Features of the pressure-induced phase transitions in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The structural changes induced by a 9-GPa pressure in Eu₂(MoO₄)₃ single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ～ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.     

The memory effect in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals exposed to high hydrostatic pressure

The influence of high-pressure hydrostatic compression and subsequent annealing on the structural properties of β’-phase single crystals of europium molybd ate was studied by IR spectroscopy and X-ray diffraction. It was demonstrated that after compression, the IR spectra exhibit drastic changes and do not depend on the initial crystallographic orientation of a single crystal. The high-pressure compression of single crystals followed by their annealing was found to result in the formation of β’-Eu₂(MoO₄)₃ single crystals with the same crystallographic orientation as that of the initial samples.     

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.     

Single-crystal structural study of the pressure-temperature-induced dimerization of C<Subscript>60</Subscript>

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C₆₀ single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C₆₀ dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C₆₀-C₆₀ dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.Received: 3 October 2003, Published online: 19 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 61.43.Bn Structural modeling: serial-addition models, computer simulation - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

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.     

An inhomogeneously distorted state of the crystal structure of a Zn<Subscript>0.95</Subscript>Fe<Subscript>0.05</Subscript>Se cubic crystal

The structural state of a bulk Zn_0.95Fe_0.05Se cubic crystal grown by the chemical transport method from the gas phase has been investigated using thermal neutron diffraction at room temperature. It has been found that the measured neutron diffraction patterns of the crystal, in addition to structural Bragg peaks, contain a clearly identified system of superstructure reflections with the wave vector k = (1/3 1/3 1/3)2π/a (where a is the parameter of the cubic unit cell), which is interpreted as a clear evidence of the incipient transition state preceding the concentration phase transformation fcc ? hcp. It has been shown that the resulting structural state includes an inhomogeneous microstrain field with the possible appearance of long-wavelength modulations based on the initial sphalerite structure.     

Two-dimensional antiferromagnetic correlations in an La<Subscript>1.4</Subscript>Sr<Subscript>1.6</Subscript>(Mn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystal

The temperature and field dependences of the magnetization, the electrical resistivity, and the magnetostriction of bilayer lanthanum manganite La_1.4Sr_1.6Mn₂O₇ single crystals and cobalt-doped La_1.4Sr_1.6(Mn_0.9Cu_0.1)₂O₇ are measured. The magnetostriction of the cobalt-doped compound increases as compared to the initial La_1.4Sr_1.6Mn₂O₇ compound, and the magnetization and the magnetoresistance of the former compound change substantially. Powder and single-crystal neutron diffraction patterns are used to detect ferromagnetic ordering in La_1.4Sr_1.6(Mn_0.9Co_0.1)₂O₇ at a temperature below T _C ~ 45(2) K, and this ordering coexists with antiferromagnetic correlations, which develop at temperatures below T _C ~ 80(5) K.     

Magnetic properties of aerugite Co<Subscript>10</Subscript>Ge<Subscript>3</Subscript>O<Subscript>16</Subscript>

This paper reports on the first study of the temperature and field dependences of the magnetization, heat capacity, and electrical properties of synthesized polycrystalline samples of aerugite Co₁₀Ge₃O₁₆, as well as on x-ray diffraction analysis of this compound. It is shown that the cobalt ions in this compound occupy three nonequivalent positions. The results of the experimental and theoretical studies suggest that aerugite is a ferrimagnet with two uncompensated magnetic moments of the cobalt atom per formula unit.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Muon-spin-relaxation investigation of EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The magnetic properties of the EuMn₂O₅ multiferroic (samples consisting of single crystals and ceramic samples) have been investigated by the muon-spin-relaxation (μSR) method in the temperature range of 10–300 K. Below the magnetic ordering temperature T _N = 40 K, the loss of the polarization of muons and the effect of the external magnetic field have been observed. Both phenomena can be explained by an additional channel of the depolarization of muons owing to the appearance of muons in a medium with a low electron density due to the charge separation process (the redistribution of the electron density in the phase transition process). The “memory” phenomenon has been revealed in a sample in the external magnetic field; the memory relaxation time depends on the size of the structure units of the samples (single crystals or ceramic grains).     

