Thermodynamic modeling of Fe–Ni pentlandite

Gibbs energy modeling of iron–nickel pentlandite has been performed using experimental data of ternary phase equilibria. A three-sublattice approach in the framework of the Compound Energy Formalism is developed to refine a two-sublattice model of pentlandite recently applied within a complete assessment of the Fe–Ni–S system. Experimental data about the iron site fraction on the octahedral sublattice at 523.15 K for the composition Fe₅Ni₄S₈ as well as the enthalpy of formation at 298.15 K for the composition Fe_4.5Ni_4.5S₈ are predicted satisfactorily by the novel model. New possibilities to interpret experimental phase equilibrium data on complex phase relations with pentlandite are discussed together on the basis of the recent extension of a second high-temperature heazlewoodite phase to a ternary solution phase.     

Na2TiGeO5: Crystal structure stability at low temperature and high pressure

The temperature evolution of the lattice parameters measured from 295 to 125 K exhibits a small instability below T_c≈278 K, indicating ferroelastic properties of Na₂TiGeO₅. The behavior is related to the specific crystal structure built of polyhedral layers with shared TiO₅ pyramids and GeO₄ tetrahedra, alternating with layers of Na⁺ cations. Antiparallel alignment of the short apical titanyl bond in adjacent rows of the polyhedral layer gives rise to spontaneous strain, when a distortion of the TiO₅ groups occurs. Single-crystal structures determined at room temperature and 120 K suggest that {1 1 0} domains, developing below T_c, entail a tetragonal-to-orthorhombic symmetry change. The mechanism is attributed to a shortening of the O–O distance between the polyhedral layers, and to minor shifts of the positions of the Ti atoms and the correlated oxygen atoms along the c-axis. The structure distortion, however, is too small to allow any unambiguous determination of the symmetry-breaking effects. The bulk modulus and its pressure derivative have been determined as B₀=89(2) GPa and . A pressure-induced phase transformation takes place at P_c≈12.5 GPa, presumably to an orthorhombic structure. The pressure effect on the transition temperature is given by ΔT_c/ΔP≈1.76 K/GPa.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Experimental study and thermodynamic calculation of Au–Bi–Sb system phase equilibria

Phase equilibria in the Au–Bi–Sb ternary system have been studied experimentally and calculated by the CALPHAD method. Three calculated isopleths with molar ratios Au:Bi=1, Bi:Sb=1 and Au:Sb=1 were compared with the DTA results from this work. The liquidus projection has been calculated. Two ternary invariant reactions were noted. Calculated phase diagram of isothermal section at 573 K was compared with the results of SEM/EDX analysis.     

Antiferromagnetism and superconductivity: Determination of the Cu spin–spin relaxation time and the spin–lattice relaxation time in Gd1.5Ce0.5Sr2Cu2RuO10

Gd_1.5Ce_0.5Sr₂Cu₂RuO₁₀ exhibits antiferromagnetic resonance at 23.9 GHz for applied fields less than 1000 Oe with a spin–spin relaxation time T₂ of approximately 0.45 ns, and with a spin–lattice relaxation time T₁ of at least 320 μs. Since in the homologue, Eu_1.5Ce0_.5Sr₂Cu₂RuO₁₀, the Ru atoms evidently fail to exhibit magnetic order, the antiferromagnetic resonance must arise from the cuprate planes. In other homologues, the cuprate planes are known to order ferromagnetically and are stacked in an antiferromagnetic configuration. The large value of T₁ suggests that phonon mediation plays no role in high temperature superconductivity. In addition, the presence of ferromagnetic cuprate planes is inconsistent with spin-fluctuation models of high temperature superconductivity.     

Magnetic properties of Lu2Co17−xSix single crystals

The Co-sublattice anisotropy in Lu₂Co₁₇ consists of four competitive contributions from Co atoms at crystallographically different sites in the Th₂Ni₁₇-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu₂Co_17−xSi_x (x=0−3.4) grown by the Czochralski method. The SRT in Lu₂Co₁₇ was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T_SR1≈680 K and “easy-cone”–“easy-axis” at T_SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu₂Co₁₆Si (T_SR1≈75 K and T_SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu₂Co_17−xSi_x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.     

Low-temperature specific heat of Ni–Mn–Ga ferromagnetic shape memory alloys

Results of the low-temperature specific heat measurements (2–80 K) for one austenitic and three martensitic Ni–Mn–Ga ferromagnetic alloys are presented. The alloy compositions are chosen to comprise a wide span of valence electron concentrations e/a=7.3–7.78. Debye temperature (261–345 K) is found to be an increasing function of e/a while the experimental values of the Sommerfeld coefficient (2.9–3.4 mJ/mol K²) appear to be increasing in the martensitic region only. Observation of those trends rekindles the discussion about the role of vibrational and electronic contributions to the lattice instability and transformation mechanism of studied alloys.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

Development of a 1K × 1K, 8–12 μm QWIP array

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1,024 × 1,024 (1K × 1K), 8–12  μm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K × 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, well-established GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 μm to beyond 15 μm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K × 1K QWIP array development including fabrication methodology, test data and imaging capabilities.     

