High-temperature phase transitions in dense germanium

Abstract

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure-temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range 64-90 GPa we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterised by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm Ge-IV becomes unstable at high-temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure-temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply.

Pressure-induced structural transformation of clathrate Ge136 via ultrafast recrystallization of an amorphous intermediate

“…The observed PIA of the clathrate has features analogous to the "cold melting" suggested to operate, e.g., in PIA of water ice resulting in the HDA phase [36,37]. The melting line of the cubic diamond phase has a negative Clapeyron slope and the melting temperature reaches a minimum of 800 K at a pressure of 10 GPa [21]. Because of a lack of a mechanism allowing a direct transformation of the clathrate to another crystalline phase, the existence of a low free-energy supercooled liquid offers a kinetic route toward a disordered structure available on compression of the clathrate, providing a rationale for the PIA.…”

“…Here we focus on the pressure-induced transformations of Ge 136 and study them by means of ab initio simulations. The phase diagrams of Si and Ge are very similar, but the range of stability of the β-tin phase in Ge is much wider than in Si [21]. The aim of our study is twofold: Find a theoretical prediction for the high-pressure behavior of Ge 136 clathrate and uncover the elusive microscopic mechanism of the transformation.…”

Pressure-induced structural transformation of clathrate Ge$_{136}$ via an ultrafast recrystallization of an amorphous intermediate

“…2 We note that the observed pressure-induced amorphization of the clathrate has features analogous to the "cold melting" suggested to operate, e.g., in pressureinduced amorphization of water ice resulting in the highdensity amorphous phase [22,23]. The melting line of the cubic diamond structure has a negative Clapeyron slope and the melting temperature reaches a minimum of 800 K at pressure of 10 GPa [15]. In lack of a mechanism allowing a direct transformation of the clathrate to another crystalline phase, the existence of a lowfree-energy supercooled liquid offers a kinetic route towards a disordered structure available upon compression of the clathrate, providing a rationale for the pressureinduced amorphization.…”

“…Here we focus on the pressure-induced transformations of Ge 136 and study them by means of ab initio simulations. The phase diagrams of Si and Ge are very similar, but the range of stability of the β-tin phase in Ge is much wider than in Si [15]. The aim of our study is twofold: find a theoretical prediction for the high-pressure behaviour of Ge 136 clathrate and uncover the elusive microscopic mechanism of the transformation.…”

Real-time (nanoseconds) determination of liquid phase growth during shock-induced melting

“…However, the lack of temperature measurements in shocked Ge requires the use of a theoretical approach to estimating temperatures along the Hugoniot. The theoretical multiphase EOS developed by Crockett et al (45) shows that the Ge Hugoniot crosses the P-T melt boundary, determined from static compression studies (46)(47)(48)(49)(50)(51), at ~50 GPa. In view of the XRD results, the multiphase Ge EOS will need to be revised.…”