Phase transition in nanostructured LaMnO<Subscript>3</Subscript>

Nanocrystalline LaMnO₃ samples have been synthesized by the coprecipitation of lanthanum and manganese salt solutions followed by annealing at 800°C in air and then in an argon flow. With the use of X-ray and neutron diffraction analysis, it has been found that the resulting samples had an orthorhombic crystal structure (space group Pbnm). In the nanocrystalline LaMnO₃ samples, an order-disorder phase transition from the low-temperature phase to the high-temperature phase occurs at (220 ± 10)°C, which is much lower than the value of 477°C for bulk LaMnO₃. The phase transition is due to the removal of the Jahn-Teller distortion of the Mn³⁺O₆ oxygen octahedrons and is accompanied by a decrease in the unit cell volume of lanthanum manganite over a narrow temperature range.     

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ?′(log f, T) and tanδ(logf, T) of a perfect BaTiO₃ single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 10⁷ Hz and T = ?80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ～79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO₃ structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO₃ as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.     

Preparation,structural characterization and conductivity of LiZr2(PO4)3

Amorphous LiZr₂(PO₄)₃ has been prepared at room temperature starting from aqueous solutions of ZrOCl₂, H₃PO₄, and LiOH and then crystallized by heating at temperatures between 600 and 900°C. The material obtained at 900°C has been characterized by X-ray powder diffractometry, DSC analysis, and ac conductivity. It is monoclinic from 20 up to about 300°C and orthorhombic at higher temperatures. A change in the activation energy for conduction (from 0.79 to 0.43 eV) and a weak endothermic effect (0.9–1.7 cal/g) are associated with the phase transition. The ac conductivity of sintered pellets is, on average, 7×10⁻⁴ S cm⁻¹ at 300°C.     

High-pressure phases in nanocrystalline Co(C) films obtained by pulsed plasma vaporization

The phase composition of nanocrystalline Co(C) films obtained by a new pulsed plasma vaporization technique was found by studying their atomic structure and magnetic properties. The films deposited at the substrate temperature T=50°C were of heterophase structure and consisted of a supersaturated solid Co(C) solution and the metastable Co₃C carbide. The films obtained at T=150°C represented a mechanical mixture of the metastable Co₃C and Co₂C carbides. The metastable Co₃C and Co₂C carbides obtained in a nanocrystalline state were high-pressure phases (～100 kbar). The thermal stability ranges of these metastable phases were determined.     

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu₂(MoO₄)₃ during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ～550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.     

Europium precipitation in monocrystalline RbCl and RbBr monitored by optical techniques

In the present paper optical absorption and photoluminescence data dealing with the different precipitated phases of divalent europium ions in rubidium chloride and rubidium bromide single crystals are presented. The europium precipitation processes which occur in these crystals in the low-temperature region (25–100°C) are quite different to those which occur at ≈ 200°C. In fact, the annealing at 70°C produces europium precipitation into a second phase precipitate which appears to be related with the Suzuki phase. However, the aging at 200°C produces the nucleation of at least two metastable precipitates from which grows the stable phase EuX₂. The optical data reported in this paper, together with those previously reported for the different precipitated phases of Eu²⁺ in the sodium and potassium halide series, give a complete picture of the different Eu-second phase precipitates which nucleate in the alkali halides.     

Structural and phase analysis of rapidly solidified Al-Fe alloys

The structure and phase composition of lightly-doped Al-Fe alloys obtained by ultrarapid quenching from the melt are investigated. The surface of foils was studied using scanning electron microscopy, atomic-force microscopy, and Rutherford backscattering technique. The variation in the phase composition of alloys during annealing was studied by x-ray diffraction technique and by resistivity and microhardness measurements. The Al-Fe alloys have microcrystalline structure with a nonuniform iron content in the near-surface region of the samples. A correlation of depth profiles of iron and phase composition of the foils is observed. It is found that decomposition of the supersaturated α solid solution proceeds in the temperature range 250–350°C. As the annealing temperature increases, a metastable Al₆Fe phase is precipitated. In the range 300–500°C, the metastable Al₆Fe phase decomposes, and a stable Al₃Fe phase is precipitated.     

Enhanced magnetization and ferrimagnetic behavior of normal spinel ZnFe2O4 thin film irradiated with femtosecond laser

We report on spatially selective change of magnetism from paramagnetic to ferrimagnetic-like behaviors in normal spinel ZnFe₂O₄ thin film under irradiation with 780 nm femtosecond laser pulses. The distribution of Zn²⁺ and Fe³⁺ ions in the irradiated region on the film surface becomes disordered because of local heating to high temperatures, and the metastable phase of ZnFe₂O₄ is frozen in by the rapid quenching after irradiation, resulting in the formation of the ferrimagnetic phase. The ferrimagnetic phase reverts to the paramagnetic state by annealing at 800°C. The present technique is useful for two-dimensional patterning of magnetic thin films.     

Synthesis hexagonal ZrW2O8 and its thermal properties

ZrW₂O₈ as the typical negative thermal expansion (NTE) material has attracted much attention for the potential applications in various fields such as tailored coefficient of thermal expansion (CTE) composites. The hexagonal ZrW₂O₈ (h-ZrW₂O₈), with the combination of ZrO₂ and WO₃ in a composite, was synthesized at a pressure of 2 GPa and the temperature between 600°C and 700°C. We found h-ZrW₂O₈ decomposes to ZrO₂+WO₃ oxides that start from 500°C and end at 800°C, and determined the CTE of h-ZrW₂O₈ is?16.3×10^?6°C^?1 in the temperature range from 150°C to 450°C. The results show that ZrW₂O₈ with a hexagonal structure is metastable and exhibits high NTE property like its cubic structure.     

High pressure-high temperature synthesis of distorted perovskite-type Cu-based oxides

Abstract

We have synthesized the rhombohedrally distorted perovskite phase of LaCuO₃ by reacting mixtures of La₂O₃ and CuO in an oxygen rich atmosphere at 1500°C and 6.5 GPa. We find this phase to be metastable; at 410°C and ambient pressure, it undergoes an irreversible transition to a tetragonal structure. By selective replacement of some or all of the La or Cu with one or more of the following elements: Ba, Ca, Cr, Ni, Pb, Sc, Sr, Ti, Y, Zn, and Zr, over 150 different alloys have been formed. Magnetic susceptibility measurements have failed to reveal the presence of superconductivity in any of these new polymorphs.     

On the low temperature phase transition in (NH4)2SO4

An ultracryostat and multidecameter were used to determine the temperature dependence of the dielectric constant ?′ and dielectric loss ?″ over a wide range of frequencies of single crystals and polycrystalline samples of (NH₄)₂SO₄ in the region of the low temperature phase transition. A sharp increase was observed in the values of ?′ and ?″ at about ?50°C. In addition, a dielectric dispersion was detected and found to be more pronounced in the high temperature phase. This dispersion was attributed to piezoelectric resonance. The observed sudden increase in the values of the dielectric constant and dielectric loss below ? 50°C was attributed to the ferroelectric nature of the low temperature phase of (NH₄)₂SO₄.A DTA thermogram showed a sharp peak at ? 50°C which indicated that the phase transition is one of first order type. A TMA thermogram showed that this transformation was associated with a rapid increase in the expansion coefficient. Such an increase in the lattice parameter might be attributed to the enhanced rotation of electric dipoles associated with the distorted NH₄⁺ and SO₄^2? ions. The distortion of both the ammonium and sulfate ions in addition to their expected orientational motion are suggested to be responsible for the ferroelectric behaviour of ammonium sulfate below ?50°C.A transition to a metastable hexagonal state at about ?40°C is thought to occur, and this transformation is found to be irreversible.     

