Pressure-induced structural change in liquid GeI₄

Abstract

The similarity in the shape of the melting curve of GeI<sub>4</sub> to that of SnI<sub>4</sub> suggests that a liquid-liquid transition as observed in liquid SnI<sub>4</sub> is also expected to occur in liquid GeI<sub>4</sub>. Because the slope of the melting curve of GeI<sub>4</sub> abruptly changes at around 3 GPa, in situ synchrotron diffraction measurements were conducted to examine closely the structural changes upon compression at around 3 GPa. The reduced radial distribution functions of the high- and low-pressure liquid states of GeI<sub>4</sub> share the same feature inherent in the high-pressure (high-density) and low-pressure (low-density) radial distribution functions of liquid SnI<sub>4</sub>. This feature allows us to introduce local order parameters that we may use to observe the transition. Unlike the transition in liquid SnI<sub>4</sub>, the transition from the low-pressure to the high-pressure structure seems sluggish. We speculate that the liquid-liquid critical point of GeI<sub>4</sub> is no longer a thermodynamically stable state and is slightly located below the melting curve. As a result, the structural change is said to be a crossover rather than a transition. The behavior of the local-order parameters implies a metastable extension of the liquid-liquid phase boundary with a negative slope.&#13.

A polymerization scenario of the liquid–liquid transition of GeI4

“…The location of the experimental melting curve [16] is also shown for reference. We have assumed that the breakpoint at ∼3 GPa on the melting curve is the triple point among Liq-II, Liq-I, and CP, through which the metastable phase boundary between Liq-II and Liq-I is expected to extend into the liquid region (as shown by a lighter curve) [17].…”

“…The experimental melting curve of crystalline GeI 4 [16] is shown by solid lines along with melting points marked by open circles. Lighter lines are metastable extensions of the melting curve and the probable Liq-II-Liq-I phase boundary [17]. A cross marks the percolation threshold of molecular networks (see section 3.3).…”

“…In contrast to the discontinuous LLT in liquid SnI 4 , the transition proceeds in a wide pressure range around the breakpoint pressure upon compression. We conjecture that the LLCP of GeI 4 is located within the crystalline phase (CP) field below the melting curve and that the LLT ends up merely a crossover [17].…”

“…Molecular crystalline GeI 4 , known to form a glass phase under high-pressure [15], has a similar melting curve to that of SnI 4 [16], but the breakpoint pressure is shifted to ∼3 GPa, reflecting the small molecular size compared with that of SnI 4 . GeI 4 also has recently been shown to undergo the pressureinduced liquid-liquid transition (LLT) from the low-pressure liquid (which we call Liq-II for GeI 4 as well) to high-pressure liquid (Liq-I) state [17]. In contrast to the discontinuous LLT in liquid SnI 4 , the transition proceeds in a wide pressure range around the breakpoint pressure upon compression.…”