed from rhombohedral for PSN to pseudo-monoclinic for the 0.75PSN–0.25PLuN compositions. The performed EDS investigations revealed that the samples PSN–PLuN are perfectly sintered. They contain a little glassy phase and their grains are well shaped. The increase of lutecium content in the examined solid solution caused downward shift of the temperature of the phase transition. The decrease of the achieved permittivity values ? was observed as well.     

Anharmonicity correction and potential constants: BH 4 − isotopic ions

Normal coordinate analysis for XY₄ type molecules of T symmetry has been carried out by the Wilson F — G matrix method. Following Dennison and making use of the spectral data the anharmonicity factors for B'H ₄ ^– , B'D ₄ ^– , B¹⁰H ₄ ^– and B¹⁰D ₄ ^– have been calculated and potential constants have been evaluated for borohydride isotopic ions.     

Vibrational-electronic coupling in IrF6, OsCl6 2− and IrCl6 2−

The results obtained in a previous paper [1] for an unsymmetric regular model are applied to solid hydrogen at low temperatures when the lattice structure and the quadrupole-quadrupole interaction make the interaction energies non-isotropic. It is shown that, as for the lattice structures with isotropic interactions, no second-order transition is likely to occur through a cooperative rotational effect. The possible occurrence of spatial ordering on sub-lattices is discussed. It is found that such ordering is unlikely to occur at low temperatures on the face-centred cubic lattice, but that on the hexagonal close-packed lattice (which is the probable crystal structure) there is a second-order transition to an ordered state. Using a zeroth-order approximation the temperature at which this transition occurs in pure ortho-hydrogen is found to be 5·8°k.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

Decay τ − → φπ − ν τ and ω−φ mixing

The decay process τ ^? → φπ ^? ν _τ is investigated on the basis of the method of chiral phenomenological Lagrangians. It is shown that the calculated value of the decay probability is very sensitive to variations in the angle of ω?φ mixing. The resulting value of this probability is compared with available experimental data and with the results of other theoretical calculations.     

Sol–gel synthesis,magnetic and magneto-optical properties of CoFe2−xTbxO4 nanocrystalline films

The nanocrystalline thin films of terbium-doped cobalt ferrite were fabricated by a sol–gel method, and the effects of crystallization conditions on the phase, morphology, magnetic and magneto-optical (MO) properties of products were investigated. Due to its large radius, the doping content, x, of Tb³⁺ ion inside cobalt spinel cannot exceed 0.2. The CoFe_2−xTb_xO₄ films consist of the grains with the average size smaller than 50 nm even annealed up to 800°C. Saturation magnetization, coercive force and MO rotation are strongly dependent on the annealing temperature.     

Quantum-chemical study of structural, spectral, and electrooptical parameters of fluorosilanes SiH4− x Fx(x=0–4)

The possibility of calculating the molecular structure, vibrational frequencies, and electrooptical parameters of the silane molecule SiH₄ and the silicon tetrafluoride molecule SiF₄ is analyzed by using the ab initio quantum-chemical method (the MP2 perturbation theory) and the density functional theory (the B3LYP functional) with a wide variation of basis sets up to the cc-pV5Z set. The results obtained are used as reference values for studing mixed fluorosilanes SiH_4?x F_x (x=1–3) and for refining their geometry, the vibrational frequencies, and the absolute intensities of absorption bands. Based on the results of high-level quantum-chemical calculation corrected against the experimental data for SiH₄ and SiF₄, the structural parameters of mixed flouro-silanes are estimated and their vibrational frequencies are calculated and assigned for the first time. The quantum-chemical estimates of the absolute intensities of IR spectra are for the first time presented for all mixed fluorosilanes.     

Pathologies of the large-N limit for RPN−1, CPN−1, QPN−1 and mixed isovector/isotensor σ-models

We compute the phase diagram in the N→∞ limit for lattice RP^N−1, CP^N−1 and QP^N−1 σ-models with the quartic action, and more generally for mixed isovector/isotensor models. We show that the N=∞ limit exhibits phase transitions that are forbidden for any finite N. We clarify the origin of these pathologies by examining the exact solution of the one-dimensional model: we find that there are complex zeros of the partition function that tend to the real axis as N→∞. We conjecture the correct phase diagram for finite N as a function of the spatial dimension d. Along the way, we prove some new correlation inequalities for a class of N-component σ-models, and we obtain some new results concerning the complex zeros of confluent hypergeometric functions.     

