The thesis begins with a brief introduction and relevant literature references. The novelty of synthesis, methodology and results of the work reported in the thesis and highlighted subsequently. The thesis consist of three parts, Part A of the thesis consist of five chapters describing new methods of synthesis of orthovanadates, mainly dealing with the structure and photocatalytic properties of synthesized materials. Part B of the thesis consist of two chapters dealing with an unique crystallization methodology for subliming and low melting organic compounds and the crystal structure determination via single crystal X-ray diffraction. Part C consists of one chapter presenting the ab-initio approach of structure determination via powder X-ray diffraction methods. Chapter 1 of the thesis presents the synthesis of three zircon type lanthanide orthovanadates, LnVO4 (Ln = Ce, Pr and Nd) via a new solution based approach at room temperature and photodegradation of a variety of water pollutants have been investigated. Chapter 2 describes the importance of microwave synthesis to produce nano particles of the zircon type lanthanide orthovanadates LnVO4. The importance of the surface area in photocatalysis is evaluated. Chapter 3 consist of the synthesis of a series of new compounds, Ln0.95φ0.05Mo0.15V0.85O4 (Ln = Ce, Pr and Nd) via solid state method. The photocatalytic activities of these compounds are investigated both under UV exposure and sunlight. Chapter 4 presents the synthesis of MxCe1-xVO4+ (M = Li, Ca and Fe) with x = 0.1, 0.25 and 0.05 respectively. Different kinds of dyes and organics are degraded under UV radiation and the specificity towards the same are evaluated. Chapter 5 describes a comparative photocatalytic conversion of cyclohexane and benzene to cyclohexanol, cyclohexanone and phenol respectively by LnVO4, LnMo0.15V0.85O4 and MxCe1-xVO4+ (Ln = Ce, Pr and Nd, M= Li, Ca and Fe with x = 0.1, 0.25 and 0.05 respectively). Kinetics of above photoconversions are established by proposing a mechanism and determining the rate constants. Chapter 6 describes the development of a novel apparatus for the crystallization of anhydrous adenine, whose structure has not been solved over the last few decades. The crystal structure is solved via single crystal X-ray diffraction. Chapter 7 presents a modified design for crystallization of low melting organic compounds and co-crystals. A new polymorph of anhydrous thymine has been grown with this apparatus and its structure has been analyzed and compared with the known form of thymine. Chapter 8 presents the ab-initio approach of structure determination via powder X-ray diffraction methods. The methodology of using the direct space approach for the determination of the crystal structure of N-(2-fluorophenyl) benzamide (a drug intermediate), which could not yield good quality single crystals, is outlined.

Les besoins en matériaux fluorescents ont entrainé l'étude de "structures d'hôte" dans lesquelles des "ions actifs" sont susceptibles de s'introduire par substitution ou plus rarement par insertion. Ces ions actifs comportent des sous-couches d ou f non saturées ; ce sont également des lanthanides ou des actinides...