Melting Curve of Black Phosphorus: Evidence for a\nSolid–Liquid–Liquid Triple Point

Black phosphorus (bP) is a crystalline\nmaterial that can be seen\nas an ordered stacking of two-dimensional layers, which results in\noutstanding anisotropic physical properties. The knowledge of its\npressure (<i>P</i>)–temperature (<i>T</i>) phase diagram, and in particular, of its melting curve is fundamental\nfor a better understanding of the synthesis and stability conditions\nof this element. Despite the numerous studies devoted to this subject,\nsignificant uncertainties remain regarding the determination of the\nposition and slope of its melting curve. Here we measured the melting\ncurve of bP in an extended <i>P</i>, <i>T</i> region\nfrom 0.10(3) to 5.05(40) GPa and from 914(25) to 1788(70) K, using\nin situ high-pressure and high-temperature synchrotron X-ray diffraction.\nWe employed an original metrology based on the anisotropic thermoelastic\nproperties of bP to accurately determine <i>P</i> and <i>T</i>. We observed a monotonic increase of the melting temperature\nwith pressure and the existence of two distinct linear regimes below\nand above 1.35(15) GPa, with respective slopes of 348 ± 21 and\nof 105 ± 12 K·GPa<sup>–1</sup>. These correspond\nto the melting of bP toward the low-density liquid and the high-density\nliquid, respectively. The triple point at which solid bP and the two\nliquids meet is located at 1.35(15) GPa and 1350(25) K. In addition,\nwe have characterized the solid phases after crystallization of the\ntwo liquids and found that, while the high-density liquid transforms\nback to solid bP, the low-density liquid crystallizes into a more\ncomplex, partly crystalline and partly amorphous solid. The X-ray\ndiffraction pattern of the crystalline component could be indexed\nas a mixture of red and violet P.