Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals

2003 ◽  
Vol 59 (1) ◽  
pp. 60-71 ◽  
Author(s):  
Markus Wunschel ◽  
Robert E. Dinnebier ◽  
Stefan Carlson ◽  
Piotr Bernatowicz ◽  
Sander van Smaalen
Keyword(s):  
High Pressure ◽  
Phase Transitions ◽  
Low Temperature ◽  
Room Temperature ◽  
Close Packing ◽  
Molecular Structures ◽  
Temperature Phase ◽  
Plastic Crystals ◽  
First Order ◽  
First Order Phase

The crystal structures of tert-butyl-tris(trimethylsilyl)silane, Si[C(CH_3)_3]_1[Si(CH_3)_3]_3 (Bu1), and di-tert-butyl-bis(trimethylsilyl)silane, Si[C(CH_3)_3]_2[Si(CH_3)_3]_2 (Bu2), at room temperature and at 105 K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5 GPa is reported. The room-temperature structures have cubic Fm\bar{3}m symmetry (Z = 4) with a = 13.2645 (2) Å, V = 2333.87 (4) Å3 for Bu1 and a = 12.9673 (1) Å, V = 2180.46 (3) Å3 for Bu2. The molecules are arranged in a cubic close packing (c.c.p.) and exhibit at least 48-fold orientational disorder. Upon cooling both compounds undergo a first-order phase transition at temperatures T_c = 230 (5) K (Bu1) and T_c = 250 (5) K (Bu2) into monoclinic structures with space group P2_1/n. The structures at 105 K have a = 17.317 (1), b = 15.598 (1), c = 16.385 (1) Å, \gamma = 109.477 (4)°, V = 4172.7 (8) Å3 and Z = 8 for Bu1and a = 17.0089 (9), b = 15.3159 (8), c = 15.9325 (8) Å, \gamma = 110.343 (3)°, V = 3891.7 (5) Å3 and Z = 8 for Bu2. The severe disorder of the room-temperature phase is significantly decreased and only a two- or threefold rotational disorder of the molecules remains at 105 K. First-order phase transitions have been observed at pressures of 0.13–0.28 GPa for Bu1 and 0.20–0.24 GPa for Bu2. The high-pressure structures are isostructural to the low-temperature structures. The pressure dependencies of the unit-cell volumes were fitted with Vinet equations of state and the bulk moduli were obtained. At still higher pressures further anomalies in the pressure dependencies of the lattice parameters were observed. These anomalies are explained as additional disorder–order phase transitions.

scholarly journals Fate of false vacua in holographic first-order phase transitions

2020 ◽  
Vol 2020 (12) ◽  
Author(s):  
Francesco Bigazzi ◽  
Alessio Caddeo ◽  
Aldo L. Cotrone ◽  
Angel Paredes
Keyword(s):  
Phase Transitions ◽  
Chiral Symmetry ◽  
Metastable Phase ◽  
Chiral Symmetry Breaking ◽  
Model Parameters ◽  
Temperature Phase ◽  
Thin Wall ◽  
Critical Temperatures ◽  
First Order ◽  
First Order Phase

Abstract Using the holographic correspondence as a tool, we study the dynamics of first-order phase transitions in strongly coupled gauge theories at finite temperature. Considering an evolution from the large to the small temperature phase, we compute the nucleation rate of bubbles of true vacuum in the metastable phase. For this purpose, we find the relevant configurations (bounces) interpolating between the vacua and we compute the related effective actions. We start by revisiting the compact Randall-Sundrum model at high temperature. Using holographic renormalization, we compute the derivative term in the effective bounce action, that was missing in the literature. Then, we address the full problem within the top-down Witten-Sakai-Sugimoto model. It displays both a confinement/deconfinement and a chiral symmetry breaking/restoration phase transition which, depending on the model parameters, can happen at different critical temperatures. For the confinement/deconfinement case we perform the numerical analysis of an effective description of the transition and also provide analytic expressions using thick and thin wall approximations. For the chiral symmetry transition, we implement a variational approach that allows us to address the challenging non-linear problem stemming from the Dirac-Born-Infeld action.

