Torsten Lundström

Raman effect in icosahedral boron-rich solids

Abstract We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badgers rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds.

On the electronic properties of β-rhombohedral boron interstitially doped with 3d transition metal atoms

Abstract The controlled modification of the p-type character of pure boron-rich solids to n-type by suitable doping is an essential prerequisite for technical applications. Interstitial doping of β-rhombohedral boron with V, Cr, Fe and Ni fulfils this requirement, while Cu and Co do not. Comparative investigations of the Seebeck coefficient, the DC electrical conductivity, the optical absorption and the dynamical conductivity of BV, BCo and BFe lead to the conclusion that donor levels positioned between the conduction band and the uppermost intrinsic electron trapping level and of sufficient density to overcompensate unoccupied valence and gap states are necessary for n-type conductivity. In contrast to Fe, the interstitial accommodation of V atoms seems to reduce the concentration of possible gap states.

The solubility of chromium in β-rhombohedral boron as determined in CrB∼41 by single-crystal diffractometry

Abstract The structure of a crystal with the composition CrB∼41 has been investigated using single-crystal diffractometry. The composition represents the maximum solubility of chromium in β-rhombohedral boron. The space group is R 3 m and the hexagonal axes are a = 10.964 A and c = 23.848 A. The three-dimensional boron network is essentially equivalent to that in β-rhombohedral boron. The chromium atoms occupy two crystallographic positions, both of which are only partially occupied. The two chromium atoms coordinate twelve and fourteen boron atoms respectively. The result of an investigation of holes in the β-rhombohedral structure is reported. There are at least eight crystallographic positions, which represent holes of sufficient size to accommodate boron or metal atoms in the structure.

Anti-Meissner effect in the BiSrCaCuO-system

Abstract The low field susceptibility of a sintered sample of Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ has been investigated in a SQUID magnetometer. In the low field FC susceptibility a new feature of high-T c superconductors is revealed, an antimeissner effect. Vortex pair fluctuations and pinning are believed to be essential for the observed behaviour.

On the solid solution of copper in β-rhombohedral boron☆

Abstract In solid solutions of copper in β-boron, the copper atoms may occupy a maximum of four non-equivalent crystallographic positions. X-ray powder methods have been used for determining the manner in which these positions are filled when the copper content is increased.

The solution hardening of β-rhombohedral boron

Abstract Solid solutions of transition metals in β-rhombohedral boron have been investigated using X-ray techniques and microhardness measurements. The variation of microhardness with unit cell volume is discussed.

Boron-rich crystals in A1-M-B (M = Li, Be, Mg) systems grown from high-temperature aluminum solutions

Abstract Crystals of icosahedral B12 compounds grown from high-temperature A1 solutions have been studied. The crystals grown are A1LiB14, A1MgB14 and A1~1.1Be~0.6B22. They are obtained as platelets or irregularly shaped polyhedra with maximum dimensions of 5 to 7 mm. The crystallography and the mechanical properties of these materials are presented.

Influence of interstitially soluted iron on structural, optical and electrical properties of β-rhombohedral boron

Abstract A systematical investigation of a series of interstitially doped B:Fe solid solutions of the β-rhombohedral boron structure with compositions up to FeB29.5, which is close to the maximum solubility, is presented. At an iron concentration of about 2.5% the conductivity character changes from the p-type behaviour of pure β-rhombohedral boron to n-type. The IR and Raman active phonons change systematically. The damping ‖ c is much stronger than ⊥ c. In the case of the stretching mode of the central boron atom in the unit cell, occupied and unoccupied unit cells can be distinguished. An additional splitting of this vibration coincides with the formation of magnetic clusters and the sign reversal of the Seebeck coefficient. Some low frequency absorption bands are attributed to atoms in the definite voids of the structure. The question of whether they are local vibrational modes or electronic transitions remains unsolved. The iron atoms lead to an electronic level at 0.133 eV above the Jahn-Teller induced split-off valence band of β-rhombohedral boron.

Crystal structure of magnesium heptaboride Mg2B14

Abstract The crystal structure of a magnesium heptaboride, whose structural formula is Mg2B14, was determined from X-ray powder patterns according to a profile refinement technique and on the basis of isotypism with MgAlB14. The symmetry is orthorhombic and the lattice parameters are a = 5.970 A , b = 8.125 A and c = 10.480 A . The refinement is based on the space group Imam. The unit cell contains five non-equivalent boron and two nonequivalent magnesium atoms. The structure consists of chains of B12 icosahedra extending in the direction of the c axis. The chains are linked laterally via inter-icosahedral bonds or BB bonds involving nonicosahedral atoms, thus forming a three-dimensional boron network. The magnesium atoms occupy two types of holes in this network, corresponding to coordination numbers 12 and 16.

On n-type β rhombohedral boron

Abstract By doping β rhombohedral boron with iron the interstitial solid solution FeB 29.5 was obtained. Investigations of the electronic transport properties between 100 and 700 K have for the first time proved that n-type β rhombohedral boron can be obtained by defined doping. Thus it was shown that a deliberate reversal of the conductivity character of β rhombohedral boron can be produced; this is a decisive prerequisite for applications within semiconductor technology.