Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Phase transition in CdHfO<Subscript>3</Subscript>

The phase transition from an orthorhombic phase (space group Pnma) to a rhombohedral phase (space group R3m) of the CdHfO₃ hafnate is investigated using methods of structural analysis. It is shown that crystal lattices of both phases contain polar structural units (octahedra, cubooctahedra). On this basis, it is assumed that the orthorhombic and rhombohedral phases of the CdHfO₃ compound are the antiferroelectric and ferroelectric phases, respectively.     

The effects of stoichiometric changes in LaMnO<Subscript>3</Subscript>

The preparation of LaMnO₃ from various precursors via a solid-state reaction combined with codeposition and sorption is found to form perovskite phases with different stoichiometric deviations, depending on the temperature of synthesis. This favors the coexistence of orthorhombic (in solid-phase synthesis) and rhombohedral (in wet synthesis) phases at room temperature.     

Shear-induced low-dimension electron transport in (LaMnO<Subscript>3</Subscript>)<Subscript>2</Subscript>/(SrMnO<Subscript>3</Subscript>)<Subscript>2</Subscript> superlattice

Effects of a mechanical shear on the electron transport properties of a (LaMnO₃)₂/(SrMnO₃)₂ superlattice are investigated using first-principle DFT calculations. While the unstrained superlattice is a 3-D conducting half metal, application of a pyramidal shear transforms it into a non-spin-polarized conductor. Depending on whether the out-of-plane component of the shear is tensile or compressive the conductivity is 1-D out-of-plane or 2-D in-plane. The shear-induced low-dimensional conductivity is also associated with the FM-AFM transition.     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Impedance investigation on porous Sr-doped LaMnO<Subscript>3</Subscript> films onto Sr-Mg-doped LaGaO<Subscript>3</Subscript> electrolyte

The use of Sr-Mg-doped LaGaO₃ (LSGM), a highly conducting oxygen ion electrolyte, in intermediate temperature solid oxide fuel cells (IT-SOFC) technology requires suitable electrode materials. Because the Sr-doped LaMnO₃ (LSM) cathode coupled with the YSZ electrolyte had shown relatively good performances, it has been also suggested for LSGM-based cells. As cathode overpotential is the main performance limitation, the optimization of the LSM/LSGM interface might be of fundamental relevance in the technology development of LSGM-based IT-SOFC. LSM films with different porosities were screen printed on both faces of the LSGM pellets; their morphology and electrical properties were investigated by scanning electron microscopy and impedance spectroscopy, respectively. Porosity was induced by the addition of icing sugar (3–5 wt%) to the LSM powder during ink preparation. Homogeneous electrode layers with a thickness of about 30 μm were obtained. The higher the sugar amount, the higher the film porosity and the lower the LSM grain size. The Nyquist plots show two low frequency arcs to which the charge transfer and mass transfer process or oxygen dissociation process were associated, respectively. One can expect that the best electrochemical performances are obtained using highly porous electrode. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Phase transition in Bi<Subscript>8</Subscript>Fe<Subscript>6</Subscript>Ti<Subscript>3</Subscript>O<Subscript>27</Subscript> multiferroic ceramics

The polycrystalline Bi₈Fe₆Ti₃O₂₇ compound was prepared by a high-temperature solid-state reaction technique. Preliminary structural analysis by X-ray diffraction (XRD) confirms the formation of a single-phase compound in an orthorhombic crystal system at room temperature. The elemental content of the compound was analyzed by EDAX microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the pellet sample. Detailed studies of temperature-dependent dielectric response at various frequencies show dielectric anomalies at 380, 389 and 403°C for 10 kHz, 100 kHz, and 1 MHz respectively. The hysteresis loop observed by applying an electric field of 12 kV/cm on the poled sample with smaller remanent polarization supports the existence of ferroelectricity in this material. The value of d₃₃ of the compound was found to be 19 pC/N.      

Phase behaviour and superionic phase transition in K<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The phase behaviour of K₃H(SeO₄)₂ (TKHSe) above room temperature has been studied by differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), simultaneous thermogravimetric and mass spectroscopy analysis (TG-MS), impedance spectroscopy (IS) and X-ray powder diffraction (XRD). Our results show that the previously claimed superionic phase transition in TKHSe at around 388 K (114.85 °C) is also the onset temperature of a slow thermal dehydration that occurs at reaction sites distributed over the surface of the crystal. That is, we propose that the TKHSe undergoes simultaneously a superionic phase transition and a decomposition process with a very slow reaction rate that is evident when the sample is pulverized to fine powder, both starting at the same temperature. As a matter of fact, we observe a decrease of the magnitude of the dc-conductivity on successive thermal runs in powdered sample attributed to sample decomposition that starts at the surface of the TKHSe grains, but the jump in conductivity is only a consequence of the order–disorder transition in the TKHSe phase that remains inside the grains.     

