1. Department of Synchrotron Radiation Research, Japan Atomic Energy Research Institute, 1-1-1 Kouto, Mikazuki, Sayo , Hyogo 679-5148, Japan
2. Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5198 , Japan

Correspondence to: Yoshinori Katayama¹ Correspondence and requests for materials should be addressed to Y.K. (e-mail: Email: katayama@spring8.or.jp).

Abstract

First-order structural phase transitions are common in crystalline solids, whereas first-order liquid–liquid phase transitions (that is, transitions between two distinct liquid forms with different density and entropy) are exceedingly rare in pure substances^{1, 2, 3, 4}. But recent theoretical and experimental studies have shown evidence for such a transition in several materials, including supercooled water^{5, 6, 7, 8} and liquid carbon^{9, 10}. Here we report an in situ X-ray diffraction observation of a liquid–liquid transition in phosphorus, involving an abrupt, pressure-induced structural change between two distinct liquid forms. In addition to a known form of liquid phosphorus—a molecular liquid comprising tetrahedral P₄ molecules—we have found a polymeric form at pressures above 1 GPa. Changing the pressure results in a reversible transformation from the low-pressure molecular form into the high-pressure polymeric form. The transformation is sharp and rapid, occurring within a few minutes over a pressure range of less than 0.02 GPa. During the transformation, the two forms of liquid coexist. These features are strongly suggestive of a first-order liquid–liquid phase transition.

1. Department of Synchrotron Radiation Research, Japan Atomic Energy Research Institute, 1-1-1 Kouto, Mikazuki, Sayo , Hyogo 679-5148, Japan
2. Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5198 , Japan

Correspondence to: Yoshinori Katayama¹ Correspondence and requests for materials should be addressed to Y.K. (e-mail: Email: katayama@spring8.or.jp).