Low temperature thermal annealing-induced α-FeSi2 derived phase in an amorphous Si matrix

Thermal annealing-induced recrystallisation in Fe ion-implanted Si was investigated by transmission electron microscopy. Single crystals of Si(111) were implanted with 120 keV Fe ions to a fluence of 1.0×10¹⁷ cm^-2 at cryogenic temperature. A buried amorphous Fe-Si layer in an amorphous Si matrix was formed in the as-implanted sample. Nanobeam electron diffraction revealed that metastable α-FeSi₂ precipitates embedded in the amorphous Si matrix were formed after annealing at 350 °C for 8 h. The formation of this α-FeSi₂-derived phase was unusual, because it has been observed only in epitaxially grown thin films. Based on the Fe_1-xSi (0<x<0.5) phase with the CsCl structure, which is another metastable phase in the Fe-Si binary system, we discuss the formation process of the metastable α-FeSi₂ in the amorphous matrix. PACS 61.43.Dq; 61.14.Lj; 61.80.Jh     

Ionized physical vapor deposited Al2O3 films: Does subplantation favor formation of α‐Al2O3?

The broad energy distributions of the condensing particles typically encountered in ion assisted vapor deposition techniques are often a drawback when attempting to understand the effect of the energetic bombardment on the film properties. In the current study, a monoenergetic Al⁺ beam generated by a filtered cathodic arc discharge is employed for the deposition of alumina (Al₂O₃) films at well defined Al⁺ ion energies between 4 eV and 200 eV at a substrate temperature of 720 °C. Structural analysis shows that Al⁺ energies of 40 eV or larger favor the formation of the thermodynamically stable α‐Al₂O₃ phase at the expense of other metastable Al₂O₃ polymorphs. The well defined ion energies are used as input for Monte‐Carlo based simulations of the ion–surface interactions. The results of these simulations reveal that the increase of the Al⁺ ion energy leads to an increase in the fraction of ions subplanted into the growing film. These findings underline the previously not considered role of subsurface processes on the phase formation of ionized physical vapor deposited Al₂O₃ films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermally stimulated luminescence from commonly occuring impurity phases in high-temperature superconductors

The insulating materials Y₂O₃, Y₂BaCuO₅, BaCO₃, Ba₃CuO₄, and BaCuO₂, which frequently occur as secondary phases in high temperature superconductors (HTS), have been investigated using thermally stimulated luminescence (TSL). When irradiated with X- or γ-rays at room temperature and subsequently heated to 400°C they produce TSL whose glow-peak maxima occur between 115 and 280°C. Analyses of the TSL glow curves reveal that the trapped charges require energies from 0.7 to 1.2 eV for their release from metastable states following mixed-order kinetics ranging from 1.3 to 3 with frequency factors varying from 10⁷ to 10¹⁴ s^-1. TSL in these nominally pure, single-phase insulating materials are attributed to impurities and radiation-induced ionic radicals.     

Fabrication and magnetic properties of ordered Fe60Pb40 nanowire arrays electrodeposited in AAO templates

Ordered ferromagnetic-nonmagnetic heterogeneous Fe₆₀Pb₄₀ nanowire arrays were successfully fabricated by alternating current (AC) electrodeposition into nanoporous alumina templates. Transmission electron microscopy (TEM) image and selected-area diffraction (SAED) pattern analysis showed that the Fe₆₀Pb₄₀ nanowires are polycrystalline with an average diameter of 22 nm and lengths up to several micrometers. X-ray diffraction (XRD) observations indicated that α-Fe and fcc Pb phase coexist and do not form metastable alloy phase. The as-deposited samples were annealed at 200, 300, 400 and 500 ^°C, respectively. Magnetic measurements showed that nanowires have high magnetic anisotropy with their easy axis parallel to the nanowire arrays, and the coercivity of the samples increased with the annealing temperature up to 400 ^°C and reached a maximum (2650 Oe). The change of magnetic properties associated with the microstructure was discussed.     

High lithium ion conducting glass-ceramics in the system Li2S–P2S5

Highly ion-conductive Li₂S–P₂S₅ glass-ceramic electrolytes were prepared by controlling the compositions and heat treatment temperatures of the glasses. The 70Li₂S·30P₂S₅ (mol%) glass-ceramic heated at 360 °C showed the highest conductivity of 3.2 × 10^− 3 S cm^− 1 at room temperature and the lowest activation energy of 12 kJ mol^− 1 for conduction in the binary system Li₂S–P₂S₅. The outstanding property was attributed to both the precipitation of the new crystal as a metastable phase and the increase in crystallinity of the phase. With increasing heat treatment temperatures, the metastable phase changed into thermodynamically stable phases such as the Li₄P₂S₆ crystal by heat treatment up to 550 °C, resulting in low conductivities of the glass-ceramics. It was, thus, found that the formation of superionic metastable phases by heating the Li₂S–P₂S₅ glasses is responsible for the marked enhancement on the conducting properties of the glass-ceramics.     

