Metals in beta-rhombohedral boron

Physica B 158 (1989) 627-628 North-Holland, Amsterdam 627 METALS IN BETA-RHOMBOHEDRAL BORON* Joe Wong and Glen A: Slack** Lawrence Livermore National Laboratory, University of California, P.O. Box 808. Livermore, CA 94550, USA **General Electric Corporate Research and Development, P.O. Box 8, Schenectady, N.Y. 12301, USA Beta-boron is a semiconductor with a band gap of 1.5 eV. It is hard and refractory with a very high melting point of 2300 K. It satisfies the "small atom and large number of atoms (105) per unit cell" criteria for potentially high thermoelectric efficiency. Crystallographically, beta-boron is made up of compact 812 and BTO polyhedral units and isolated B atoms, all linked together by strong, directional covalent bonds. These structural and bonding features create a very open structure in beta-boron. Assuming RB = 0.8811, the degree of space filling is estimated to be only 36%. Hence, the structure contains many holes that can interstitially accommodate a variety of metal atoms in the Periodic Table. In Fig. 1, the normalized XANES spectra are shown for a series of 3d metals, 1 atomic % in concentration, doped in beta-boron. For the early members, Ti, V, and Cr in particular, a number of well-resolved transitions up to 24 eV are evident. The pre-edge feature at l-2 eV is a 1s -f 3d transition, which normally is diople forbidden, but is rather strong, indicative of appreciable overlap with the p-states of the metal dopants. Two well-defined bound states at 7 and 11 eV are also evident. As the d-states are progressively filled up in going across the 3d series from Ti to Cu, the pre-edge feature at l-2 eV becomes less well-defined as Is clear the XANES of Mn, Fe and almost disappears in the spectra of Co, Ni and Cu. This is again evident from the spectra of Mn through Cu. The features at high energies are due to lattice scattering. Concentration variations and EXAFS analysis show that both V and Cr occupy the more syrrmetric AT site with 12 boron neighbors at a bond distance of 2.15 II up to a doping concentration of 1X, while Ti, Fe and Mn occupy both AT and D sites, the latter has 14 boron atoms as neighbors. Above 0.5% Ni goes into a second site, whereas Cu undergoes multi-site occupancy at 1 atom X. In two systems doubly doped with 1% each of V and Cu in one case, and 1% V and 3% Cu in another, the V and Cu XANES spectra of the double doped materials are identical to the respective singly doped boron materials. This indicates that the local structure and electronic configuration around the V atom is little affected by the presence of Cu in the boron lattice and vice versa. This result demonstrates conclusively that there is no interaction between two dissimilar dopants in the boron lattice (Figs. 2 & 3). *Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. 0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Transcript of Metals in beta-rhombohedral boron

Page 1: Metals in beta-rhombohedral boron

Physica B 158 (1989) 627-628 North-Holland, Amsterdam

627

METALS IN BETA-RHOMBOHEDRAL BORON*

Joe Wong and Glen A: Slack**

Lawrence Livermore National Laboratory, University of California, P.O. Box 808. Livermore, CA 94550, USA

**General Electric Corporate Research and Development, P.O. Box 8, Schenectady, N.Y. 12301, USA

Beta-boron is a semiconductor with a band gap of 1.5 eV. It is hard and refractory with a very high melting point of 2300 K. It satisfies the "small atom and large number of atoms (105) per unit cell" criteria for potentially high thermoelectric efficiency. Crystallographically, beta-boron is made up of compact 812 and BTO polyhedral units and isolated B atoms, all linked together by strong, directional covalent bonds. These structural and bonding features create a very open structure in beta-boron. Assuming RB = 0.8811, the degree of space filling is estimated to be only 36%. Hence, the structure contains many holes that can interstitially accommodate a variety of metal atoms in the Periodic Table.

In Fig. 1, the normalized XANES spectra are shown for a series of 3d metals, 1 atomic % in concentration, doped in beta-boron. For the early members, Ti, V, and Cr in particular, a number of well-resolved transitions up to 24 eV are evident. The pre-edge feature at l-2 eV is a 1s -f 3d transition, which normally is diople forbidden, but is rather strong, indicative of appreciable overlap with the p-states of the metal dopants. Two well-defined bound states at 7 and 11 eV are also evident. As the d-states are progressively filled up in going across the 3d series from Ti to Cu, the pre-edge feature at l-2 eV becomes less well-defined as Is clear the XANES of Mn, Fe and almost disappears in the spectra of Co, Ni and Cu. This is again evident from the spectra of Mn through Cu. The features at high energies are due to lattice scattering.

Concentration variations and EXAFS analysis show that both V and Cr occupy the more syrrmetric AT site with 12 boron neighbors at a bond distance of 2.15 II up to a doping concentration of 1X, while Ti, Fe and Mn occupy both AT and D sites, the latter has 14 boron atoms as neighbors. Above 0.5% Ni goes into a second site, whereas Cu undergoes multi-site occupancy at 1 atom X.

In two systems doubly doped with 1% each of V and Cu in one case, and 1% V and 3% Cu in another, the V and Cu XANES spectra of the double doped materials are identical to the respective singly doped boron materials. This indicates that the local structure and electronic configuration around the V atom is little affected by the presence of Cu in the boron lattice and vice versa. This result demonstrates conclusively that there is no interaction between two dissimilar dopants in the boron lattice (Figs. 2 & 3).

*Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)