Symmetry relationship in electron beam induced phase transitions

1989 ◽  
Vol 47 ◽  
pp. 500-501
Author(s):  
J. P. Zhang ◽  
L. D. Marks
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Oxygen Content ◽  
Transition Metal Oxides ◽  
Selection Rule ◽  
Point Group ◽  
Group Symmetry ◽  
Lower Oxygen ◽  
Image Position ◽  
New Phase

In this work we present a symmetry rule for the phase transitions in transition metal oxides driven by DIET of oxygen in an electron microscope. The phase transitions are to a structure with a higher point group symmetry. The new phase with a lower oxygen content is either one with a supergroup symmetry with respect to the original phase, or an amorphous intermediary. If a possible lower oxygen content phase does not have the correct supergroup symmetry, it is not formed. Of this type are the phase transitions in TiO2, MoO3, WO3, V2O5 and Ta2o5 of which the experimental results are listed in the first section of Table 1. It is also found that the point group is conserved during the phase transition if the oxide belongs to the highest groups Oh or D6h, for instance NiO, CoO and ZnO. This symmetry selection rule can therefore be used to predict the route of the phase transition. The selection rule is:

Phase Transitions, Superconductivity, and Ferromagnetism in Fullerene Systems

MRS Bulletin ◽  
1994 ◽  
Vol 19 (11) ◽  
pp. 28-30 ◽  
Author(s):  
C.N.R. Rao ◽  
Ram Seshadri
Keyword(s):  
Solid State ◽  
Alkaline Earth ◽  
Point Group ◽  
Group Symmetry ◽  
High Electron ◽  
Chemical Transformations ◽  
Disordered Solids ◽  
High Electron Affinity ◽  
Centers Of Mass ◽  
Image Position

By virtue of their unique structures, fullerenes exhibit novel chemical transformations. Particularly pertinent to this article are the interesting properties exhibited by fullerenes in the solid state. These molecules are spherical or near-spherical in shape. Molecules with high point-group symmetry, which are not bound strongly in the solid state, tend to crystallize into structures with long-range periodicity of the molecular centers of mass, but the molecular orientations are random or even dynamically disordered. When dynamically disordered, themolecules rotate about some preferred axis. C60 and C70 satisfy the criteria for such orientationally disordered solids and exhibit rich phase behavior in the solid state. Since C60 has high electron affinity, it forms anion salts with alkali and alkaline-earth metals as well as with strong organic donor molecules. With tetrakis dimethylaminoethylene (TDAE), which is a very powerful electron donor, C60 forms a 1:1 solid that is ferromagnetic. C60-TDAE is the molecular organic ferromagnet with the highest Tc (of 16 K) known to date. Some of the alkali and alkaline-earth fullerides, on the other hand, show superconductivity, with transition temperatures going up to 33K. We shall briefly examine some of these solid-state properties.

X-ray diffraction and Raman study of DL-alanine at high pressure: revision of phase transitions

2012 ◽  
Vol 68 (4) ◽  
pp. 412-423 ◽  
Author(s):  
Nikolay A. Tumanov ◽  
Elena V. Boldyreva
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Structural Phase ◽  
Group Symmetry ◽  
Second Phase ◽  
Structural Phase Transitions ◽  
X Ray Diffraction ◽  
X Ray ◽  
Cell Parameters ◽  
Raman Study

The effect of pressure on DL-alanine has been studied by X-ray powder diffraction (up to 8.3 GPa), single-crystal X-ray diffraction and Raman spectroscopy (up to ∼ 6 GPa). No structural phase transitions have been observed. At ∼ 1.5–2 GPa, cell parameters b and c become accidentally equal to each other, but the space-group symmetry does not change. There is no phase transition between 1.7 and 2.3 GPa, contrary to what has been reported earlier [Belo et al. (2010). Vibr. Spectrosc. 54, 107–111]. The presence of the second phase transition, which was claimed to appear within the pressure range from 6.0 to 7.3 GPa (Belo et al., 2010), is also argued. The changes in the Raman spectra have been shown to be continuous in all the pressure ranges studied.

