Fluctuations and Phase Separation in a Quasi-One-Dimensional System

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 adsorbate−substrate and adsorbate−adsorbate interactions. DFT results are in striking agreement with experimental results for the c(2 × 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).

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“…The latter are known from Br/Pt(110) as well. 24 distinctly fewer linear defects is obtained, as shown in Figure 5.…”

Section: ■ Results and Discussionmentioning

confidence: 81%

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

et al. 2014

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“…Here we investigate an order-order phase transition in the Br/Pt(110) adsorbate system at half-monolayer (ML) coverage from a (2 × 1) rectangular phase at low temperature to a c(2 × 2) checkerboard structure at room temperature. Previous analysis yielded the following results5678: In both ordered phases every second short bridge site on the substrate close-packed rows is occupied, but with either zero phase difference from row to row ((2 × 1) structure) or with a phase difference of π (c(2 × 2); see Figure 1). The energetic preference for the (2 × 1) structure at low temperature is due to a coupling between charge density wave fluctuations associated with a periodic lattice distortion (CDW/PLD) and the Br adlayer.…”

Section: Resultsmentioning

confidence: 93%

“…fluctuations of the order parameter m which occur around the CDW phase transition temperature. With increasing temperature the fluctuations become faster and the atomic resolution is lost5. As T exceeds 200 K, however, the CDW/PLD fluctuations die out, since m → 0 and, owing to the repulsive Br-Br interaction represented by the first term in equation (1), a static c(2 × 2) Br pattern develops.…”

Section: Discussionmentioning

confidence: 99%

Experimental observation of defect pair separation triggering phase transitions

Cordin

Lechner

Duerrbeck

et al. 2014

Sci Rep

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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.

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“…Further evidence of 1D behaviour stems from a peak profile analysis of low-energy electron diffraction spots as a function of temperature [22]. Diffraction spot profiles of fluctuating materials are typically composed of a Gaussian contribution, which is proportional to the square of the long-range order parameter, and a Lorentzian contribution, which arises from short-range correlation in the fluctuating system.…”

Section: Quasi-1d Behaviour Of the C(2 × 2)-br/pt(110)mentioning

confidence: 99%

“…Instead a glassy phase develops with c(2 × 2) and (2 × 1) domains alternating randomly in [001] direction. Occasionally, (3 × 2) structural elements with 3 atoms per unit cell (yielding again a local coverage of Θ Br = 0.5 ML) are observed [22]. On virtually defect-free preparations no (3×2) structural elements appear and the domains are extended over hundreds of Angstroms along the [110] direction.…”

Section: Anomalous Phase Transition Due To Competing Electronic Intermentioning

confidence: 99%

Electronically driven phase transitions in a quasi-one-dimensional adsorbate system

et al. 2010

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A quasi-1D system is prepared using the Pt(110) surface as a template. The electronic surface resonance structure is studied by angle-resolved photoemission spectroscopy for the clean surface as well as for different Bromine coverages. A Fermi surface mapping reveals saddle points at the Fermi level in the interior of the surface Brillouin zone. Correspondingly, a maximum in the static response function χ(q, 0) at the connecting vector q is expected. With 1/2Gx < q < 2/3Gx one observes indeed a 3-fold periodicity around defects and a 2-fold periodicity at low temperature for ΘBr = 0.5 ML. Cooling of a defect-free c(2 × 2)−Br/Pt(110) preparation counter-intuitively results in a loss of long-range order. Motivated by DFT calculations this is attributed to an anomalous order-order phase transition into the (2 × 1) phase accompanied by intense, strongly anisotropic fluctuations within a temperature range of ∼200 K. The peculiar behaviour is rationalised in terms of a competition between inter-adsorbate repulsion and an adsorbate triggered 2kF interaction in the substrate.

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Fluctuations and Phase Separation in a Quasi-One-Dimensional System

Cited by 13 publications

References 18 publications

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

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

Experimental observation of defect pair separation triggering phase transitions

Electronically driven phase transitions in a quasi-one-dimensional adsorbate system

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