Josef Bauer

High-temperature reactive phase formation in the Nb-N system

Abstract The formation of niobium nitrides was investigated in the temperature range 1400–1800°C by reaction of niobium sheet with nitrogen (1–30 bar N2). Equilibrated samples yielded the p−T−[N]/[Nb] relationship of δ-NbN1−x: [N]/[Nb]=1.585−4.148·10−4·T−0.05176·ln(pN2)+4.984·10−5·T·ln(pN2). In diffusion couples the formation of the phases δ-NbN1−x, γ-Nb4N3±x and β-Nb2N was observed. Between δ-NbN1−x and γ-Nb4N3±x a concentration discontinuity was absent, which is due to the fact that γ-Nb4N3±x is not stable in the investigated temperature range but forms upon cooling. This is visible by the typical hatched microstructure at the nitrogen-poor boundary of δ-NbN1−x. The c/a ratio of the tetragonal cell of γ-Nb4N3±x increases with increasing nitrogen content. The homogeneity ranges of phases were determined from the diffusion bands by wavelength-dispersive EPMA. A portion of the phase diagram is presented. The thickness ratio of the β-Nb2N phase band vs. the δ-NbN1−x/γ-Nb4N3±x phase band was found to be dependent on the applied nitrogen pressure. This is due to the different surface concentration of nitrogen in the δ-NbN1−x phase and the influence of the homogeneity range on the layer growth rate.

The X-ray and electronic structures of GdB4

Abstract The structure of GdB 4 has been determined by single crystal X-ray diffraction. The compound adopts the ThB 4 structural type (space group P 4/ mbm ; a =7.1316(2) (A); c =4.0505(2) (A); 222 reflections with F o >4 σ ( F o ); R =0.0196; wR 2 =0.0424). Tight-binding calculations indicate that the bonding within this compound is best described as resulting from the interaction between (Gd 2+ ) 2 and B 4− 8 subnets. The latter is made of B 2− 6 octahedra linked by B 2− 2 units and satisfies the closed-shell requirement. A strong covalent interaction occurs between both subnets in such a way that the formal Bue5fbB double bond existing in the B 2− 2 units is considerably weakened by π -donation to the metals.

First-principles study of ternary metal borocarbide compounds containing finite linear BC2 units

Abstract Electronic structures of the ternary metal borocarbide compounds Sc2BC2, Al3BC3 and Lu3BC3 containing linear BC2 units are compared using density functional calculations. Results reveal a covalent bonding between the metallic matrix and the formally BC25− nonmetal anions which is stronger for the aluminum compound than for the two others.

Structural and magnetic properties of the rare-earth-borocarbides, R5B2C6 (R=Y, Ce-Tm)

Abstract The compounds R 5 B 2 C 6 , R=Y, Ce–Tm, were prepared by arc melting the elements, rare earth ingots, boron and carbon. From X-ray powder analyses these compounds were found to crystallize essentially with tetragonal symmetry, space group P4 (No. 75). The yttrium containing sample is a Pauli type paramagnet below room temperature down to 4.2 K. The magnetic properties of Ce 5 B 2 C 6 are characterized by an intermediate valence behavior. Gd 5 B 2 C 6 orders antiferromagnetically at a Neel temperature T N =26 K and reveals a metamagnetic transition above B =0.5 T. All the remaining samples investigated undergo ferromagnetic transitions in the temperature regime below T =40 K. These borocarbides exhibit a magnetic behavior typical for narrow domain wall ferromagnets. Both the Curie temperatures, T C , as well as the paramagnetic Curie temperatures, θ P , scale roughly with the de Gennes factor and hence the indirect exchange interaction via the conduction electrons (RKKY-interacting) is the dominating force of the R–R coupling mechanism.

Thermal expansion of β-rhombohedral boron

Abstract The cell dimensions of β-rhombohedral boron have been measured in the temperature range 10–1273 K, using X-ray diffraction. The linear thermal expansion coefficients ( α l =( l − l 298 )/ l 298 ( T − T 298 )) were determined along the a and c directions. It was found that β- rhombohedral boron is anisotropic, displaying a significantly larger thermal expansion coefficient in the c direction than in the a direction. The average linear thermal expansion coefficient is 6.4×10 −6 K −1 in the temperature range 293–1023 K.

