A. Nørlund Christensen

The crystal growth of the transition metal compounds TiC, TiN, and ZrN by a floating zone technique

Abstract Single crystals of TiC, TiN, and ZrN have been prepared using a floating zone technique. The carbide has been grown in a He ambient gas pressure of up to 20 atm, while the nitrides have been grown in a N2 gas pressure of up to 20 atm. A technique has been developed which allows preparation of the nitrides from the elements and subsequent crystal growth in a single operation.

The crystal structure of Cr8O21 determined from powder diffraction data: Thermal transformation and magnetic properties of a chromium-chromate-tetrachromate

Abstract Thermal decomposition of CrO3 was utilized to prepare a powder sample of the chromium oxide usually designated Cr3O8. Combined information from powder diffraction data using synchrotron, conventional X-ray, and neutron radiation allowed determination of the structure. The structure is triclinic (a = 5.433(1), b = 6.557(1), c = 12.117(2) A, α = 106.36(1), β = 95.73(1) and γ = 77.96(1)°) and was refined in the space group P 1 . The true composition of the compound is Cr8O21. There are two distinct types of chromium atoms in the structure, which may be designated the oxidation numbers (III) and (VI), respectively. The structure is built from pairs of edge-sharing Cr(III)O6 octahedra linked together by Cr(VI)O4 tetrahedra to form sheets. The sheets are then linked together by tetrachromate groups (Cr(VI)4O13) to form a three-dimensional structure. Thus, the chromium oxide may be described as Cr(III)2(Cr(VI)O4)2(Cr(VI)4O13). The magnetic properties of Cr8O21 were investigated in the temperature range 5 to 300 K. Above 100 K the compound is paramagnetic. Magnetic susceptibility data indicate a transition to antiferromagnetism around 100 K, but only vague indications for additional magnetic reflections were found with neutron powder diffraction.

Isotope effects in the bonds of β-CrOOH and β-CrOOD

Samples of orthorhombic chromium oxide hydroxide, β-CrOOH, and the deuterated compound, β-CrOOD, were prepared hydrothermally. The crystal structures were determined by powder profile refinement technique using neutron diffraction data. Unit cells are: β-CrOOH: a = 4.862(2)A, b = 4.298(2) A, c = 2.995(1) A; β-CrOOD: a = 4.873(5) A, b = 4.332(7) A, c = 2.963(2) A, with Z = 2. The space group is P21nm or Pnnm. Distances found for the hydrogen atoms using space group P21nm are: OD···O: 2.58(2) A, OD: 1.07(2) A, OH···O: 2.46(2) A, and OH: 1.10(2) A, showing large isotope effects.

Hydrothermal and high-pressure preparation of some BaMnO3 modifications and low-temperature magnetic properties of BaMnO3(2H)

Three modifications of barium manganese oxide, BaMnO3(2H), BaMnO3(9R), and BaMnO3(4H), were prepared using hydrothermal and high-pressure techniques. The crystal structure of BaMnO3(2H) was refined using X-ray and neutron-diffraction powder patterns. The structure is hexagonal with space group P63mmc, and a = 5.694, A, c = 4.806, A. The magnetic properties of BaMnO3(2H) were investigated between 2.4° and 273°K in a magnetic field of 9750 Oe. The compound is antiferromagnetic with a Neel temperature of less than 2.4°K. Neutron-diffraction powder patterns were obtained at 1.8°K and 300°K. Three weak magnetic superlattice reflections were indexed on the basis of a hexagonal unit cell with the dimensions aH = a√3, and cH = c, where a and c are the dimensions of the chemical unit cell. A collinear antiferromagnetic arrangement of the spins parallel to the [001] direction describes the magnetic structure.

Crystal growth and characterization of the transition metal silicides MoSi2 and WSi2

Abstract Single crystals of molybdenum silicide, MoSi 2 , and tungsten silicide, WSi 2 , were grown using the floating zone technique. The crystals were grown at a He ambient gas pressure of up to 1 MPa. Characterization was made using X-ray powder diffraction and single crystal neutron diffraction analysis.

Hydrothermal preparation and low temperature magnetic properties of Mn(OH)2

Abstract Manganese hydroxide,Mn(OH)2, was prepared using hydrothermal techniques. The magnetic properties of the compound were investigated between 4.2 and 273 K. Mn(OH)2 is antiferromagnetic with a Neel temperature of 12 K. Neutron diffraction powder patterns were obtained at 4.2 K and at 300 K. The magnetic superlattice reflections were indexed on the basis of a hexagonal unit cell with the dimensions a M = a (3) , and cM = 2c where a = 3.322 A and c = 4.734 A are the dimensions of the chemical unit cell. An antiferromagnetic sequence of the spins along the c axis, and a screw spiral propagating in the basal plane describes the magnetic structure. The hydrogen atom position of the crystal structure is determined.

Preparation and crystal structure of β-Ta2N

Abstract Samples of the tantalum nitride β-Ta2N were obtained from the elements by the sintering of tantalum in nitrogen at 2000°C. In the zone melting of tantalum in nitrogen, samples with a lamella structure of β-Ta2N and e-TaN were formed. The crystal structure of β-Ta2N was refined from a neutron diffraction powder pattern using the profile analysis method and the trigonal space group P 3 1m (No. 162). Unite cell parameters are a = 5.285 (5) Ac = 4.919(3) A, with Z = 3. The composition of the sample investigated was TaN0.43(1).

Rate of reactions between D2O and CaxAlyOz

The rate of the reaction between D/sub 2/O and the calcium aluminum oxides Ca/sub 3/Al/sub 2/O/sub 6/, Ca/sub 5/Al/sub 6/O/sub 14/, CaAl/sub 2/O/sub 4/, and CaAl/sub 4/O/sub 7/ was investigated by on-line neutron diffraction powder methods at temperatures from room temperature to 100/sup 0/C. The rate of the reaction increases with increasing calcium content of the compounds and with increasing temperature for each of the compounds. The crystallographic stable hydrate Ca/sub 3/Al/sub 2/(OD)/sub 12/ is obtained from CaAl/sub 4/O/sub 7/ and CaAl/sub 2/O/sub 4/ at temperatures above 63/sup 0/C, from Ca/sub 5/Al/sub 6/O/sub 14/ at temperatures above 49/sup 0/C, and from Ca/sub 3/Al/sub 2/O/sub 6/ at temperatures as low as 7/sup 0/C.

Photoemission studies of single crystals of titanium carbide

Abstract The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3 d and carbon 2 p states at ca. 3–4-eV binding energy, and a single carbon 2 s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2 p band at − 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented. The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV. By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3 d and C 2 p states. The C 2 p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW 5–9 , MAPW 10 and EPM 11 band structure calculations predict a flat band of p -character between the symmetry points X ′ 4 and K 3 , most likely responsible for the majority of C 2 p excitations observed. The C 2 s states, on the other hand, form a single band centered around −10.4 eV. The results obtained are consistent with several recent energy band structure calculations 5–11, 13 that predict a combined bonding of covalent, ionic and metallic nature.

The crystal growth of δ-VN by floating zone and zone annealing techniques

Single crystals of δ-VN have been prepared using a floating zone technique. Crystals grown in a nitrogen ambient gas pressure of up to 2 MPa had a composition in the range VN0.74 to VN0.86. The nitrogen content of the crystals was increased by zone annealing at 2000°C in 2 MPa nitrogen gas to the composition range VN0.84 to VN1.0.