Rare-Earth Tetraborides RB$_4$: Analysis of Trends in the Electronic Structure

Abstract

Both the basic electronic structure of tetraborides, and the changes across the lanthanide series in RB4 (R = rare earth) compounds, are studied using the correlated band theory LDA+U method in the all-electron Full Potential Local Orbital (FPLO) code. A set of boron bonding bands can be identified that are well separated from the antibonding bands. Separately, the "dimer B" 2pz orbital is non-bonding (viz. graphite and MgB2), and mixes strongly with the metal 4d or 5d states that form the conduction states. The bonding bands are not entirely filled even for the trivalent compounds (thus the cation d bands have some filling), which accounts for the lack of stability of this structure when the cations are divalent (with more bonding states unfilled). The trends in the mean 4f level for both majority and minority, and occupied and unoccupied, states are presented and interpreted. I. BACKGROUND AND MOTIVATION