Mechanisms and Nucleation Characteristics of the Pressure-Induced B1−B2 Transition in Potassium Halides: A Question of Ion Hardness and Softness

Single-crystal Brillouin spectroscopy with CO2 laser heating and variable q

We describe a Brillouin spectroscopy system integrated with CO2 laser-heating and Raman spectroscopic capabilities. Temperature is determined by measurements of the grey-body thermal radiation emitted by the hot sample, with the system response calibrated relative to a standard tungsten ribbon lamp. High-pressure laser-heating Brillouin scattering measurements of acoustic velocities on liquid water and ice compressed in a diamond-anvil cell were performed at temperatures up to 2500 ± 150 K at high pressure. Single-crystal laser-heating Brillouin measurements were made on the (111) plane of San Carlos olivine at ∼13 GPa, 1300 ± 200 K. The pressure as measured by ruby fluorescence is shown to be within ±0.5 GPa of the pressure on the olivine sample during laser heating when KCl and KBr are used as pressure-transmitting media. In addition, the system is designed for continuously variable scattering angles from forward scattering (near 0° scattering angle) up to near back scattering (∼141°). This novel setup allows us to probe a wide range of wave vectors q for investigation of phonon dispersion on, for example, crystals with large unit cells (on the scale of hundreds of nm).

Study of the B1-B2 transition in colloidal clusters

The possible mechanisms for the B1 (NaCl-type) to B2 (CsCl-type) transition in crystalline colloidal clusters of equally sized particles are studied by means of two computational techniques: metadynamics and nudged elastic band calculations. The system is modelled by a screened Coulomb potential. Different interaction ranges are considered. The transition from a perfect NaCl cubic cluster to a full CsCl cluster is forced by metadynamics, revealing a transition path with intermediate metastable configurations in which planes are shifted one by one. The presence of metastable configurations in the transition path, corresponding to a certain number of NaCl planes turned into CsCl, has clear analogies with the known Hyde and O'Keeffe mechanism for ionic crystals, with some important differences due to finite-size effects. These comprise the fact that the transition starts by shifting a surface plane by means of a row-by-row mechanism that has no analog in bulk crystals. The energy barriers between the local minima in the transition path are calculated, showing that the barriers strongly depend on the screening length, in such a way that the B1 metastable phase can have very long lifetimes when the interaction is sufficiently long-ranged.

Walking the path from B4- to B1-type structures in GaN

Molecular dynamics simulations are performed on the wurtzite-type structure (B4) to the rocksalt-type structure (B1) pressure-induced phase transition in GaN. From this, a nucleation and growth mechanism through a tetragonal metastable configuration is found. An intermediate of h-MgO type structure suggested from static calculations is ruled out. However, the pathway through the tetragonal intermediate may be altered by defect incorporation. While the overall transformation mechanism is preserved for both vacancies and Ga substitution by indium, already a 5% aluminum substitution establishes a transition route which avoids the tetragonal structure. Changes in the transformation mechanism and the resulting stabilization of the previously metastable high-pressure modification is elaborated by tracing the interplay of phase nucleation and growth and defect incorporation.