Layered Oxides And Phosphates Of Bismuth: New Structural Types And Related Properties
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The thesis entitled "Layered Oxides and Phosphates of Bismuth: New Structural Types and Related Properties" consists of three parts and six chapters. It begins with an introductory note providing a brief overview of the existing literature on bismuth based layered oxides. Part A of the thesis describes novel variants of the Aurivillius, Scheelite and Fluorite related phases with special emphasis on photocatalytic behavior of these materials. Chapter 1 describes two new compounds, LiBi4Nb3O14 and LiBi4Ta3O14 characterized by single crystal X-ray diffraction to depict a unique structure, which have similarities with both an ideal Aurivillius phase and a typical hexagonal tungsten bronze. Photo-degradation of a wide range of water pollutants, which include commonly, used dyes in textile industry and phenols suggest selectivity of LiBi4Nb3O14 towards hydroxyl group containing aromatic compounds. Chapter 2, Section 1 presents the structure of BaBi2Mo4O16 as a new compound among Scheelites, which shows high selectivity towards photo-degradation of non- chloro-containing aromatic systems. Further, the degradation rates are found to be significantly higher than the commercial TiO2 catalyst for nitro and methyl phenols. The solid solution, BaBi2Mo4-xWxO16 (0.25 ≤ x ≤ 1) has been analyzed to obtain insights into the compositional aspects. Section 2 has a detailed discussion on the structural stability of the Scheelite BaBi2Mo4O16 upon substitution of divalent cations, Ca, Sr and Pb at the barium site. The potential of these compounds as photocatalytic materials has been investigated. Chapter 3 describes a detailed kinetic analysis along with a single crystal X-ray structure determination of Ba2Bi24Mo10O68. Substitution at the Bi site by Ba and La results in differences in photocatalytic activity. All variants are observed to show specificity towards nitro- substitution at the ortho- position in phenols. Chapter 4 is a re-determination of the crystal structures of the triclinic polymorphs of BiNbO4 and BiTaO4 since one of the earlier reports is on a twinned crystal with large, unacceptable thermal ellipsoids and the other is a powder data -based Rietveld refinement of this model. Further BiNbO4 has been investigated to evaluate selectivity in photo degradation of dyes. In Part B, an attempt to correlate the crystal structure with relaxor ferroelectric behavior is outlined. Chapter 5 investigates a bismuth based tetratungsten bronze (TTB) relaxor material for the first time. Single crystal X-ray diffraction studies on the composition, x = 0.77 of the solid solution, Ba5x/2Bi(1-x)5/3Nb5O15 (0.52 ≤ x ≤ 0.8) depicts disorder in the occupancies of barium and bismuth atoms which is suggested to be the possible origin for relaxor behavior. Dielectric measurements correlate the structural features. Part C consists of a structure-based analysis on possible correlations with ionic conductivity. Chapter 6 describes two new crystal structures, Bi17P8O45.5 and Bi20P8O50, and the evolution of the series, Bi16+xP8O45+(3x-2)/2 (0.7 ≤ x ≤ 4.0). A rationale for the low ionic conductivity exhibited in these materials is arrived at from the arrangement of atoms in the crystal structure. Appendix A consists of a description of the crystal structure and ionic conductivity of Bi3W2O10.5.
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