Two-dimensional electron gas at the interface of Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> ferroelectric and LaMnO<Subscript>3</Subscript> antiferomagnet

The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO₃ single crystals of different orientations with epitaxial films of ferroelectric Ba_0.8Sr_0.2TiO₃ deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO₃ single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K. The numerical simulations of the structural and electronic characteristics of the BaTiO₃/LaMnO₃ ferroelectric?antiferromagnet heterostructure has been performed. The transition to the state with two-dimensional electron gas at the interface is demonstrated.     

Phase transition in a KPb<Subscript>2</Subscript>Br<Subscript>5</Subscript> crystal

Crystals of the KPb₂Br₅compound are investigated using polarized light microscopy and calorimetry. The birefringence and the angle of rotation of the optical indicatrix are measured in the temperature range 270–620 K. It is found that the KPb₂Br₅ crystal undergoes a first-order ferroelastic phase transition at temperatures T_0↑ = 519.5 K and T_0↓ = 518.5 K with a change in the enthalpy ΔH = 1300 ± 200 J/mol. This transition is accompanied by both twinning and the symmetry change mmm ? P2₁/c. It is revealed that the angle of rotation of the optical indicatrix exhibits an unusual behavior under variations in the temperature due to a strong temperature dependence of the birefringence.     

Anomalous slowing down of the longitudinal spin relaxation near the antiferromagnetic phase transition in LaMnO<Subscript>3</Subscript> manganite

It has been found that, instead of the expected critical acceleration of the longitudinal spin relaxation near the Néel temperature in stoichiometric LaMnO₃ samples, the relaxation is sharply retarded. This slowing down is similar to that observed earlier in doped manganites with ferromagnetic ordering.     

A study of magnetic ordering in LaMnO<Subscript>3+δ</Subscript> single crystals

The magnetic state of LaMnO_3+δ single crystals with different contents of nonstoichiometric oxygen grown by electric deposition is studied by measuring the temperature and field dependences of magnetization for different orientations of a magnetic field. The results obtained are analyzed in terms of a two-phase magnetic-state model.     

Phase transition effects in polycrystalline Hg<Subscript>2</Subscript>Br<Subscript>2</Subscript> samples

The effects accompanying the ferroelastic phase transition in Hg₂Br₂ polycrystalline samples are compared in an x-ray diffraction study with similar effects observed to occur in Hg₂Br₂ single crystals. In particular, an analysis is made of the “orthorhombic” splitting of the basal plane reflections and the behavior with temperature of the Bragg and diffuse reflections from the X points of the Brillouin zone, which characterize the behavior of the order parameter and its fluctuations, respectively. Polycrystalline samples exhibit strong smearing of the phase transition effects originating from the existence of damaged surface layers and elastic and plastic strain fields which induce order parameter fluctuations over a wide temperature range.     

Thermal engineering of lead-free nanostructured CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>SnCl<Subscript>3</Subscript> perovskite material for thin-film solar cell

Perovskite solar cell is a kind of revolutionary investigation in the field of renewable energy which is capable of mitigates the deficiencies of silicon solar cell and its uprising efficiency can bring blessing to society. The presence of lead (Pb) in perovskite solar cell can make worst and negative impact on environment and is not desirable for our society. In this paper, general plans are anticipated by replacement of Pb with tin (Sn) in open atmosphere to fabricate the CH₃NH₃SnCl₃ photovoltaic cells in chlorine (Cl)-rich environment. Excess uses of Cl has positive influences on morphological growth of the film and it also suppresses the oxidation tendency of tin (Sn) with existing oxygen in atmosphere and maintains same chemical atmosphere as bulk. Various characterization tools like X-ray diffraction, scanning electron microscope (SEM) have been used to study the effect of annealing temperature on crystal stricture, phase formation, impurities, and morphologies of the film. Finally, photovoltaic performance was reported using the solar simulator under 1.5 sun illumination.

