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Abstract

Powder diffraction is a common technique used for the structure characterization of solid crystalline materials since many years in various research fields from physics to materials science. It is a powerful technique as one can obtain wealth of information about crystal structure, identify and quantify the phase composition of a sample, as well as analyze microstructure, evaluate the success of a synthesis method whether the desired sample is produced, crystallinity of a sample and so forth. Over years, instrumental developments such as detector systems and computer technology carried powder diffraction technique forward in terms of data collection time, accelerated data processing, etc. Particularly with the development of high energy radiation sources, i.e., synchrotron and neutrons, it became possible to study structures of materials not only under ambient, but also under non-ambient conditions which generated a powerful technique known as in situ powder diffraction. This was a relatively important development from structure characterization viewpoint because in situ powder diffraction provides direct insight into the structural changes of a material when studied under reaction and/or operating conditions. As a result, structure-property relationship of the material of interest can be established. For example, particle growth, crystallization, formation, reduction and/or segregation of phases, catalytic reactions, alloying and/or dealloying, structure transitions upon heating, cooling, etc. of a sample can directly be monitored and studied via in situ powder diffraction. Although X-ray powder diffraction (XRPD) provides beneficial information about average bulk crystal structure of a material under ambient and/or non-ambient conditions, the technique is limited when amorphous and/or disordered structures as well as materials on nanoscale are in question. Therefore, it is critical to take into account not only the Bragg reflections, but also diffuse scattering arising from disordered structures. As the contribution from diffuse scattering is hidden under the Bragg reflections, i.e., in the background of an XRPD pattern, a wise strategy for accurate structure characterization in this case is working on total scattering which consists of both Bragg reflections and diffuse scattering. To achieve this, pair distribution function (PDF) which is obtained via Fourier transform of total scattering data plays an important role. Not only the local structure of ordered, but also disordered, amorphous or nanosized materials can be studied via PDF analysis. This technique was initially developed and used by crystallographers and physicists to study the structures of amorphous materials such as carbon as well as liquids. Popularity and applications of the technique have increased in recent years which made PDF analysis also an interdisciplinary technique for local structure analysis of wide range of materials, though it is still not as common as powder diffraction. Moreover, thanks to synchrotron and neutron radiation facilities, PDF is also studied under non-ambient conditions to reveal local structural changes and their relation to properties of materials. Therefore, PDF can be applied as a complementary technique to XRPD. In this thesis, characterization of various inorganic materials was performed by use of X-ray powder diffraction (XRPD) and/or pair distribution function (PDF) techniques under ex situ and/or in situ conditions. Especially, the application of in situ XRPD is highlighted in this thesis. Depending on the research objectives, XRPD and PDF experiments were performed at synchrotron radiation facilities or on laboratory diffractometers. PtM (M = Ni, Co) alloys and perovskites with various compositions are the inorganic materials of interest in this thesis which have important energy-oriented applications in science and technology. In the first part of the thesis (Subchapter 3.1) the main focus is on the formation and the local structure of PtM (M = Ni, Co) alloys supported on hollow graphitic spheres (HGSs) which was studied by the combination of ex situ/in situ XRPD and PDF techniques. In order to shed light on the alloying/dealloying as well as ordering/disordering phenomena of the local structure of the above-mentioned alloys, synchrotron radiation was used under in situ conditions. In situ XRPD and PDF showed that ordering of the local structure of PtNi alloy requires longer time compared to PtCo alloy. As the local structure of PtNi alloy was not completely ordered, while the local structure of PtCo was completely ordered after heat treatment at 800 °C and cooling to room temperature. It was found that combination of in situ XRPD and PDF techniques provides more comprehensive information about the local and bulk average structure of the alloys. During this PhD work, not only the structure characterization of inorganic materials was employed, but also synthesis of various perovskites was systematically studied (Subchapter 3.2). Usually perovskites are synthesized via solid-state reaction at elevated temperatures (≥900 °C). However, here it was found that among various synthesis methods such as ball-milling and solid-state reaction, incipient wetness impregnation of porous carbon is a very beneficial method as wide range of phase pure perovskites could be synthesized from simple to substituted compounds at 700 °C. In addition, some compounds were synthesized even at lower temperature, 550 °C. However, further decrease in the synthesis temperature (450 °C) did not lead to the formation of perovskites. The impact of synthesis temperature on the bulk and local structure of perovskites was revealed by ex situ XRPD and PDF data, respectively, collected on laboratory diffractometers. Besides structure characterization and synthesis, also the application of perovskites in heterogeneous catalysis as catalyst precursors was investigated in Subchapter 3.3. The catalytic performances of perovskites for NH₃ decomposition reaction was tested in a laboratory reactor. In order to establish the relationship between the chemical composition and the catalytic performance, in situ XRPD was studied under catalytic reaction conditions on a laboratory diffractometer. In the final part of the thesis, formation of some simple layered perovskites is studied by in situ XRPD (Subchapter 3.4). Structural changes of the as-synthesized LaCoO₃, LaNiO₃ and LaFeO₃ under H₂ flow were monitored by in situ XRPD on a laboratory diffractometer. As-synthesized perovskites were heated under H₂ flow at 650 °C in the in situ XRPD reaction chamber and data were collected accordingly. It was found that layered La₂CoO_4±δ perovskite was formed already at 650 °C in ~50 min, while the formation of the layered La₂NiO_4±δ perovskite occurred much faster (in ~20 min) at 650 °C compared to La₂CoO_4±δ. In situ XRPD experiments provided an in-depth insight into the formation of the layered perovskites which enabled the design of a practical and direct ex situ synthesis method for the production of a layered La₂CoO_4±δ perovskite in powder form in a tube furnace. The time used for H₂ dosing was found to play a critical role in the accomplishment of the ex situ synthesis of La₂CoO_4±δ. When H₂ was overdosed, decomposition of the sample into rare-earth oxide and reduction to transition metal was observed. In the literature, synthesis methods employed for the production of La₂CoO_4±δ perovskite usually end up with a calcination at high temperatures (>800 °C). This study shows that La₂CoO_4±δ can be ex situ synthesized using H₂ already at 650 °C avoiding implication of prolonged calcinations at high temperatures which might be important for various applications of La₂CoO_4±δ.