Single-Crystal X-Ray Scattering Study of Superstructures and Phase Transitions in Graphite-Hno3 and Graphite-Br2 Intercalation Compounds

1982 ◽  
Vol 20 ◽  
Author(s):  
R. Moret ◽  
R. Comes ◽  
G. Furdin ◽  
H. Fuzellier ◽  
F. Rousseaux
Keyword(s):  
Phase Transitions ◽  
Low Temperature ◽  
Room Temperature ◽  
Temperature Phase ◽  
Temperature Structure ◽  
X Ray ◽  
X Ray Scattering ◽  
Scattering Study ◽  
Temperature Liquid ◽  
Low Temperature Phase

ABSTRACTIn α-C5n-HNO3 the condensation of the room-temperature liquid-like diffuse ring associated with the disorder-order transition around 250 K is studied and the low-temperature. superstructure is examined.It is found that β-C8n-HNO3 exhibits an in-plane incommensurate order at room temperature.Two types of graphite-Br2 are found. Low-temperature phase transitions in C8Br are observed at T1 ≍ 277 K and T2 ≍ 297 K. The room-temperature structure of C14Br is reexamined. Special attention is given to diffuse scattering and incommensurability.

scholarly journals 127I NQR and Crystal Structure Studies of [N(CH3)4]2 CdI4

2000 ◽  
Vol 55 (1-2) ◽  
pp. 225-229 ◽  
Author(s):  
Hideta Ishihara ◽  
Keizo Horiuchi ◽  
Thorsten M. Gesing ◽  
Shi-qi Dou ◽  
J.-Christian Buhl ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Phase Transition ◽  
Order Phase Transition ◽  
Intensity Ratio ◽  
Room Temperature ◽  
Liquid Nitrogen Temperature ◽  
Temperature Phase ◽  
First Order ◽  
First Order Phase Transition ◽  
First Order Phase

The temperature dependence of 127I NQR and DSC as well as the crystal structure at room temperature of the title compound were determined. This compound shows a first-order phase transition of an order-disorder type at 245 K. Eight 127I(v1:m = ±1/2 ↔ ±3/2) NQR lines of 79.57, 81.86, 82.56, 83.36, 84.68, 87.72, 88.34, and 88.86 MHz, and corresponding eight 127I(v2: m = ±3/2 ↔±5/2) NQR lines were observed at liquid nitrogen temperature. Three 127I(υi) NQR lines wfth an intensity ratio of 1:1:2 in the order of decreasing frequency were observed just above the transition point and two NQR lines except for the middle-frequency line disappeared around room temperature. This temperature behavior of NQR lines is very similar to that observed in [N(CH3)4]2Hgl4. Another first-order phase transition takes place at 527 K. The structure of the room-temperature phase was redetermined: orthorhombic, Pnma, Z = 4, a = 1342.8(3), b = 975.7(2), c = 1696.5(3) pm. The NQR result of three lines with an intensity ratio of 1:1:2 is in agreement with this structure. The thermal displacement parameters of atoms in both cations and anions are large.

Determination of the low-temperature structure of hexamethylbenzene

2005 ◽  
Vol 61 (2) ◽  
pp. 200-206 ◽  
Author(s):  
John A. Stride
Keyword(s):  
Low Temperature ◽  
Temperature Phase ◽  
Temperature Structure ◽  
Thermally Induced ◽  
First Order ◽  
Neutron Powder Diffraction Data ◽  
First Order Phase Transition ◽  
First Order Phase

The low-temperature structure of hexamethylbenzene has been determined from neutron powder diffraction data and found to differ from the room-temperature phase predominantly by a translation of molecular planes to a form a cubic close-packed type structure. By performing measurements as a function of temperature, the role of thermally induced agitation of the molecular units in the first-order phase transition is clearly demonstrated.

