Quantum-chemical features of Ln—O interatomic interactions in LnNbxOy (Ln=La, Ce, Pr, Nd, Pm, Sm, Eu) systems

Properties of the lanthanide—oxygen bonds in lanthanide niobates LnNb₇O₁₂(Ln=La, Ce, Pr, Nd, Pm, Sm, Eu) have been studied using the ab initio cluster X _α discrete variational method with two different basis sets of molecular orbitals. It has been found that the structural instability of lanthanides LnNb₇O₁₂ (Ln=Nd, Sm) observed in experiments is due to the combined effect of decrease in the stability of [LnO₈] cluster and increase in quantum-mechanical Ln—O repulsion when the minimum near neodymium (in the first model of the basis set of molecular orbitals) or praseodymium (in the second model of the basis set of molecular orbitals) atoms is passed.     

Crystal structure and electrical properties of GdxMn1 ? xS and TixMn1 ? xSe solid solutions

Regions of the existence of sulfide Gd _x Mn_{1 − x} S and selenide Ti _x Mn_{1 − x} Se solid solutions have been identified. Their electrical and thermoelectric properties have been studied in the temperature range 80–900 K. It has been established that the substitution of Gd²⁺ and Ti²⁺ ions for Mn²⁺ cations initiates reversal of the type of charge carrier with respect to the starting compounds MnS and MnSe. The cation substitution in solid solutions brings about a change from the hole conduction (α > 0) characteristic of the manganese monosulfide and monoselenide to the electronic conduction (α < 0).     

Synthesis and characterization of ZnxHg1?xSeyS1?y quantum dots

This article describes the synthesis of highly water-soluble Zn _x Hg_1−x Se _y S_1−y quantum dots (QDs) in aqueous solution through a simple photo-assisted reaction between ZnSe QDs and mercury(I) nitrate dihydrate [Hg₂(NO₃)₂·2H₂O]. In order to deduce the optimal synthesis conditions, we varied several parameters, including the concentrations of mercaptosuccinic acid (MSA) and Hg₂(NO₃)₂·2H₂O, the illumination time, and the reaction temperature. When irradiated at temperatures below 80 °C, the ZnSe QDs reacted with the S²⁻ ions formed rapidly from MSA and the Hg²⁺ ions formed from Hg₂ ²⁺ ions to form Zn _x Hg_1−x Se _y S_1−y QDs through a process of photo-etching and surface combination. Under different conditions, we prepared a series of Zn _x Hg_1−x Se _y S_1−y QDs that emit fluorescence at the maximum wavelengths ranging from 405 to 760 nm. Inductively coupled plasma-mass spectrometry and transmission electron microscopy/energy dispersive spectrometry revealed that the content of Hg in the Zn _x Hg_1−x Se _y S_1−y QDs was greater when the synthesis was conducted at higher temperature. The Zn_0.88Hg_0.12Se_0.44S_0.56 QDs exhibit improved photostability than crude ZnSe QDs and possess long lifetimes (τ₁ ~ 38 ns and τ₂ ~ 158 ns).     

Thermal conductivity of single crystals of Ca1 ? xErxF2 + x and Ca1 ? xTmxF2 + x solid solutions

The thermal conductivity of single crystals of Ca_{1 − x} Er _x F_{2 + x} (x = 0.01, 0.05, 0.07, and 0.10) and Ca_{1 − x} Tm _x F_{2 + x} (x = 0.02, 0.04, and 0.06) solid solutions is studied in the temperature ranges 50–300 and 298–673 K. With increasing content of rare-earth elements, the behavior of thermal conductivity in these solid solutions changes from the characteristic of defect single crystals to glasslike. The concentration dependences of thermal conductivity for the two systems differ insignificantly.     

Ab initio study of structural, electronic, optical, and vibrational properties of ZnxSy (x?+?y?=?2 to 5) nanoclusters

