Synthesis and Tuning of Multifunctional Materials at High Pressure

Abstract: At the present stage, human society is developing at an unprecedented speed, facing an emergence of highly pressing challenges, e.g., information explosion, energy production problems, environmental pollution, climate problems. Functional materials with tailored properties are considered as holding a key to solving these problems. In this thesis, high-pressure techniques were employed to synthesize and tune the properties of multiferroic materials relevant to spintronic and light-harvesting applications, and multifunctional high-entropy alloys.Melanostibite (Mn2FeSbO6, MFSO) is a very rare mineral discovered in Sweden. Previous studies indicate it is a potential multiferroic material with foreseen applications in information storage and spintronic devices. However, its multiferroic phase has not been synthesized yet. Herein, the structural evolution of MFSO was studied up to ~50 GPa, and the LiNbO3-type MFSO was synthesized at high pressure and moderate temperature. As a polar structure material, the LiNbO3-type MFSO represents a promising candidate for multiferroic materials. The double perovskite, Pb2CoTeO6, was also compressed to ~60 GPa, while no polar phase was discovered. The obtained results provide guidance to the synthesis of new multiferroic double perovskite.Solar energy is a promising alternative to fossil fuels and thus a viable solution to the global energy problem. Light-harvesting materials, which absorb sunlight and transform it into electricity by the photovoltaic effect, represent the core part of solar cells. Currently, the dominant commercial light-harvesting material is silicon. However, silicon and recently emerged organic-inorganic perovskites have several drawbacks. Multiferroic oxides are considered as stable and nontoxic light-harvesting materials. But, their bandgap energies are generally too large for photovoltaic applications. Herein, high-pressure technique was applied to treat Mn3TeO6, and a quenchable phase of Mn3TeO6 displaying a greatly narrowed bandgap was synthesized. The measured absorption spectrum of the quenched phase reveals that it may be suitable for photovoltaic applications. The present research opens a green way to tune the bandgap energy of multiferroic.High-entropy alloys (HEAs) were first synthesized in 2004. However, knowledge of this new class of promising alloys is still very limited, even in very fundamental aspects. The present results reveal that lattice distortion plays important roles in the phase transition of HEAs, and demonstrate the future possibility of designing the Invar high-entropy alloy, a promising structural material. The results show that it is possible to combine several practical properties in a single alloy, which will widen the range of applications of HEAs. The presented research demonstrates that high-pressure represents an effective way to tune various properties of materials, as well as can be applied for the synthesis of materials with exotic properties which are usually not stable or attainable at ambient conditions.