Structural Transitions

Mechanisms of Phase Transitions

The detailed investigation of structural reconstruction, in pressure or temperature induced phase transitions, is a major challenge in modern material sciences. The combined use of structural modeling approaches and of different advanced numerical tools allows for a detailed understanding of phase nucleation and growth in the solid state. Therein, intermediate reconstruction steps can be elucidated in detail.

Pressure Induced Phase Transitions

Investigation of pressure induced phase transitions and of the final morphology of pressurized materials

Figure 1. Nanodomain fragmentation of CdSe under pressure. a,d HR-TEM images, b,c Results from molecular dynamics simulations.

Under normal conditions, CdSe crystallizes in the wurtzite structure type (B4). Applying moderate pressure (2.5-3.5 GPa) it transforms into the rock-sal structure type (B1). As a third polymorph, zinc blende (B3 structure type), albeit metastable, does exist. The appearance of the pressurized material is markedly different from the starting sample. Our simulations reveal initial nuclei formation in the B1 structure matrix, which are of B4 type, followed by regions of B3 structural motifs, which grow between already defined B4 regions. The final (sub) nanodomains result from further growth of this initial configuration.

Ferroelectrics

Investigation of ferroelectric domain nucleation and growth

Figure 1. Polarized clusters in the paraelectric phase of barium titanate

Local structures in the paraelectric phase of different perowskite compounds are investigated, as well as characteristic correlation patterns and length rescaling across the paraelectric to ferroelectric phase transiton. Different atomistic simulation techniques are employed, e.g. ab-initio (Born-Oppenheimer) molecular dynamics (MD) and classical MD. Fourier transforms of resulting structures are directly compared to x-ray diffuse scattering patterns.

Topology

Use of minimal surfaces approximants for the description and understanding of crystalline matter.

Figure 1. Set of interpenetrating nets obtained froma hyperbolic tiling of a minimal surface approximant

The use of minimal surface approximants (Fourier summations) allows for the generation of triply periodic, bicontinuous surfaces. They have the properties of carving space into labyrinths, and can be associated with space group symmetries. They allow for a better understanding of crystal structures. In the context of structural phase transitions, they can be used to model transition paths between limiting structures.

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