Title:	Screening multifunctional transition-metal borides under high-pressure: An ab initio approach
DNr:	SNIC 2018/3-113
Project Type:	SNIC Medium Compute
Principal Investigator:	Wei Luo <wei.luo@physics.uu.se>
Affiliation:	Uppsala universitet
Duration:	2018-03-01 – 2019-03-01
Classification:	10304 10403
Homepage:	http://www.physics.uu.se
Keywords:

Abstract

The aim of this application is to expand and accelerate our understanding of multifunctional materials. The relation between electronic structure and the crystallographic atomic arrangement is one of the fundamental questions in condensed matter physics and inorganic chemistry.. Solids have been mainly studied at ambient conditions, i.e. at room temperature and zero pressure. However it was realized early that there is also a fundamental relation between volume and structure, and that this dependence could be most fruitfully studied by means of high pressure experimental techniques. From a theoretical point of view this is an ideal type of experiment, since only the volume is changed, which is a very clean variation of the external conditions. With the significant advancement of computing resources, computational investigations of materials under extreme conditions have become a routine affair. Within the framework of ab initio density functional theory (DFT), the high-pressure experiments with systematic variation of volume can also be theoretically calculated. Therefore, a direct corroboration of the equation of states obtained from experiment and theory can be possible. It is pertinent to emphasize that the predictive power of computation goes beyond merely supplementing the experimental results of pressure-volume relationships. Transition-metal boride compounds (TM–B) are a kind of materials belonging to the family of superhard materials (Hv ≥ 40 GPa) because of the degree of directional covalent bonding of three dimensional (3D) boron network. For instance, our recent studies show that FeB4 could be transformed from metallic phase to semiconducting phase under pressure which is predicted to be a superhard material with hardness of 43 GPa as well as the energy gap (Eg) of 2 eV. Hence, this example indicates that TM-B compounds might be one of the promising muti-functional materials. This initial discovery suggests that it can be a potential replacement for diamond and there can be a host of exciting and unexplored applications. However, still, there are some questions and controversies by previous investigations for TM-B materials. Those issues are open the researchers, for example; (i) CrB4 has the same ambient phase with FeB4 but why do they have different high-pressure pathways? (ii) WB4 has been proposed to be a superhard material with crystal structure of P63/mmc symmetry, but some studies have reported the instability of that structure. (iii) What is the effects of number and energy level of d electrons of transition metals to crystal structure and relating properties of TMB4 under high pressure? Therefore, this class of materials certainly deserves further investigation to address the relevant questions and get deeper understanding. This application aims at theoretically investigating phase behaviors of transition metal borides by considering effects of different transition metals (i.e. Cr, Mn, Fe, Mo, Tc, Ru, W, Re and Os) on crystal structure, mechanical properties, electronic and superconducting properties and high-pressure phase transitions. Based on thorough investigations of the phase behaviors at ambient and extreme conditions, it is possible to guide new directions for potential applications of this material family.