Jürgen Evers

Electrical properties of alkaline earth disilicides and digermanides

Abstract The electrical resistivity of high purity CaSi2, CaGe2, SrSi2, SrGe2, BaSi2 and BaGe2 was measured in the temperature range from 20°C to 800°C by a 4-point technique. CaSi2, CaGe2 and SrSi2 show metallic, SrGe2, BaSi2 and BaGe2 semiconducting behaviour. This difference can be explained by the change of the structure of the “anion”-sublattices and the change of the metal-distances in the metal-sublattices.

Non-stoichiometric metals with strongly varying electron concentration: Gd-monochalcogenides and LaS

Abstract The reflectivity of single crystals of Gd monochalcogenides and of LaS has been measured at 300 K in the spectral region between 0.03 and 12eV. Special attention has been given to the chemical analysis of the materials and the determination of the deviation from stoichiometry. The optical constants have been determined by means of a Kramers-Kronig relation. A coupled mode of plasmons with interband transitions has been observed. The separation into interband transitions and free electron behaviour permitted the determination of the number of free carriers and their effective mass. It can be shown that the carrier concentration changes much more drastically than the stoichiometry. By comparison with LaS the position of the 4 f 7 levels of GdS was found to be 9 eV below the Fermi level.

Effect of pressure on the structures of divalent metal disilicides MSi2 (M = Ca, Eu, Sr)

Abstract After high-pressure/high-temperature treatment (40 kbar at 1000–1500°C) and quenching to ambient conditions CaSi2, EuSi2, and SrSi2 crystallize in the α-ThSi2 type of structure. Lattice constants and positional parameters have been determined by X-ray powder technique. Structural relations are discussed.

Transformation of three-connected silicon in BaSi2

Abstract Solid-solid transitions in trimorphic BaSi 2 have been investigated up to 40 kbar and 1000°C by X-ray powder technique in quenched samples. All transformations between orthorhombic BaSi 2 I with isolated Si-tetrahedra, trigonal BaSi 2 II with corrugated Si-layers, and cubic BaSi 2 III with a three-dimensional three-connected Si-net can be performed in approximately 5 min at high-pressure-high-temperature conditions. At ambient conditions the difference in molar volume between BaSI 2 I and BaSi 2 III is relatively large ( ΔV I–III = −6.79 cm 3 /mole) and that between BaSi 2 III and BaSi 2 II very small ( ΔV III-II = −0.05 cm 3 /mole). Consequently in the pressure-temperature phase diagram the boundary (I–III) shows a strong pressure dependence contrary to that of (III-II) which is less dependent on variation of pressure. The triple point between the three solid phases is near 11 kbar and 925°C. Substitution of divalent metal and quadrivalent metalloid can easily influence the phase relations in BaSi 2 .

Transformation of three-connected silicon nets in CaSi2

Abstract Up to 40 kbar and 1100°C, CaSi2 is dimorphic. Trigonal/rhombohedral CaSi2I (CaSi2-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi2II (α-ThSi2-type structure) with a three-dimensional net of three-connected Si atoms. The silicon net of CaSi2II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis. In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi2 has been studied at equilibrium and nonequilibrium conditions. The pressure-temperature phase diagram of CaSi2 has been investigated by X-ray technique in quenched samples. From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi2(I-II) have been determined ΔS = 3.2 e.u., ΔU = 4.9 kcal/mole. Under nonequilibrium conditions the transformation CaSi2(II-I) yielded ΔH = −4.2 kcal/mole at 500°C and ambient pressure in a DTA apparatus. Complete transformation of metastable CaSi2II can be achieved within 5 min at a heating rate of 20°C/min. Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi2 are discussed.

Molecular Structure of Hydrazoic Acid with Hydrogen-Bonded Tetramers in Nearly Planar Layers

Hydrazoic acid (HN(3))--potentially explosive, highly toxic, and very hygroscopic--is the simplest covalent azide and contains 97.7 wt % nitrogen. Although its molecular structure was established decades ago, its crystal structure has now been solved by X-ray diffraction for the first time. Molecules of HN(3) are connected to each other by hydrogen bonds in nearly planar layers parallel to (001) with stacking sequence A, B, ... The layer distance, at 2.950(1) Å, is shorter than that in 2H-graphite [3.355(2) Å]. The hydrogen bonds N-H···N are of great interest, since the azido group consists of three homonuclear atoms with identical electronegativity, but different formal charges. These hydrogen bonds are bifurcated into moderate ones with ≈2.0 Å and into weak ones with ≈2.6 Å. The moderate ones build up tetramers (HN(3))(4) in a nearly planar net of eight-membered rings. To the best of our knowledge, such a network of tetramers of a simple molecule is unique.

Transformation of three-dimensional three-connected silicon nets in SrSi2

Abstract Dimorphic SrSi 2 is the first compound for which the two simplest three-dimensional three-connected nets are found in its polymorphs. The cubic net of three-connected silicon atoms (SrSi 2 type of structure) can be transformed into the tetragonal one (α-ThSi 2 type of structure) by a high-pressure-high-temperature treatment. The tetragonal phase is quenchable. Heating of this phase to 600–700°C at ambient pressure results in transformation into the cubic one. At a heating rate of 20°C/min complete transformation can be achieved within 5 min in a DTA apparatus. The energy of transformation has been obtained from the peak areas of the DTA curves to −1.6 ± 0.3 kcal/mole. Although the transformation between the three-dimensional three-connected sets in SrSi 2 must be formally classified as a reconstructive one, a relatively small entropy change ( ΔS = 1 · 1 cal/deg · mole) has been calculated from the change in molar volume and p -T equilibrium conditions. Therefore, structural relations between the cubic and the tetragonal nets are discussed.

Purification of calcium and barium by reactive distillation

Abstract The purification of calcium and barium has been carried out by double ultra high vacuum “reactive distillation” of these metals in a molybdenum column. The distributions of all elemental impurities and molecular species in the starting materials and in various fractions of the distillate were determined with spark source mass spectrographic analysis. The residual gases before, during and after distillation were analyzed by a quadrupole mass spectrometer. The total contents of metallic impurities (carbon and silicon included) of the double distilled metals were, in calcium 124 at. ppm and in barium 69 at. ppm.

A review on fulminating gold (Knallgold)

Expensive and explosive! The reaction of ammonia and gold(III) compounds leads to explosive products known as “fulminating gold”. Gold scientists should be aware of the potential hazards involved in the reaction of these starting materials. Herein, a historic and scientific review on this fascinating substance is presented. Fulminating gold has been known since medieval times, but cannot be fully characterized due to its polymeric and heterogeneous character. Magnetic measurements confirm previous EXAFS analyses that the gold atoms have a square planar coordination environment (AuN4-moieties that are party connected and form a 3-dimensional network). The explosive properties and resulting safety instructions are discussed in detail as well.

Semiconducting behaviour of RuGa2

RuGa2 with TiSi2-type structure was prepared by inductively heating ruthenium and gallium in a water-cooled copper boat under argon atmosphere. The electrical conductivity of a polycrystalline sample of nearly rectangular shape (7 × 5 × 4 mm3) was measured in the temperature range from 20 °C to 400 °C by the four-point technique. RuGa2 is a semiconductor with an electrical resistivity of 0.2 ohm · cm at room temperature and a bandgap of ∼ 0.42 eV. Semiconducting properties have been qualitatively demonstrated for RuAl2 (TiSi2-type structure) and for Os2Si3 (Ru2Si3-type structure, defect TiSi2-structure).