![]() ![]() This physical picture of liquids has now been upended by numerous examples of distinct locally favorable structures separated by first-order phase transitions arising in single-component liquid systems ranging from atomic liquids, like silicon ( 1) or phosphorous ( 2), to molecular liquids, such as water ( 3, 4) or triphenyl phosphite ( 5– 7), to metallic glass-formers ( 8). The isotropic liquid state has been typically regarded as singular and related to the gas state, differing only by a change in density. The experimental access to the evolution of local order and structural dynamics across a liquid–liquid transition opens up unprecedented possibilities to understand the nature of the liquid state. We independently corroborate these changes in local organization using Raman spectroscopy. Furthermore, the sizes of nonpolar local domains and ion-coordination numbers deduced from wide-angle X-ray scattering also change abruptly at the LLT. We observe a step-like increase in the static dielectric permittivity at the transition. Here, we show evidence of an LLT in a glass-forming trihexyltetradecylphosphonium borohydride ionic liquid that shows no tendency to crystallize under normal laboratory conditions. However, the experimental evidence for the existence of an LLT in many molecular liquids remains controversial, due to the prevalence and high propensity of the materials to crystallize. Furthermore, it has been suggested that the unique properties of materials such as water, which is critical for life on the planet, are linked to the existence of the LLT. The LLT is fundamental to the understanding of the liquid state and has been reported in a few materials such as silicon, phosphorus, triphenyl phosphite, and water. A liquid–liquid transition (LLT) is a transformation from one liquid to another through a first-order transition. ![]()
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