Hidden electronic rule in the "cluster-plus-glue-Atom" model

Electrons and their interactions are intrinsic factors to affect the structure and properties of materials. Based on the “cluster-cluster-plus-glue-atom” model, an electron counting rule for complex metallic alloys (CMAs) has been revealed in this work (i. e. the CPGAMEC rule). Our results on the cluster structure and electron concentration of CMAs with apparent cluster features, indicate that the valence electrons’ number per unit cluster formula for these CMAs are specific constants of eight-multiples and twelve-multiples. It is thus termed as specific electrons cluster formula. This CPGAMEC rule has been demonstrated as a useful guidance to direct the design of CMAs with desired properties, while its practical applications and underlying mechanism have been illustrated on the basis of CMAs’ cluster structural features. Our investigation provides an aggregate picture with intriguing electronic rule and atomic structural features of CMAs

Structural studies on Functional Materials using Solid-State NMR, Powder X-ray Diffraction and DFT Calculations

Analytical and theoretical techniques were used in this work for structural studies of framework materials. One and two dimensional 31P and 17O solid state NMR experiments highlight subtle thermally induced structural changes in (MoO2)2P2O7 pyrophosphate, tungsten trioxide WO3 and negative thermal expansion ZrW2O8. DFT methods using CASTEP software to calculate 31P and 17O NMR parameters are performed on these structures and discussed in comparison to experimental results, published structures and thermal mechanisms

Metallacarboranes derived from 1,1′-bis(o-carborane)

Chapter one gives an overview of single cage heteroborane chemistry, particularly the areas of 12- and 13-vertex metallacarboranes and their isomerisations. Also included is the chemistry of bis(carboranes), with recent developments on chelating derivatives of 1,1'-bis(o-carborane), as well as reduction/metallation of bis(o-carborane). Chapter two describes metallation of the [7-(1′-1′,2′-closo-C2B10H11)-7,8-nido- C2B9H10]2– dianion with various {NiPP}2+, {PdNN}2+ or {NiP2}2+ fragments (PP = chelating diphosphine; NN = chelating diamine; P = monodentate phosphine or phosphite) and which leads either to unisomerised 3,1,2-MC2B9 species or to isomerised 4,1,2-MC2B9 or 2,1,8-MC2B9 species, all with a pendant C2B10 substituent. The products were fully characterised spectroscopically and crystallographically as appropriate. Overall the results suggest that an important factor in a 3,1,2 to 4,1,2 isomerisation is the relief gained from steric crowding, whereas a 3,1,2 to 2,1,8 isomerisation appears to be favoured by strongly electron-donating ligands on the metal. Chapter three describes capitation of the [7-(7′-7′,8′-nido-C2B9H10)-7,8-nido-C2B9H10]4– tetraanion with {NiPP}2+ and {BX}2+ fragments (PP = chelating diphosphine; X = Br, I, Ph). This results in examples of MC2B9-MC2B9 architectures (3′,1′,2′-3,1,2; 4′,1′,2′-3,1,2) or bis(carborane) derivatives. Thermal isomerisation of the bis(nickelacarboranes) is studied. Unexpected interconversion is observed between bis(nickelacarborane) diastereoisomers (rac and meso isomers of 3′,1′,2′-NiC2B9-3,1,2-NiC2B9) ligated by dmpe and the mechanism of this interconvesion is considered. A stereospecific product was observed in the nickelacarborane ligated by dppe and this is rationalised by DHB. Formation of 4′,1′,2′-NiC2B9-4,1,2-NiC2B9 and 2′,1′,8′-NiC2B9-4,1,2-NiC2B9 isomers on thermolysis of 3′,1′,2′-3,1,2 or 4′,1′,2′-3,1,2 MC2B9-MC2B9 precursors is elucidated in terms of DHB. Finally a new naming convention is introduced for these 12-vertex/12- vertex bis(nickelacarboranes) to distinguish the chirality between cages. Chapter four elaborates the thermolysis of a rac/meso mixture of the species [1-(1′-4′-Cp- 4′,1′,6′-closo-CoC2B10H11)-4-Cp-4,1,6-closo-CoC2B10H11] to yield a rac/meso mixture of [1-(1′-4′-Cp-4′,1′,12′-closo-CoC2B10H11)-4-Cp-4,1,12-closo-CoC2B10H11]. Cage carbonatom identification is accomplished by both the VCD and BHD methods. Polyhedral expansion of the rac and meso isomers of 4′,1′,12′-CoC2B10-4,1,12-CoC2B10 was also attempted targeting 14-vertex metallacarborane/14-vertex metallacarborane derivatives. Chapter five contains the experimental procedures leading to, and characterisation details for, all new compounds reported herein. Crystallographic data is listed in the Appendix together with structure solution and refinement details

Order and Disorder in Mixed (Si, P)–N Networks Sr2SiP2N6:Eu2+ and Sr5Si2P6N16:Eu2+

