Browsing by Author "Neelamraju, Sridhar"

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Open Access
Modeling the rational synthesis of magnesium difluoride via the low-temperature atom beam deposition method
(2013) Neelamraju, Sridhar; Jansen, Martin (Prof. Dr.)
A given chemical system, in general, will realize many (meta)stable structures, some of which might be observable on an experimentally viable timescale. Some of these polymorphs could have novel properties waiting to be exploited. However, addressing the problem of directing solid state synthesis towards such unknown polymorphs remains a major challenge. The prediction of new compounds using various theoretical methods is not usually followed up by an actual synthesis and planning the synthesis of novel inorganic solids often requires recourse to theoretical methods that can not only predict the thermodynamic stability of possible structure candidates but also model the kinetic behavior of atoms during the experimental synthesis. Here, we strive to fill this gap between knowledge derived from structure prediction methods and performing the actual synthesis of new structures experimentally by using tools available to the theoretical chemist that include energy landscape search algorithms, ab initio spectroscopy calculations and molecular dynamics simulations with the synthesis of MgF2 via the low-temperature atom beam deposition (LT-ABD) method as the model system. Hence, in a first step, in order to understand the possible cluster modifications of MgF2 that can exist in the vapor phase, we perform global optimizations on neutral and charged clusters using Monte-Carlo simulated annealing and find many possible structures. We also explore the energy landscape of (MgF2)3 and (MgF2)4 using the threshold algorithm in order to be able to estimate the stability and dynamics of these clusters. This method allows us to determine not only the stable and metastable isomers but also the barriers separating these isomers and the probability flows among them, yielding estimates of the stability of all the isomers found. We find that there is reasonable qualitative agreement between the ab initio and empirical potential energy landscapes, and important features such as sub-basins and energetic barriers follow similar trends. However, we observe that the energies are systematically different for the less compact clusters, when comparing empirical and ab initio energies. Furthermore, we employ the same procedure to additionally investigate the energy landscape of the tetramer. For this case, however, we use only the empirical potential due to computational limitations. This is followed by the calculation of Raman and IR spectra including the phonon modes and their intensities, for all the clusters found from the above study. We also calculate IR intensities and phonon modes for all bulk polymorphs of MgF2. This way, we provide the synthetic chemist with a means to observe possible (meta)stable phases of this system in both the vapor phase and the deposit while performing a deposition experiment on MgF2. The calculated data are compared with in-situ measurements in the LT-ABD apparatus. The MgF2 vapor and film are characterized via Raman spectroscopy of the MgF2 gas phase species embedded in an Ar-matrix and of the MgF2-films deposited onto a cooled substrate, respectively. We find that, in the vapor phase, there are mostly monomers and dimers of the neutral and charged species present in our experimental setup. Furthermore, the results suggest that in the amorphous bulk MgF2, rutile-like domains are present and MgF2 clusters similar to those in the matrix. Finally, peaks at about 800 cm-1, which are in the same range as the Ag modes of clusters with dangling fluorine atoms connected to three-coordinated Mg atoms, indicate that such dangling bonds are also present in amorphous MgF2 and can be used to track the amorphous to crystalline transition in this system. Finally, we model the growth of solid MgF2 from the gas-phase on an Al2O3 substrate as it occurs in a real LT-ABD experiment, a hypothetical MgF2-anatase substrate and a MgF2-rutile substrate. The process is studied in all its stages, from the dynamics of MgF2 clusters in the gas phase, over their impact on the surface of the cold and hot substrates, and their diffusion on the substrate, to the formation of crystallites. The growth process was analyzed as a function of synthesis parameters including the substrate temperature, deposition rate and types of clusters deposited. Both high and low rates resulted in the formation of amorphous MgF2 deposits. On annealing, we discovered a possible mechanism for the stabilization of the CaCl2-type structure. We find two competing structures in the first few nanoseconds of the deposition related to the CaCl2 and CdI2 structure types and argue that this competition stabilizes the CaCl2-type structure long enough for experimental observations to take place. Furthermore, the atom arrangements found in our simulations are in good agreement with existing experimental observations based on TEM and XRD measurements, for both the amorphous and the partly ordered metastable phase.