The preparation and characterisation of mixed-anion and non-oxide materials

• Main Author: Headspith, David Andrew
• Other Authors: Francesconi, M. Grazia (supervisor)
• Published: University of Hull 2009
• Subjects: 546.3; Chemistry Physical Sciences
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.518617

Traditionally, research in solid-state chemistry has focused largely on the chemistry of oxides and on chemical tailoring of the structure and physical properties via cationic substitutions. Consequentially, the chemistry of non-oxide ompounds and other means of chemical tailoring, such as anionic substitutions, have been comparatively overlooked by the scientific community. Non-oxides offer a wide diversity of chemistry, most noticeable in unusual oxidation states and coordination geometries found for metals in these compounds. Furthermore, the development of anionic substitutions could open up an alternative avenue for the modification of the structure and properties in solids and the preparation of novel compounds. The work reported here covers: (1) the preparation of non-oxide compounds (Ba2CoS3, Ba2MnS3, Ce2MnN3, Ce2-xLaxMnN3) and mixed-anion compounds (Ce2MnN3F2-δ, apatite oxide-nitrides) via direct synthesis and/or cationic and anionic substitutions; (2) the characterisation of their structure and (3) the characterisation of selected physical properties. The three-dimensional magnetic cell of Ba2CoS3, which undergoes a transition to long-range order at 46 K, was found to be double the size of the crystallographic unit cell, along the c-axis. A conclusive representation of the magnetic cell of Ba2MnS3, with similar structure to Ba2CoS3, could not be achieved and two possible models are proposed. A one-step synthetic route for Ce2MnN3, more convenient than the route reported in the literature, was developed in this work. Cationic substitutions led to the preparation of the solid solution Ce2-xLaxMnN3 and anionic manipulation of the lattice, via fluorination, led to the preparation of the first example of a quaternary nitride-fluoride Ce2MnN3F2-δ. The structure of Ce2MnN3F2-δshowed layers of distorted MnN5F octahedra and staged fluorine occupancy of the interstitial sites. A range of novel oxide-nitrides were prepared via reaction of the apatite-type oxides La9.33Si6O26, La8+xSr2−xM6O26+x/2 (M = Si, Ge; 0 less than or equal to x less than or equal to 2), La9.67Si5CoO26 and La10M5CoO26.5 (M = Si, Ge) with gaseous ammonia at temperatures above 700°C, most retaining the apatite structure with nitrogen located in interstitial sites.