scholarly journals A positive to negative uniaxial thermal expansion crossover in an organic benzothienobenzothiophene structure

2020 ◽  
Vol 76 (4) ◽  
pp. 661-673
Author(s):  
Dan G. Dumitrescu ◽  
Gilles H. Roche ◽  
Joël J. E. Moreau ◽  
Olivier J. Dautel ◽  
Arie van der Lee
Keyword(s):  
Phase Transition ◽  
Thermal Expansion ◽  
Atomic Displacement ◽  
Continuous Change ◽  
Hydrogen Atoms ◽  
First Order ◽  
Continuous Type ◽  
Supercritical Phase ◽  
The Difference ◽  
Atomic Displacement Parameters

Compound 6,6′-([1]benzothieno[3,2-b][1]benzothiophene-2,7-diyl)bis(butan-1-ol) (BTBT-C4OH) displays a continuous type 0 first-order isosymmetric phase transition at 200 K which is accompanied by a continuous change of the thermal expansion along the b axis from positive to negative. The equivalent isotropic atomic displacement parameters for all non-hydrogen atoms as well as all the eigenvalues of the anisotropic atomic displacement tensor show discontinuous behavior at the phase transition. The eigenvalues of the translational tensor in a rigid-body description of the molecule are all discontinuous at the phase transition, but the librational eigenvalues are discontinuous only in their temperature derivative. BTBT-C4OH displays a similar type of quasi-supercritical phase transition as bis(hydroxyhexyl)[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C6OH), despite the difference in molecular packing and the very large difference in thermal expansion magnitudes.

Energetic materials: variable-temperature crystal structures of γ- and ∊-HNIW polymorphs

2004 ◽  
Vol 37 (5) ◽  
pp. 808-814 ◽  
Author(s):  
Nadezhda B. Bolotina ◽  
Michaele J. Hardie ◽  
Richard L. Speer Jr ◽  
A. Alan Pinkerton
Keyword(s):  
Thermal Expansion ◽  
Energetic Materials ◽  
Variable Temperature ◽  
Mean Field ◽  
Field Potential ◽  
Atomic Displacement ◽  
Linear Temperature Dependence ◽  
Linear Temperature ◽  
The Difference ◽  
Atomic Displacement Parameters

Crystals of γ- and ∊-hexanitrohexaazaisowurtzitane (γ- and ∊-HNIW, space groupP21/nfor both crystals) have been investigated in the 100–298 K temperature range using single-crystal X-ray diffraction techniques. Temperature-dependent changes of their crystal lattices have been evaluated from the second-rank thermal expansion tensors. Both lattices undergo anisotropic thermal expansion, that of γ-HNIW being more anisotropic than that of the ∊ phase. Comparison with previously reported predictions from molecular dynamics calculations indicates significant differences. Although there are many short (less than van der Waals) intermolecular interactions in both polymorphs, there is no obvious relationship between the short distances and the difference in thermal expansion behavior. Non-linear temperature dependence of the atomic displacement parameters is indicative of anharmonicity of the crystal mean field potential.

Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)

2012 ◽  
Vol 68 (2) ◽  
pp. 101-106 ◽  
Author(s):  
Heribert A. Graetsch ◽  
Chandra Shekhar Pandey ◽  
Jürgen Schreuer ◽  
Manfred Burianek ◽  
Manfred Mühlberg
Keyword(s):  
Crystal Structures ◽  
Tungsten Bronze ◽  
Atomic Displacement ◽  
Type Structure ◽  
Formula Unit ◽  
Tetragonal Tungsten Bronze ◽  
First Order ◽  
Oxygen Coordination ◽  
Incommensurate Modulation ◽  
Atomic Displacement Parameters

The incommensurately modulated crystal structures of Ca0.28Ba0.72Nb2O6 (CBN28) and Ce0.02Ca0.25Ba0.72Nb2O6 (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,−AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,−aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO6 octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.

ON THE STRUCTURE OF SUPERCRITICAL PHASE TRANSITION

1990 ◽  
Vol 05 (14) ◽  
pp. 1081-1087 ◽  
Author(s):  
YUMI S. HIRATA ◽  
HISAKAZU MINAKATA
Keyword(s):  
Phase Transition ◽  
Statistical Mechanics ◽  
Order Phase Transition ◽  
False Vacuum ◽  
Close Analogy ◽  
First Order ◽  
First Order Phase Transition ◽  
Supercritical Phase ◽  
First Order Phase ◽  
Irreversible Nature

A novel physical picture is presented for the normal-to-supercritical "phase" transition in QED around a large-Z nucleus. The process is described as the decay of the false vacuum in close analogy to the first-order phase transition in statistical mechanics. The irreversible nature of the transition is pointed out and the physical implications of this picture are discussed.

scholarly journals Crystal Structure of Urotropine Under High Pressure and Non-Hydrostatic Stress-Induced Phase Transitions in Cage Compounds

