Ulrich Häussermann

Thermal decomposition of ammonia borane at high pressures

The effects of high pressure (up to 9 GPa) on the thermal decomposition of ammonia borane, BH3NH3, were studied in situ by Raman spectroscopy in a diamond anvil cell. In contrast with the three-step decomposition at ambient pressure, thermolysis under pressure releases almost the entire hydrogen content of the molecule in two distinct steps. The residual of the first decomposition is polymeric aminoborane, (BH2NH2)x, which is also observed at ambient pressure. The residual after the second decomposition is unique to high pressure. Presumably it corresponds to a precursor to hexagonal BN where macromolecular fragments of planar hexagon layers formed by B and N atoms are terminated by H atoms. Increasing pressure increases the temperature of both decomposition steps. Due to the increased first decomposition temperature it becomes possible to observe a new high pressure, high temperature phase of BH3NH3 which may represent melting.

Optical and electronic properties of metal doped thermoelectric Zn4Sb3

Optical and electronic properties of metal (Pb, Bi, Sn, and In) doped Zn4Sb3 are reported in the temperature range 80–300 K, which covers the β, α, and α′ structural phases of this thermoelectric material. Metal doping alters the subtle balance between Zn disorder and Zn deficiency present in β-Zn4Sb3 and changes its low temperature structural behavior. Analysis of infrared reflection data shows that the formation of ordered α′-Zn4Sb3 is accompanied by a substantial increase in the free charge-carrier concentration. In contrast, for samples where doping suppresses the occurrence of the low temperature α′-phase, the free charge-carrier concentration is only weakly temperature dependent. Different degrees of structural disorder in doped β-Zn4Sb3 and the ordering processes at low temperatures leading to α- and α′-Zn4Sb3 are also recognized in the charge-carrier dynamics.

Polyanionic gallium hydrides from AlB2-type precursors AeGaE (Ae = Ca, Sr, Ba; E = Si, Ge, Sn).

The quaternary hydrides (deuterides) SrGaGeH(D), BaGaSiH(D), BaGaGeH(D), and BaGaSnH(D) were obtained by investigating the hydrogenation behavior of AeGaE intermetallic compounds (Ae = Ca, Sr, Ba; E = Si, Ge, Sn), and structurally characterized by powder X-ray and neutron diffraction as well as solid state (1)H NMR investigations. The new main group metal/semimetal hydrides were found to crystallize with the simple trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.22-4.56 A, c = 4.97-5.30 A) and feature a two-dimensional polyanion [GaEH](2-) that corresponds to a corrugated hexagon layer built from three-bonded Ga and E atoms. H is terminally attached to Ga. In BaGaSiD, a considerable degree of stacking disorder could be detected. Polyanions [GaEH](2-) are electron precise, and the hydrides AeGaEH display small band gaps in the range of 0.1-0.6 eV at the Fermi level. This is in contrast to the metallic precursor phases AeGaE, which are representatives of the AlB2 structure type or variants of it. Hydrogenation has only minor consequences for the metal/semimetal atom arrangement, and the induced metal-nonmetal transition is reversible for SrGaGe, BaGaSi, and BaGaGe. BaGaSnH partially decomposes into a mixture of intermetallic compounds upon hydrogen release. Desorption temperatures are above 400 degrees C.

Transport properties of the II–V semiconductor ZnSb

The intermetallic compound ZnSb is an electron poor (II–V) semiconductor with interesting thermoelectric properties. Electrical resistivity, thermopower and thermal conductivity were measured on single crystalline and various polycrystalline specimens. The work establishes the presence of impurity band conduction as an intrinsic phenomenon of ZnSb. The impurity band governs electrical transport properties at temperatures up to 300–400 K after which ZnSb becomes an intrinsic conductor. Furthermore this work establishes an inherently low lattice thermal conductivity of ZnSb, which is comparable to the state-of-the-art thermoelectric material PbTe. It is argued that the impurity band relates to the presence of Zn defects and the low thermal conductivity to the electron-poor bonding properties of ZnSb.