Lattice Instabilities, Anharmonicity and Phase Transitions in PbTiO3 and PbZrO3

1995 ◽  
Vol 408 ◽  
Author(s):  
K. M. Rabe ◽  
U. V. Waghmare
Keyword(s):  
Phase Transitions ◽  
Low Temperature ◽  
Structural Phase ◽  
Orthorhombic Phase ◽  
Cubic Phase ◽  
Temperature Phase ◽  
Structural Phase Transitions ◽  
Phonon Modes ◽  
First Order ◽  
Low Temperature Phase

AbstractMost perovskite structure oxides exhibit structural phase transitions from a hightemperature cubic phase to a distorted low-temperature phase which can be described by the freezing-in of one or more phonon modes of the cubic structure [1]. The first-order cubic-tetragonal ferroelectric transition in PbTiO3 at Tc = 763 K involves the freezing-in of a single F15 polar mode. In PbZrO3 , the structure of the antiferroelectric low-temperature orthorhombic phase is far more complicated, with forty atoms per unit cell and the freezing-in of R25 and Σ3 modes, perhaps accompanied by other modes as well [2][3].

scholarly journals Twinning and Pseudosymmetry at High Pressures

2014 ◽  
Vol 70 (a1) ◽  
pp. C260-C260 ◽  
Author(s):  
Karen Friese ◽  
Andrzej Grzechnik
Keyword(s):  
High Pressure ◽  
Phase Transitions ◽  
Single Crystal ◽  
High Pressures ◽  
Rotational Symmetry ◽  
Type I ◽  
Experimental Conditions ◽  
First Order ◽  
Volume Fractions ◽  
First Order Phase

Twinning is a common known problem in the study of crystal structures from single-crystal data and often related to a high degree of pseudosymmetry of the structure with respect to a higher symmetrical parent one. With the rising popularity of in situ single-crystal diffraction studies under high pressure, the occurrence of twinned structures is more and more frequently reported. In this contribution, we review the available information on merohedral and pseudomerohedral twinning as well as pseudosymmetry under high pressures [1]. For twinning by merohedry type I (inversion twinning), a reliable characterization of the twin domains and volume fractions is difficult and largely depends on the experimental conditions, i.e., on the number of measured Friedel pairs and the chosen wavelength. For twinning by merohedry (type II) and for twinning by pseudomerohedry, twin volume fractions could be reliably determined from high-pressure data for several cases. In none of these, a significant influence of hydrostatic pressure on the volume fractions of the individuals was observed. Pressure-induced twinning has also been observed for compounds which undergo first-order phase transitions. It is remarkable that the twinning operation in such cases is related to the loss of rotational symmetry elements of the higher symmetrical polymorph, although the high- and low-pressure phases are not in the group-subgroup relationship. The analysis of pseudosymmetry of several compounds as a function of pressure suggests that this parameter can be used to predict the (in)stability of compounds. In particular, a decrease in pseudosymmetry seems to be strongly correlated with the occurrence of first-order phase transitions in which the crystals break or amorphize.

Distortions of [Sb2Cl10]4– Bioctahedra and Phase Transitions in the Chloroantimonate(III) (C3H5NH3)2[SbCl5] · (C3H5NH3)Cl

2007 ◽  
Vol 62 (1) ◽  
pp. 44-50 ◽  
Author(s):  
Bartosz Zarychta ◽  
Maciej Bujak ◽  
Jacek Zaleski
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Low Temperature ◽  
Room Temperature ◽  
Lone Pair ◽  
Inorganic Salts ◽  
Organic Cations ◽  
Temperature Phase ◽  
Dsc Studies ◽  
Low Temperature Phase

Bis(allylammonium)pentachloroantimonate(III) - allylammonium chloride, (C3H5NH3)2[SbCl5] · (C3H5NH3)Cl, belongs to the chloroantimonate(III) organic-inorganic salts family. The DSC studies of this compound showed two anomalies at 181 K and at 223 K. Both are associated with phase transitions, which mainly occur due to ordering-disordering processes of the organic cations. Between 181 and 223 K the structure is incommensurate. The crystal structure was determined at 298 and 86 K. At both temperatures the crystal structure consists of (C3H5NH3)+ cations, anionic distorted [Sb2Cl10]4− units and isolated Cl− ions. In the room-temperature phase two out of three, and in the low-temperature phase two out of six allylammonium cations were found to be disordered. The deformations of the [Sb2Cl10]4− moieties in both phases are discussed and explained by the deviation of the SbIII 5s electron lone pair from its spherical symmetry and the influence of N-H···Cl hydrogen bonds, which join together the organic and inorganic sublattices.

