Isomorph invariance of the structure and dynamics of classical crystals

Albrechtsen, Dan; Olsen, Anne-Marie Schjerning; Pedersen, Ulf R.; Schrøder, Thomas B.; Dyre, Jeppe C.

doi:10.1103/physrevb.90.094106

Cited by 38 publications

(60 citation statements)

References 66 publications

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“…In particular, these systems are a unique opportunity to elucidate the isomorph invariance of the structure and dynamics theory recently generalized for classical crystals. 48,49 …”

Section: Discussionmentioning

confidence: 99%

Structure and reorientational dynamics of 1-F-adamantane

Hassine

Negrier

Romanini

et al. 2016

Phys. Chem. Chem. Phys.

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The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of xray powder diffraction and broadband dielectric spectroscopy. At temperatures below the melting, the molecule forms an orientationally disordered phase I with cubic-centered structure (Phase I, Fm 3 m, Z=4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancies factors of 1/8. A solid-solid phase transition to a lowtemperature tetragonal phase (Phase II, P 4 21c, Z=2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two-and three-fold axes. In the dielectric loss spectra, the cooperative (α) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to such "bimodal" relaxation, a secondary Johari-Goldstein relaxation is detected at lower temperatures.

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“…In particular, these systems are a unique opportunity to elucidate the isomorph invariance of the structure and dynamics theory recently generalized for classical crystals. 48,49 …”

Section: Discussionmentioning

confidence: 99%

Structure and reorientational dynamics of 1-F-adamantane

Hassine

Negrier

Romanini

et al. 2016

Phys. Chem. Chem. Phys.

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“…While most metals and van der Waals bonded liquids are believed to have isomorphs in the condensed-phase part of their phase diagram, systems with significant directional bonding like covalently or hydrogen-bonded systems are not expected to have isomorphs. 32,37,38 The Lennard-Jones (LJ) liquid belongs to the former class of the so-called Roskilde (R)-simple systems, 31,[39][40][41][42][43] which includes most systems with more or less spherical interaction symmetry, the property that traditionally defines a "simple" system. 8 We use below the isomorph theoretical framework as a guide to arrive at an expression for how simple liquids' viscosity varies throughout the high-density part of the thermodynamic phase diagram.…”

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confidence: 99%

Communication: Simple liquids’ high-density viscosity

Costigliola

Pedersen

Heyes

et al. 2018

The Journal of Chemical Physics

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“…Many model systems studied so far have been shown to obey the isomorph theory, including atomic liquids with a range of different pair potentials [26][27][28][29], crystals [30], as well as rigid [31] and flexible molecular liquids [32]. Experimental evidence for the isomorph theory proved harder to get, but has been provided as well [33][34][35].…”

Section: It Is Still An Open Question What Controls the Dynamics Of Vmentioning

confidence: 99%

“…Interestingly, most models that have been simulated show that power-law density scaling does not work, i.e., the scaling function h(ρ) which describes the shape of the isomorphs, is not a power law [26][27][28][29][30][31][32]. Instead, the scaling function h(ρ), which describes the isomorphs via the equation h(ρ)/T = Const., is a more general function of density, and depends on the pair potential.…”

Section: Arxiv:160405763v2 [Cond-matsoft] 18 Aug 2016mentioning

confidence: 99%

Pair Potential That Reproduces the Shape of Isochrones in Molecular Liquids

2016

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Many liquids have curves (isomorphs) in their phase diagrams along which structure, dynamics, and some thermodynamic quantities are invariant in reduced units. A substantial part of their phase diagrams is thus effectively one dimensional. The shape of these isomorphs is described by a material-dependent function of density h(ρ), which for real liquids is well approximated by a power law ρ γ . However, in simulations, a power law is not adequate when density changes are large; typical models such as the Lennard-Jones liquid show that γ(ρ) ≡ d ln h(ρ)/ d ln ρ is a decreasing function of density. This paper presents results from computer simulations using a new pair potential that diverges at a nonzero distance and can be tuned to give a more realistic shape of γ(ρ). Our results indicate that the finite size of molecules is an important factor to take into account when modeling liquids over a large density range.It is still an open question what controls the dynamics of viscous, glass-forming liquids [1][2][3][4][5][6]. Although the dynamics in general depends on both temperature T and density ρ, the dynamics of many organic supercooled liquids can be collapsed onto a single curve when plotted against a combined, material-specific variable h(ρ)/T [7][8][9]. It was found in many experiments that the scaling function h(ρ) is generally well approximated by a power law as h(ρ) = ρ γ , with γ being the materialspecific density-scaling exponent [10,11]; we refer to this as power-law density scaling. Another important development was the discovery that the dynamics of liquids are a function of the excess entropy [12,13].The isomorph theory [14] explains why both density scaling and excess-entropy scaling work for some liquids. Liquids that obey the isomorph theory have curves in their phase diagram, so-called isomorphs, along which not only the dynamics, but also the structure, excess entropy and other thermodynamic quantities are invariant. The development of the isomorph theory was initiated by the observation that in computer simulations some liquids have strongly correlated fluctuations in their energy and pressure. More specifically, if the energy E and pressure p are split in a kinetic part and a configurational part that only depends on the particle positions R ≡ (r 1 , . . . , r N ), as followsthe strong correlations are found between the thermal equilibrium fluctuations of the potential energy U and the virial W in the N V T ensemble [15], although strong correlations have also been found at high pressures in * a.a.veldhorst@gmail.com † tbs@ruc.dk ‡ dyre@ruc.dk the N pT ensemble [16]. Indeed, the standard correlation coefficientindicates whether a liquid obeys the isomorph theory: this is the case whenever R > 0.9 (although this value is of course somewhat arbitrary). The standard linear regression "slope" of the fluctuationsis the density-scaling exponent [14], and the theory thus provides a convenient way to determine the densityscaling exponent in computer simulations. Another empirical observation that can b...

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Isomorph invariance of the structure and dynamics of classical crystals

Cited by 38 publications

References 66 publications

Structure and reorientational dynamics of 1-F-adamantane

Structure and reorientational dynamics of 1-F-adamantane

Communication: Simple liquids’ high-density viscosity

Pair Potential That Reproduces the Shape of Isochrones in Molecular Liquids

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