The role of structural and nonstructural water in oxyfluoride K<Subscript>2</Subscript>WO<Subscript>2</Subscript>F<Subscript>4</Subscript> · H<Subscript>2</Subscript>O

X-ray structural and polarization optical investigations have been performed, and birefringence and rotation angles of the optical indicatrix φ _b and φ _c of the K₂WO₂F₄ · H₂O crystal have been measured in the temperature range of 100–600 K. The structure and symmetry of compounds at room temperature have been refined. It has been established that the layered crystal K₂WO₂F₄ · H₂O can exist in two states (A and B) depending on the atmospheric humidity and undergoes the sequence of reversible and irreversible phase transformations G ₃ ↔ G ₂ → G ₁ → G ₀. The sequences of changes in the phase symmetry P [`1]\bar 1 ↔ C2/m → P4/nmm for samples A and m ↔ C2/m → P4/nmm for samples B have been found. The second-order proper ferroelastic phase transition (P [`1]\bar 1 ↔ C2/m) at T ₀₃ = 270–290 K (G ₃ ↔ G ₂) is accompanied by twinning and appearance of the shift deformation x ₆. The crystal system of the substance for the B crystals remains invariable after the second-order phase transition G ₃ ↔ G ₂. The irreversible first-order phase transition G ₂ → G ₁ occurs in a temperature range T ₀₂ ≈ 350–380 K; it is accompanied by the loss of the crystallization water, which then is reduced easily from the atmosphere for a day. The substance decomposes at T ₀₁ ≈ 510 K (G ₁ → G ₀). The distinction between the A and B crystals has been explained by the presence or absence of free water in interlayer spacings.     

Hyperfine Interactions in Pb<Superscript>3+</Superscript>F<Stack><Subscript>8</Subscript><Superscript>−</Superscript></Stack>F<Stack><Subscript>a</Subscript><Superscript>−</Superscript></Stack> clusters in fluorite crystals

The parameters of hyperfine interactions in Pb³⁺F ₈ ^? F _a ^? tetragonal clusters of MeF₂ crystals (Me=Ca, Sr, Ba) are interpreted. The contributions of the spin polarization to the parameters of the proper hyperfine interaction and additional (ligand) hyperfine interactions are calculated in the approximation of weak binding between a charge-compensating ion F _a ^? and a cubic fragment in the tetragonal cluster. It is demonstrated that correct inclusion of the contributions from the spin polarization to the ligand isotropic hyperfine interaction for the F _a ^? ion leads to anomalously large parameters of this interaction for MeF₂ crystals. These results are in agreement with experimental data.     

12.95 mW sixth harmonic generation with KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystal

We obtained the second harmonic of a frequency-tripled Nd:YVO₄ laser at wavelength of 177.3 nm with 12.95 mW output by using an optically contacted KBe₂BO₃F₂-CaF₂ prism-coupled device with a 2.1 mm thick KBe₂BO₃F₂ crystal, which is to our knowledge the best result at this wavelength. PACS 42.65.Ky; 42.70.Mp; 42.55.Xi     

Sequence of phase transitions in a quasi-one-dimensional β-Na<Subscript>0.33</Subscript>V<Subscript>2</Subscript>O<Subscript>5</Subscript> compound with variable valence

The thermal properties—specific heat, thermal conductivity, and thermal expansion coefficients—of a single crystal of quasi-one-dimensional variable-valence β-Na_0.33V₂O₅ compound were studied. With lowering temperature, it sequentially undergoes the structural (T _S ～ 230 K), charge (T _C ～ 136 K), and magnetic (T _N ～ 22 K) phase transitions. The structural transition at T _S , resulting in the ordering of the Na ions, and the charge ordering at T _C , resulting in the charge redistribution over the positions of V ions, are accompanied by the anomalies in the temperature dependences of all the studied properties. The magnetic ordering at T _N results in the appearance of the canted antiferromagnetic structure and manifests itself only in the anomaly in the temperature dependences of the thermal expansion coefficients.     

F<Stack> <Subscript>3</Subscript> <Superscript>−</Superscript> </Stack> molecular ions in fluoride crystals

The UV absorption spectra of F₃^? molecular ions in LaF₃, SrF₂, CaF₂, and BaF₂ crystals doped with rare-earth elements are studied. Comparison of radiation-colored and additively colored crystals reveals the absorption bands of F₃^? hole centers in the region near 6 eV. Nonempirical calculations of optical transitions agree well with experimental results.     