Oxygen non-stoichiometry, redox thermodynamics, and structure of LaFe1 − x Co x O3 − δ

The oxygen non-stoichiometry and redox thermodynamic properties of the LaFe_{1 ? x} Co _x O_{3 ? δ} system (x?=?0.25 and 0.75) are studied. At low temperatures, the LaCoO₃ and LaFeO₃ systems show partial solid solubility. At 1,273 K (in air), both compounds are single phases and are orthorhombic and rhombohedral for x?=?0.25 and 0.75, respectively. Thermogravimetry has been used to measure the oxygen non-stoichiometry versus oxygen partial pressure at three temperatures, 1,223, 1,273, and 1,323 K. Redox thermodynamic quantities are extracted directly from the oxygen non-stoichiometry curves. The extracted enthalpies of oxidation do not vary significantly with stoichiometry, and for x?=?0.25 and 0.75, they are ?640?±?60 and ?440?±?60 kJ (mol O₂)^?1, respectively. Ideal solid solution thermodynamic models are used to analyze the redox mechanisms.     

Domain structure and phase transitions of LiKSO4:EPR of SO−4 centers

Analysis of the EPR of SO⁻₄ centers in LiKSO₄ crystals indicates that the SO⁻₄ centers are associated with local positive-ion vacancies. Twin-domains of the 〉110〈 types are observed to be temperature-dependent. The anomaly at 215 K on cooling is associated with a sudden growth of the twins and the anomaly at 190 K is associated with a p6₃ to p6₃P6,mc structural phase transition.     

Structure and magnetic properties of the intermetallic La2Co17−xMox (x=0.5, 1, 1.5, 2) and La2Co16−yFeyMo (y=0, 1, 2, 3, 4, 6) compounds

The effect of Mo and Fe atoms on the crystal structure and magnetic properties of the intermetallic La₂Co_17−xMo_x (x=0.5, 1, 1.5, 2), and La₂Co_16−yFe_yMo (y=0, 1, 2, 3, 4, 6) compounds have been studied by X-ray diffractometry, magnetic measurements and Mössbauer spectroscopy. All samples belong to the rhombohedral Th₂Zn₁₇-type structure and their lattice parameters a and c increase both with Mo and Fe content. From the La–Co–Mo samples only the one with x=0.5 presents planar anisotropy, whereas from the La–Co–Fe–Mo samples only the y=1 has uniaxial anisotropy. The magnetization M_S and the Curie temperature T_C decrease upon Mo substitution, whereas the anisotropy field H_A does not change significantly. In the Fe-substituted compounds M_S increases, but the Curie temperature increases slightly for 0⩽y⩽4 but decreases in y=6. The low temperature M–T curve shows that the samples La₂Co_16.5Mo_0.5, and La₂Co₁₀Fe₆Mo present a spin reorientation transitions at 70 and 260 K, respectively. Mössbauer samples were obtained for all Fe-containing samples in the temperature range 20–300 K. Above 260 K a jump in the values of the hyperfine fields and quadrupole splitting parameters is observed which can be associated to the spin reorientation.     

Studying the dielectric, electromechanical, and elastic properties of K1 − x (NH4) x H2PO4 mixed crystals

Temperature dependences of the longitudinal permittivity, piezoelectric coefficient d ₃₆, and elastic constant c ₆₆ ^E of K_{1 ? x} (NH₄) _x H₂PO₄ mixed crystals are studied experimentally. A microscopic model is proposed for crystals of the K_{1 ? x} (NH₄) _x H₂PO₄ type that includes the piezoelectric contribution to the effective pseudospin cluster Hamiltonian. Dielectric elastic and electromechanical properties of these crystals with ammonium concentrations x below 0.40 are calculated in a wide temperature range using the cluster approach. The calculation results are in qualitative agreement with experimental data.