2020 ◽  
Author(s):  
Piotr A. Guńka ◽  
Anna Olejniczak ◽  
Samuele Fanetti ◽  
Roberto Bini ◽  
Ines E. Collings ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Phase Transition ◽  
High Pressure ◽  
Structural Changes ◽  
Hydrostatic Stress ◽  
Atomic Displacement ◽  
Deviatoric Stress ◽  
Cage Compounds ◽  
First Order Phase Transition ◽  
Atomic Displacement Parameters

High-pressure behavior of hexamthyleneteramine (urotropine) was studied <i>in situ</i> using angle-dispersive single-crystal synchrotron X-ray diffraction (XRD) and Fourier transform infrared absorption (FTIR) spectroscopy. Experiments were conducted in various pressure transmitting media (helium and neon for XRD, nitrogen and KBr for FTIR experiments) to study the effect of deviatoric stress on phase transformations. Contrary to As<sub>4</sub>O<sub>6</sub> arsenolite, a material of similar cage-like molecular structure, no pressure-induced helium penetration into the crystal structure was observed. Instead, two pressure-induced structural changes are observed. The first one is suggested by the following occurrences: (i) gradual quenching of the magnitudes of atomic displacement parameters, (ii) diminishing libration contribution to the experimental C−N bond length, (iii) discontinuity in calculated IR-active vibrational modes and (iv) asymptotically vanishing discrepancy between the experimental and DFT‑calculated unit cell volume. All these features reach a plateau at ~4 GPa and can be attributed to a damping of molecular librations and atomic thermal motion, pointing to the existence of a second-order isostructural phase transition. The second transformation, with an onset at ~12.5 GPa is a first-order phase transition to a tetragonal structure, characterized by sluggish kinetics and considerable hysteresis upon decompression. However, it occurs only in non-hydrostatic conditions, induced by a deviatoric stress in the sample. This behavior finds analogies in similar cubic crystals built of highly symmetric cage-like molecules and <a>may be considered a common feature</a> of such systems. Last but not least, it is worth noting successful Hirshfeld atom refinements, carried out for the incomplete high-pressure diffraction data up to 14 GPa, yielded more realistic C−H bond lengths than the independent atom model.

Phase Transition Studies in PMBAB and PEBAB

1984 ◽  
Vol 39 (7) ◽  
pp. 696-699 ◽  
Author(s):  
V. G. K. M. Pisipati ◽  
N. V. S. Rao
Keyword(s):  
Phase Transition ◽  
Thermal Expansion ◽  
Molar Volume ◽  
Ultrasonic Velocity ◽  
Adiabatic Compressibility ◽  
First Order ◽  
Thermal Expansion Coefficients ◽  
Methylene Unit ◽  
Expansion Coefficients ◽  
Transition Studies

The variation of density and ultrasonic velocity with temperature in N-(p-methoxybenzylidene)- p-aminobenzonitrile (PMBAB) and N-(p-ethoxybenzylidene)-p-aminobenzonitrile (PEBAB) are presented. The density jumps and thermal expansion coefficients suggest the nematic-isotropic transition of both compounds to be of first order. The adiabatic compressibility βad, molar sound velocity Rn and molar compressibility Aw are computed. The contribution of a methylene unit to the molar volume is found to be higher than literature values. The anomalous ultrasonic velocity dip at the nematic-isotropic transition for both compounds is found to be high; the value is many times those found for other nematic-isotropic or nematicsmectic A transitions.

Crystal structure and phase transition of thermoelectric SnSe

2016 ◽  
Vol 72 (3) ◽  
pp. 310-316 ◽  
Author(s):  
Mattia Sist ◽  
Jiawei Zhang ◽  
Bo Brummerstedt Iversen
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Site Occupancy ◽  
Functional Materials ◽  
Atomic Displacement ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Lattice Constants ◽  
Temperature Phase Transition ◽  
Atomic Displacement Parameters

Tin selenide-based functional materials are extensively studied in the field of optoelectronic, photovoltaic and thermoelectric devices. Specifically, SnSe has been reported to have an ultrahigh thermoelectric figure of merit of 2.6 ± 0.3 in the high-temperature phase. Here we report the evolution of lattice constants, fractional coordinates, site occupancy factors and atomic displacement factors with temperature by means of high-resolution synchrotron powder X-ray diffraction measured from 100 to 855 K. The structure is shown to be cation defective with a Sn content of 0.982 (4). The anisotropy of the thermal parameters of Sn becomes more pronounced approaching the high-temperature phase transition (∼ 810 K). Anharmonic Gram–Charlier parameters have been refined, but data from single-crystal diffraction appear to be needed to firmly quantify anharmonic features. Based on modelling of the atomic displacement parameters the Debye temperature is found to be 175 (4) K. Conflicting reports concerning the different coordinate system settings in the low-temperature and high-temperature phases are discussed. It is also shown that the high-temperatureCmcmphase is not pseudo-tetragonal as commonly assumed.

