The effect of spin dilution on magnetism of the linear chain system β-Cu2?xZnxV2O7

We have measured the magnetic susceptibility (χ) and heat capacity (C _p) of β-Cu_2−x Zn _x V₂O₇ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 2) in the temperature range 2–300 K. A one-dimensional alternating exchange Heisenberg antiferromagnetism (HAF) is observed in all compositions with chains of infinite length. The intra-chain exchange remains uniform and decreases marginally with dilution of the magnetic state. A cooperative ordering is seen in the magnetic chains for all Zn concentrations (x ≤ 0.3). The temperature of occurrence of this transition decreases with increasing Zn concentration. Though the conventional spin-wave theory has been used here to describe the properties of the ordered phase, the presence of some contributions like the lattice heat capacity in C _p and the Curie-Weiss term in susceptibility introduces some uncertainties in the estimation of the proportions contributed by the spin system. Therefore, the nature of the ordered phase could not be ascertained unambiguously.     

Electronic properties of LiMn2?xTixO4

Ti-substituted LiMn₂O₄ (LiMn_2−x Ti _x O₄, x=0, 0.15, 0.30, 0.45, 0.60, and 0.75) has been synthesized using solid-state reactions. Their crystal and electronic structures were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD data suggested that the lattice parameters of LiMn_2−x Ti _x O₄ increase due to the replacement of Mn by Ti ions. XPS results indicated that the substituted Ti ions were in +4 oxidation state; consequently, the normal oxidation state of Mn ions has been detected by measuring the binding energy splitting of Mn 3s states, which decreases with the content of substituted Ti. The valence band spectra suggested that the intensity of e _g level of Mn 3d orbitals increased due to the increase of the Mn³⁺/Mn⁴⁺ ratio.     

Ion-electron transfer in solid solutions Rb2 ? 2xFe2 ? xPxO4

The rubidium monoferrite RbFeO₂-based solid solutions with the composition Rb_{2 − 2x} Fe_{2 − x} P _x O₄ have been synthesized, and their crystal structure and the temperature and concentration dependences of the total and electron conductivities have been studied. The introduction of P⁵⁺ ions has been found to sharply decrease the electron conductivity that prevails in pure rubidium monoferrite and, at the same time, to increase the ionic conductivity. The latter becomes dominant as the phosphorus concentration increases. The maximum rubidium-cation conductivity of the materials under study is ∼3 × 10⁻² S/cm at 300°C and ∼3 × 10⁻¹ S/cm at 700°C. The results have been compared with the previously obtained data for similar solid solutions based on rubidium monogallate and monoaluminate.     

Correlation of magnetic properties with the local features of the electronic and crystal structure in the Ce2Fe17 ? xMnx intermetallide: XAFS data analysis

The rearrangement of the local environment of cerium has been examined using extended X-ray absorption fine structure spectroscopy above the K-Ce absorption edge for various manganese contents and temperatures in Ce₂Fe_{17 − x} Mn _x intermetallics (x = 0, 1, 2). The valence state of cerium has been studied simultaneously by X-ray absorption near-edge structure spectroscopy above the L ₃-Ce absorption under the same conditions. A correlation has been revealed between the changes observed in the local electronic and crystal structure of Ce₂Fe_{17 − x} Mn _x compounds and the types of magnetic states in them.     

Structures and magnetic anisotropy of β-Mn2V2O7 crystals synthesized by the molten salt method

β-Mn2V2O7 crystals with strip shape are successfully prepared by the molten salt method in a closed crucible,and are characterized by x-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM),selected area of electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM).The results indicate that the sample is of the β-Mn2V2O7 crystal with monoclinic symmetry,level natural cleavage facets and directional growth.Magnetic properties are measured by vibration sample magnetometry (VSM) at room temperature,and the magnetic hysteresis loop indicates that the β-Mn2V2O7 has anti-ferromagnetic properties with low coercive force and remnant magnetization.The magnetic measurement results in different directions exhibit that the β-Mn2V2O7 has magnetic anisotropy,which is due to the fact that the magnetic interaction energy of the β-Mn2V2O7 is lowest only when the electron configuration is in a certain direction.     

