Variants Of Complex Bismuth And Zirconium Oxides : Structure-Property Correlation Studies

Abstract

The thesis entitled “Variants of Complex Bismuth and Zirconium Oxides: Structure-Property Correlation Studies” consists of five chapters. A short introductory note outlines the synthetic procedures, characterization techniques and evaluated properties such as photocatalysis, second harmonic generation, ionic conductivity and thermal expansion in these materials. Chapter 1 deals with a new solid solution Pb3-xBi2x/3V2O8 (0.20 ≤ x ≤ 0.50), stabilizing the high temperature γ form of Pb3V2O8 in the system Pb3V2O8−BiVO4. Single-crystals of the composition x = 0.50 were grown and the structure is a new variant in palmierite structural type as determined by both single crystal X-ray and powder neutron diffraction. Several refinement strategies backed up by difference Fourier methods were used to arrive at the final crystal structure. ac impedance studies indicate conductivity of the order of 10-4 Ω-1 cm-1 for Pb2.5Bi1/3V2O8. Chapter 2 has two sections and describes the structure property correlation in bismuth based vanadate and phosphate eulytites. Section 2.1 discusses the crystal structure of Pb3Bi(VO4)3, the first eulytite compound containing [VO4]3- moieties. The compound displays incongruent melting behavior. Single-crystals were grown by melt-cool technique adding excess amount of BiVO4. The crystal structure has been characterized by both X-ray and neutron diffraction studies. Section 2.2 describes the crystal structures of four phosphate eulytites A3Bi(PO4)3 ( A = Ca, Cd, Sr, Pb). The crystals were grown from melt-cool technique with considerable difficulty as the compounds melt incongruently. While Pb3Bi(VO4)3 and Pb3Bi(PO4)3 have an unique position for one of the oxygen atoms, Sr3Bi(PO4)3, Ca3Bi(PO4)3, Cd3Bi(PO4)3 display split oxygen atomic sites. The SHG efficiencies measured on polycrystalline samples were 5.3, 3.8, 2.85, 1.21 and 0.64 times that of KDP (KH2PO4) for Pb3Bi(VO4)3, Cd3Bi(PO4)3, Sr3Bi(PO4)3, Pb3Bi(PO4)3 and Ca3Bi(PO4)3 respectively. Chapter 3 describes the isolation of the compound Sr2Bi2/3V2O8, a variant palmierite, in the phase diagram of SrO-Bi2O3-V2O5. The compound was synthesized by ceramic method and it is of interest to note that the Sr(1) site also accommodates Bi as found by single crystal X-ray studies unlike that found in the case described in chapter 1. Chapter 4 has two sections, dealing with synthesis, characterization and photocatalytic properties of trigonal and monoclinic polymorphs of ZrMo2O8, a negative thermal expansion material in its cubic form. Section 4.1 describes the synthesis of trigonal polymorph of ZrMo2O8 by both ceramic and combustion synthesis methods. SEM images show a particle size of 40-50 nm for combustion synthesized samples and 8-10 μm for solid state synthesized ZrMo2O8. The band gap obtained by UV-visible diffuse reflectance spectra for the combustion synthesized and solid state synthesized samples were 2.70 and 2.74 eV and the BET surface area were 1.0 m2/g and 10.0 m2/g. DFT electronic structure calculations reveal the indirect band gap nature of this polymorph. Photo-degradation studies performed on pollutant water show specific affinity to degrade dyes which do not possess anthraquinonic moieties. Section 4.2 describes the single-crystal structure determination and catalytic properties of monoclinic polymorph of ZrMo2O8. The band gap measured for the monoclinic form by UV-visible diffuse reflectance spectra was 2.57 eV. This polymorph was found to be specific towards the degradation of cationic dyes. Chapter 5 discusses a new solid solution ZrV2-xMo5x/6O7 (0 ≤ x ≤ 0.8) identified in the phase diagram of ZrO2−V2O5−MoO3. These compounds were synthesized via the solution combustion method. The resulting products were characterized by powder X-ray diffraction, solid-state UV-visible diffuse reflectance spectra, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photo-catalytic activity shows specificity towards the degradation of non-azo dyes. Single-crystals were grown by melt-cool technique from the starting materials with twice the MoO3 quantity. Since, these crystals belong to a cubic system, space group Pa 3, they were tested for negative thermal expansion using variable temperature single-crystal XRD and indeed they exhibit this property above 370 K.