ABSTRACT
We report experimental evidence for a crossover between a liquidlike state and a gaslike state in fluid methane (CH_{4}). This crossover is observed in all of our experiments, up to a temperature of 397 K, 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and x-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line-a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquidlike and gaslike properties. On the liquidlike side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gaslike side this competes with the attractive van der Waals potential leading the vibration frequency to decrease as pressure is increased.
ABSTRACT
We report studies of pressure-induced phase transitions of ordered and disordered ternary tetradymite-like Bi2Te2Se by synchrotron powder X-ray diffraction (PXRD) in diamond anvil cells (DACs) for pressures up to 59 and 49 GPa, respectively. The first sample (SB) was prepared from a single crystal with ordered Se/Te sites while the second sample (Q) was prepared from a quenched melt resulting in disordered Se/Te. This allows for an investigation of the effect of disorder on the phase transitions and the equation of states (EoS) of the tetradymite-like α phase. Fitting of a third order Birch-Murnaghan EoS to the α phases yielded bulk moduli K0 of 34.5(10) and 38.3(17) GPa and K' of 6.2(3) and 5.0(5) for the SB and Q samples, respectively. An electronic topological transition (ETT) was identified in both samples at pressures of 4.4 and 3.1 GPa, respectively. This was followed by a transition near 11 GPa to a phase that is isostructural with the ß phase of Bi2Te3. The Se/Te ordering only affects the transition pressure to a small extent. A cubic phase that resembles the δ phase observed in high-pressure studies of Bi2Te3 appears at 17-20 GPa, but the ternary composition leads to a more complex structure. The presence of a low angle diffraction peak in the δ phase demonstrates that the true structure is not simply body-centred cubic. In this way the samples resemble Bi2Se3 where Bi and Se show a high degree of ordering, but the proposed structure of δ-Bi2Se3 also does not fully describe the data for δ-Bi2Te2Se.
