The local structure of transition metal doped semiconducting boron carbides

Abstract

Brand, Jennifer I.; and Dowben, Peter A., "The local structure of transition metal doped semiconducting boron carbides" (2010 Abstract Transition metal doped boron carbides produced by plasma enhanced chemical vapour deposition of orthocarborane (closo-1,2-C 2 B 10 H 12 ) and 3d metal metallocenes were investigated by performing K-edge extended x-ray absorption fine structure and x-ray absorption near edge structure measurements. The 3d transition metal atom occupies one of the icosahedral boron or carbon atomic sites within the icosahedral cage. Good agreement was obtained between experiment and models for Mn, Fe and Co doping, based on the model structures of two adjoined vertex sharing carborane cages, each containing a transition metal. The local spin configurations of all the 3d transition metal doped boron carbides, Ti through Cu, are compared using cluster and/or icosahedral chain calculations, where the latter have periodic boundary conditions.

Insights into the local electronic structure of semiconducting boron carbides in the vicinity of transition metal dopants

“…1, and more extensively seen in the supplementary material. The experimentally determined separation between paired Fe ions was about 5.03 ± 0.03 Å [32,37] while the Mn-Mn separation was found to be close to 5.15 ± 0.03 Å [32], and that between Co-Co was found to be about 5.28 ± 0.02 Å [32,36,39]. We found that this model (Fig.…”

“…This understanding of the structure of boron carbides has been obtained from the analysis of experimental extended X-ray absorption fine structure (EXAFS) [32,36,37,39] and near edge X-ray adsorption fine structure [36,39]. The analysis providing the best fit of the unfiltered EXAFS data, even with doping levels of 50-200 ppm, is consistent with a model that has the transition metal Co [32,36,39], Mn [32] or Fe [32,37] atoms preferentially occupying positions in two adjoined icosahedral cages at the apical sites, likely at a boron or carbon site for each cage, as schematically shown in Fig. 1, and more extensively seen in the supplementary material.…”

“…Semiconducting boron carbides are based on the packing of icosahedra [32], and may contain a significant amount of hydrogen [33], so defects are expected in this material. Successful boron carbide homojunctions have, nonetheless, been fabricated by plasma enhanced chemical vapor deposition (PECVD) of carboranes, with Ni [34,35], Co [32,36], Fe [37] and Mn [32] doping. Indeed, nickel doping of boron carbide has opened a route to the fabrication of the boron carbide based Esaki tunnel diode [34,35].…”

“…The latter has been one of the great unsolved problems of condensed matter theory [38], and remains a challenge because of the very small difference in scattering cross-section between boron and carbon, while neutron scattering experiments would require studying 11 B enriched materials with nearly a complete depletion of the 10 B isotope. Our approach is to tackle the electronic structure based on our knowledge of the structure in the region of transition metal impurities [32,36,37,39]. We have used a cluster or a repeating chain of icosahedra to model just the region in the vicinity of the transition metal impurities, where the structure semiconducting boron carbide is known most reliably.…”

The chromium site in doped glassy lithium tetraborate

“…Chromium is in many respects very similar, able to adopt a number of nominal valencies from zero valent chromium to chromium þ6 (i.e. [14,15], with very different consequences [15]. It is also known that the electronic properties of lithium borates may be manipulated by the direct inclusion of dopants, as well as, by the presence of Li þ , O, and dopant vacancies to serve as trap sites [10,16].…”