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.     

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.     

Low-field magnetization of ludwigites Co3O2BO3 and Co3 ? xFexO2BO3 (x ≈ 0.14)

Ludwigite single crystals of compositions Co₃O₂BO₃ and Co_{3 − x} Fe _x O₂BO₃ (x ≈ 0.14) have been synthesized. The crystal structure is investigated at room temperature, and the magnetization is studied in the temperature range T = 4.2–100 K in magnetic fields of up to 600 Oe. The orthorhombic symmetry is revealed, and the unit cell parameters are determined. A number of features are established for the temperature dependence of the magnetization. In unsubstituted Co₃O₂BO₃, two magnetic transitions are found at T _C1 = 43 K and T _C2 = 15 K. At temperatures below 40 K, spin-glass state is revealed. Substitution of iron ions for cobalt ions leads to a noticeable shift in the magnetic transitions toward the high-temperature range: T _C1 = 83 K and T _C2 = 74 K. A ferromagnetic ordering of the P type is found in the Co_{3 − x} Fe _x O₂BO₃ (x ≈ 0.14) compound. Original Russian Text ? N.V. Kazak, N.B. Ivanova, V.V. Rudenko, A.D. Vasil’ev, D.A. Velikanov, S.G. Ovchinnikov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 5, pp. 916–919.     

Mechanism of NdSr2 ? xCaxMn2O7 (0 ≤ x ≤ 2.0) solid solution formation

The mechanisms of formation of NdSr_{2− x} Ca _x Mn₂O₇ (0 ≤ x ≤ 2.0) solid solution have been studied. The dependence of its volume magnetic susceptibility on the calcium concentration is obtained. The stability of this solid solution is compared with that of similar solid solutions in lanthanum-containing systems.     

Specific features of the structural and magnetic states of a Zn1 ? x Ni x Se crystal ( x = 0.0025) at low temperatures

The magnetic state and the structure of a Zn_{1 − x} Ni _x Se (x = 0.0025) bulk crystal were studied at low temperatures. It is revealed that the magnetic and crystal structures below T ≅ 15 K are dependent on the cooling rate of this dilute semiconductor. For example, on fast cooling to 4.2 K, about 10% hexagonal ferromagnetic phase is formed in the crystal. During heating, the phase disappears at T ≅ 15 K. The results obtained are discussed with allowance for the specific features of the Jahn-Teller distortions in this compound. Original Russian Text ? S.F. Dubinin, V.I. Sokolov, V.D. Parkhomenko, S.G. Teploukhov, N.B. Gruzdev, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 12, pp. 2188–2191.     

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.     

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.     

Profile of the refractive index distribution over the depth of (HfO2)x(Al2O3)1 ? x films

Capabilities of the method of laser null ellipsometry for the study of optical parameters of thin films of multicomponent alloys are demonstrated. Films of alloys (HfO₂) _x (Al₂O₃)_{1 − x} on single-crystal silicon are obtained by the chemical vapor deposition with the use of the following volatile compounds: hafnium dipivaloylmethanate (IV) and aluminum acetylacetonate (III). The selection of initial sets for their optical parameters is justified. The selected model is shown to correspond to experimental data. In films deposited from the mixture of precursors, the refractive index increases from a substrate to a film surface. Due to the separation of sources, films described by a single-layer model were deposited. This fact is indicative of the homogeneous distribution of components over the film thickness.     