An ab initio study of the stability, structural, electronic. and optical properties has been performed for 46 zinc sulfide nanoclusters Zn _x S _y (x + y = n = 2 to 5). Five out of them are seen to be unstable as their vibrational frequencies are found to be imaginary. A B3LYP-DFT/6-311G(3df) method is employed to optimize the geometries and a TDDFT method is used for the study of the optical properties. The binding energies (BE), HOMO-LUMO gaps and the bond lengths have been obtained for all the clusters. For the ZnS₂, ZnS₃, and ZnS₄ nanoclusters, our stable structures are seen to be different from those obtained earlier by using the effective core potentials. We have also considered the zero point energy (ZPE) corrections ignored by the earlier workers. For a fixed value of n, we designate the most stable structure the one, which has maximum final binding energy per atom. The adiabatic and vertical ionization potentials (IP) and electron affinities (EA), charges on the atoms, dipole moments, optical properties, vibrational frequencies, infrared intensities, relative infrared intensities, and Raman scattering activities have been investigated for the most stable structures. The nanoclusters containing large number of S atoms for each n is found to be most stable. The HOMO-LUMO gap decreases from n = 2–3 and then increases above n = 3. The IP and EA both fluctuate with the cluster size n. The optical absorption is quite weak in visible region but strong in the ultraviolet region in most of the nanoclusters except a few. The optical absorption spectrum or electron energy loss spectrum (EELS) is unique for every nanocluster and may be used to characterize a specific nanocluster. The growth of most stable nanoclusters may be possible in the experiments.     

Charge state of manganese ions and the nonstoichiometry of Ca1 ? xLayMnO3 ? δ single crystals

The charge states and effective spins of manganese ions in nonstoichiometric single crystals of Ca_{1 − x} La _y MnO_{3 − δ} (x = 0, 0.05, 1) manganites have been determined from the exchange splitting of Mn 3s X-ray photoelectron spectra. The chemical composition of the manganite samples under investigation, including the concentration of oxygen, has been determined using the data of the X-ray microanalysis of cations. It has been found that the manganite single crystals under investigation have large defectiveness in both the cation and oxygen sublattices. The efficiency of the complex use of 3s photoelectron spectroscopy and X-ray microanalysis in the determination of the charge state of manganese ions and violations of stoichiometry in doped and defected manganites has been demonstrated.     

Effects of strontium and calcium simultaneous substitution on electrical and structural properties of Pb1?x?yCaxSryTiO3 thin films

Ferroelectric Pb_1−x−y Ca _x Sr _y TiO₃ thin films (denoted by PCST90, PCST70, and PCST30) were deposited on the Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. Their properties were investigated from the viewpoint of crystal structure, microstructure, dielectric, and ferroelectric properties. X-ray diffraction patterns revealed the formation of PCST90, PCST70, and PCST30 thin films without any secondary phases. Infrared and Raman spectroscopy results show that a gradual phase transition from tetragonal to pseudocubic or cubic perovskite structure may occur in PCST thin films with the simultaneous increase of Ca²⁺ and Sr²⁺ contents. Both substitution of isovalent Ca²⁺ and Sr²⁺ at Pb²⁺-site enhanced the dielectric constant and reduced the remnant polarization. In addition, ferroelectric test analyses show that the PCST thin films undergo a ferroelectric-to-paraelectric phase transformation with an amount of Pb²⁺, Ca²⁺, and Sr²⁺ at 30%, 35%, and 35% mol, respectively. Hence, the absence of ferroelectric property may be attributed to a decreasing of the octahedron distortion in the perovskite structure accompanied by a weakening of long-range ferroelectric order.     

Effect of the spin and valence states of cobalt ions on the kinetic properties of Ho1 ? xSrxCoO3 ? δ and Er1 ? xSrxCoO3 ? δ compounds

The electrical conductivity and Seebeck effect in ceramics based on cobaltites Ho_{1 − x} Sr _x CoO_{3 − δ} (x = 0.65, 0.75, 0.85, 0.95) and Er_{1 − x} Sr _x CoO_{3 − δ} (x = 0.75, 0.85, 0.95) with a perovskite-like structure have been investigated in the temperature range T > 77 K. All the compounds under study are characterized by the variable-range-hopping conductivity with the temperature dependence of the electrical resistivity corresponding to the Mott law. It has been found that, in the Ho_0.35Sr_0.65CoO_{3 − δ} compound, thermally excited Co³⁺ ions contribute to the electrical conductivity with an increase in temperature to 250 K. The Seebeck coefficient of the systems studied decreases as the strontium concentration and temperature increase. It has been shown that, for an adequate explanation of this behavior, proper allowance must be made for the splitting of the 3d levels, as well as for the charge disproportionation of the cobalt ions.     

Electrical conductivity and magnetic properties of La1 ? xCaxMn1 ? yFeyO3 ceramic samples (x = 0.67, y = 0, 0.05)

The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La_{1 − x} Ca _x MnO₃ with x = 0.67 (LCMO) and La_{1 − x} Ca _x Mn_{1 − y} Fe _y O₃ with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–10⁵ G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures T _CO ≈ 272 K for LCMO and T _CO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers p _eff = 4.83 μ_B (LCMO) and 4.77 μ_B (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < T _CO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and a _und for LCMO is a/a _und = 0.88. Original Russian Text ? V.S. Zakhvalinskiĭ, R. Laiho, T.S. Orlova, A.V. Khokhulin, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 1, pp. 61–68.     

