Synthesis and Structure-Property Relationships of a Few Fluoride Compounds: (I) Mixed-Metal Fluorocarbonates and (Ii) a Reduced Niobium Oxyfluoride
Tran, Thanh Thao 1985-
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This dissertation focuses on the synthesis and structure-property relationships of new complex metal fluorides of (i) alkaline-metal lead fluorocarbonates, alkaline-metal magnesium fluorocarbontes and (ii) a reduced niobium oxyfluoride. We have designed new structure building units, PbO6F and PbO6F2, in layered compounds of KPb2(CO3)2F and K2.70Pb5.15(CO3)5F3, that were then developed to three-dimensional noncentrosymmetric RbPbCO3F and CsPbCO3F. Powder second-harmonic generation (SHG) measurements on K2.70Pb5.15(CO3)5F3, RbPbCO3F and CsPbCO3F using 1064 nm radiation revealed SHG efficiencies of approximately 40, 250 and 300 x a-SiO2, respectively. The Mg2+ substitution of the Pb2+ cation in APbCO3F (A = alkaline-metal) resulted in KMgCO3F and RbMgCO3F with unique coordination environments of magnesium, i.e., pentagonal arrangements in the ab-plane. These compounds are second-harmonic generation (SHG) phase-matching at both 1064 nm (120 and 160 x a-SiO2) and 532 nm (0.33 and 0.6 x b-BaB2O4) with short UV cut-off edges, below 200 and 190 nm for KMgCO3F and RbMgCO3F, respectively. These mixed-metal fluorocarbonates were also characterized by infrared spectroscopy, thermogravimetric and differential thermal analysis. In addition, we report a new highly correlated electron system, Nb2O2F3. It is a new low-valent niobium (III/IV) oxyfluoride that features [Nb2X10] units (X = O, F) with short (2.5739(1) Å) Nb-Nb metal-metal bonds. The [Nb2X10] units are connected through shared O/F vertices to form a 3D structure. Nb2O2F3 undergoes a structural transition (below 90 K), from a monoclinic to a triclinic structure, which can be described as a charge ordering or disproportionnation of [Nb2]7+dimers. That resulted in a doubly (2.5000(9) Å) and a singly bonded (2.6560(9) Å) Nb2 dimers. The structural change is associated with unusual field-independent ‘spin-gap-like’ magnetic and electronic phase transition.