Energetic materials: variable-temperature crystal structures of two biguanidinium dinitramides

2003 ◽  
Vol 36 (6) ◽  
pp. 1334-1341 ◽  
Author(s):  
Nadezhda B. Bolotina ◽  
Michaele J. Hardie ◽  
A. Alan Pinkerton
Keyword(s):  
Thermal Expansion ◽  
Crystal Structures ◽  
Energetic Materials ◽  
Variable Temperature ◽  
Atomic Displacement ◽  
Thermal Motion ◽  
Linear Temperature Dependence ◽  
Linear Temperature ◽  
Thermal Displacements ◽  
Atomic Displacement Parameters

The crystal structures of the energetic materials biguanidinium mono-dinitramide C2H8N{}_{5}^{\,+}.N3O{}_{4}^{\,-}, (BIGH)(DN), and biguanidinium bis-dinitramide C2H9N{}_{5}^{\,2+}.2N3O{}_{4}^{\,-}, (BIGH2)(DN)2, have been determined at several temperatures in the range 85–298 K using single-crystal X-ray diffraction techniques. The thermal expansion second-rank tensors have been determined to describe the thermal behavior of the crystals studied. Strongly anisotropic thermal expansion is most important in the direction perpendicular to the least-squares planes of the dinitramide ions in both cases, suggesting that the atomic thermal motion is significantly anharmonic in these crystals. Anharmonicity of thermal motion is also evident from the non-linear temperature dependence of the atomic displacement parameters. Rigid-body analysis of thermal motion both of dinitramide anions and of biguanidinium cations was performed using the libration and translation second-rank tensors. For both compounds, the libration thermal motion is strongly anisotropic with the dominating libration axes oriented in a similar manner in both anions and cations. Although the translation motion of the ions is not strongly anisotropic, the axes of largest thermal displacements are close to the directions of greatest thermal expansion of the crystals.

Phase Transition Studies across Isotropic-Smectic A and Smectic A-Smectic F Phases in 90.4

1989 ◽  
Vol 44 (1) ◽  
pp. 23-25
Author(s):  
D. M. Potukuchi ◽  
P. Bhaskara Rao ◽  
N. V. S. Rao ◽  
V.G. K. M. Pisipati
Keyword(s):  
Phase Transition ◽  
Differential Scanning Calorimetry ◽  
Thermal Expansion ◽  
Phase Transitions ◽  
Pressure Dependence ◽  
Scanning Calorimetry ◽  
First Order ◽  
Smectic A ◽  
Density Results ◽  
Transition Studies

Abstract The temperature dependence of the density of N(p-n-nonyloxy benzylidene)p-n-butylaniline (90.4) is studied. The compound exhibits smectic A, smectic F and smetic G phases. The isotropicsmectic A (IA) and smetic A-smectic F phase transitions are found to be first order. The calculated thermal expansion coefficient and differential scanning calorimetry support the density results. The density jumps, associated enthalpies and the estimated pressure dependence of the transition temperatures for IA transitions in other compounds are also presented.

scholarly journals Thermal expansion and atomic displacement parameters of cubic KMgF3perovskite determined by high-resolution neutron powder diffraction

2002 ◽  
Vol 35 (3) ◽  
pp. 291-295 ◽  
Author(s):  
I. G. Wood ◽  
K. S. Knight ◽  
G. D. Price ◽  
J. A. Stuart
Keyword(s):  
Thermal Expansion ◽  
High Resolution ◽  
Powder Diffraction ◽  
Room Temperature ◽  
Coefficient Of Thermal Expansion ◽  
Atomic Displacement ◽  
Debye Model ◽  
Neutron Powder Diffraction ◽  
First Derivative ◽  
Atomic Displacement Parameters

The structure of KMgF3has been determined by high-resolution neutron powder diffraction at 4.2 K, room temperature and at 10 K intervals from 373 K to 1223 K. The material remains cubic at all temperatures. The average volumetric coefficient of thermal expansion in the range 373–1223 K was found to be 7.11 (3) × 10−5 K−1. For temperatures between 4.2 and 1223 K, a second-order Grüneisen approximation to the zero-pressure equation of state, with the internal energy calculatedviaa Debye model, was found to fit well, with the following parameters: θD= 536 (9) K,Vo= 62.876 (6) Å3, K_{o}^{\,\prime} = 6.5 (1) and (VoKo/γ′) = 3.40 (2) × 10−18 J, where θDis the Debye temperature,Vois the volume atT= 0, K_{o}^{\,\prime} is the first derivative with respect to pressure of the incompressibility (Ko) and γ′ is a Grüneisen parameter. The atomic displacement parameters were found to increase smoothly withTand could be fitted using Debye models with θDin the range 305–581 K. At 1223 K, the displacement of the F ions was found to be much less anisotropic than that in NaMgF3at this temperature.

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