Molecular dynamics simulation of the phase transition in adamantane

1988 ◽  
Vol 66 (4) ◽  
pp. 1018-1025 ◽  
Author(s):  
A. S. Trew ◽  
G. S. Pawley
Keyword(s):  
Orientational Order ◽  
Significant Degree ◽  
Phase Changes ◽  
Dynamics Simulation ◽  
Local Order ◽  
Temperature Phase ◽  
Orientational Distribution ◽  
Molecule Model ◽  
Low Temperature Phase ◽  
Degree Of Order

Phase changes in adamantane have been studied by MD simulation on the DAP computers, using a zero-pressure technique to simulate clusters of 128 and 256 molecules where each member interacts with all others via the rigid molecule model and the 6-exp atom–atom potential. The form of the potential has been modified to permit the use of the 16 hydrogen sites only, giving a 65% saving in the calculation times. This model is shown to give lattice dynamics of adamantane closely similar to results with potentials which are generally accepted.Using this potential the system equilibrates into the correct low temperature phase [Formula: see text] and on heating, a transition is observed at 210 ± 10 K to an Fm3m phase where the molecules lie preferentially in the Td orientations, as expected. Further heating beyond 240 ± 15 K removes all apparent orientational order, though the underlying lattice is still fcc. On recooling the cluster from 300 to 100 K the orientational distribution function developed a significant degree of order as determined through the calculation of a correlation function designed to show any local order. This order is consistent with the lowest phase structure, but would in itself be insufficient to suggest a particular crystal structure.

Formation process of periodic non-conservative antiphase boundary structure in L-Pd5Ce

1990 ◽  
Vol 48 (4) ◽  
pp. 162-163
Author(s):  
Masaru Itakura ◽  
Noriyuki Kuwano ◽  
Kensuke Oki
Keyword(s):  
Formation Process ◽  
Domain Size ◽  
Antiphase Boundary ◽  
Boundary Structure ◽  
Temperature Phase ◽  
Arc Melting ◽  
Iced Water ◽  
Antiphase Domains ◽  
Low Temperature Phase ◽  
Degree Of Order

The low temperature phase of Pd5Ce (L-Pd5Ce) has a one-dimensional long period superstructure (1D-LPS) derived from Ll2. The periodic antiphase boundaries (APBs) are parallel to (110) planes and have a shift vector of 1/2[110]. Hereafter, the indices are referred to the basic lattices of Ll2 As insertion of the APB causes a change in composition, such an APB is called “non-conservative”. Then, a domain size M depends upon the Ce concentration in the alloy. It was found that M increases also with temperature. The temperature dependency of M is attributed to a change of the degree of order within the antiphase domains. In this work, morphology of the non-conservative APBs is observed to clarify the formation process of the 1D-LPS.The alloy of Pd-16.7 at%Ce was prepared by arc melting in argon atmosphere. Disc specimens made from the alloy ingot were first held at 985 K for 260 ks and quenched in iced water to obtain the state of M=∞ or Ll2, followed by annealing for various lengths of time. The annealing temperature was 873 K where the equilibrium value for M is about 3 in unit of (110) lattice spacing of Ll2. Observation was carried out using microscopes JEM-2000FX, JEM-4000EX (HVEM Lab., Kyushu Univ.) and JEM-2000EX (Dept. of Mater. Sci. Tech., Kyushu Univ.).

New High- and Low-Temperature Phase Changes of ZIF-7: Elucidation and Prediction of the Thermodynamics of Transitions

2015 ◽  
Vol 137 (42) ◽  
pp. 13603-13611 ◽  
Author(s):  
Yi Du ◽  
Bradley Wooler ◽  
Meghan Nines ◽  
Pavel Kortunov ◽  
Charanjit S. Paur ◽  
...  
Keyword(s):  
Low Temperature ◽  
Phase Changes ◽  
Temperature Phase ◽  
Low Temperature Phase

35Cl NQR Studies of Hexachlorometallates(IV) with Organic Cations, A2 [MCl6] and A'[MCl6], M=Sn, Te, Pb and A+, A'2+ = Organic Cation

1990 ◽  
Vol 45 (3-4) ◽  
pp. 293-302 ◽  
Author(s):  
Dirk Borchers ◽  
Alarich Weiss
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Low Temperature ◽  
Resonance Line ◽  
Organic Cation ◽  
Phase Changes ◽  
Organic Cations ◽  
Temperature Phase ◽  
35Cl Nqr ◽  
Low Temperature Phase

