Prediction of Phase Diagrams for Hard Materials: Application to Boron Crystals

Hard materials and refractory materials, such as diamond, are generally stable and have high melting points. The phase diagrams of these materials seem to be relatively simple. However, recent progress of high-pressure experiments along with theoretical predictions by density-functional theoretical (DFT) methods has disclosed the richness of new structures, which has attracted the attention of material researchers. An elemental crystal of boron is an extreme case; there are many polymorphic modifications, and it is only recently that the phase diagram has been established. In the course of studying the phase diagram, a new structure of the γ -phase has been discovered. Now, we have to admit that hard materials are rich sources for materials research. Many metastable phases are hidden behind the widely accepted phase diagrams. How to discover these hidden phases is one of the central issues for materials research. In this paper, experiences with predicting the phase diagram of boron by a theoretical approach are described and impacts on materials research, by taking superconductivity research as a working example, are shown. A combination of the microscopic method of DFT and the macroscopic approach of thermodynamics is an extremely powerful tool.