The crystal structures of (NH4)2TeBr6 and Ce2TeBr6 have been reinvestigated using X-ray diffraction from powders and single crystals. Three dimensional single crystal intensity data obtained photographically have been used to refine these structures, which are of the cubic K2PtCl6 type, by a full matrix least squares analysis. The Te-Br bond is found to be 2.70* in both crystals after correction for the thermal notion of the atoms. A phase transition in (NH4)2TeBr6 has been observed at about 183°K. Pros an examination of Weissenberg photographs taken at 163°K, the low temperature phase is found to be tetragonal, space group p4/mnc (D64h) with a « 7.501+/-0.005* and c =10.765*+/-0.005*. The transformation in this case is probably similar to that reported in other isomorphous crystals, e.g., K2snBr6. A review of other crystals with similar structure is included in this thesis and the theory of X-ray diffraction as applicable to the present problem is discussed briefly. / Thesis / Master of Science (MSc)

The application of the entropy concept to stabilize oxide systems opens the possibility of discovering new materials with unique structural and functional properties. High-entropy alloys and oxides, which are based on the entropy stabilization concept and composed of multi-principal elements, have the potential to tailor structural and functional properties to meet specific needs. The study of lanthanum-based perovskite materials that benefit from the entropy stabilization approach is a promising area of research.However, the inherent randomness of multi-principal elements presents new challenges, making it difficult to predict their behavior. To understand these difficulties, we have initiated a methodical investigation of La-based medium- and high-entropy perovskite oxides. This study focuses on the synthesis, characterization, sintering mechanism, and electrical conductivity properties of nine La1-xCax(A1/3, B1/3, C1/3)O3 medium-entropy perovskite oxide systems (A, B, and C = three combination of Cr or Co or Fe or Ni or Mn) and one La1-xCax(Cr0.2Co0.2Fe0.2Ni0.2Mn0.2)O3 high-entropy perovskite oxide system (for x = 0.1 to 0.3). This research aims to provide better understanding of: (1) synthesis process, (2) temperature of single-phase formation, (3) the impact of various combinations of multiple B-site transitional elements and Ca doping on crystal structure, and microstructure (4) sintering mechanism and (5) electrical conductivity properties.

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