Phase formation in near-morphotropic region of the PbZr1 ? xTixO3 system, structural defects, and electromechanical properties of the solid solutions

Ceramics of PbZr_{1 − x} Ti _x O₃ solid solutions were systematically studied in the range 0.37 ≤ x ≤ 0.57. The x-T phase diagram (T = 25°C) is constructed containing in single-phase regions isosymmetric states characterized by different concentration dependences of the structural and electrophysical parameters. There are also regions where these states coexist, with their unit cell volume remaining unchanged. The existence of these states is interpreted in terms of the real (defect) structure of the ceramics. Several reasons are indicated that are responsible for the appearance of the phase states and related to the crystallochemical peculiarities of the solid solutions: the alternate valence of titanium ions; the infinitely adaptive structure of titanium dioxide; and the formation, ordering, and rotation of the planes of crystallographic shear. It is shown that the transition from the rhombohedral to tetragonal phase occurs indirectly, through two intermediate phases with lower symmetry whose appearance is favored by the defects in the solid solutions. Original Russian Text ? L.A. Reznichenko, L.A. Shilkina, O.N. Razumovskaya, E.A. Yaroslavtseva, S.I. Dudkina, O.A. Demchenko, Yu.I. Yurasov, A.A. Esis, I.N. Andryushina, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 5, pp. 958–965.     

Electrical conductivity and chemical stability of perovskite-type BaCe0.8-xTixY0.2O3-δ

Here we report the synthesis, chemical stability, and electrical conductivity of Ti-doped perovskite-type BaCe_0.8-x Ti _x Y_0.2O_3-δ (x = 0.05, 0.1, 0.2, and 0.3; BCTY). Samples were synthesized by conventional solid state (ceramic) reaction from corresponding metal salts and oxides at elevated temperature of 1,300–1,500 °C in air. The powder X-ray diffraction confirmed the formation of a simple cubic perovskite-type structure with a lattice constant of a = 4.374(1), 4.377(1), and 4.332(1) ? for x = 0.05, 0.1, and 0.2 members of BCTY, respectively. Like BaCe_0.8Y_0.2O_3-δ (BCY), Ti substituted BCTY was found to be chemically not stable in 100% CO₂ and form BaCO₃ at elevated temperature. The bulk electrical conductivity of BCTY decreased with increasing Ti content and the x = 0.05 member exhibited the highest conductivity of 2.3 × 10⁻³ S cm⁻¹ at 650 °C in air, while a slight increase in the conductivity, especially at low temperatures (below 600 °C), was observed in humidified atmospheres.     

M?ssbauer study of compounds of Cu3 ? xFexSnS4 and Cu2Fe1 ?xZnxSnS4 systems

A M?ssbauer study of the structural and charge states of ⁵⁷Fe and ¹¹⁹Sn atoms in the compounds of Cu_{3 −x} Fe _x SnS₄ and Cu₂Fe_{1 − x} Zn _x SnS₄ systems was performed. It was shown that the iron atoms in the compounds of both systems were in the divalent and trivalent states occupying the tetrahedral positions of the structure. The character of the changes of the degree of covalency of the Fe²⁺-S, Fe³⁺-S and Sn⁴⁺-S bonds during the isomorphic substitution in the systems was established.     

Noncentrosymmetric cubic helical ferromagnets Mn1 ? yFeySi and Fe1 ? xCoxSi

Two systems of noncentrosymmetric cubic helical magnets Mn_{1 − y} Fe _y Si (y = 0.06, 0.08, 0.10) and Fe_{1 − x} Co _x Si (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.50) have been compared. The concentration dependences of the critical temperature and magnetic field have been obtained using small-angle polarized-neutron scattering and analyzed in the framework of the Bak-Jensen model. It has been established that, among the two interactions that play the main role in these systems, i.e., the isotropic symmetric ferromagnetic exchange and the Dzyaloshinskii-Moriya isotropic antisymmetric interaction, the former interaction determines the critical temperature in the Mn_{1 − y} Fe _y Si system and the latter interaction determines this temperature in the Fe_{1 − x} Co _x Si system.     