Effect of charge carriers on the magnetic properties of the CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript> ferromagnetic semiconductor

The results of an investigation into how the composition of annealing mixtures influences the Curie temperature of single crystals of the CdCr₂Se₄ ferromagnetic semiconductor are reported. The mechanism of doping of single crystals is analyzed, and the role played by each of the components of the annealing batch is established. It is concluded that the indirect exchange involving n-type carriers near impurity gallium ions leads to a sharp increase in the phase transition temperature (from 130 to 172 K) of the samples. The dependences of the magnetization of samples with different phase transition temperatures on the temperature and magnetic field are compared.     

Low-temperature neutron diffraction study of La<Subscript>0.95</Subscript>Nd<Subscript>0.05</Subscript>CrO<Subscript>3</Subscript>

We have synthesized polycrystalline La_0.95Nd_0.05CrO₃ sample by doping the La-site of LaCrO₃ with Nd and its magnetic properties have been studied using DC magnetization and neutron diffraction techniques. DC magnetization study shows a paramagnetic to a weak ferromagnetic-like transition at ∼295 K followed by signatures of a spin reorientation phenomenon at 233 and 166 K and, finally a transition to an antiferromagnetic-like phase at ∼21 K. Low-temperature neutron diffraction measurements confirm a weak ferrimagnetic ordering of Cr³⁺ moments at all temperatures below 295 K.     

HgO-added YB<Subscript>a2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>−δ superconductors

The HgO-added YB_a2Cu₃O_7−δ (YBCO) superconductor has been studied for its structural and superconducting properties. Polycrystalline YBCO samples were synthesized through solid-state reaction method by adding HgO in different concentrations without using oxygen annealing. All the samples showed a sharp superconducting transition temperature around 90 K. The X-ray diffraction patterns of all the samples revealed monophasic Y-123 nature. The structural studies were carried out by neutron scattering and Rietveld analysis. The neutron scattering revealed that Hg is not incorporated in the Y-123 system and has shown optimum oxygen concentration. The significant role played by the HgO is to provide oxygen ambient through its decomposition, thus changing the oxygen balance in favour of high Cu-valence state.     

X-ray study of microcrystalline Hg<Subscript>2</Subscript><Emphasis Type="Italic">Hal</Emphasis><Subscript>2</Subscript> ferroelastics

The rhombic splitting of basal plane reflections and the thermal behavior of fundamental and diffuse reflections from X-points of the Brillouin zone characterizing the behavior of an order parameter and its fluctuations, respectively, were studied in polycrystalline Hg₂Cl₂ and Hg₂Br₂ samples. In the case of polycrystalline samples, a strong broadening of phase transition effects was observed, due to damaged surface layers and elastic and plastic stress fields inducing order parameter fluctuations over a wide temperature range.     

PLD and RF-PLD synthesis of Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films for electrically controlled devices

Ba_0.6Sr_0.4TiO₃ (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance–voltage (C–V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.     

A novel synthesis of FeNbO<Subscript>4</Subscript> nanorod by hydrothermal process

This study reports a facile, gram-scale synthesis of FeNbO₄ nanorods via hydrothermal route, using iron nitrate [Fe(NO₃)₃] and niobium tartarate (Nb tartarate) in presence of potassium peroxosulfate. The formation of single phase, polycrystalline orthorhombic structure of FeNbO₄ was confirmed by the careful analysis of the X-ray diffraction (XRD) pattern. The average crystallite size, calculated using a few XRD peaks, was found to be 12.8 nm. As indicated by transmission electron microscopy (TEM) and field emission scanning electron microscopy, the average length and diameter of the rods were found to be only 25 × 7 nm and 47 × 14 nm, respectively. The selected area electron diffraction and high-resolution transmission electron microscopy (HRTEM) data of the single rod implied that FeNbO₄ nanorods were polycrystalline in nature and grew up along the c-axis. HRTEM also revealed that the fringes are equidistant with a lattice separation of 0.91 Ǻ, which corresponded to the (111) plane of the FeNbO₄ crystal. Elemental composition of the nanorods was confirmed using electron dispersive X-ray spectroscopy analysis while binding state of the surface was intervened through X-ray photoelectron spectroscopy. Mechanistic investigations suggested that potassium peroxosulfate played a crucial role in the unidirectional growth of particles. The synthetic method is simple, amenable to scale up and contributes a new tool box for the development of FeNbO₄-based one-dimensional (1D) structures that appears to be more promising for a myriad of applications, compared to their 3D counterparts.     

