Lanthanum Magnesium Research Articles

Glassy dynamics in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer system

The dynamic processes in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], copolymer with a VDF content of 55 mol % have been studied by measurements of the temperature and frequency-dependent linear $({\ensuremath{\varepsilon}}_{1})$ and third-order nonlinear $({\ensuremath{\varepsilon}}_{3})$ dielectric constants. Analysis of the complex linear dielectric response by a temperature-frequency plot has revealed that the longest relaxation time diverges at $T=277\mathrm{K},$ while the bulk of the relaxation times remains finite below this freezing temperature. Such an asymmetric behavior is, together with the temperature dependence of the static field-cooled dielectric constant, very similar to that observed in classical relaxor systems, such as lead magnesium niobate and lanthanum lead zirconate titanate, and is reminiscent of the dynamic behavior observed in various spin glasses. In addition, a paraelectric-to-glass crossover in the temperature dependence of the dielectric nonlinearity ${a}_{3}={\ensuremath{\varepsilon}}_{3}/{\ensuremath{\varepsilon}}_{0}^{3}{\ensuremath{\varepsilon}}_{1}^{4},$ typical for relaxors, has been observed.

Calorimetric investigations on kinetics of the dehydrogenation process in the system lanthanum–magnesium–hydrogen

A kinetic study of the dehydrogenation of several magnesium-based alloys/compounds with lanthanum is presented and the results are referred to the kinetics of the dehydrogenation process of pure magnesium hydride. Measurements were performed in a differential scanning calorimeter (DSC) by applying various pressure steps to the samples at isothermal conditions. Different thermokinetic models were fitted to the experimental obtained DSC curves as well as the power curves, calculated from the DSC curves, to evaluate the kinetic parameters. The derived dehydrogenation rate is discussed in terms of their dependence on hydrogen pressure and temperature. Under certain conditions, it is possible to visualise individual rate-determining steps. Considering these results first mechanistic considerations on the dehydrogenation reaction of the magnesium-based alloys/compounds can be drawn.

Structural, optical and electronic properties of LaMgH x switchable mirrors

Structural, optical and electronic properties of lanthanum magnesium alloy thin films are studied in situ in real time during hydrogenation. X-ray data show that the as-deposited films contain the intermetallic phase LaMg with CsCl structure as well as fcc β-La and fcc LaH x . Hydrogenation initiates a transformation of β-La to β-LaH x , while the intermetallic phase disproportionates. The fcc structure stays intact during further uptake of hydrogen up to LaH 3, and Mg transforms to the rutile structure of MgH 2. During hydrogen absorption the alloys change from a good conducting, metallic state with high reflectance, to a transparent color neutral insulator with a band-gap of about 3 eV.

The perovskite system La(Mg 2/3Nb 1/3)O 3: II. A neutron powder diffraction study

The crystal structure of the perovskite compound, lanthanum magnesium niobate [La(Mg 2/3Nb 1/3)O 3 (LMN)] has been determined at 300 K by means of neutron powder diffraction. Subsequent Rietveld refinement indicates that the crystal structure has monoclinic P2 1 /n symmetry, which is relatively uncommon for oxide perovskites, ABO 3. An analysis of the structural data obtained reveals that this space group is uniquely capable of accommodating both BO 6-octahedral tilting and cationic ordering. Consideration is also given to the likelihood of further compositions adopting this symmetry, with some suggestions for future work on the pseudo-binary system Pb(Mg 1/3Nb 2/3)O 3–La(Mg 2/3Nb 1/3)O 3 (i.e. PMN–LMN or, equivalently, PLMN).

The perovskite system La(Mg 2/3Nb 1/3)O 3: I. Synthesis, formation and characterisation of powders

A perovskite-like phase of lanthanum magnesium niobate, La(Mg 2/3Nb 1/3)O 3 (LMN) has been synthesised by a mixed oxide synthetic route. Both magnesium oxide and magnesium carbonate hydroxide pentahydrate have been investigated as magnesium precursors, with the formation of the LMN phase investigated as a function of calcination temperature and dwell time by DTA and XRD. Morever, particle size distribution, morphology, phase composition and crystal structure have been determined via laser diffraction, SEM and TEM/EDX, respectively. A combination of X-ray and electron diffraction studies has demonstrated that heterogeneous powders are produced, with the presence of an MgO-rich phase and varying Mg/Nb ratios within the parent phase of LMN. Minor phases have also been identified as LaNbO 4, La 3NbO 7, MgO, Mg 4Nb 2O 9 and La 2O 3, with their concentrations decreasing with rising calcination temperature. X-ray and electron diffraction data are supportive of monoclinic symmetry in the parent LMN phase.

