Polyamorphism Mirrors Polymorphism in the Liquid-Liquid Transition of a Molecular Liquid.

Finlay Walton,Christopher D Syme,Gianfelice Cinque,Jamie Macewen,John Bolling,Andrew Farrell,Klaas Wynne,Mario González Jiménez,Hans M Senn,Claire Wilson

doi:10.1021/jacs.0c01712

Finlay Walton, Christopher D Syme + Show 8 more

Open Access

https://doi.org/10.1021/jacs.0c01712

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Abstract

Liquid–liquid transitions between two amorphous phases in a single-component liquid have courted controversy. All known examples of liquid–liquid transitions in molecular liquids have been observed in the supercooled state, suggesting an intimate connection with vitrification and locally favored structures inhibiting crystallization. However, there is precious little information about the local molecular packing in supercooled liquids, meaning that the order parameter of the transition is still unknown. Here, we investigate the liquid–liquid transition in triphenyl phosphite and show that it is caused by the competition between liquid structures that mirror two crystal polymorphs. The liquid–liquid transition is found to be between a geometrically frustrated liquid and a dynamically frustrated glass. These results indicate a general link between polymorphism and polyamorphism and will lead to a much greater understanding of the physical basis of liquid–liquid transitions and allow the systematic discovery of other examples.

Highlights

It is well known that substances have mesophases: phases in between the crystal and the isotropic liquid in which the liquid has partial orientational or translational ordering
On quenching the samples to temperatures in the range 216−226 K, triphenyl phosphite (TPP) undergoes an liquid−liquid transition (LLT) from the high-temperature liquid 1 by nucleating droplets of liquid 2, which continue to grow until the entire sample is converted to the new phase
As the temperature is lowered, the probability for nucleation of crystal 2 increases; the viscosity increases on approaching the glass transition temperature, resulting in “a heavily nucleated state composed of nanocrystals of the stable crystalline phase embedded in the matrix of non-transformed supercooled liquid”

Summary

Introduction

It is well known that substances have mesophases: phases in between the crystal and the isotropic liquid in which the liquid has partial orientational or translational ordering. The LLT in TPP has been studied previously using microscopy, and the observations here (see Figure 1(a)) are consistent with previous work.[12,13,23] The thermodynamically stable crystalline phase of TPP has a melting temperature of Tm = 298 K, and the liquid is readily supercooled.

Results

Conclusion