Abstract35Cl NQR spectra of eleven hexachlorometallates (IV) A1 [MCl6] and A'[MCl6], M = Sn, Te, Pb, and A+ =ethylammonium, 4-picolinium, anilinium, triethylammonium, chinoxalinium, and piperi-dinium ions, and A'2+ = 1.3-propylenediammonium ion, have been observed as a function of tem-perature. The ethylammonium hexachlorometallates(IV) (C2H5 NH3)2 [MCl6], M = Sn, Te, show a phase transition at 128.8 K and 204 K, respectively. Both compounds yield a single resonance line in their high temperature phases. In case of the stannate this single 35Cl resonance line splits up into two lines at Tc = 128.8 K, whereas for the tellurate no 35Cl NQR signals could be found in the low temperature phase. A phase transition was also found for the 1.3-propylenediammonium hexa-chlorostannate(IV) at 287 K where the six line NQR spectrum of the low temperature phase changes into a four line spectrum. In contrast, the corresponding plumbate shows no transition. All other compounds studied contain distorted [MCl6]2- octahedra, and therefore they yield more than one 35Cl resonance line. The complexes have been investigated in the temperature range temperature where the lines fade out. In case of the hexachlorostannates(IV) with the ethylammonium, the 1.3-propylenediammonium and the triethylammonium ions, the crystal structures of the compounds are known and compared with the results of the 35Cl NQR spectroscopy.

Topological polymorphism and temperature-driven topotactic transitions of metal–organic coordination polymers

CrystEngComm ◽  
2020 ◽  
Vol 22 (38) ◽  
pp. 6295-6301
Author(s):  
Vadim A. Dubskikh ◽  
Anna A. Lysova ◽  
Denis G. Samsonenko ◽  
Danil N. Dybtsev ◽  
Vladimir P. Fedin
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Solid State ◽  
Low Temperature ◽  
High Temperature Phase ◽  
Phase Changes ◽  
Organic Ligands ◽  
Temperature Phase ◽  
Metal Organic ◽  
Low Temperature Phase

A facile crystal-to-crystal solid-state phase transition between a low-temperature phase and a high temperature phase changes the MOF topology and involves a significant rearrangement of bulky organic ligands.

The local order and structural evolution of amorphous PdAg alloy during isothermal annealing under high pressure: A molecular dynamics study

2015 ◽  
Vol 93 (1) ◽  
pp. 7-13
Author(s):  
Fatih Ahmet Celik ◽  
Sefa Kazanc
Keyword(s):  
Molecular Dynamics ◽  
Isothermal Annealing ◽  
Orientational Order ◽  
Structural Evolution ◽  
Alloy System ◽  
Order Parameters ◽  
Dynamics Simulation ◽  
Model Alloy ◽  
Local Order ◽  
Bond Orientational Order

In this study, the local order and the structural evolution of the PdAg binary alloy system during the crystallization process from the amorphous phase under different pressures was investigated using the molecular dynamics simulation method. The structural development and phase transformation of the model alloy system for pressures of 0, 1, and 5 GPa are analysed based on variations of the radial distribution function, the bond orientational order parameters, and Honeycutt–Andersen bond-type index method. The simulation results showed that higher pressures favoured amorphous-type atomic clusters and had an important effect on the bond orientational order parameters of the model system of the transformation from amorphous state to stable crystal phase during the isothermal annealing.

Stress-Induced Birefringence of Swollen Polymer Networks

1970 ◽  
Vol 43 (4) ◽  
pp. 829-835 ◽  
Author(s):  
A. N. Gent
Keyword(s):  
Strong Dependence ◽  
Orientational Order ◽  
Polymer Networks ◽  
Solvent Molecule ◽  
Significant Degree ◽  
Local Order ◽  
Nonpolar Solvents ◽  
Repeat Units ◽  
Cis And Trans ◽  
High Degree

Abstract Stress-optical coefficients have been determined for lightly crosslinked samples of cis- and trans-polyisoprene, swollen to a high degree in a variety of nonpolar solvents. A strong dependence on the geometrical asymmetry of the solvent molecule has been observed. Solvents with long, straight molecules gave large values of the stress-optical coefficient whereas those with compact, symmetrical molecules gave minimum values. This effect is attributed to short-range orientational order, due to packing effects in a molecularly anisotropic medium. The large values obtained for unswollen polymers correspond to those in relatively anisotropic solvents, and thus indicate a significant degree of local order in the bulk phase. The minimum values are interpreted, using calculated optical anisotropies of chain repeat units, as measures of the stiffness of isolated chains.

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