Polyamorphism in tin tetraiodide

Abstract

The discovery of a first-order phase transition in fluid phosphorus aroused renewed interest in polyamorphism in liquids with a locally tetrahedral molecular structure. We have performed in situ synchrotron x-ray diffraction measurements on tin tetraiodide, which consists of SnI4 tetrahedral molecules at ambient pressure, and established that the liquid forms existing above and below 1.5 GPa, where the slope of the melting curve of the solid phase changes abruptly, have different structures. This discovery offers evidence of thermodynamically stable polyamorphism in general compounds as well as in elements. A possible phase diagram that includes the two amorphous states already found is proposed based on the pseudobinary regular solution model. The vertex-to-face orientation between the nearest molecules plays a key role in the transition from the low-pressure to the high-pressure liquid phase.

Communication: Probable scenario of the liquid–liquid phase transition of SnI4

“…This finding then conflicts with the content of the phase diagram proposed in Ref. 1; it implies that the LLCP, if it exists, should be located on the high-pressure side of the path.…”

“…1 With the help of molecular dynamics (MD) simulations, we conjectured that the vertex-toface (called type-2 hereafter) orientation between the nearest molecules plays a key role in the formation of the structure of the high-density liquid (HDL) from that of the low-density liquid (LDL). 1 A phase diagram was proposed on the basis of the pseudo-binary regular-solution (pBRS) model.…”

“…1 With the help of molecular dynamics (MD) simulations, we conjectured that the vertex-toface (called type-2 hereafter) orientation between the nearest molecules plays a key role in the formation of the structure of the high-density liquid (HDL) from that of the low-density liquid (LDL). 1 A phase diagram was proposed on the basis of the pseudo-binary regular-solution (pBRS) model. 2 That is, among four possible scenarios, 3 the liquid-liquid critical point (LLCP) scenario, 4 which could quite naturally explain the relation between the two liquid states and the low-density amorphous (LDA) and high-density amorphous (HDA) states discovered earlier, 5 was chosen.…”

“…10 That the peak located at ∼7.5 Å −1 (which moves to a smaller wavenumber position in the HDL state) remains unmoved is also a characteristic feature of the LDL structure. 1 The SF was then inverted to the reduced radial distribution function (RRDF), 8 G, which indicates the distribution of atoms in the radial direction, and is shown in Fig. 2(b).…”

“…11) of the SF depicted in Fig. 2(a) was fitted to the molecular form factor (MFF), 1 which was calculated by treating the molecule as a rigid tetrahedron. The distance r Sn−I from the center of mass (the location of the Sn atom) of the tetrahedron to its vertex (the location of an I atom) was regarded as a fitting parameter.…”