Björn Winkler

Calculation of the elastic constants of the Al2SiO5 polymorphs andalusite, sillimanite and kyanite

Abstract Based on quantum mechanical calculations we predict the elastic constants of kyanite at 0 K. The reliability of the prediction has been evaluated by computing the elastic constants of andalusite and of sillimanite and comparing them to experimentally determined values. The computed bulk moduli of andalusite (145 GPa) and of sillimanite (159 GPa) are constistent with experimental values. Only two of the computed elastic contants c13 of andalusite and c23 of sillimanite differ from the experimental values by more than 11%. As the parameter-free model is transferable, the predictions for the bulk modulus, B = 178 GPa, and the elastic constants of kyanite are believed to be equally reliable. In contrast to the promising results of our quantum mechanical calculations, the agreement with experimental values is poor for elastic properties derived from a transferable empirical core-shell model.

Synthesis of Binary Transition Metal Nitrides, Carbides and Borides from the Elements in the Laser-Heated Diamond Anvil Cell and Their Structure-Property Relations

Transition metal nitrides, carbides and borides have a high potential for industrial applications as they not only have a high melting point but are generally harder and less compressible than the pure metals. Here we summarize recent advances in the synthesis of binary transition metal nitrides, carbides and borides focusing on the reaction of the elements at extreme conditions generated within the laser-heated diamond anvil cell. The current knowledge of their structures and high-pressure properties like high-(p,T) stability, compressibility and hardness is described as obtained from experiments.

Systematic prediction of crystal structures

A generally applicable and systematic prediction of crystal structures and their properties has been an important goal of crystallography and materials science. Here we present such a general and systematic approach. This approach is based on a combination of graph theory with quantum mechanics. As an application, structures, properties and relative stabilities of small hypothetical carbon polymorphs with up to six atoms per unit cell are presented

Applicability of a quantum mechanical `virtual crystal approximation' to study Al/Si-disorder

Abstract We show that a quantum mechanical version of the virtual crystal approximation can be used to study Al/Si-disorder in silicates. Full geometry optimizations show that this approach reproduces disordered crystal structures with an accuracy equal to that of density functional calculations for fully ordered structures. As this approach is based on density functional theory in conjunction with a plane wave basis set and ultrasoft pseudopotentials, it is possible to study large and complex crystal structures. As first examples we present calculations for hollandite-type KAlSi 3 O 8 , where Si is octahedrally coordinated, and gehlenite, Ca 2 Al 2 SiO 7 , with tetrahedrally coordinated Al and Si-atoms.

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3

Synchrotron powder diffraction data from methylammonium tin bromide, CH(3)NH(3)SnBr(3), taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc2(1), a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) A. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s(2) lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost).

Reaction of rhenium and carbon at high pressures and temperatures

Abstract A study of the reaction of rhenium with carbon at high-(P, T) conditions up to Pmax = 67 GPa and Tmax = 3800 K is presented. A hexagonal ReCx was identified as the stable phase at high-(P, T) conditions. A composition of ReC0.5 is proposed. No evidence for a cubic ReC polymorph with rocksalt structure, as suggested in the literature, or for any other phase was found at the P-T conditions explored. A preliminary P-T rhenium-carbon phase diagram has been derived and properties such as bulk moduli and elastic stiffness coefficients were obtained.

Crystal growth and elastic properties of orthorhombic Bi2Ga4O9

The combination of favourable oxygen conductivity at high temperatures with mechanical strength make Bi-containing compounds with mullite-type crystal structures strong candidates for use as electrolytes of solid fuel cells. Large single crystals of orthorhombic Bi2Ga4O9 with dimensions up to 20 × 20 × 10 mm3 were grown by the top-seeded solution growth technique. Their elastic constants at room temperature were determined for the first time using resonant ultrasound spectroscopy. The values given in GPa are c11 = 143.4(2), c22 = 161.7(2), c33 = 224.2(3), c44 = 69.6(1), c55 = 49.2(1), c66 = 76.5(2), c12 = 73.7(2), c13 = 62.2(3) and c23 = 70.3(3). Further, the crystal structure and the elastic properties of Bi2Ga4O9 were studied at 0 K by parameter-free ab initio calculations based on density-functional theory. On average the computed elastic constants differ from the experimental values by about 10%, indicating the reliability of the theoretical approach. Like in other mullite-type compounds the anisotropy of the longitudinal elastic stiffness is clearly controlled by the structurally dominant octahedral chains running parallel to [001]. The deviations from Cauchy relations show a significant anisotropy of the type g22>g11≈g33 which is related to the covalent character of the bonding interactions within the infinite –Bi–O–Bi–O– bond chains parallel to [010]. The mean elastic stiffness of Bi2Ga4O9 is about 40% smaller than for 2/1-mullite and sillimanite. This discrepancy can be attributed to the mechanically very soft behaviour of the Bi 6s2 lone electron pair. Its stereochemical activity is clearly evident from both the asymmetry of the bismuth coordination polyhedron and the calculated electron density maps.

Thermodynamic properties of MgSiO3 perovskite derived from large scale molecular dynamics simulations

A molecular dynamics simulation study of MgSiO3 has been performed using a large sample containing 4096 unit cells. Thermodynamic properties have been extracted using a semiclassical approximation to the correct quantum mechanical treatment, using the calculated density of states and the quantum harmonic formalism for thermodynamic functions. Simulations performed at different temperatures and volumes have given an estimate of the relative contributions due to thermal expansion (quasi-harmonic effects) and direct anharmonic interactions. Comparison of results for mean square atomic displacements with results on smaller samples have shown the limitations of smaller sample sizes.

Single-crystal structure refinement of diaspore at 50 GPa

Abstract The crystal structure of diaspore, AlO(OH), has been investigated by in situ single-crystal synchrotron X-ray diffraction at ~50 GPa using the diamond-anvil cell technique. Diaspore is found to retain its structure up to 51.5 GPa at room temperature, which is more than 30 GPa above the transition pressure to δ-AlO(OH) found in quenched high-temperature experiments and derived from density functional theory calculations. The compression is anisotropic and largest for the a axis. This can be explained by the fact that the structural response to pressure is mainly due to the shortening of the hydrogen bond, which is oriented nearly parallel to the a axis. The hydrogen bond becomes significantly more symmetric with pressure up to 50 GPa.

Anomalous inelastic neutron scattering from calcite

Inelastic neutron scattering measurements on calcite (CaCO3) in its low temperature phase have revealed the existence of an unusual column of inelastic scattering at the wavevector corresponding to the F point of the high temperature Brillouin zone. At the same wavevector there is also a transverse acoustic soft mode and the column of scattering ranges in energy from zero up to the soft mode. The intensity of the anomalous scattering increases rapidly with temperature, and is consistent with an Arrhenius relation the form exp(-T*/T) where T*=1035 K. The authors speculate that this scattering arises from thermal fluctuations of the calcite structure into a different ordered structure, which is related to an ordering instability at the F point. Evidence for this possibility has also been obtained from lattice energy calculations.