Johanna Nylén

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.

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.

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.

Synthesis, structure, and electronic properties of 4H-germanium

Reinvestigation of the reaction of Li7Ge12 with benzophenone in tetrahydrofuran solution affords the metastable crystalline germanium allotrope allo-Ge, which transforms into another allotrope, 4H-Ge, upon annealing at temperatures between 150 and 300 °C. When annealing 4H-Ge above 400 °C the ground state modification α-Ge is obtained. The crystal structure of 4H-Ge was refined from powder X-ray diffraction data (space group P63/mmc (no. 194), a = 3.99019(4) and c = 13.1070(2) A, Z = 8) and the sequence of phase transitions from allo-Ge to α-Ge was monitored by temperature-dependent powder X-ray diffraction experiments. Electrical resistivity measurements and quantum-mechanical calculations show that 4H-Ge is a semiconductor, which is in contrast to previous theoretical predictions. The Raman spectrum of 4H-Ge displays three bands at 299, 291, and 245 cm−1 which are assigned to E1g, E2g and A1g modes, respectively, and relate to the optic mode in α-Ge.

Characterization of a high pressure, high temperature modification of ammonia borane (BH3NH3)

At elevated pressures (above 1.5 GPa) dihydrogen bonded ammonia borane, BH3NH3, undergoes a solid-solid phase transition with increasing temperature. The high pressure, high temperature (HPHT) phase precedes decomposition and evolves from the known high pressure, low temperature form with space group symmetry Cmc21 (Z = 4). Structural changes of BH3NH3 with temperature were studied at around 6 GPa in a diamond anvil cell by synchrotron powder diffraction. At this pressure the Cmc21 phase transforms into the HPHT phase at around 140 °C. The crystal system, unit cell, and B and N atom position parameters of the HPHT phase were extracted from diffraction data, and a hydrogen ordered model with space group symmetry Pnma (Z = 4) subsequently established from density functional calculations. However, there is strong experimental evidence that HPHT-BH3NH3 is a hydrogen disordered rotator phase. A reverse transition to the Cmc21 phase is not observed. When releasing pressure at room temperature to below 1.5 GPa the ambient pressure (hydrogen disordered) I4mm phase of BH3NH3 is obtained.

Optical and electronic properties of thermoelectric Zn4Sb3 across the low-temperature phase transitions

Optical and electronic properties of thermoelectric Zn4Sb3 across the low-temperature phase transitions

Formation of hydrous stishovite from coesite in high-pressure hydrothermal environments

Abstract In low-temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO2 source at temperatures of 350–550 °C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H2O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 °C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H+ for Si4+ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm–1, indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm–1, respectively. Protons of the octahedral hydrogarnet defect produce