Search results for "Phase Transition"

High-pressure lattice-dynamics of NdVO4

2017

High-pressure Raman-scattering measurements and ab initio calculations on NdVO4 have been carried out up to 30 GPa. Our combined experimental and theoretical study confirms that beyond 5.9 GPa NdVO4 undergoes an irreversible zircon to monazite transition. The coexistence of zircon and monazite phases is experimentally observed up to ~8 GPa (which agrees with the theoretical transition pressure), stabilizing the monazite phase as a single phase around 10 GPa. Calculations additionally predict the existence of a second high-pressure phase transition at 12.4 GPa. This reversible phase transition has been experimentally observed beyond 18.1 GPa and remains stable up to 30 GPa. The post-monazite…

Experimental and Theoretical Investigations on Structural and Vibrational Properties of Melilite-Type Sr2ZnGe2O7 at High Pressure and Delineation of …

2015

We report a combined experimental and theoretical study of melilite-type germanate, Sr2ZnGe2O7, under compression. In situ high-pressure X-ray diffraction and Raman scattering measurements up to 22 GPa were complemented with first-principles theoretical calculations of structural and lattice dynamics properties. Our experiments show that the tetragonal structure of Sr2ZnGe2O7 at ambient conditions transforms reversibly to a monoclinic phase above 12.2 Gpa with similar to 1% volume drop at the phase transition pressure. Density functional calculations indicate the transition pressure at, similar to 13 GPa, which agrees well with the experimental value. The structure of the high-pressure mono…

K+ and Na+ effects on the gelation properties of κ-Carrageenan

2004

The effects of K(+), Na(+) ions and their mixture on the conformational transition and macroscopic gel properties of kappa-Carrageenan system have been studied using different experimental techniques. The macroscopic gelation properties of kappa-Carrageenan were found to be dependent upon cosolute type. Indeed, a more ordered and strong gel was obtained in the presence of K(+) with respect to Na(+) ions. The gel properties obtained using mixtures of two cosolutes are shown to depend on the [K(+)]/[Na(+)] ratio.

Exploring the high-pressure behavior of the three known polymorphs of BiPO4: Discovery of a new polymorph

2015

We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs: trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (P < 0.2-0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneo…

Critical Phenomena at the Surface of Systems Undergoing a Bulk First Order Transition: Are They Understood?

2002

Systems that exhibit a first-order phase transition in the bulk, such as binary alloys where the order parameter vanishes discontinuously at some critical value of a control parameter, may show a continuous vanishing of the local order parameter at the surface. This “surface-induced disordering” is described theoretically as a variant of critical wetting, where an interface between the locally disordered surface and the ordered bulk gradually moves towards the bulk. We test this description by Monte Carlo simulations for a body centered cubic model alloy, with interactions between nearest and next nearest neighbors, for which the phase diagram in the bulk has been calculated very accurately…

Magnetism and variable temperature and pressure crystal structures of a linear oligonuclear cobalt bis-semiquinonate

2016

The crystal structure of the first oligomeric cobalt dioxolene complex, Co3(3,5-DBSQ)2((t)BuCOO)4(NEt3)2, 1, where DBSQ is 3,5-di-tert-butyl-semiquinonate, has been studied at various temperatures between 20 and 200 K. Despite cobalt-dioxolene complexes being generally known for their extensive ability to exhibit valence tautomerism (VT), we show here that the molecular geometry of compound 1 is essentially unchanged over the full temperature range, indicating the complete absence of electron transfer between ligand and metal. Magnetic susceptibility measurements clearly support the lack of VT between 8 and 300 K. The crystal structure is also determined at elevated pressures in the range f…

Lattice dynamics of CuAlO2 under high pressure fromab initio calculations

2007

The density functional perturbation theory is employed to study the vibrational properties of CuAlO 2 under pressure. The calculations are preformed using the pseudopotential wave method and the local density approximation for the exchange-correlation (XC) potential. The d electrons of Cu are treated as valence states. We present the phonon dispersion curves. Our results are in good agreement with the available experimental Raman scattering experiments. Ab initio calculations show the presence of a dynamical instability, possibly related with the experimentally observed phase transition.

Effect of Variable Valence Ion Doping on the Dielectric Properties of BaTiO3–Based Materials

2014

Dielectric properties of BaTiO3, BaTiO3 + 0.1 wt.% Fe2O3 and BaTiO3 + 1 wt.% Fe2O3 ceramics were studied. The dielectric measurements were performed at the temperature ranging from 130 K to 500 K and at the frequency ranging from 0.1 Hz to 10 MHz. Phase transitions were also determined by a thermal analysis in a wide temperature range. Both thermal analysis and electrical characterization techniques show that the temperature of phase transition is shifted towards lower temperatures with increasing Fe2O3 content. The changes and diversity of the observed phase transition temperatures occurred as a consequence of the ion substitution. Such behaviour of the investigated polycrystalline materia…

Critical wetting in the square Ising model with a boundary field

1990

The Ising square lattice with nearest-neighbor exchangeJ>0 and a free surface at which a boundary magnetic fieldH1 acts has a second-order wetting transition. We study the surface excess magnetization and the susceptibility ofL×M lattices by Monte Carlo simulation and probe the critical behavior of this wetting transition, applying finite-size scaling methods. For the cases studied, the results are not consistent with the presumably exactly known values of the critical exponents, because the asymptotic critical region has not yet been reached. Implication of our results for critical wetting in three dimensions and for the application of the present model to adsorbed wetting layers at surfac…

2020

Abstract The structural stability and physical properties of CrVO4 under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressu…