High-pressure behaviour of GeO2: a simulation study

In this work we study the high pressure behaviour of liquid and glassy GeO2 by means of molecular dynamics simulations. The interaction potential, which includes dipole polarization effects, was parameterized from first-principles calculations. Our simulations reproduce the most recent experimental data to a high degree of precision. The proportion of the various GeOn polyhedra is determined as a function of the pressure: a smooth transition from tetrahedral to octahedral network is observed. Finally, the study of high-pressure, liquid germania confirms that this material presents an anomalous behaviour of the diffusivity as observed in analog systems such as silica and water. The importance of penta-coordinated germanium ions for such behaviour is stressed.Comment: 16 pages, 4 figures, accepted as a Fast Track Communication on Journal of Physics: Condensed Matte

The construction of a reliable potential for GeO2 from first-principles

The construction of a reliable potential for GeO2, from first-principles, is described. The obtained potential, which includes dipole polarization effects, is able to reproduce all the studied properties (structural, dynamical and vibrational) to a high degree of precision with a single set of parameters. In particular, the infrared spectrum was obtained with the expression proposed for the dielectric function of polarizable ionic solutions by Weis et al. [J.M. Caillol, D. Levesque and J.J. Weis, J. Chem. Phys. 91, 5544 (1989)]. The agreement with the experimental spectrum is very good, with three main bands that are associated to tetrahedral modes of the GeO2 network. Finally, we give a comparison with a simpler pair-additive potential.Comment: 9 pages, 8 figure

Deformation mechanisms leading to auxetic behaviour in the α-cristobalite and α-quartz structures of both silica and germania

Analytical expressions have been developed in which the elastic behaviour of the α-quartz and α-cristobalite molecular tetrahedral frameworks of both silica and germania are modelled by rotation, or dilation or concurrent rotation and dilation of the tetrahedra. Rotation and dilation of the tetrahedra both produce negative Poisson’s ratios (auxetic behaviour), whereas both positive and negative values are possible when these mechanisms act concurrently. Concurrent rotation and dilation of the tetrahedra reproduces with remarkable accuracy both the positive and negative nu31 Poisson’s ratios observed in silica α-quartz and α-cristobalite, respectively, when loaded in the x3 direction. A parametric fit of the concurrent model to the germania α-quartz experimental nu31 Poisson’s ratio is used to predict nu31 for germania α-cristobalite, for which no experimental value exists. This is predicted to be +0.007. Strain-dependent nu31 trends, due to concurrent rotation and dilation in the silica structures, are in broad agreement with those predicted from pair-potential calculations, although significant differences do occur in the absolute values. Concurrent dilation and rotation of the tetrahedra predicts that an alternative uniaxial stress (sigma3)-induced phase exists for both silica α-quartz and α-cristobalite and germania α-cristobalite, having geometries in reasonable agreement with beta-quartz and idealised beta-cristobalite, respectively

b-Initio Molecular Dynamics Simulation of High Temperature GeO2

We made a series of molecular dynamic simulations to get a representative, high temperature GeO2 system to eventually be quenched down to room temperature so as to obtain an amorphous system to be studied under pressure for comparison to experimental results. We carefully generated an initial configuration such that all simulations have entirely been carried out by ab-initio methods in the range 10K-3000K

Liquid-liquid phase transition in Stillinger-Weber silicon

It was recently demonstrated that the Stillinger-Weber silicon undergoes a liquid-liquid first-order phase transition deep into the supercooled region (Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects of perturbations on this phase transition. We show that the order of the liquid-liquid transition changes with negative pressure. We also find that the liquid-liquid transition disappears when the three-body term of the potential is strengthened by as little as 5 %. This implies that the details of the potential could affect strongly the nature and even the existence of the liquid-liquid phase.Comment: 13 page

Dynamic compression experiments of SiO2 and GeO2 at synchrotron- and X-ray free electron laser (XFEL) light sources

In this work, the high pressure behavior of four major SiO2 polymorphs were investigated by means of dynamic compression: α-quartz, fused silica, stishovite and α-cristobalite. Here, laser shock compression and dynamic diamond anvil cell (dDAC) techniques were applied and the concomitant use of hard X-ray radiation at synchrotron- and X-ray free electron laser (XFEL) facilities made a time-resolved investigation of the lattice response at high pressures possible.In dieser Arbeit wurde das Hochdruckverhalten von vier SiO2 Modifikationen mit Hilfe dynamischer Kompression durch Laser-induzierten Schock und der dynamischen Diamantstempelzelle (dDAC) untersucht: α-Quarz, Quarzglas, Stishovit und α-Cristobalit. Die gleichzeitige Verwendung harter Röntgenstrahlung eines Synchrotrons oder Freie-Elektronen-Lasers (XFEL) ermöglicht eine zeitaufgelöste Untersuchung der Gitterstruktur dieser Materialien