Thermodynamic properties of the clathrate-type silver-oxide Ag6O8AgHF2

We report normal-state and superconducting properties of the clathrate-type silver-oxide Ag₆O₈AgHF₂. We present electrical resistivity, DC- and AC-susceptibility and specific-heat measurements of single crystalline Ag₆O₈AgHF₂. In the normal state, Ag₆O₈AgHF₂ exhibits metallic conductivity and a phase transition near 110 K, possibly a structural phase transition as observed in the related compound Ag₆O₈AgNO₃. The onset of superconductivity of our samples is observed around 1.2–1.5 K, and the H–T phase diagram is determined for the first time. The upper critical field H_c2(0) is estimated to be about 2000–2200 Oe and the coherence length ξ_GL(0) to be 40 nm.     

Solid-state Raman laser generating <1 ns, multi-kilohertz pulses at 1096 nm

A new Raman oscillator is described based on a diode end-pumped Nd:YAG laser and an intra-cavity rubidium titanyl phosphate (RTP) crystal that serves as the active Q-switch. This compact and robust laser generates <1 ns, kilohertz pulses at 1096 nm with a 1064 nm pump and has potential for 1480 nm eye-safe operation using a 1340 nm pump. Based on rate equation modelling a Raman gain of ≈1.7–2.2 cm/GW at 1064 nm is estimated for RTP.     

The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

On the origin of 1.5 μm luminescence in porous silicon coated with sol–gel derived erbium-doped Fe2O3 films

Sol–gel derived Fe₂O₃ films containing about 10 wt% of Er₂O₃ were deposited on porous silicon by dipping or by a spin-on technique followed by thermal processing at 1073 K for 15 min. The samples were characterized by means of PL, SEM and X-ray diffraction analyses. They exhibit strong room-temperature luminescence at 1.5 μm related to erbium in the sol–gel derived host. The luminescence intensity increases by a factor of 1000 when the samples are cooled from 300 to 4.2 K. After complete removal of the erbium-doped film by etching and partial etching the porous silicon, the erbium-related luminescence disappears. Following this, luminescence at 1.5 μm originating from optically active dislocations (“D-lines”) in porous silicon was detected. The influence of the conditions of synthesis on luminescence at 1.5 μm is discussed.     

Luminescence of defect centers in yttrium–aluminum garnet crystals

Spectral–kinetic properties of the 2.38 eV emission in neutron-irradiated and as-received yttrium aluminum garnet (Y₃Al₅O₁₂) crystals are investigated. This luminescence band as well as its lifetime and the temperature dependence are analyzed. It is suggested that the 2.38 eV luminescence band is due to the strongly allowed transition of the F⁺-center in garnet crystals. Possible reasons that may explain the observed temperature dependence of this emission intensity are discussed.     

Magnetization enhancement at low temperature due to surface ordering in Fe–Ni–B amorphous nanoparticles

We report magnetization measurements up to 5 T as a function of temperature from 5 to 300 K in an amorphous Fe–Ni–B system nanoparticle (average diameter 3 nm). These nanoparticles were dispersed in a non-magnetic polymer matrix to minimize interparticle interactions and the results were contrasted with a concentrated powder sample. The saturation magnetization shows a sudden increase below 40 K. This behavior is observed in both the dispersed and the concentrated systems, being noticeably more pronounced in the first. This result can be explained considering that the large magnetic contribution at low T is due to the magnetic ordering of the particle surface.     

Surface diffusion of dysprosium on the W(1 1 1) facet

Diffusion of dysprosium on the (1 1 1) facet of a tungsten micromonocrystal was investigated by means of spectral analysis of field emission current fluctuations. The experimental spectral density functions of the current fluctuations were analysed by using Gesley and Swanson’s theoretical spectral density function, which enables to determine the surface diffusion coefficient D for dysprosium. Derived from the temperature dependence of D, the diffusion activation energy E is presented for some Dy coverages θ_(1 1 1). In the temperature range 400–600 K, the E first drops from 1.25 eV per atom at θ₍₁₁₁₎≈0.25 ML to 0.48 eV per atom at θ₍₁₁₁₎≈1 ML (corresponding to the minimum of the work function of the system), then increases to 1.03 eV per atom at θ₍₁₁₁₎≈1.3 ML. The results are discussed from the aspects of the substrate structure and interaction in the adsorbed layer.     