Order–disorder phenomena determined by high-resolution powder diffraction: the structures of tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 and tetrakis(trimethylsilyl)silane Si[Si(CH3)3]4

1999 ◽  
Vol 55 (6) ◽  
pp. 1014-1029 ◽  
Author(s):  
Robert E. Dinnebier ◽  
Wayne A. Dollase ◽  
Xavier Helluy ◽  
Jörg Kümmerlen ◽  
Angelika Sebald ◽  
...  
Keyword(s):  
Phase Transition ◽  
Low Temperature ◽  
Intermediate Phase ◽  
Lattice Parameter ◽  
Temperature Phase ◽  
Space Group ◽  
First Order ◽  
First Order Phase Transition ◽  
First Order Phase ◽  
Low Temperature Phase

The compounds tetrakis(trimethylsilyl)methane C[Si(CH3)3]4 (TC) and tetrakis(trimethylsilyl)silane Si[Si(CH3)3]4 (TSi) have crystal structures with the molecules in a cubic closed-packed (c.c.p.) stacking. At room temperature both structures have space group Fm{\bar 3}m (Z = 4) with a = 13.5218 (1) Å, V = 2472.3 (1) Å3 for TSi, and a = 12.8902 (2) Å, V = 2141.8 (1) Å3 for TC. X-ray scattering data can be described by a molecule with approximately sixfold orientational disorder, ruling out a structure with free rotating molecules. Upon cooling, TSi exhibits a first-order phase transition at T c = 225 K, as is characterized by a jump of the lattice parameter of Δa = 0.182 Å and by an exothermal maximum in differential scanning calorimetry (DSC) with ΔH = 11.7 kJ mol−1 and ΔS = 50.0 J mol−1 K−1. The structure of the low-temperature phase is refined against X-ray powder data measured at 200 K. It has space group P2 13 (Z = 4), a = 13.17158 (6) Å and V = 2285.15 (2) Å3. The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as is shown by analyzing distances between atoms and by calculations of the lattice energy in dependence on the orientations of the molecules. TC has a phase transition at T c1 = 268 K, with Δa 1 = 0.065 Å, ΔH 1 = 3.63 kJ mol−1 and ΔS 1 = 13.0 J mol−1 K−1. A second first-order phase transition occurs at T c2 = 225 K, characterized by Δa 2 = 0.073 Å, ΔH 2 = 6.9 kJ mol−1 and ΔS 2 = 30.0 J mol−1 K−1. The phase transition at higher temperature has not been reported previously. New NMR experiments show a small anomaly in the temperature dependence of the peak positions in NMR to occur at T c2. Rietveld refinements were performed for the low-temperature phase measured at T = 150 K [space group P2 13, lattice parameter a = 12.609 (3) Å], and for the intermediate phase measured at T = 260 K [space group Pa{\bar 3}, lattice parameter a = 12.7876 (1) Å]. The low-temperature phase of TC is formed isostructural to the low-temperature phase of TSi. In the intermediate phase the molecules exhibit a twofold orientational disorder.

scholarly journals Break of symmetry at the surface of IrTe2 upon phase transition measured by X-ray photoelectron diffraction

2021 ◽  
Author(s):  
Maxime Rumo ◽  
Aki Pulkkinen ◽  
KeYuan Ma ◽  
Fabian von Rohr ◽  
Matthias Muntwiler ◽  
...  
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Room Temperature ◽  
Local Environment ◽  
Atomic Position ◽  
X Ray ◽  
Photoelectron Diffraction ◽  
First Order ◽  
First Order Phase Transition ◽  
First Order Phase

Abstract IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T−IrTe2 in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2 trigonal to a monoclonic unit cell and confirm the occurence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

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