Ce3+在Ln2SO6中的发光(Ln=La,Gd或Y)

Ce3+在La2SO6中的发光光谱和激发光谱与在Gd2SO6和Y2SO6中的比较有显著差别。从被Ce3+敏化的中心的激发光谱上观察到在La基质中于300和330nm附近还有二个激发峰,显示出在这三个基质中Ce3+有很接近的激发态情况,从漫反射光谱也得到证实。不过在La2SO6中Ce3+的发光起源于次最低的而不是最低的5d能级。     

Ternary alloys CdyZn1 ? yO and MgxZn1 ? xO as materials for optoelectronics

Thin films of Cd _y Zn_{1 − y} O and Mg _x Zn_{1 − x} O (y = 0−0.35, x = 0−0.45) ternary alloys have been grown by pulsed laser deposition onto sapphire substrates. The record solubility limits of Cd (y = 0.3) and Mg (x = 0.35) have been achieved in hexagonal zinc oxide. The mismatch of the lattice parameters a of Cd_0.2Zn_0.8O and Mg_0.35Zn_0.65O does not exceed 1%; in this case, the band gap discontinuity is 1.3 eV. The surface roughness of the films does not exceed 2.5 nm at x = 0−0.27 and y = 0−0.20.     

Study of high-temperature oxygen permeability in Sr1 ? xLaxCo0.8 ? yNbyFe0.2O3 ? z perovskites

The oxygen permeability of ceramic SrCo_{0.8 − y} Fe_0.2Nb _y O_{3 − z} (0 ≤ y ≤ 0.2) and La_0.3Sr_0.7Co_0.6Fe_0.2Nb_0.2O_{3 − z} disc membranes as a function of temperature and oxygen partial pressure was studied. Kinetic analysis was performed based on the experimental data on oxygen permeability as a function of oxygen partial pressure.     

Structure, ferroelectric and piezoelectric properties of?(Bi0.98?xLa0.02Na1?x)0.5BaxTiO3 lead-free ceramics

Lead-free (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics have been prepared by an ordinary sintering technique and their structure, ferroelectric and piezoelectric properties have been studied. The results of X-ray diffraction show that La²⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.04<x<0.10. Compared with pure Bi_0.5Na_0.5TiO₃ ceramics, the (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics possess much smaller coercive field E _c and larger remanent polarization P _r. Because of the low E _c (3.38 kV/mm), large P _r (46.2 μC/cm²) and the formation of the MPB of rhombohedral and tetragonal phases, the piezoelectric properties of the ceramics are significantly enhanced at x=0.06: d ₃₃=181 pC/N and k _p=36.3%. The depolarization temperature T _d reaches a minimum value near the MPB. The ceramics exhibit relaxor characteristic, which is probably a result from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both polar and non-polar regions at the temperatures above T _d.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

Synthesis and study of photocatalytic activity of nanoscale quasi-one-dimensional Ti1 ? xVxO2 (0 ≤ x ≤ 0.13) and Zn1 ? xCoxO (0 ≤ x ≤ 0.3) oxides

A original method of synthesis of quasi-one-dimensional Ti_{1 − x} V _x (OCH₂CH₂O)₂ (0 ≤ x ≤ 0.13) and Zn_{1 − x} Co _x (HCOO)(HOCH₂CH₂O) (0 ≤ x ≤ 0.3) has been developed. The synthesis products were used as a basis for nanoscale extended Ti_{1 − x} V _x O₂ and Zn_{1 − x} Co _x O oxides. The compounds obtained and products of their thermolysis were investigated by X-ray diffraction, microscopy, IR spectroscopy, thermogravimetry, and chemical analysis; the shape and size of particles were determined by scanning electron microscopy. The good prospects of Ti_{1 − x} V _x O₂ and Zn_{1 − x} Co _x O solid solutions as photocatalysts for hydroquinone oxidation are shown.     

Characterization of Cd1?xMgxTe film and plasma diagnostic studies by optical emission spectroscopy during sputtering

Cd_1−x Mg _x Te is an attractive II–VI semiconductor alloy candidate for obtaining energy gaps wider than 1.5 eV of CdTe needed for the top junction absorber layer in a polycrystalline thin-film tandem solar cell. We have shown that these alloy films can easily be prepared by sputtering, however, the sputtering rate is substantially lower than for CdTe and the attainable cell performance has been poor, both before and after activation treatments with chlorine-containing vapors. In this work we have applied optical emission spectroscopy (OES) using selected peaks of ArII, CdII, MgI, TeII and correlated the results with crystallographic and morphological characteristics of the deposited films. Results were obtained as a function of rf sputter power and sputter gas pressure.