Structure and thermoresponsive behaviour of porous and non-porous borophosphates
Date
2016
Authors
Mogodi, Mashikoane Wilson
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Abstract
In this PhD thesis, the synthesis, crystal chemistry and thermoresponsive behaviour of non-porous borophosphates [ABPO5 (A= Ca, Sr, Ba) and BPO4] and porous borophosphates [NH4Fe(III)[BP2O8(OH)] and MIxMIIz(H2O)2[BP2O8].zH2O (MI = Na, NH4 and MII = Mn, Co)] phases were investigated. Understanding the crystal structure dynamics as a function of temperature of the selected porous and non-porous borophosphates revealed the thermal stability of the studied compounds, while serving as a predictive measure of the effects of temperature on other materials properties and subsequent applications.
The non-porous borophosphates were synthesized using the solid state method, whereas the porous borophosphates were synthesized using the hydrothermal method. The powder X-ray diffraction (PXRD) technique, along with the application of the Rietveld refinement method, was the principle characterisation technique employed for the non-destructive and non-invasive thermoresponsive characterisation of the studied borophosphate phases.
This thesis consists of seven chapters, four of which are independent papers corresponding to four chapters. Chapter 1 reviews the relevant scientific literature, while chapter 2 describes the methods of characterisation used in this thesis. Concerning the highly thermally stable non-porous borophosphates, chapter 3 presents the thermal expansion behaviour of ABPO5 compounds, which have been determined from the sequential application of the Rietveld refinement method of variable temperature powder X-ray diffraction (VT-PXRD) data. For trigonal ABPO5 compounds, a near linear expansion of the unit cell axes was found for all structures as a function of temperature. The variation of the crystal structure with temperature of ABPO5 compounds was also established.
With the general understanding that borophosphates display intriguing crystal structure architectures, chapter 4 describes the synthesis and characterisation of four metal borophosphate hydrates: NaMII(H2O)2[BP2O8](H2O); MII = Co (I), Mn (II) and (NH4)0.5MII1.25(H2O)2[BP2O8](H2O)0.5; MII = Co (III), Mn (IV). The structures refined at room temperature from PXRD data revealed that isostructural phases I and II have an ordered arrangement of water molecules in the voids, whereas isostructural phases III and IV have fractional and disordered distribution of water molecules in the voids. Scanning electron microscope (SEM), fourier transform infrared spectroscopy (FT-IR), variable temperature powder X-ray diffraction (VT-PXRD) and thermogravimetric analysis (TGA) for all
compounds are also presented. Chapter 5 was an investigation into the synthesis, crystal structure and thermal properties of the porous iron borophosphate NH4Fe(III)[BP2O8(OH)]. Variable temperature PXRD and thermogravimetric analysis were used to investigate the compounds thermal stability and expansion behaviour. Thermal investigations indicated that the compound is stable up to 470 °C.
Of great importance to the accurate and/or precise measurement of the crystal structures and lattice parameters of the phases investigated using powder diffraction was the correct application of the Rietveld refinement method to the measured diffraction data. Therefore, chapter 6 is an investigation into a number of different Rietveld refinement approaches, which were aimed at modelling the changes in the atomic coordinates of BPO4 as a function of temperature. Parametric Rietveld refinements and rigid body Rietveld refinements were among the refinement strategies employed.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016.
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Citation
Mogodi, Mashikoane Wilson (2016) Structure and thermoresponsive behaviour of porous and non-porous borophosphates, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/21714>