Charge dynamics in quasi-one dimensional β-Sr<Subscript>1/6</Subscript>V<Subscript>2</Subscript>O<Subscript>5</Subscript>

Polarized infrared reflectivity was measured as a function of temperature on a quasi-one dimensional β-Sr_0.17V₂O₅ single crystal. Along the conduction direction, the optical conductivity exhibits a weak electronic background in the far infrared and a large mid infrared band. Sum rule analysis shows that both far infrared and midinfrared bands arise from 3d vanadium electrons. As temperature is decreased, the mid infrared band slightly narrows and redshifts, but does not exhibit drastic change. In contrast, the spectral weight grows at low frequencies and peaks appear in the phonon energy range. In the transverse direction, β-Sr_0.17V₂O₅ shows an insulator-like behaviour at all temperatures. The optical response is compared with the iso-electronic and iso-structural compound β-Na_0.33V₂O₅. Although spectra are very similar in both compounds at room temperature, their temperature dependence differs. However, some general trends indicate that charge carriers are of the same nature in both compounds. The results suggest that electron- phonon coupling is relevant to explain the optical properties of β-Sr_0.17V₂O₅. The optical response along the conducting direction is assigned to an adiabatic small polaron absorption, in intermediate or strong electron-phonon coupling regime. In this framework, the optical conductivity is in qualitative agreement with recent Dynamical Mean Field Theory results over a wide temperature range.     

Current dependent fluctuations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6+δ</Subscript> thin film

The current dependence of the excess conductivity is measured up to ≃3T_c for a Bi₂Sr₂CuO_6+δ thin film, as a function of doping. It is found to be anomalously sensitive to the transport current and to behave as a universal function of T/T_c in the whole doping range. We discuss these results in the perspective of a granular superconductor with a gapless-like behavior.     

O<Subscript>2</Subscript> photodesorption from AuO<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> and Au<Subscript>2</Subscript>O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>

Using time-resolved photoelectron spectroscopy, the decay channels of AuO₂⁻ and Au₂O₂⁻ following photoexcitation with 3.1-eV photons have been studied. For AuO₂⁻, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au₂O₂⁻, the spectra indicate O₂ desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au _n O₂⁻ with analogous data on Ag _n O₂⁻ clusters, a discernible pattern emerges: for dissociatively bound O₂(AuO₂⁻, Ag₃O₂⁻), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au₂O₂⁻, Ag₂O₂⁻, Ag₄O₂⁻, Ag₈O₂⁻), the 3.1-eV photoexcitation triggers fast O₂ desorption with a high quantum yield.     

Effect of hydrostatic pressure on the superconductivity in fluorinated Hg<Subscript>0.8</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3.2</Subscript>O<Subscript>8+δ</Subscript>

The effect of hydrostatic pressure on the superconducting transition temperature was measured for the Hg-1223 phase of a fluorinated mercury cuprate high-temperature superconductor with T_c(optim)=38 K. The value of the T_c derivative with respect to pressure was found to be rather high (11.0 K/GPa); at P=1.5 GPa, T_c=153.5 K. The results obtained are discussed in connection with works on the synthesis of such samples.     

Cluster spin glass and superparamagnetism in RuSr<Subscript>2</Subscript>Eu<Subscript>1.5</Subscript>Ce<Subscript>0.5</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10-</Subscript><Subscript>δ</Subscript>

We investigate in detail the dc magnetization and nonlinear ac susceptibility behavior of the superconducting ferromagnet RuSr₂Eu_1.5Ce_0.5Cu₂O_{10- δ} (Ru1222) to develop a comprehensive understanding of the spin glass and superparamagnetism in this material. The structural properties of the system result in the formation of magnetic (ferromagnetic) clusters of different sizes, shapes and properties. The magnetic clustering of the system leads to observation of various features in dc magnetization and ac susceptibility consistent with superparamagnetism and cluster spin glass states, which can coexist or stand alone, depending on the temperature range considered. Experimental results of magnetic measurements in combination with their analysis have enabled us to explain and distinguish these phenomena, as well as to propose a temperature dependent scenario of the system behavior.     

Specific heat of the [NH<Subscript>2</Subscript>(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystal in the temperature region 80–300 K

The specific heat of [NH₂(CH₃)₂]₂ZnCl₄ was measured calorimetrically in the temperature region 80–300 K. As the temperature T decreases, the C _p (T) dependence indicates a phase transition sequence, with the phase transition at T₆=151 K observed for the first time. The thermodynamic characteristics of the crystal were refined. The transformation occurring at T₂=298.3 K is shown to be an incommensurate-commensurate phase transition.     

High-Temperature Heat Capacity of Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>–Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> Solid Solutions

Differential scanning calorimetry has been used to study the influence of temperature on the heat capacity of synthesized vanadates Zn₂V₂O₇, (Cu_0.56Zn_1.44)V₂O₇, and (Cu_1.0Zn_1.0)V₂O₇. It is found that dependences C_p = f(T) have extremes. The thermodynamic properties of Zn₂V₂O₇ have been determined.     