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Phase transitions in the oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>NbOF<Subscript>6</Subscript>

(NH₄)₃NbOF₆ single crystals were grown, polarization-optical studies were performed, and birefringence was measured over the temperature range 90–500 K. A sequence of first-order structural phase transitions was found at temperatures T _1↓ = 259.7 K and T _2↓ = 257.7 K with temperature hysteresis δT ₁ = 0.9 K and δT ₂ = 1.9 K. The transitions are accompanied by twinning and the following change in the crystal symmetry: cubic ? tetragonal ? monoclinic. Optical second harmonic generation is found to occur at room temperature, which indicates that the cubic phase is not centrosymmetric. It is assumed that the phase transitions are ferroelastic and ferroelectric in nature.     

Galvanomagnetic transport properties of nitrogenated (La,Sr)MnO<Subscript>3-δ</Subscript> and LaMnO<Subscript>3-δ</Subscript>

Epitaxial thin films of nitrogenated La_0.65Sr_0.30MnO₃ were grown on MgO(100) substrates by pulsed laser deposition (PLD). The nitrogenation was achieved by a continuous nitrogen flow in the PLD chamber with pressures of up to 0.12 mbar. The chemical analysis of the samples regarding the exchange of oxygen by nitrogen was achieved by time of flight secondary ion mass spectrometry, sputtered neutral mass spectrometry (SNMS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and yielded a content of incorporated nitrogen ranging from 0.6% to 3.8%. Without nitrogenation the electrical resistivity of La_0.65Sr_0.30MnO₃ exhibited a metal–insulator (MI) transition at about 180 K. The magnetoresistance (MR) effect (ΔR/R(0)) was about -50% at the transition temperature. Our nitrogen contents affected the MI transition so as to completely disappear and resulted in a resistivity increase of more than three orders of magnitude as well. By carefully reoxidizing the samples with subsequent heat treatments in air the MI transition reappeared at lower temperatures and we found a continuously enhanced MR ratio for decreasing temperatures. MR ratios of more than -99% were observed for a magnetic field of 10 T. The results are interpreted as a percolation phenomenon of ferromagnetic–metallic domains within an antiferromagnetic–semiconducting matrix. PACS 75.47.Gk; 75.47.Lx; 74.62.Dh; 81.15.Fg     

Electrochemical behavior of carbonic precursor with Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>nanostructured material in hybrid battery system

The nanostructured Na₃V₂(PO₄)₃ (NVP) cathode material has been synthesized using the sol-gel route for different molar fractions of citric acid as a carbon source during the synthesis. The nanostructured NVP as cores with carbonic shell structures are fabricated with two different citric acid molar ratios. The thermal treatment has been optimized to convert the amorphous carbon shell into graphitic carbon to realize the better electrical conductivity and thus effective electron transfer during the electrochemical charge transfer process. The X-ray diffraction measurements confirmed the rhombohedral crystallographic phase (space group R-3c) with average crystallite size ~28 ± 5 nm. The coin cells are assembled in a hybrid rechargeable electrochemical cell configuration with lithium as a counter electrode and LiPF₆-EC:DEC:DMC (1:1:1 ratio) as the electrolyte. The electrochemical charge/discharge measurements are carried out at C/10 and C/20 rates and the measured specific capacities are 80 and 120 mAhg^?1 for samples with lower and higher citric acid molar ratios, respectively. The studies suggest that NVP can be used as an effective cathode material in hybrid electrochemical cells, and a higher concentration of citric acid may result in the effective carbonic shell for optimal electron transfer and thus enhanced electrochemical performance.     

Phase transition with suppression of magnetism in BiFeO<Subscript>3</Subscript> at high pressure

The magnetic behavior of a Bi⁵⁷FeO₃ powdered sample was studied at high pressures by the method of nuclear forward scattering (NFS) of synchrotron radiation. The NFS spectra from ⁵⁷Fe nuclei were recorded at room temperature under high pressures up to 61.4 GPa, which were created in a diamond anvil cell. In the pressure interval 0 < P < 47 GPa, the magnetic hyperfine field H^Fe at the ⁵⁷Fe nuclei increased reaching a value of ～52.5 T at 30 GPa, and then it slightly decreased to ～49.6 T at P = 47 GPa. As the pressure was increased further, the field H^Fe abruptly dropped to zero testifying a transition from the antiferromagnetic to a nonmagnetic state (magnetic collapse). In the pressure interval 47 < P < 61.4 GPa, the value of H^Fe remained zero. The field H^Fe recovered to the low-pressure values during decompression.