We compare the wurtzite (WZ) → rock salt (RS) transition mechanisms seen in a molecular dynamics simulation at elevated pressure and in a metadynamics simulation close to the experimental transition pressure. The mechanism seen in the molecular dynamics is characterized by a direct compression parallel to the direction, while in the metadynamics it is seen as a sliding of layers parallel to the WZ direction. The h-MgO structure is seen as an intermediate in both cases, although it is only stable at the lower pressure. The RS structure produced by the direct compression and that which results from the sliding-layer mechanism make an angle of 15° with each other. Our metadynamics simulations along with other results suggest that sliding-layer mechanisms govern the transition close to the experimental transition pressure.
Keywords: Solid–solid transitions; Semiconductors; Molecular dynamics; Structure of bulk crystals; Crystallographic aspects of phase transformations; Pressure effects
Fig. 1. Evolution of the coordination of the 576-atom CdSe sample during the MD. The fractions of four- (black line), five- (red line), and sixfold (blue line) coordinated atoms are shown. The pressure is increased to P = 10.5 GPa at time t = 500 ps. At t = 508 ps five-coordination briefly takes over, when the boat- and chair-like structures in the c-direction collapse, forming a new Cd–Se bond. The five-coordination is short-lived however, and the transformation to six-coordinated RS is complete before t = 509 ps.
Fig. 2. Evolution of the enthalpy at 300 K during the MD and for the structures obtained by quenching the room temperature structures to 0 K at the hydrostatic external pressure of 10.5 GPa.
Fig. 3. Mechanism of the WZ to RS transformation in the 576 atom CdSe sample raised to pressure of P = 10.5 GPa. The hexagonal WZ six-membered rings are compressed along the direction and expanded along the WZ direction such that a new Cd–Se bond is formed, creating the four-membered rings of RS. The initial direction is parallel to the final RS direction.
Fig. 4. Coordination of the sample during the metadynamics. The fractions of four- (black line), five- (red line), and sixfold (blue line) coordinated atoms are shown. A change in coordination from fourfold to fivefold occurs at metastep 274, and the five-coordinated HS structure persists until metastep 290 at which point the sample transforms to the six-coordinated RS structure.
Fig. 5. Evolution of the enthalpy at 300 K during the metadynamics and for the structures obtained by quenching the room temperature structures to 0 K at the hydrostatic external pressure of 2.5 GPa.
Fig. 6. Conversion of the HS structure to the RS structure during the metadynamics. (a) The sample during metastep 285. The transition to HS has already taken place. (b) The sample during metastep 288. An alternate pair of the layers remain stationary while the adjacent layers have begun to slide in the ±HS directions, converting most of the sample to RS. (c) During metastep 290. A layer is now sliding in the HS direction relative to the adjacent layers, which completes the transformation. (d) Later during metastep 290. The RS direction lies parallel to the original WZ direction.
Fig. 7. Schematics illustrating four possible transition routes from WZ-type to RS-type CdSe. (a) M I: layers (shaded blue) slide in the direction, with each layer sliding by a/2 relative to the layer below. (b) M II: Alternating layers (shaded green) slide in the WZ direction by a/2, relative to the stationary layers in-between (red). (c) M III: Alternate layers remain stationary relative to one another (shaded red), while the layers in-between slide alternately in the (blue) and WZ (green) directions by a/2. (d) Compression mechanism: the hexagonal six-membered rings are compressed along the direction, and expanded along the orthogonal WZ direction. The RS structure shown in (d) makes an angle of 15° with those shown in (a)–(c).
Solid State Sciences
Volume 12, Issue 2, February 2010, Pages 157-162
Morphology and dynamics of nanostructures and disordered systems via atomic-scale modelling