Speaker
Description
A reliable description of surfaces structures phase equilibria in a reactive environment is a prerequisite for understanding mechanism of, e.g., heterogeneous catalysis. However, studying phase equilibria at ab initio level, is a formidable challenge, especially for highly anharmonic systems with sluggish barriers. Pioneering techniques require the prior knowledge of relevant phases and do not yield to unexpected phases and phase transitions. Furthermore, only free energy differences among the considered phases have been used to assess the phase stability regions, whereas the modelling of the singularities of a response function (e.g., heat capacity) is needed to locate phase boundaries and distinguish phase transitions from smooth transitions.
In this work, we introduce a fully ab initio approach to determine temperature-pressure (T, p) surface phase diagram and apply it to evaluate phase equilibria of surfaces in a reactive environment [1]. For this purpose, our replica-exchange grand-canonical (REGC) method [2] is extended by evaluating the heat capacity, Cv(T, p), as function of T and p, thus locating phase boundaries including triple and critical points where Cv(T, p) shows ridges. Furthermore, we show how the crucial limitation of the GC approach, formally defined only for a constant-volume ensemble, is circumvented by sampling different simulations cells and connecting all the phases in both cells via a reference phase. The approach is demonstrated by addressing open questions for the Si(100) surface in a hydrogen gas phase. By defining microscopic descriptors, 25 distinct thermodynamically stable surface phases are identified, most of which including few order-disorder phase transitions, have not been observed experimentally, so far. The results also show that Si-Si-bonds forming/breaking is the driving force behind the phase transition between the experimentally confirmed 3×1 and 2×1 adsorption patterns.
The REGC approach yields an ab initio description of surface restructuring as well as phase equilibria at technologically relevant (T, p) conditions. This constitutes an important advancement in the field of surface science and relevant for important applications, such as heterogeneous catalysis.
References
[1] Y. Zhou, C. Zhu, M. Scheffler, and L. M. Ghiringhelli, Ab Initio Approach for Thermodynamic Surface Phases with Full Consideration of Anharmonic Effects: The Example of Hydrogen at Si(100), Phys. Rev. Lett. 128, 246101 (2022). https://doi.org/10.1103/PhysRevLett.128.246101
[2] Y. Zhou, M. Scheffler, and L. M. Ghiringhelli, Determining surface phase diagrams including anharmonic effects, Phys. Rev. B 100, 174106 (2019). https://doi.org/10.1103/PhysRevB.100.174106
Addresses
(a) Present address: Department of Physics, Technical University of Denmark, Lyngby, Denmark
(b) School of Advanced Manufacturing, Guangdong University of Technology, Jieyang 515200, China
(c) Present address: The FAIRmat Consortium of the NFDI, Humboldt-Universität zu Berlin, Berlin, Germany
| Abstract Number (department-wise) | SG 11 |
|---|---|
| Department | Scheffler Group |