A structural phase transition in NaTaOGeO4 and its relation to phase transitions in titanite

2007 ◽  
Vol 63 (4) ◽  
pp. 545-550 ◽  
Author(s):  
Thomas Malcherek
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Transition Metal ◽  
Structural Phase Transition ◽  
Order Parameter ◽  
Diffuse Scattering ◽  
Structural Phase ◽  
Group Symmetry ◽  
Critical Temperatures ◽  
Symmetry Properties

A structural phase transition from space-group symmetry P21/c to C2/c is reported for NaTaOGeO4 (NTGO). The critical temperature has been located at T c = 116 K, based on the appearance of sharp diffraction maxima at positions h + k = 2n + 1 of reciprocal space on cooling below this temperature. Strongly anisotropic diffuse scattering in sheets normal to [001] is observable for T > T c and persists up to ambient temperature. Similarities to phase transitions observed in other compounds of the titanite structure type are discussed. The symmetry properties of these phase transitions are reassessed on the basis of the structural data available. The primary order parameter is identified with the displacement of the transition metal cation M (M = Ta in NTGO) away from the centre of symmetry that it nominally occupies in the paraphase. The order parameter transforms as the Y_{2}^{-} representation. The anisotropic diffuse scattering is attributed to the one-dimensional correlation of local M displacements parallel to the direction of chains of trans-corner-sharing MO6 octahedra. The critical temperatures of the isomorphous phase transitions in various titanite-type compounds depend linearly on the squared transition-metal displacement measured in the ordered P21/c phase.

Entropy diagnosis for phase transitions occurring in functional materials

2005 ◽  
Vol 77 (8) ◽  
pp. 1331-1343 ◽  
Author(s):  
Michio Sorai
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Selection Rule ◽  
Functional Materials ◽  
Condensed State ◽  
Molecular Motions ◽  
Good Tool ◽  
The Stability ◽  
Calorimetric Investigations ◽  
Good Probe

Functionalities of materials manifest themselves as the result of a concerted effect among molecular structure, intermolecular interactions, and molecular motions. Since the entropy of substance directly reflects the degree of molecular motions, the entropy plays a crucial role when one discusses the stability of a given phase at finite temperatures. When a delicate balance of these three factors is broken, the condensed state faces a catastrophe and is transformed into another phase. Therefore, phase transition is a good probe for elucidation of the interplay between these three factors. As there is no selection rule in thermodynamics, the entropy gain at the phase transition is a good tool to diagnose the mechanism of phase transition. In this presentation, calorimetric investigations aimed at the elucidation of the mechanisms governing phase transitions occurring in molecule-based functional materials are reviewed.

scholarly journals Phase transition sequences in tetramethylammonium tetrachlorometallates by X-ray diffraction and spectroscopic measurements

2017 ◽  
Vol 73 (5) ◽  
pp. 844-855 ◽  
Author(s):  
Jack Binns ◽  
Garry J McIntyre ◽  
José A. Barreda-Argüeso ◽  
Jesús González ◽  
Fernando Aguado ◽  
...  
Keyword(s):  
Phase Transition ◽  
Point Group ◽  
Lone Pair ◽  
Group Symmetry ◽  
First Principles Calculations ◽  
X Ray Diffraction ◽  
Intermediate Phases ◽  
Increasing Temperature ◽  
First Time ◽  
Prototype Phase

The phase transition sequences of two members of the tetramethylammonium tetrachlorometallate(III) family of hybrid organic–inorganic salts have been determined and structurally characterized as a function of temperature for the first time. Unusually, a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase is observed. Two additional intermediate phases are observed for Fe3+. First-principles calculations and the presence of short Cl...Cl contacts for Ga3+suggest the [GaCl4]−anion to be conformationally hindered due to stronger lone-pair–σ-hole interactions. The conformationally more flexible Fe3+structures show sublattice melting with the onset of rotational disorder in the [NMe4]+cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl4]−sublattice.

PHASE TRANSITION DUE TO NON-CONTRACTIBLE LOOP IN HETEROTIC STRING D<10

1990 ◽  
Vol 05 (04) ◽  
pp. 771-787
Author(s):  
NORIJI KATO ◽  
IKUO SENDA
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Effective Potential ◽  
Finite Temperature ◽  
Space Time ◽  
Heterotic String ◽  
First Order ◽  
New Phase ◽  
Compactified Space

The heterotic string of the uncompactified dimensions less than ten, D<10, without space-time supersymmetry is considered. The states which have excitations only in the instanton sectors appear in the spectrum. These states become tachyonic below or above a certain scale of the compactified space and make the vacuum unstable. These phenomena are understood as phase transitions due to noncontractible loop on the compactified space. The investigation into the effective potential tells that the phase transition is of first order. The properties of the new phase are studied both in the field theoretic and the stringy manners. The heterotic string at finite temperature in D<10 is also discussed.