In the field of nitride phosphors, which are crucial for phosphor-converted light-emitting diodes, mixed tetrahedral networks hold a significant position. With respect to the wide range of compositions, the largely unexplored (Si, P)–N networks are investigated as potential host structures. In this work, two highly condensed structures, namely Sr2SiP2N6 and Sr5Si2P6N16 are reported to address the challenges that arise from the similarities of the network-forming cations Si4+ and P5+ in terms of charge, ionic radius, and atomic scattering factor, a multistep workflow is employed to elucidate their structure. Using single-crystal X-ray diffraction, energy-dispersive X-ray spectroscopy (EDX), atomic-resolution scanning transmission electron microscopy (STEM)-EDX maps, and straightforward crystallographic calculations, it is found that Sr2SiP2N6 is the first ordered, and Sr5Si2P6N16 the first disordered, anionic tetrahedral (Si, P)–N network. After doping with Eu2+, Sr2SiP2N6:Eu2+ shows narrow cyan emission (λmax = 506 nm, fwhm = 60 nm/2311 cm−1), while for Sr5Si2P6N16:Eu2+ a broad emission with three maxima at 534, 662, and 745 nm upon irradiation with ultraviolet light is observed. An assignment of Sr sites as probable positions for Eu2+ and their relation to the emission bands of Sr5Si2P6N16:Eu2+ is discussed

DFT Study on the Li Mobility in Li-Ion-Based Solid-State Electrolytes

I have investigated the diffusion mechanisms of Li-ion in amorphous lithium phosphite (LiPO3) with addition of sulphur. By applying the nudge elastic band (NEB) method in crystal LiPO3 and Li3PO4, I confirmed the easing of diffusion pathways for Li ion in LiPO3 which is consistent with the previous theoretical finding[1]. From the diffusion study in 0.5 Li2O- 0.5 P2O5 and 0.4 Li2SO4 – 0.6 (Li2O-P2O5) melts above 3000K performed with ab-initio molecular dynamics (AIMD), produces identical outcome as experimental at lower temperatures. I demonstrated the benefit of addition of S in increasing Li+ mobility. I also found that the Li2SO4 addition into the glass results in a characteristic shift in Li-ion vibration to a lower vibrational frequency. In addition, the presence of oxygen surrounding the diffusion pathways appears to be essential in assisting the Li+ mobility in both crystalline and amorphous structures. The activation energy barrier shows similar pattern as reported in experimental analysis with LiPO3[2]

A structural study of high temperature metal-rich titanium sulfide phases

Ti/sub 2/S and Ti/sub 8/S/sub 3/ have been prepared by high temperature annealing techniques. The crystal structures of these two phases have been determined from single crystal x-ray diffraction data. Both structures were refined using a full-matrix least-squares treatment of positional parameters and isotropic temperature factor coefficients. Ti/sub 2/S crystallizes with orthorhombic symmetry, space group Pnnm, having unit cell dimensions a = 11.367A, b= 14.060A, and c = 3.326A. Ti/sub 2/S is isostructural with Ta/sub 2/P. Ti/sub 8/S/sub 3/ crystallizes with monoclinic symmetry, space group C2/m, a = 32.69A, b = 3.327A, c = 19.35A, ..beta.. = 139.9/sup 0/ (b - unique). Ti/sub 2/S and Ti/sub 8/S/sub 3/ have structural features similar to the features of a large number of metal-rich transition-metal chalcogenides and pnictides. These various structure types have been characterized in terms of nonmetal trigonal prismatic coordination polyhedra, eight different metal partial coordination polyhedra, a short (approximately equal to 3.4A) crystallographic axis, two unique layers of atoms containing both metal and nonmetal atom positions, and mirror planes coincident with the two layers of atom positions. The existence of a variety of structures with these structural features has led to their consideration as a unique structural class. The structural similarities and differences between the structure types of this class have been discussed in detail. Comparison of different structure types emphasized the importance of the metal bonding contribution in understanding the structural features and suggested limitations on qualitative bonding models used to understand the structural-chemical principles underlying structure stability

Lattice relaxation in solid solutions: long-range vs. short-range structure around Cr3+ and Co2+ in oxides and silicates

This dissertation reports the results derived from the 3-year doctoral thesis project aimed at exploring some oxide and silicate structures as promising ceramic pigments with enhanced colorimetric properties with respect to the traditional colorants. Solid solutions of perovskite, alumoniobite, and melilite compounds were obtained by doping octahedral and tetrahedral coordination sites with transition metal ions (e.g. Cr3+, Co2+, and Zn2) through a solid-state synthesis performed by means of an industrial-like process. The analytical techniques adopted to investigate the synthesized compounds allowed the determination of the "averaged" crystal structure, or the so termed long-range properties, and the short-range properties (i.e. the local structure around the substituting ions) through X-ray powder diffraction and electron absorption spectroscopy (EAS), respectively. As stated by Geiger (2001) "an understanding of the microscopic, mesoscopic and macroscopic properties and of the behaviour of solid solutions under different conditions is a challenge for all disciplines concerned with the solid state". As a matter of fact, the precise determination of a structure around impurities results fundamental to provide detailed information on their incorporation and on physical properties. For instance, in the case of the solid solutions here reported, the lattice incorporation of transition metal ions as impurities is the cause of their gradual coloration. Most of the times, such a coloration is more intense as greater is the impurity amount. The final goal of this work, was attained by calculating the structural relaxation coefficient for each studied solid solution by combining the mean with the local bond distances achieved by XRPD and EAS, respectively