Behavior of Disordered Materials under Extreme Conditions

A joint experimental and theoretical study on the behaviors of disorderedchalcogenides (i.e., amorphous As2Se3 and amorphous AsSe) under high-pressure,behaviors of chemically disordered high entropy alloys (HEAs) under high-pressure andhigh-temperature, and low temperature dynamics of Zr-based metallic glasses (MGs) ispresented. A brief introduction, experimental methods and behavior of the studiedmaterials under extreme conditions of temperature and pressure are documented.A reversible breakdown of intermediate range ordering (IRO) and associated networktransition is observed under pressure in amorphous As2Se3. Such a networktransformation is found to be gradual without any sudden jump in density.A reversible pressure-induced crystallization is observed in amorphous As2Se3. Thehigh pressure FCC phase is found to be metastable due to possessing excess amount offree energy, and upon decompression the amorphous phase is found to be retrieved.Surprisingly, the as-prepared amorphous phase and the amorphous phase recovered fromthe complete decompression of the high-pressure crystalline phase are found to beidentical within the theoretical and experimental uncertainty. This is first time that anamorphous material, after going through pressure-induced crystallization, is seen torecover its virgin local structure upon complete decompression, which is the exactnovelty in the current thesis.Similar to amorphous As2Se3, a reversible breakdown of intermediate range ordering(IRO) and associated network transition is observed under pressure in amorphous AsSe.Such a network transformation is found to be gradual without any sudden jump indensity.Behaviors of chemically disordered high entropy alloys (HEAs) under high-pressureand high-temperature have been studied. For the studied HEAs, equations of state aredeveloped under high-pressure, and linear and volumetric thermal expansions aredocumented under high-temperature. The HEAs are found to be stable under both thehigh-pressure and high-temperature, and no phase transition is seen to occur up to thehighest pressure and temperature achieved in the current study.The low temperature dynamics and the possible origin of boson peak in the Zr-basedmetallic glasses have also been described in the current thesis. A universal correlationbetween the local structure and boson peak is established for the Zr-based metallicglasses. The boson peak is found to be originated from the vibrations caused in thedensity fluctuation regions in the metallic glasses

Temperature- and Pressure-Induced Polyamorphic Transitions in AuCuSi Alloy

Temperature-induced liquid–liquid phase transition (LLPT) and pressure-induced amorphous–amorphous phase transition (AAPT) have never been simultaneously reported in any single metallic system. In an Au$_{55}$Cu$_{25}$Si$_{20}$ alloy, however, we discovered a temperature-induced LLPT by detecting “reversible λ-anomalies” of the thermal expansion coefficient between two liquid states at ambient pressure, while a pressure-induced AAPT in Au$_{55}$Cu$_{25}$Si$_{20}$ metallic glass (MG) occurs upon compression at ambient temperature. Both LLPT and AAPT are reversible with a hysteresis in temperature and pressure, respectively. Using molecular dynamics simulations and synchrotron X-ray techniques, we elucidate structural differences in both low- and high-pressure Au$_{55}$Cu$_{25}$Si$_{20}$ MG phases and low- and high-temperature Au$_{55}$Cu$_{25}$Si$_{20}$ liquid phases. Electronic transfer between Si and Au or/and Cu atoms occurs in both temperature-induced LLPT and pressure-induced AAPT in the Au$_{55}$Cu$_{25}$Si$_{20}$ alloy

Isosymmetric phase transitions, ultrahigh ductility, and topological nodal lines in α- A g2 S

We report two reversible pressure-induced isosymmetric phase transitions in $α−Ag_{2}S$ that are accompanied by two compressive anomalies at 7.5 and 16 GPa, respectively. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in $α−Ag_{2}S$. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. The band-crossing points near the Fermi energy form a nodal-line structure due to the preservation of the time-reversal and space-inversion symmetries under pressure. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals