ESR line broadening of Fe3+ in Fe/MgO at low temperatures

Electron spin resonance linewidths of Fe³⁺ in single crystal MgO were examined at 9 GHz for crystals having iron concentrations of 2300 ppm and 4300 ppm over the temperature range 293 to 4.2 K. For the + 1/2 –1/2 transition the linewidth remains constant from 293 to about 15 K, below which line broadening occurs. From 15 to 4.2 K the linewidth increases steadily without any change in the g value of the line. At 4.2 K the width of the line is about 3.7 times larger than at 77 K for each concentration. The lineshapes are Lorentzian over the whole temperature range examined and the data suggested that the effects observed below 15 K were due to non-secular line broadening mechanisms (the 10/3 effect) originally proposed by Anderson and Weiss.     

Amplifier based on a field-effect triode with negative current feedback

Conclusions A field-effect triode amplifier with series negative current feedback allows a voltage gain of the order of 200–300 to be obtained for a load resistance R_s1 M. The coefficient K_u begins to decrease noticeably only for a feedback resistance above 500 .The current gain reaches (8–10)·10³. Increasing the resistances R_s and R_L to hundreds of ohms has practically no effect on K_i. For a further increase of R_s and R_L the coefficient K_i decreases.The power gain reaches its maximum value (of the order of 10⁴ or more) for R_s100 and R_L=10–100 k. An increase in R_s leads to a reduction of K_pmax and to a shift of the extremum of the function K_p=f(R_L) into the range of higher values of R_L.A large input resistance of the amplifier (tens of megohms and higher) is obtained when R_s increases to 10–100 M. The maximum input resistance is obtained for R_L and R_s and may exceed values of from hundreds of megohms to several gigaohms. The minimum input resistance is hundreds of kilohms for R_L and R_s0.The minimum input resistance (5–10 k or less) is ensured for R_g and R_L0. An increase of the output resistance to hundreds of megohms or higher occurs for R_g and R_s.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 67–70, September, 1971.     

X-ray diffraction and Ir-absorption characteristics of lanthanide orthophosphates obtained by crystallisation from phosphoric acid solution

Powders of LnPO₄ · H₂O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) prepared by crystallisation from boiling phosphoric acid (2 M H₃PO₄/1) solution were characterised by X-ray diffraction and FTIR-spectroscopy. Hexagonal LnPO₄ · H₂O (La Tb), tetragonal (Ho Lu and Y) and orthorombic DyPO₄ · H₂O crystalline modifications were identified. Ir-spectra of the hydrated hexagonal, anhydrous tetragonal LnPO₄ · H₂O (Dy, Ho, Er, Tm, Yb, Lu and Y) and anhydrous monoclinic (La Tb) are consistent with those reported in the literature. However, the hydrated tetragonal LnPO₄ · H₂O (Ho, Er, Tm, Yb, Lu and Y) display a surplus band (625 cm^–1) in the region of ₄, which was not reported in the literature. The band disappears after ignition at 950°C, while the tetragonal structure is still maintained, which may imply that it is attributed to hydrogen bonding of H₂O molecules to the phosphate oxygen in hydrated salts. Some of the phosphates, after ignition at 950°C, display additional P₂O₇ ^4– band at 1265–1267 cm^–1. That may be resulted from HPO₄ ^2– for PO₄ ^3– substitution in the phosphates crystallised in acidic (2 M H₃PO₄/1) solution.     

Magnetic field and temperature dependence of the Knight shift in superconductors of small dimensions

The magnetic field dependence of the paramagnetic Knight shift in weak-coupling superconductors is evaluated within the framework of the BCS-Gor'kov theory for samples of small dimensions. The behavior of the monotonic and oscillatory parts of the Knight shift are examined in detail in the regimesT0 andTT _c ^– .     

Kinetics of Reactions of Np and Pu Ions with Hydrazine Derivatives: XVII. Reaction of Pu(VI) with Hydroxyethylhydrazine

Reduction of Pu(VI) to Pu(III) with hydroxyethylhydrazine (HOC₂H₄N₂H₃) in HNO₃ solutions involves the following consecutive steps²: Pu(VI) + HOC₂H₄N₂H₄ Pu(V) + ...; Pu(V) + HOC₂H₄N₂H₄ ⁺ Pu(IV) + ...; Pu(V) + Pu(III) 2Pu(IV); and Pu(IV) + HOC_2H4N₂H₄ ⁺ Pu(III) + .... The overall kinetic equations of these steps were suggested, and their rate constants and activation energies were determined. The mechanisms of the four reaction steps, consistent with the experimental kinetic data, are discussed.     