Phase Transformation in Titanium Nanoparticles from First Principles

2014 ◽  
Vol 213 ◽  
pp. 42-46
Author(s):  
Andrey N. Chibisov
Keyword(s):  
Phase Transition ◽  
Molecular Dynamics ◽  
Phase Transformation ◽  
First Principles ◽  
Point Group ◽  
Group Symmetry ◽  
Point Group Symmetry ◽  
First Principles Calculations ◽  
Titanium Nanoparticles

We have used molecular dynamics and first-principles calculations to investigate the structure and agglomeration of Ti nanoparticles. The results indicate that Ti nanoclusters undergo a phase transition with a change of point group symmetry. In addition, we found that titanium nanocluster growth occurred along the [0001] direction.

Thermodynamic study at high temperature of the superconducting system YBa2Cu3Oz with 6

1987 ◽  
Vol 2 (6) ◽  
pp. 757-761 ◽  
Author(s):  
J. F. Marucco ◽  
C. Noguera ◽  
P. Garoche ◽  
G. Collin
Keyword(s):  
High Temperature ◽  
Large Deviations ◽  
Oxygen Content ◽  
Phase Stability ◽  
Stability Studies ◽  
Β Phase ◽  
Α Phase ◽  
Lower Oxygen ◽  
New Phase ◽  
Partial Oxygen

The YBa2Cu3Oz system has been investigated by thermogravimetry in a controlled partial oxygen pressure PO2 ranging from 105-10 Pa. A compound with z = 6.290 was prepared at T = 950°C and PO2 = 105 Pa and identified as a metastable β phase. Subsequent isothermal or isobar phase stability studies were performed. This β phase has a narrow metastability domain 6.25 <z < 6.50 at high temperature. By decreasing the temperature or increasing the PO2, the β phase transforms irreversibly into an α phase characterized by large deviations 7—x from stoichiometry. Near the stoichiometry z = 7, the structure of the a phase is orthorhombic, and fully ionized vacancies are likely to be predominant. By decreasing the PO2, the β phase transforms into a new phase with a lower oxygen content (z < 6.1).

scholarly journals Structure cristalline de type alluaudite KNa5Mn3(MoO4)6

2015 ◽  
Vol 71 (1) ◽  
pp. 69-72 ◽  
Author(s):  
Chahira Bouzidi ◽  
Wafa Frigui ◽  
Mohamed Faouzi Zid
Keyword(s):  
Solid State ◽  
Point Group ◽  
Three Dimensional ◽  
Group Symmetry ◽  
Dimensional Structure ◽  
Site Symmetry ◽  
Structural Description ◽  
Point Group Symmetry ◽  
The Third ◽  
New Phase

The new phase potassium pentasodium trimanganese hexakis(molybdate), KNa5Mn3Mo6O24, has been synthesized using solid-state methods. The structure is composed ofM2O10(M= Mn, Na) dimers and MoO4tetrahedra (point group symmetry 2) sharing corners and forming layers parallel to (100), which are linkedviacommon corners of another type ofMO4tetrahedra, forming a three-dimensional structure with two types of large channels along [001] in which two types of Na+cations (one with site symmetry 2, one with -1) and K+cations (site symmetry 2, half-occupation) are located. Mn2+and the third type of Na+cations are located at the same siteMwith occupancies of 0.75 and 0.25, respectively. A comparative structural description is provided between the structure of the title compound and those of the related phases Cu1.35Fe3(PO4)3and NaAgFeMn2(PO4)3.

X-ray studies of the phase transitions of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate, (C(NH2)3)2Zr(N(CH2COO)3)2 · H2O

1999 ◽  
Vol 214 (3) ◽  
Author(s):  
J. Schreuer ◽  
E. Haussühl
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Phase I ◽  
Phase Iii ◽  
Group Symmetry ◽  
Translational Symmetry ◽  
X Ray Diffraction ◽  
Space Group Symmetry ◽  
X Ray ◽  
Structural Differences

AbstractThe structural differences of phase I (at 193 K) and phase III (293 K) of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate were investigated by means of X-ray diffraction. The phase transition III → I is characterised by a loss of translational symmetry as it is indicated by the change of space group symmetry from

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