Two Types of Domain Boundaries in Lanthanum Magnesium Niobate

Microstructural studies of the domain boundaries in the complex perovskite compound lanthanum magnesium niobate (La[Mg2/3Nb1/3]O3, LMN) were conducted using transmission electron microscopy. Both the 1:1 chemical ordering of B‐site cations and the tilting of oxygen octahedra affected the domain boundaries. Two types of domain boundaries were observed. In addition to the presence of antiphase boundaries, which were insensitive to the crystallographic planes, ferroelastic domain boundaries that were caused by the phase transition due to the tilting of oxygen octahedra also were present. In some grains, only one type of oxygen tilting was present, which resulted in a single domain in one grain. Two or three domains were observed in a grain where the walls were parallel to the {110} plane. Many domains also were observed in a grain that had boundaries whose linear characteristics were gradually reduced.

99/01474 Modeling fly ash generation for pulverized coal combustion : Wigley, F. and Williamson, J. Prog. Energy Combust. Sci., 1998, 24, (4), 337–343

Yttria-stabilized zirconia (YSZ) has been widely used as an environmental and thermal barrier coating material. However, above 1200 °C, its thermo-physical properties degrade because of undesirable microstructural and phase modifications. On the other hand, rare-earth hexaaluminates, having magnetoplumbite structure, exhibit high-temperature phase stability and it possess excellent thermo-physical properties at temperatures above 1200 °C. This work highlights the development of lanthanum magnesium hexaaluminate (LaMgAl11O19) ETBC material via wet-chemistry route, the effects on its crystal growth as a result of N, N-dimethylformamide (DMF) organic addition, and its hot corrosion studies. The as-synthesized doped DMF xerogel powders were calcined at 1100 °C–1400 °C, subjected to x-ray diffraction (XRD) and scanning electron microscopy (SEM) for their structural and morphological characterization. The average crystallite size of LMA decreased up to 14% (with addition of DMF) at the tested temperatures as confirmed by XRD. Whereas, SEM micrographs confirmed the platelet like microstructure of LMA powder. Hot corrosion studies at 700 °C, 800 °C and 900 °C for 5 h have revealed the degradation mechanism of LMA in V2O5 environment. At 700 °C, AlVO4, and at 800 °C, LaVO4, MgAl2O4 and AlVO4, whereas at 900 °C, α-Al2O3, MgAl2O4 & LaVO4 were confirmed as the main corrosion products.

Concentration Effects and Energy Transfer in the Cathodoluminescence of Europium–Manganese-Doped Lanthanum Magnesium Hexaaluminate

We have examined the cathodoluminescence of lanthanum magnesium hexaaluminate heavily doped with europium and manganese. Emission peaks characteristic of manganese, europium and an unidentified defect have been observed. No evidence of energy transfer between europium and manganese ions has been measured.

Defect Energetics and Nonstoichiometry in Lanthanum Magnesium Hexaaluminate

Computer-based atomistic simulation methods are applied to address quantitatively the defect energetics and crystal chemistry of lanthanum magnesium hexaaluminate (LMA). The tetrahedral site preference of Mg in the magnetoplumbite structure is determined by calculating lattice energies for different Mg ion distributions. It is revealed that the intrinsic and extrinsic disorders are much influenced by the distribution of Mg in the structure. Our calculations show that oxygen Frenkel disorder is the dominant defect mode to be expected, even though Schottky disorder may also exist. Several feasible defect processes in nonstoichiometric LMA are determined from the enthalpies of the quasi-chemical reactions for the processes with simple point defect energies. We have also modeled some defect complexes in the mirror plane regions. It is found that the Mg ions positioned in the tetrahedral sites suppress the formation of cation vacancies in the mirror plane, through hindering the relaxation of the 12kAl ions by which the vacancies are stabilized. In Mg-deficient nonstoichiometric LMA, however, it is expected that the defect complex [VLa+VAl+2(VAl+Ali)] will be formed in the interspinel layer. Our calculations also indicate that the OLadefect is improbable in LMA not only as a simple point defect but also as a member of a defect complex.

The hydriding-dehydriding characteristics of La 2Mg 17

Lanthanum magnesium, La 2Mg 17, was investigated to determine its hydrogenation-dehydrogenation behavior. The hydride was formed by two methods, metallurgical and chemical, and the two were compared. It was concluded that the metallurgical process was preferred for this particular alloy. An unexpected result of the study was the determination of a direct correlation between the hydrogenation conditions and the temperature required for the subsequent release of the hydrogen.

Cathodoluminescent measurements of europium—manganese-doped lanthanum magnesium hexaaluminate

Cathodoluminescent measurements have been taken on europium—manganese-doped lanthanum magnesium hexaaluminate. Three emission peaks have been observed, corresponding to (in order of increasing energy) manganese, europpium and a defect emission. The temperature dependence of the emissions from both as-grown and annealed crystals is determined.

Time decay of the excited states of Eu+2 in europium-doped LMA

Optical measurements of the luminescence of Eu +2 -doped lanthanum magnesium aluminate (LMA or LaMgAl 11 O 19 ) after high-temperature annealing in various atmospheres are reported. Changes in the time decay can be related to the reversible conversion of Eu +2 into Eu +3

Converting Eu2+ to Eu3+ in europium-doped LMA

Optical measurements of the luminescence of Eu2+-doped lanthanum magnesium aluminate (LMA or LaMgAl11O19) after high-temperature annealing in various atmospheres are reported. Major differences in the excitation spectra are explained by the reversible conversion of Eu2+ into Eu3+.

Polarized targets in biology

Abstract Polarized nuclear spin targets are playing an important role in high energy physics experiments. In many cases they are hydrogenous solids, the nuclear spins of which are almost completely aligned with respect to an external magnetic field. In the seventies, polarized targets started exciting the interest of neutron physicists, e.g. at the Institut Laue-Langevin (ILL), Grenoble, the reason for this being the sensitivity of the amplitude of the interaction of neutron spins with proton spins to their mutual spin orientation. Polarized neutrons are an ideal probe of polarized nuclear spin densities in solids. In a first experiment at the ILL, Hayter, Jenkin and White (1974) obtained a proton density distribution in the unit cell of a lanthanum magnesium nitrate crystal, which because of its high water content was assumed to reflect some properties of biological material.

Precipitation of copper and chromium impurities in lanthanum magnesium aluminate crystals during thermochemical reduction

Thermochemical reduction at high temperatures has been performed on chromium-doped lanthanum magnesium aluminate crystals. Analytical transmission-electron-microscopy and optical-absorption techniques were used to characterize the crystals. Copper impurities inherently present in the crystals began to aggregate and form Cu-rich particles at 1500 K near the surface region. Below 1870 K there was no evidence of chromium precipitates being formed. However, Cr-rich particles were formed at 2020 K. The distribution of the precipitates was very inhomogeneous and the precipitates were also concentrated near the surface region.

Spin-polarizing 3He nuclei with an arc-lamp-pumped neodymium-doped lanthanum magnesium hexaluminate laser.

We report a systematic study of optical pumping of $^{3}\mathrm{He}$ nuclei by metastability exchange using an arc-lamp-pumped neodymium-doped lanthanum magnesium hexaluminate (Nd:LMA) laser. The laser produces 6 W at 1083 nm in a 1.5-GHz bandwidth and yields a substantial improvement in attainable nuclear polarization and pumping rates. Spin-polarized $^{3}\mathrm{He}$ has applications in several fields of physics; the focus in this article is on targets for nuclear physics, which require high pumping rates to maintain the highest polarization. We have studied the influence of the $^{3}\mathrm{He}$ pressure, pumping transition, discharge intensity and frequency, cell size and shape, and laser power on both the polarization and pumping rate. Under ideal conditions, we obtain 85% nuclear polarization. With pumping rates of ${10}^{18}$ (${10}^{19}$) atoms/s, 82% (50%) polarization is obtained. We have found a pressure-dependent saturation of the increase in pumping rate with laser power. Based on our results, we discuss projections for the performance of both internal and external targets.

Lanthanum magnesium aluminate (LMA) and its derivatives: An example of laser materials engineering

A new family of lanthanide-activated laser materials based on magnetoplumbite structural type is studied. By some convenient chemical modifications it is possible to improve the laser characteristics of the starting material such as laser efficiency, thermal behavior, tunability range, and wavelengths of emission. A new way of activating solids using ionic exchange is also described and applied to a set of compounds close to the magnetoplumbite family.

Characterization and laser properties of lanthanum magnesium hexa-aluminate activated by neodymium and chromium

We report here the first results on laser action obtained with a 10 cm long rod of LaMgAl 11O 19:Cr, Nd (LMA:Cr, Nd) in transverse pumping with continuous Kr lamps. The composition of the crystal was found to be approximately 4% Nd and 1% Cr. Flourescence decay measurements gave the time constant and the efficiency of the energy transfer to Nd ions from excited Cr ions located in different sites of the lattice. A laser output power of 4 W was obtained. The laser properties of LMA:Cr, Nd were compared to those of LMA:Nd for different optical resonators and line narrowing techniques. Preliminary results show that the addition of Cr to the lattice decreases the thermal lensing problems.

Transition-metal and rare-earth ions in beta-alumina type materials

The spectroscopic and optical properties of transition-metal and rare-earth ion doped hexagonal aluminates are reviewed. Materials discussed include sodium beta- and beta″-alumina as well as lanthanum magnesium aluminate and related materials. The optical properties of the activator ions are discussed in terms of the properties and structure of the host lattice, i.e. sites occupied, symmetry, and defects in particular. Some of the ways these activators have been used to probe the structures of these materials are also discussed.

Crystal growth of LaMgAI 11O 19: Nd

Single crystals of neodymium-doped lanthanum magnesium hexaluminates up to 50 mm in diameter and 80 mm length have been grown by the Czochralski technique along the optical e axis. These single crystals exhibit a core effect which seems to be correlated with a change in the chemical composition between the core and the matrix of the crystal.