Simulation of the electronic structure of simple oxides BeO and SiO2 and complex oxides Be2SiO4 and Be2SixGe1 ? xO4 with the phenacite structure

The ab initio numerical calculations of the electronic structure of simple oxides BeO and SiO₂ and complex oxides Be₂SiO₄ and Be₂Si _x Ge_{1 − x} O₄ with the phenacite structure have been performed using the electron density functional theory. The calculations indicate that the main feature of the systems under investigation is the presence of oxygen states in both the valence and conduction bands. The splitting of the bottom of the conduction band has been revealed in the electronic structure of the Be₂Si _x Ge_{1 − x} O₄ system. The splitting width is about 1.5 eV. The main contribution to the formation of a narrow subband of the conduction band comes from the 2s and 2p states of oxygen and the 4d state of germanium. Microscopic models of the spatial localization of the electron density on lower energy states of the conduction band of oxide crystals have been developed using the Wannier function technique. The reflection spectra of BeO, SiO₂, and Be₂SiO₄ have been analyzed. The reported calculations of the electronic structure imply the exciton nature of the 9.7-eV reflection peak in the Be₂SiO₄ crystal.     

Optical properties of the HfO2 ? x N x and TiO2 ? x N x films prepared by ion beam sputtering

Optical characteristics of the HfO_{2 − x} N _x and TiO_{2 − x} N _x films obtained by reactive ion beam sputtering have been investigated by spectral ellipsometry. The chemical composition of the films was determined using X-ray photoelectron spectroscopy. The nitrogen content in the oxynitride films (determined by the N₂/O₂ ratio in the gas mixture during synthesis) reached ≈9 at % for TiO_{2 − x} N _x and ≈ 6 at % for HfO_{2 − x} N _x . It is found that the dispersion relations n(λ) and k(λ) for the TiO_{2 − x} N _x films change from those characteristic of titanium dioxide to those typical of titanium nitride with an increase in the nitrogen content from 0 to ≈9 at %. The optical parameters of the HfO_{2 − x} N _x films depend weakly on the nitrogen content in the range 0–6 at %. Original Russian Text ? V.V. Atuchin, V.N. Kruchinin, A.V. Kalinkin, V.Sh. Aliev, S.V. Rykhlitskiĭ, V.A. Shvets, E.V. Spesivtsev, 2009, published in Optika i Spektroskopiya, 2009, Vol. 106, No. 1, pp. 77–82.     

Dynamics and distribution of doped holes in the CuO2 plane of slightly doped antiferromagnetic YBa2(Cu1 ? zLiz)3O6 + x (x < 0.1) studied by Cu(1) NQR

Incidence of doped holes in the CuO₂ plane on the AF state was studied by Cu(1) nuclear quadrupole resonance (NQR) in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} compounds. Inhomogeneous distribution of doped holes in the plane was detected in the low temperature measurements of transverse (1/T ₂) and longitudinal (1/T ₁) relaxation rates. We establish that at lower T the holes motion slows down and we estimate that the holes localize finally in restricted regions (∼3 lattice constants) in the Coulomb potential of the Li+ ions. Also we compared the hole behavior in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} samples with that in slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆. A stronger trapping potential of the in-plane Li⁺ impurities was concluded as compared to slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆ compound with out-of-plane Ca²⁺ impurities.     

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.     

M?ssbauer and X-ray studies of the structural phase transformations and suppression of polymorphism in La1 ? xSrxMn0.98Fe0.02O3 + δ (x = 0.05?0.30)

The specific features of the structural transformations in La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} (x = 0.05−0.30) as functions of the Sr content have been investigated using M?ssbauer spectroscopy and X-ray diffraction. It has been shown that, when the Sr content is lower than 10%, three phases are formed: PnmaII, PnmaI, and R-3c. In the Sr concentration range from 10 to 20%, the PnmaI and R-3c phases are formed. At a strontium content of 20%, the rhombohedral phase is formed. When the strontium content increases to 30%, the presumably monoclinic (distorted cubic) phase is formed in addition to the rhombohedral phase. The specific features of the formation and suppression of the phases in the LaMn_0.98Fe_0.02O_{3 + δ} compound are compared with those observed in the strontium-doped La_{1 − x} Sr _x Mn_0.98Fe_0.02O_{3 + δ} compound. The general regularities of the structural transformations and substantial differences between them have been revealed.     

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.