Structural relaxation in the CoP-CoNiP system upon low-temperature annealing

The atomic structure of alloys in the CoP-CoNiP system in the initial state and its behavior upon low-temperature annealing is investigated. It is shown that structural relaxation starts at temperatures of 150–200°C and results in local atomic ordering at the network boundaries. Crystals 2–5 nm in size start to undergo nucleation at the boundaries of structural heterogeneities when heated further to 250–300°C. The nanocrystal structure corresponds to the metastable phase delta-Co (ICSD 42684) and the unknown phase Co_{1 ? x} P _x . The estimated diffusion coefficient for CoP alloy is 10^?14 m² s^?1, according to the experimental data.     

Ni51Zr49非晶合金晶化过程的电子显微镜研究

用透射电子显微镜对Ni₅₁Zr₄₉非晶合金的晶化过程进行了研究,发现晶化初期首先从非晶基体上析出的是具有C心单斜点阵的NiZr＇相,a=0.3268nm,b=0.4101nm,c=0.5224nm,β=71.8°。与稳定相NiZr有很明确的取向关系:[100]_NiZr＇∥[100]_NiZr [001]_NiZr＇∥[110]_NiZr。它可以看成是NiZr相的亚稳态或超结构。随温度升高,逐渐转变成NiZr相。温度再高时,在非晶基体上还会析出少量Ni₁₀Zr₇相。晶化初期,无论是NiZr还是NiZr＇相,都是以多次孪晶的形式析出。孪晶有这两种基本形式,一种是以NiZr或NiZr＇的[021]方向为轴的180°孪晶,一种是以NiZr的[001]或NiZr＇的[010]方向为轴的36°旋转孪晶,其它孪晶为这两种孪晶方式的重复与组合。随着温度的升高,孪晶逐渐减少。 关键词：     

Complexes d'autoassociation du cyclohexanol. I - modele thermodynamique des solutions diluees.

Activity coefficients for cyclohexanol in diluted isooctane solutions at 25°C and 45°C are obtained from vapour pressure measurements, x_u = 10^?3 to 5.10^?2 (x_u mole fraction of cyclohexanol) ; enthalpies of mixing for the same composition range are obtained at 25°C. The results are interpreted in terms of the model of ideal association where open dimer and trimer, open and cyclic other i-mers are considered in a six parameters model : two standard enthalpies for formation of the H-bonds (Δ$\text{H}$_2o for the dimer and h for the stepwise formation for other bonds), four equilibrium constants (K_2o and K_3o for the dimer and trimer formation from the monomer, K_c for the cyclisation of an open i-mer and k for the stepwise formation of an open, or cyclic, i-mer higher than the trimer). The cyclic form of the tetramer is shown to be the predominent one.     

Synthesis and electrochemical properties of P2-Na<Subscript>2/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>2/3</Subscript>O<Subscript>2</Subscript>

The pure phase P2-Na_2/3Ni_1/3Mn_2/3O₂ was synthesized by a solid reaction process. The optimum calcination temperature was 850 °C. The as-prepared product delivered a capacity of 158 mAh g^?1 in the voltage range of 2–4.5 V, and there was a phase transition from P2 to O2 at about 4.2 V in the charge process. The P2 phase exhibited excellent intercalation behavior of Na ions. The reversible capacity is about 88.5 mAh g^?1 at 0.1 C in the voltage range of 2–4 V at room temperature. At an elevated temperature of 55 °C, it could remain as an excellent capacity retention at low current rates. The P2-Na_2/3Ni_1/3Mn_2/3O₂ is a potential cathode material for sodium-ion batteries.