Features of the structure and properties of β-FeSi<Subscript>2</Subscript> nanofilms and a β-FeSi<Subscript>2</Subscript>/Si interface

The electronic, geometric, and magnetic structure of nanofilms of the β phase of iron disilicide FeSi₂ with the (001), (100), and (010) surfaces have been simulated through density functional calculations. A substantial reconstruction of the (001) surface terminated with silicon atoms has been observed, which was accompanied by an increase in the surface symmetry and appearance of “squares” of silicon atoms. Analysis of the electron density of states (DOS) and spin DOS projected on the contributions of layers of atoms (LSDOS) indicates that all plates have metallic properties. The main contribution near the Fermi level comes from the surface iron layers and it decreases rapidly with an increase in the distance from the surface of the plate. Analysis of the calculated effective magnetic moments of atoms shows that the surface layers in the plates have a significant magnetic moment, in particular, iron layers on the (001) surface (1.89 μ_B/atom). The moments of atoms decrease rapidly with an increase in their distance from the surface. The electron and geometric regions of a (001)Si/FeSi₂ interface have been studied. Analysis of the LSDOS shows that the surface conducting state mainly determined by the contribution from the near-surface silicide layers is implemented in this region. The possibility of the formation of the perfect and sharp Si/FeSi₂ interface has been demonstrated.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Determination of Montelukast in Plasma Using β − Cyclodextrins Coated on CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Magnetic Nanoparticles in Luminol–H<Subscript>2</Subscript>O<Subscript>2</Subscript> Chemiluminescence System Optimized by Doehlert Design

A novel chemiluminescence method using β ? cyclodextrins coated on CoFe₂O₄ magnetic nanoparticles is proposed for the chemiluminometric determination of montelukast in plasma. The effect of coated β ? cyclodexterinon CoFe₂O₄ magnetic nanoparticles in the chemiluminescence of luminol–H₂O₂ system was investigated. It was found that β ? cyclodexterin coated on CoFe₂O₄ magnetic nanoparticles could greatly enhance the chemiluminescence of the luminol–H₂O₂ system. Doehlert design was applied in order to optimize the number of experiments to be carried out to ascertain the possible interactions between the parameters and their effects on the chemiluminescence emission intensity. This design was selected because the levels of each variable may vary in a very efficient way with few experiments. Doehlert design and response surface methodology have been employed for optimization pH and concentrations of the components. Results showed under the optimized experimental conditions, the relative CL intensity (ΔI) is increased linearly in the concentration range of 0.003–0.586 μgml^?1 of montelukast with limit of detection (LOD) 1.09 × 10^?4 μgml^?1 at S/N ratio of 3, limit of quantitative (LOQ) 3.59 × 10^?4 μgml^?1 and the relative standard deviation 2_.63 %. The method has been successfully applied to the determination of montelukast in plasma of human body. Results specified that relative chemiluminescence intensity (ΔI) has good proportional with the montelukast concentration with R² = 0.99979. The test of the recovery efficiency for known amounts of montelukast was also performed, the recoveries range obtained from 98.2 to 103.3 %, with RSDs of <4 % indicated that the proposed method was reliable.     

Tunneling spectra of submicron Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+</Subscript><Subscript>δ</Subscript> intrinsic Josephson junctions: evolution from superconducting gap to pseudogap

The electronic, structural, and magnetic properties of Ru and Rh thin films on Ag(001) substrate are investigated by means of density functional calculations. The generalized gradient approximation is used to treat the exchange correlation potential. Alloying, burying, and fully relaxing effects are considered for different degrees of coverage: 0.25, 0.50, 1 and 2 ML. Alloying and burying effects reduce the magnetic moments while fully relaxing effects enlarge them. For Ru, the magnetic moment is high for 0.25 ML and vanishes for 2 ML; however, for Rh, the magnetic moment remains high even for 2 ML. Nevertheless, when cluster formation is analysed we conclude that the absence of magnetism in a number of previous experimental works could be attributed to the formation of big size clusters.     

Effect of γ irradiation on the thermochromic phase transition in [(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystals (as derived from heat capacity measurements)

The heat capacity of [(C₂H₅)₂NH₂]₂CuCl₄ crystals, both nonirradiated and γ-irradiated to a dose of 10⁷R, was studied in the temperature interval 90–330 K by adiabatic calorimetry. The temperature dependence of C_p(T) was found to have a peak-shaped anomaly in the region of the thermochromic phase transition (PT) at T = 322.7 K. Smoothened experimental heat capacity data were used to calculate the changes in the thermodynamic functions. The changes in the entropy and enthalpy of the thermochromic PT were determined to be ΔS = 42 J K^?1 mol^?1 and ΔH = 13653 J mol^?1 for the nonirradiated crystals and ΔS = 39 J K^?1 mol^?1 and ΔH = 12120 J mol^?1 for the irradiated crystals, respectively. Irradiation of a [(C₂H₅)₂NH₂]₂CuCl₄ crystal by γ rays to a dose of 10⁷ R was shown to shift the PT point toward lower temperatures by ΔT ≈ 1.7 K.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.