Structural behavior of the acetylide carbides Li2C2 and CaC2 at high pressure

The effects of high pressure (up to 30 GPa) on the structural properties of lithium and calcium carbide, Li(2)C(2) and CaC(2), were studied at room temperature by Raman spectroscopy in a diamond anvil cell. Both carbides consist of C(2) dumbbells which are coordinated by metal atoms. At standard pressure and temperature two forms of CaC(2) co-exist. Monoclinic CaC(2)-II is not stable at pressures above 2 GPa and tetragonal CaC(2)-I possibly undergoes a minor structural change between 10 and 12 GPa. Orthorhombic Li(2)C(2) transforms to a new structure type at around 15 GPa. At pressures above 18 GPa (CaC(2)) and 25 GPa (Li(2)C(2)) Raman spectra become featureless, and remain featureless upon decompression which suggests an irreversible amorphization of the acetylide carbides. First principles calculations were used to analyze the pressure dependence of Raman mode frequencies and structural stability of Li(2)C(2) and CaC(2). A structure model for the high pressure phase of Li(2)C(2) was searched by applying an evolutionary algorithm.

TlF and PbO under High Pressure: Unexpected Persistence of the Stereochemically Active Electron Pair

Even under a pressure of 46 GPa, the low-symmetry lone-pair structures of isoelectronic TIF and PbO (see picture for β-PbO), classic examples of systems with a stereochemically active lone pair, resist transformation into the corresponding high-symmetry NaCl and CsCl structures. Ab initio calculations allowed a simple bonding picture for lone-pair structures involving inert-pair elements to be developed.

Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO2

Stishovite (SiO2 with the rutile structure and octahedrally coordinated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350–550 °C near 10 GPa produces stishovite with significant amounts of H2O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 ± 0.2 wt % H2O and NMR suggests that the primary mechanism for the H2O uptake is a direct hydrogarnet-like substitution of 4H+ for Si4+, with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO2 + H2O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H2O at convergent plate boundaries, and in other relatively cool high-pressure environments.

Large-volume multianvil cells designed for chemical synthesis at high pressures

Two multianvil cell assemblies with octahedral edge lengths (OEL) of 18 and 25 mm have been developed, which allow sample volumes of up to 153 and 387 mm3, respectively. Pressure calibrations were performed at room temperature and 1200 °C using quenched samples, and at variable temperature using in situ synchrotron measurements. These calibrations employed 25.4 mm tungsten carbide anvils with a truncation edge length of 12 mm for the 18 mm OEL cell (18/12 configuration) and 15 mm for the 25 mm OEL cell (25/15 configuration) and yielded sample pressures of up to 10 and 7.5 GPa, respectively, at loads approaching 800 t. The measured thermal gradients at 1200 °C vary between 5–10 °C/mm (18/12) and 15–20 °C/mm (25/15). By combining large sample/reaction volumes with minimal thermal gradients, the new multianvil cell assemblies are especially useful for chemical synthesis at high-pressure, high-temperature conditions.

Coexistence of hydrogen and polyanions in multinary main group element hydrides

Abstract Multinary hydrides based on mixtures of s- and p-block metals are often not fully hydrogenated, but possess a polymeric anion which is formed by the p-block component upon reduction by the s-block metals. Hydrogen may occur hydridic and exclusively coordinated by s-block metal ions (Zintl phase hydrides), or be incorporated in the polymeric anion where it acts as a covalently bonded terminating ligand (polyanionic hydrides). The current state of Zintl phase hydrides and polyanionic hydrides is reviewed with an emphasis on crystal structures and bonding principles.

Thermal and vibrational properties of thermoelectric ZnSb : Exploring the origin of low thermal conductivity

The intermetallic compound ZnSb is an interesting thermoelectric material, largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multi-temperature single crystal X-ray diffraction (9 - 400 K) and powder X-ray diffraction (300 - 725 K) measurements we characterized the volume expansion and the evolution of structural properties with temperature and identify an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons we consolidate the presence of low-energy optic modes with wavenumbers below 60 cm-1. Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures {\Theta}D = 195K, {\Theta}E1 = 78 K and {\Theta}E2 = 277 K as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of electron poor semiconductors, such as Zn4Sb3, CdSb, Cd4Sb3, Cd13-xInyZn10, and Zn5Sb4In2-x.