Formation of nanodomain ensembles during polarization reversal in Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript>: Ce single crystals

The results of the study of nano- and microdomain structure evolution in single crystals of relaxor ferroelectric strontium barium niobate Sr_0.61Ba_0.39Nb₂O₆ doped by cerium are presented. It was shown that the initial nanodomain structure represents a self-similar three-dimensional maze. The fractal dimension and average period were revealed. It was demonstrated that application of series of alternating electric field pulses makes it possible to produce a single-domain state in the surface layer. The features of the growth and “merging,” as well as the shape of ensembles of isolated nanodomains formed during switching from the single-domain state, were investigated. The formation of the nanodomain ensembles was considered as a result of the self-organized discrete switching controlled by determined nucleation.     

Improvement of the intergranular pinning energy in uniaxially compacting (Bi-Pb)<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10+δ</Subscript> ceramic samples

Measurements of the electrical resistivity as a function of temperature, ρ(T), for different values of applied magnetic field, B_a (0 ≤ B_a ≤ 50 mT), were performed in polycrystalline samples of Bi_1.65Pb_0.35Sr₂Ca₂ Cu₃O_10+δ subjected to different uniaxial compacting pressure (UCP). We have found appreciable differences in the grain orientation between samples by using X-ray diffractometry. From the X-ray diffraction patterns performed, in powder and pellet samples, we have estimated the Lotgering factor along the (00l) direction, F_(00l). The results indicate that F_(00l) increases ~23% with increasing UCP suggesting that grains of these samples are preferentially aligned along the c-axis, which is parallel to the compacting direction. The resistive transition of the samples have been interpreted in terms of the thermally activated flux-creep model. In addition, the effective intergranular pinning energy, U₀, have been determined for different applied magnetic field. The magnetic field dependence of U₀, for B_a > 8 mT, was found to follow a H^{- α} dependence with α = 0.5 for all samples. The analysis of the experimental data strongly suggested that increasing UCP results in appreciable changes in both the grain alignment and the grain connectivity of the samples. We have successfully interpreted the data by considering the existence of three different superconducting levels within the samples: the superconducting grains, the weak-links, and the superconducting clusters.     

Dielectric and optical properties of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)<Subscript>0.8</Subscript>Ti<Subscript>0.2</Subscript>O<Subscript>3</Subscript> (PMNT-0.2) ferroelectric relaxor single crystals

The dielectric and optical (optical transmission, small-angle light scattering, birefringence) properties of PMNT-0.2 single crystals and their variation induced by a dc electric field have been studied. The birefringence was found to increase anomalously at the transition from the rhombohedral ferroelectric to the inhomogeneous relaxor phase (the spontaneous ferroelectric transition temperature T_sp). Below T_sp, the dielectric and optical properties were observed to exhibit anomalies originating from reorientation and growth of domains in size. Unlike ferroelectric relaxors of the type of PbB′_1/3B′_2/3O₃ and PbB′_1/2B′_1/2O₃, in PMNT-0.2 neither induction of the ferroelectric phase by an electric field nor thermally stimulated destruction of the ferroelectric state occurs through the percolation mechanism (i.e., they are not accompanied by anomalously narrow maxima in small-angle light scattering). This is attributed to the inhomogeneous structure of the relaxor phase, as a result of which the phase transition does not take place simultaneously in various regions of the crystal.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.