Browsing by Author "Zagorac, Dejan"

Now showing 1 - 2 of 2

Open Access
Energy landscape investigations of chemical matter and structure prediction of binary inorganic solids
(2012) Zagorac, Dejan; Jansen, Martin (Prof. Dr.)
Recent developments in experimental solid state chemistry have offered us new possibilities in controlling synthesis routes, especially by varying the pressure and temperature. Similarly, the development of new theoretical methods and tools makes it worthwhile to reinvestigate apparently well-known systems. In this thesis several binary systems, in particular PbS and ZnO, have been investigated using several different theoretical methods. Since lead sulfide is an important semiconductor, it is of great interest to identify possible (meta)stable modifications, both at standard and elevated pressures. To predict such structures, global optimizations on the energy landscape of PbS were performed for several pressures, using simulated annealing followed by local optimizations of the candidates found. In order to be able to take the effect of the non-bonding valence electrons of Pb into account, ab initio energies were employed for both the global and local optimization. Besides the experimentally known modifications (NaCl structure type at standard pressure and the CsCl structure type at high pressure), several additional promising structures, which might be accessible synthetically, were found; exhibiting e.g. the TlI type, the FeB type, and the alpha-GeTe-type. E(V ) curves and H(p) curves were calculated and analyzed, and further properties of the new modifications were investigated by performing band structure and phonon calculations. In order to gain new insights in the ZnO system, we have performed global explorations of the energy landscape using simulated annealing with an empirical potential, both at standard and elevated pressure (up to 100 GPa). Besides the well-known structure types (wurtzite, sphalerite and rock-salt), many new interesting modifications were found in different regions of the energy landscape. Furthermore, we observed many distorted variations of these main types, in particular new structures built-up from various combinations of structure elements of these types, exhibiting a variety of stacking orders. Furthermore, a search with the threshold algorithm for different numbers of formula units was performed. For each relevant structure type and number of formula units, the local energy landscape was analyzed. In addition to the threshold algorithm runs, we also performed prescribed path studies where we analyzed pairs of structures and the barriers separating them on the landscape as function of pressure and temperature. Additionally, we employed newly developed methods for the energy landscape representation. Energy landscapes are used to understand the behaviour of a chemical system, but problems exist with the visualization of high-dimensional spaces and the complexity of the structures. Therefore, finding low-dimensional representations of the energy landscape in order to analyze specific properties would be a significant step. Here, we present multidimensional representations of the energy landscapes of PbS and ZnO with various approximations. Finally, in order to find a suitable AB compound for synthesis of the "5-5" structure type, we performed constrained explorations for binary compounds with elements from groups V, IV - VI, and III - VII. Among others, TlF, SnO, SnS, SnSe, GeS, GeSe, PbO, PbS, ZnO and ZnS, were chosen for the study. For all compounds a local optimization on ab initio level with LDA functional was performed for the 5-5 structure type and other experimentally known or promising structure types. Afterwards, the results were compared with earlier theoretical work involving the 5-5 structure type in the earth alkaline metal oxides and the alkali metal halides. We suggest the GeSe and the ZnO systems as the most suitable ones for synthesizing the 5-5 structure type. In both cases, the synthesis of this 5-fold coordinated structure might be possible in a small pressure range between the 6-fold coordinated NaCl and the 4-fold coordinated wurtzite structure type.
Open Access
Raman characterization of dioxygen species as defects in single-crystal ZnO including their pressure dependence
(2025) Fischer, Dieter; Bloos, Dominik; Krajewska, Aleksandra; McNally, Graham M.; Zagorac, Dejan; Schön, Johann Christian
The defects in zinc oxide crystals are of crucial importance for their usability in many applications and are not yet fully understood. Here, we demonstrate that dioxygen species are present as defects in the grown ZnO, resulting in a bending of the atom layers that lie perpendicular to the c-axis. In the Raman spectra, these defects cause the appearance of bands different from the known bands of perfect ZnO crystals allowed by symmetry. These additional Raman bands, which have been frequently reported for ZnO in the past, can thus be fully explained by the presence of dioxygen species, and the widespread assumption of second-order modes for the assignments of these bands is not necessary. Furthermore, the Raman spectrum belonging to perfect zinc oxide in the ideal wurtzite structure is presented, obtained from small domains in ZnO(0001) crystals exposed to pressures up to 2 GPa. The dependence of the O-O stretching modes on the applied pressure proves the presence of dioxygen species in ZnO, which is also confirmed by phonon calculations of structure models with embedded dioxygen species. The surface quality of the ZnO crystals studied is also reflected in the Raman spectra and is included in the analysis.