EXAFS analysis of Er sites in Er–O and Er–F co-doped crystalline Si

We present extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) analyses of Er–O and Er–F co-doped Si. Samples were prepared by multiple implants at 77 K of Er and co-dopant (O or F) ions resulting in the formation of a2 μm thick amorphous layer uniformly doped with 1×10¹⁹ Er/cm³ and 3×10¹⁹ O/cm³, 1×10²⁰ O/cm³ or 1×10²⁰ F/cm³. EXAFS measurements show that the local environment of the Er sites in the amorphous layers consists of 6 Si first neighbors. After epitaxial regrowth at 620°C for 3 h, Er is fully coordinated with 8 F ions in the Er–F samples, while Si and O ions are concomitantly present in the first shell of O co-doped samples. Post regrowth thermal treatments at 900°C leave the coordination unchanged in the Er+F, while the Er+O (ratio 1 : 10) doped samples present Er sites with a fully O coordinated shell with an average of 5 O atoms and 4 O atoms after 30 s and 12 h, respectively. We have also found that the fine structure and intensity of the high-resolution PL spectra are strongly dependent on the Er-impurity ratio and on thermal process parameters in the Er–O co-doped samples, while this is not observed for the F-doped samples. The most intense PL response at 15 K was obtained for the 1 : 3 E : O ratio, suggesting that an incomplete O shell around Er is particularly suitable for optical excitation.     

Enhanced luminescence from transparent Ni-doped MgO–Al2O3–SiO2 glass ceramics by Ga2O3 addition

Transparent Ni²⁺-doped MgO–Al₂O₃–SiO₂ glass ceramics without and with Ga₂O₃ were synthetized. The precipitation of spinel nanocrystals, which was identified as solid solutions in the glass ceramics, could be favored by Ga₂O₃ addition and their sizes were about 7.6 nm in diameter. The luminescent intensity of the Ni²⁺-doped glass ceramics was largely enhanced by Ga₂O₃ addition which could mainly be caused by increasing of Ni²⁺ in the octahedral sites and the reduction of the mean frequency of phonon density of states in the spinel nanocrystals of solid solutions. The full width at half maximum (FWHM) of emissions for the glass ceramics with different Ga₂O₃ content was all more than 200 nm. The emission lifetime increased with the Ga₂O₃ content and the longest lifetime is about 250 μs. The Ni²⁺-doped transparent glass ceramics with Ga₂O₃ addition have potential application as broadband optical amplifier and laser materials.     

Structural evolution of low-pressure polymerised C60 with polymerisation conditions

We show in this paper that characteristic features in the Raman spectra, especially the frequency of the pentagonal pinch mode, can give information about the polymeric structure of pressure polymerised C₆₀. High-pressure treatment at 1 GPa below 510 K for 3 h results in the formation of a low fraction of dimers only, while treatment at the same pressure and time above 540 K affords a fully polymerised material. In the latter case, different relative fractions of dimers and polymer chains are obtained depending on whether the final reaction conditions were reached by isobaric or isothermal path. We suggest that this difference results from different reaction dynamics in the two cases. The polymerisation rate depends on T and p and on the rotational and orientational states of the molecules. At 1 GPa no polymerisation is observed in sc C₆₀, while in “hexagon” oriented sc C₆₀ at 1.7 GPa dimers are already formed 175 K below the fcc–sc transition and a fully polymerised material is obtained just below the transition to the fcc phase.     

Electronic conduction properties of Au/C60/p-Si and C60/Au/p-Si sandwich structures: I–V and transducer characteristics

Gold-fullerite [C₆₀]-silicon (p-type) sandwich structures have been fabricated in order to investigate intrinsic cross-sectional and planar electronic conductive properties, in particular the C₆₀/p-Si p–n heterojunction. The turn-on voltage of this p–n heterojunction lies in the range 0.25–0.27 V. The I–V characteristics of the Au/C₆₀/p-Si structure are mostly defined by the bulk specific resistance of the fullerite crystal film itself (6×10⁷ Ω cm). I–V curves in the C₆₀/Au/p-Si structure are shown to be ohmic. Au/C₆₀/p-Si sandwiches irradiated with swift (300 MeV) heavy ions, (⁸⁴Kr¹⁴⁺) to a total fluence 10¹⁰ ion/cm² yield structures which are sensitive to ambient air pressure, specifically in the case of a transverse contact configuration, and if one of the contacts is located on the irradiated part of the fullerite film. The sandwich-structure sensitivity to pressure is 5×10⁻⁶ Pa⁻¹. This exceeds the sensitivity of conventional silicon pressure transducers by almost three orders of magnitude.