Experimental observation of defect pair separation triggering phase transitions

First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e. g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality

Degenerate Phases of Iodine on Pt(110) at Half-Monolayer Coverage

Halogens, including iodine, are commonly used as additives to enhance the selectivity of catalytic processes. In sustainable-energy applications, such as dye-sensitized solar cells or photocatalytic water splitting, iodine is often applied as redox shuttle. Since platinum is both a prominent catalyst as well as electrode material, we investigated the phases of iodine on Pt(110) at a coverage of 0.5 monolayers by scanning tunneling microscopy, low-energy electron diffraction, and by density functional theory (DFT) calculations. Three distinctly different phases occur at room temperature, involving occupation of two different binding sites. Preferred binding sites and phase stability ranges are different from the ones reported for Br on Pt(110), reflecting a different balance of adsorbatesubstrate and adsorbateadsorbate interactions. DFT results are in striking agreement with experimental results for the c(2 x 2) phase, where the substrate remains planar, but disagree with experiment for phases involving a surface buckling. This is attributed to a preference of DFT for local over nonlocal interactions, i.e., CDW/PLD correlations. Comparison with literature data reveals a trend for increasingly unspecific iodine bonding to the substrate in the sequence Pt(110), Pd(110), Au(110)