Raman scattering studies on phase transition in barium sodium niobate at high pressure

Phase transition in barium sodium niobate at high pressure is investigated by Raman scattering experiments. Polarized Raman spectra from polished single domain crystal plates of definite orientation are obtained under pressure produced by a diamond anvil device. Anomalous hardening and intensity decrease of 32 cm^-1B₂ mode are found with increasing pressure. A remarkable pressure dependence of the frequencies of the bending modes of NbO₆ octahedrons is observed.     

P-T diagram in the As-S system

Abstract

Chalcogenide glasses in the system As-S are candidates for very low loss fiber-optic materials. In order to modulate physical properties of such glassese by wellaimed pressure variation the knowledge of the P-T diagram is a precondition. Therefore the aim of this investigation was to cheque the energy-dispersive X-ray diffraction method for low symmetry materials like Realgar (AsS) in order to elaborate P-T diagrams in a time reducing technique.     

Phase diagram and physical properties of the sodium niobate-lithium niobate-cadmium niobate three-component system

The phase diagram of the (Na,Li,Cd_0.5)NbO₃ ternary system is refined. The electrical properties of solid solutions over a broad range of the component concentrations were studied. The compositions promising for high-temperature transducers were obtained.     

Diffused ferroelastic phase transition in barium sodium niobate

We report the temperature dependence of the birefringence Δn_ab across the ferroelastic phase transition of Ba₂ NaNb₅O₁₅. The recorded variations correspond to a transition of the diffused type, with an inflection point at 275°C. This behaviour is consistent with the local disorder induced by a well-known lack of stoechiometry of the available crystals. A large thermal hysteresis is also pointed out.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Structural phase diagram of rare-earth-cadmium equiatomic compounds

A structural phase diagram is proposed for rare-earth-cadmium equiatomic compounds essentially established by means of magnetic susceptibility in a SQUID susceptometer. A structural transition occurs at about 210 K throughout the series; for light rare-earth compounds, a second transition line delimits an intermediate phase between 210 K and low temperatures. Both phases are unknown.     

Incommensurate phases in lead potassium niobate and lead sodium niobate,homologues of barium sodium niobate.

Electron diffraction and high resolution electron microscopy reveal the presence of incommensurate structures in lead potassium niobate, lead sodium niobate and in mixed crystals. The incommensurability can be described either as being due to the presence of quasi-periodic “diffuse” anti-phase boundaries or as being due to the presence of deformation waves, the deformation mode being shearing of the NbO₆ octahedra. The incommensurability depends on composition as well as on temperature.     

Mott transition,compressibility divergence,and the P-T phase diagram of layered organic superconductors: an ultrasonic investigation

The phase diagram of the organic superconductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl has been investigated by ultrasonic velocity measurements under helium gas pressure. Different phase transitions were identified through several elastic anomalies characterized from isobaric and isothermal sweeps. Our data reveal two crossover lines that end on the critical point terminating the first-order Mott transition line. When the critical point is approached along these lines, we observe a dramatic anomaly of the sound velocity which is consistent with a diverging compressibility of the electronic degrees of freedom.     

New phase transition with increasing symmetry on cooling in barium sodium niobate

A new and unsual transition is reported in Ba₂NaNb₅O₁₅: the crystal, which was previously known to undergo at 573 K a ferroelastic transition 4mm → mm2 (C_4v → C_2v), reverts back at 110 ± 5 K to a low temperature phase of 4mm (C_4v) symmetry. Thus this crystal presents a unique example of a ferroelastic (mm2) phase stable in a wide temperature range (470 K) and sandwiched between two phases possessing the same higher point symmetry.     

Properties of nonstoichiometric sodium niobate

Stoichiometric and nonstoichiometric sodium niobate of the composition Na_1−x NbO_3−x/2 (0≤x≤0.2) is obtained by the two-stage solid-phase reaction technique. X-ray diffraction study reveals the homogeneity region for compositions below x=0.1. At higher x, the samples become two-phase. The phase transition is found to occur at x=0.04. It is accompanied by a change in the multiplicity of the monoclinic subcell of the sodium niobate rhombic unit cell, as well as by the anomalous behavior of its structural and electrophysical characteristics. From the study of the dielectric and piezoelectric properties of the material, it is concluded that the stability of the antiferroelectric phase is enhanced, whereas the ferroelectric phase becomes less stable, as the nonstoichiometry in terms of Na₂O increases.     

Modeling the phase diagram of carbon

We determined the phase diagram involving diamond, graphite, and liquid carbon using a recently developed semiempirical potential. Using accurate free-energy calculations, we computed the solid-solid and solid-liquid phase boundaries for pressures and temperatures up to 400 GPa and 12 000 K, respectively. The graphite-diamond transition line that we computed is in good agreement with experimental data, confirming the accuracy of the employed empirical potential. On the basis of the computed slope of the graphite melting line, we rule out the hotly debated liquid-liquid phase transition of carbon. Our simulations allow us to give accurate estimates of the location of the diamond melting curve and of the graphite-diamond-liquid triple point.     

Tuning the orthorhombic–rhombohedral phase transition temperature in sodium potassium niobate by incorporating barium zirconate

We report that the orthorhombic–rhombohedral phase transition temperature in sodium potassium niobate could be tuned from lower to higher than room temperature by incorporating barium zirconate into it. It is revealed that when barium zirconate is between 8 mol% and 15 mol%, the solid solution exhibits rhombohedral symmetry at room temperature. Rietveld analysis shows that the space group of the rhombohedral phase is R3m. It is suggested that a relatively larger B‐cation favors the low‐temperature phase which is a common phenomenon for perovskite ferroelectrics. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The ground-state phase diagram of the spinless 1D falicov-kimball model: Numerical studies

Extrapolation of small-cluster exact-diagonalization calculations is used to study the ground state phase diagram of the spinless one-dimensional Falicov-Kimball model at half filling. Our results show that the phase diagram has an extremely simple structure for the Coulomb interactionsU≥2. Here the ground states are the most homogeneous configurations (mhc) with the smallest periods. Valence transitions are discontinuous and only of the type insulator-insulator. In this region the finite size effects are negligible and thus the picture of valence transitions is definitive. ForU<2 the phase diagram exhibits a more complicated structure. Here we have specified a domain where the ground states are the mhc and a metallic domain where the ground states are mixtures of configurations with the empty configuration. The boundary between these two domains is the boundary of discontinuous insulator-metal transitions. Unlike the caseU≥2 the valence transitions are gradual in the weak coupling limit. This work was supported by the Slovak scientific grant agency VEGA, contract No. 4177/97.     

Structural phase diagram for ZnS nanocrystalline thin films under swift heavy ion irradiation

The synthesis of nanocrystalline ZnS thin films by pulsed laser deposition and their modification by swift heavy ions are presented. The irradiations with 150 MeV Ni ions at fluences of 1×10¹¹, 1×10¹² and 1×10¹³ ions/cm² have been used for these studies. Irradiation results in structural phase transformation and bandgap modification of these films are investigated by using X-ray diffraction and UV-visible absorption measurements, respectively. Since stoichiometry changes induced by irradiation can contribute to the modification of these properties, elastic recoil detection analysis has been performed on pristine and 150 MeV Ni ions irradiated ZnS thin films using a 120 MeV Ag ion beam. The stoichiometry of the films has been found to be similar for pristine and ion irradiated samples. A structural phase diagram based on thermal and pressure spikes has been constructed to explain the structural phase transformation.