Magnetic properties of the borocarbides R3BxC4−x (R=Gd, Ho; x<1) with Sc3C4-type structure

Abstract X-ray powder diffractograms proved the occurrence of the Sc 3 C 4 -type of structure for Ho 3 B x C 4− x for 0≤ x ≤0.8, whereas the gadolinium containing compound could only be stabilized by adding 7.5 at.xa0% boron to the sample. Ac as well as dc magnetic susceptibilities of the pure binary Ho 3 C 4 exhibit an antiferromagnetic transition at a Neel temperature of T N =10 K, followed by a metamagnetic transition in external fields of H >0.1 T. The holmium borocarbides undergo ferromagnetic transitions below T C =35 K. The feature of the magnetization curves is reminiscent of a system composed of narrow domain walls. The gadolinium-borocarbide also orders ferromagnetically at a Curie temperature of T C =159 K.

Magnetic properties and structural chemistry of the solid solution La5B2C6-Gd5B2C6

Abstract The borocarbides (La 1− x Gd x ) 5 B 2 C 6 with 0≤ x ≤1 were prepared by arc melting elemental lanthanum, gadolinium, boron and carbon. From X-ray powder diffraction analysis, all alloys were found to be isostructural with the Ce 5 B 2 C 6 type. (La 1− x Gd x ) 5 B 2 C 6 (with 0≤ x ≤1) forms a continuous solid solution. La 5 B 2 C 6 is a type I superconductor at T H crit =34 mT) or by replacing lanthanum by paramagnetic gadolinium (Gd 3+ ) ( n crit 5 B 2 C 6 is an antiferromagnet with a Neel temperature T N =26 K. Within the solid solution (La 1− x Gd x ) 5 B 2 C 6 the ordering temperature decreases gradually with increasing x . The positive θ p values suggest that the spin structure contains ferromagnetic sheets which are antiferromagnetically coupled. For x T >4.2 K.

Electron probe microanalysis in the ternary Gd–B–C system

Abstract EPMA exploration of the Gd–B–C system in the region “Gd–GdB2–GdBC” and in the neighborhood of the recently described Gd4B3C4 compound led to the identification of 9 new ternary phases, which allows to clear up the phase diagram of this ternary system. A structural description of the bonding between the non-metal atoms in most of the identified compounds is proposed, on the basis of simple electron counting rules and using the planar repeat units or the finite linear anions which have been shown to exist in the structurally characterized rare-earth borocarbide compounds.

Twinning and intergrowth of rare earth boride carbides

Twins and intergrown crystals of tetragonal rare earth boride carbides, especially those with the La(5)B(2)C(6) structure type, have been investigated by high-resolution electron microscopy and X-ray diffraction. The structure of the twin interface has been determined. It provides an explanation for coherently intergrown domains of different structure. The Sc(3)C(4) structure type is remarkable because it is frequently intergrown with La(5)B(2)C(6)-type phases. It provides, for instance, a model for the intergrowth of other types, e.g. Gd(4)B(3)C(4) and Gd(5)B(2)C(5). The presence of metal-atom square nets in different orientations in the structures accounts for a number of intergrowth phenomena. The possibilities and limitations of X-ray structure determinations are discussed with respect to actual examples.

Nitridation of Ti/Nb alloys and solid-state properties of δ-(Ti,Nb)N

Abstract The formation of microstructures and diffusion layers in the Tiue5f8Nbue5f8N system was investigated by annealing compact Ti/Nb alloys (0–100 at% Ti) in a high-purity nitrogen atmosphere (3 and 30 bar N 2 ) in the temperature range 1300–1600°C. The alloy starting material was in the form of wedge-type samples as well as in the form of plane sheets. After nitridation all samples showed a yellow colour which is characteristic for the fee δ-(Ti,Nb)N phase, the layer growth rate of which was a minimum at 75 at% Ti. The nitride layer as well as the precipitates were rich in Ti whereas in the β alloy an Nb increase was observed. During nitridation needles with different length and width grew from the inner layer boundary into the original alloy. Electron probe microanalysis (EPMA) was performed to gain insight into the time evolution of the diffusion profiles of Ti, Nb and N. Homogeneous plane-sheet samples were used for measurements of the microhardness, the superconducting transition temperature and the lattice parameter of δ-(Ti,Nb)N.