Search results for "Solid"
showing 10 items of 3575 documents
Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe
2007
Phase-change materials are of immense importance for optical recording and computer memory, but the structure of the amorphous phases and the nature of the phase transition in the nanoscale bits pose continuing challenges. Massively parallel density functional simulations have been used to characterize the amorphous structure of the prototype materials ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and GeTe. In both, there is long-ranged order among Te atoms and the crucial structural motif is a four-membered ring with alternating atoms of types $A$ (Ge and Sb) and $B$ (Te), an ``$ABAB$ square.'' The rapid amorphous-to-crystalline phase change is a reorientation of disordered $ABAB$ …
Nature of the phase transition in spin crossover compounds
1990
Abstract Starting from the phenomenological free energy describing the spin equilibrium of continuous or gradual high spin (HS) ⇌ low spin (LS) transitions a reduced equation of state has been derived which is of the type known in mean field theories. The continuous HS ⇌ LS transitions of [Fe(2-pic-ND 2 ) 3 ]Cl 2 ·EtOD (2-pic = 2-picolylamine) at ambient pressure and p = 1200 bar and of [Fe(2-pic) 3 ]Cl 2 -MeOH can be classified as isobars above the critical point of the system. Around and below the critical point a complex behaviour is expected for thermodynamic reasons combined with the consequences of an elastic interaction mechanism between the HS and LS complex molecules in the crystal…
Electrical conductivity and micro-Raman scattering studies of ionic conduction in Li1−xHxIO3 solid solutions
2002
Abstract Li 1− x H x IO 3 solid solutions have been investigated by ac electrical conductivity and micro-Raman techniques, for x ≤0.32. The presence of protons leads to a continuous reduction of the anisotropic intrinsic conduction of the system. The in-plane conduction mechanism would be by Li + vacancy hopping, while a mixed process by interstitial Li + and H + would account for the conduction along the c -axis. The solid solutions undergo the same phase transition sequence as the pure crystal, but the presence of the protons shifts the transition temperatures to lower values. Micro-Raman spectroscopy has been successfully introduced to demonstrate that proton mobility occurs preferential…
Phase transitions in modified Na 1/2 Bi 1/2 TiO 3 ‐SrTiO 3 solid solutions
2009
Continuous transfer from relaxor to normal ferroelectric phase transition is found in Na1/2Bi1/2TiO3-SrTiO3-PbTiO3 if concentration of PbTiO3 increases. The relaxor properties in NBT solid solutions are described by the power law and Vogel-Fulcher relationship, and concentration dependence of the relevant parameters is discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Self-Ordered Second-Component Nb Clusters in KNbxTa1?xO3 Solid Solutions and Their Physical Properties
1999
Semi-empirical Hartree-Fock calculations using the intermediate neglect of the differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO 3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that a seven-particle cluster in KTaO 3 :Nb has two types of states with different nature. Namely, a state with a dilatation combined with an off-center displacement of the central Nb ion in [111] directions in the cluster, and a state with a full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-…
Crystal structure and dielectric properties of the [(CH3)2NH2]3Sb2(1-x)Bi2xCl9(DMACAB) mixed crystals
2001
Phase transitions in [(CH3)2NH2]3Sb2(1-x)Bi2xCl9 (DMACAB) mixed salts in the composition range 0≤x≤0.41 have been investigated by the pyroelectric method and dielectric measurements over the frequency range from 75 kHz to 900 MHz. The phase situation is additionally confirmed by the differential scanning calorimetry (DSC) and dilatometric techniques. A transition from the paraelectric (PE) to the ferroelectric (FE) phase is observed for crystals with 0≤x≤0.14. Pyroelectric measurements support the presence of polar phases. The dynamic dielectric behaviour of ferroelectric systems is found to be determined by the existence of two independent relaxators. The low-frequency relaxator reveals a …
Observation of random-field behavior in (CO)1−x(N2)xmixtures physisorbed on graphite
1993
Heat-capacity studies have been carried out on submonolayers of (CO${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(${\mathrm{N}}_{2}$${)}_{\mathit{x}}$ mixtures physisorbed on graphite. We find that pure CO undergoes an end-to-end ordering transition at ${\mathit{T}}_{\mathit{c}}$=5.18 K, which is shown to belong to the two-dimensional Ising universality class. In diluting CO with ${\mathrm{N}}_{2}$ the heat-capacity peaks become progressively rounded until the phase transition is completely suppressed at a ${\mathrm{N}}_{2}$ concentration of only x=0.07. The analysis of the data shows that this behavior is in quantitative agreement with predictions of the random-field theory applied to adsorbed…
Analytical solution of 1D Ising-like systems modified by weak long range interaction
1999
It is well-known that 1D systems with only nearest neighbour interaction exhibit no phase transition. It is shown that the presence of a small long range interaction treated by the mean field approximation in addition to strong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling leads to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values for nearest neighbour interaction constants. For this interaction scheme an analytical solution has been derived and the interplay betwe…
Alkaline cavities and tetrahedra in and solid solutions
1997
Seven different crystals were grown with 0 < x < 2 while seven different crystals were grown with , the limit value x = 1 corresponding to the clearly defined compound. A crystallographic study of these solid solutions was performed by comparison with the pure crystal. Structures were solved in the Pnma space group for the rubidium solid solutions and in the space group for the potassium ones. It was shown that the distribution of Cs and Rb, or Cs and K, among the two cationic sites called and was not random: Rb and K exhibit a higher affinity for the smallest cavity, the site. While the phase transition of and can be interpreted as rotations of the rigid tetrahedra mainly around the a dire…
ChemInform Abstract: Chemistry and Properties of Solids with the (NZP) Skeleton
2010
Abstract Solids with an [NZP] skeleton in their structure from a very broad family presenting very different properties with varying chemical composition. These properties range from radioactive nuclide immobilization and insulation to ionic and/or electronic conduction, extending to the field of very low thermal expansion among others. The open [NZP] skeleton provides the stability and flexibility necessary to form continuous ranges of solid solution, allowing the tailoring of the best composition-property-structure material. Chemical activity may become particularly high at the surface and absorption reactions and catalytic properties have the highest interest at present. In this paper, c…