Dynamical instability of the chain oxygen with softened bonds in the 123 — System

Dynamical stability of the chain oxygen with softened effective force constants is investigated. Low-frequency b_1u resonance reaches zero at 29% bond softening and O(1) will be displaced out of the chain in the a — direction. Decrease of the oxygen radius (O^2– O^–) induces also localized electronic states.     

Mullite formation from xerogels of (0.84–2.2) Al2O3·1SiO2

Xerogels of (0.84–2.2) Al₂O₃·1SiO₂ prepared by chemical coprecipitation of Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ experience three thermal reaction paths for mullite formation. Those with pseudoboehmite are found to form mullite via the paths of either Al-Si spinel mullite transformation or -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite, depending upon the ratios of Al₂O₃/SiO₂. Higher SiO₂ content may prefer the former reaction. Xerogels composed of bayerite form mullite via the route -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite. Mullite thus formed exhibits a different crystal size, being 20–25 nm for that obtained from pseudoboehmite and around 37 nm for bayerite. The highest yield of mullite formation may be achieved with xerogels of pseudoboehmite with the stoichiometric mullite compositions, 3Al₂O₃·SiO₂.     

Effects Induced by 16O–18O Oxygen Isotope Exchange in Manganite Ceramics and Films

Unique sensitivity of electrical resistivity to the ¹⁶O¹⁸O isotope exchange was observed for (La_1–y Pr _y )_0.7Ca_0.3MnO₃ ceramics and epitaxial thin films. At y=0.7, substitution of ¹⁶O by ¹⁸O in ceramic samples results in the reversible transition from a metal-like to insulating state. In thin films, this metal–insulator transition occurs at y=0.5 due to the strains induced by substrate. The applied magnetic field (H2 T) transforms the sample with ¹⁸O back to the metallic state. In such magnetic fields the temperature dependence of resistivity exhibits a peak corresponding to the onset of metallic state. The shift of the peak toward low temperatures after the ¹⁶O¹⁸O exchange exceeds 50 K at 3 T for both ceramic and thin film samples. The results are interpreted in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the charge ordered insulating and the ferromagnetic metal-like states.     

The hP1-type phases in alloys of cadmium,mercury, and indium with tin

The hP1-type phases in alloys of Cd, Hg, and In with Sn are stable in the valence electron concentration interval 3.80–3.95. Following a rule of Raynor, the axial ratioc/a decreases with increasing valence electron concentration,N _b ^/at . Using lattice constant measurements the rule is confirmed, and by using density measurements the absence of constitutional vacancies is verified. Thec/aN _b ^/at relation of the hP1 phases is similar toc/aN _b ^/at of InSn _m (03Sn. A bonding-type proposal, explaining the stability of these structures and the dependence of their axial ratio on valence electron concentration, is derived.     

A novel method of introducing PbO dopant and (the study of) its influence on the electrical properties of semiconducting Cd2−x Pb x SnO4 thick films

The sheet resistance of Cd₂SnO₄ thick films was reduced from 15580 to 0.09 k with respect to dopant concentration and peak firing temperature (600 to 900° C). Distinct colour changes were observed in these films. The inorganic binder introduced an impurity which greatly induce changes in its electrical properties. The Arrhenius relation (logR–10³/T) generally indicated slopes of 2 to 3 for all the compositions of Cd_2–x Pb _x SnO₄ (x=0.002, 0.01, 0.02, 0.04 and 0.1). The donor ionization energies (E _d) varied from 0.01 to 0.76 eV. Resistance measurements during heating-cooling cycles indicated the possible presence of structural defects such as oxygen vacancies and cadmium interstitials. The oxidation of dopant (Pb²⁺Pb⁴⁺+2e) contributed in a major way to the overall conductivity. Scanning electron micrographs showed a progressive network formation due to sintering, thus contributing to the carrier mobility.     

Far- and mid-infrared spectrum of YBa2Cu3O6.0 in high magnetic fields

The far- and mid-infrared spectrum of antiferromagnetic YBa₂Cu₃O_6.0 was investigated by infrared transmission measurements (kc-axis) in high magnetic fields up to 16.5Tesla at T=1K. A peak at 1436cm ^–1 which previously was assigned to the excitation of single optical magnons did not show a measurable shift with magnetic field. In the far-infrared, no signature of acoustic magnon absorption has been observed in the magneto transmittance. However, at 83 cm^–1 the sixth phonon of symmetry E_u in YBa₂ Cu₃ O_6.0 has been found.     

Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

Reduction of interlayer Co2+ ions in fluorine mica using diethylene glycol

Reduction of interlayer Co²⁺ ions in expandable fluorine mica has been attempted. The polyol process using diethylene glycol as a reducing agent was employed. The co²⁺ -exchanged mica was refluxed at about 225–235 °C in liquid diethylene glycol for 10–120 minutes. Consequently, zero-valence Co metal (Co⁰) intercalated mica having a metallic grey colour was obtained by in situ reduction of interlayer Co²⁺ ions, showing different properties from Co²⁺ -exchanged mica as a precursor. The layer charge of Co-metal-intercalated mica is compensated by protons which are produced through the course of reduction of interlayer Co²⁺ ions. The reduced sample heated at 350 °C, which had no organic molecules, exhibited a basal spacing of 1.28 nm, indicating the presence of 0.34 nm cobalt metal clusters after subtracting the thickness of the host silicate layer. During the process of reduction in diethylene glycol, cobalt metal particles were expelled from the interlayers which grew to about 0.5 m onto the external surfaces of host mica crystals with increasing refluxing time.     

Oxygen Stoichiometry and Phase Transitions of SrCo1– xFexO3 – δ

The oxygen stoichiometry of SrCo_{1 – x} Fe _x O_{3 –} (0.1 x 0.9) solid solutions was determined by solid-electrolyte coulometry as a function of temperature (20–1000°C) and oxygen partial pressure (1–100 Pa). The results, together with X-ray diffraction studies between 400 and 900°C, indicated a sequence of phase transitions: perovskite brownmillerite high-temperature perovskite. The composition stability region of the parent perovskite structure was shown to be 0.2 <x 1.0. The Co-rich samples (x 0.2) consisted of perovskite and hexagonal phases.     

Optical and esr spectra of titanium (III) in Na2O-B2O3 and Na2O-P2O5 glasses

The optical absorption and the esr spectra of titanium(III) in binary Na₂O-B₂O₃ and Na₂O-P₂O₅ glasses have been studied. Titanium(III) produces two optical absorption bands around 20 000 and 14 000 cm^–1 which are assigned to the²B_2g ²B_1g and²B_2g ²A_1g transitions respectively of Ti³⁺ in a tetragonally distorted octahedral environment. The absorption bands in phosphate glasses are narrower and absorption coefficients higher than those in borate glasses. The esr spectrum of titanium(III) in all the glasses consists of a broad asymmetric line withg 1.94 in borate glasses, andg 1.92 in phosphate glasses; no hyperfine structure has been observed.     

Influence of the technological conditions on the luminescence of Eu3+ ions in Sr2SnO4

Different synthesis conditions have been used to prepare the stoichiometric compounds of the Sr2–xEuxSnO4 with x=0.00, 0.01, 0.02 and 0.03. As the 5D07F0 transition is forbidden by a selection rule of the Judd–Ofelt theory, the J=0J=0 level gains some intensity from the mixing of J states in the Sr2–xEuxKxSnO4 and Sr1–xEuxSnO4 samples. For Sr2–xEuxSnO4 samples prepared under a slightly reducing atmosphere, the J=0J=0 transition is split into two peaks, which confirms the substitution of Eu3+ ions for different crystallographic sites. © 1998 Chapman & Hall     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

Quantum evaporation of 4He

Quantum evaporation is the process in which an excitation is annihilated at the free surface and an atom is ejected. With liquid ⁴He the excitation is a phonon or a R ⁺ or R ^– roton. The conditions necessary to observe this phenomenon are discussed and the various experiments that demonstrate the conservation of energy and parallel component of momentum are reviewed. It is shown that phonons, with energy >10K, and rotons have long lifetimes, tending to as T 0. These excitations can be injected into liquid He and collimated into beams so that angular and scattering measurements can be performed.     

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa₂Cu₃O_6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm^–1, O(2)-O(3) at 338 cm^–1, and O(4) at 488 cm^–1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies, which is in agreement with the enhancement of the static conductivity(0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.