Molecule-like Structural Units In Typical Inorganic Materials

Inorganic nonmetallic materials,originated from silicates,is one of the three materials species,together with oganic polymers and metals.To meet the increasing demands for service properties,materials compositions become more and more complex and specific.While the specific compositions should be based on specific structural precursors,their structural origin is for long missing and is hindering the materials research and development,for there is no molecular concept to be used as the structural units.Therefore,practical composition design methods from atomic or molecular structural levels and summarizing existing composition rules are presently a major target of researchers to assist new materials development.The objective of the present thesis is just focusing on the structural precursor of chemical compositions in inorganic materials.Considering the limitation in conventional crystallography in describing atomic structures and the missing molecules in inorganic compounds,we first introduce the cluster-plus-glue-atom model to construct the basic molecule-like structural unit.By doing so we propose a new structural concept for inorganic materials and provide a promising tool to explore the composition rule of inorganic materials.Next we obtain the molecule-like units for typical inorganic compounds and for silica-related oxides,justifying the universality of the cluster-plus-glue-atom model.Last,we analyze the cluster structures in hard borides,as exemplified by MgB12C2 and validate the relevant composition formula for the good glass-forming composition in this system via thin film deposition,which provides a new route for developing hard coating.The major contents and conclusions are the following.(1)Extension of the cluster-plus-glue-atom model,successfully used in metals already,to inorganic materials.First,by analyzing the structure of crystalline silica SiO2,we construct the cluster-plus-glue-atom structural unit and examine its molecule-like properties.We propose that each unit acts like a 'molecule',containing information on composition,electronic structure,and atomic structure.In other words,the use of the cluster model to address phase structures is not only simpler than conventional crystallography but also construct the relationship between composition and structure formation via the cluster formula.Accordingly we surveyed typical structural types of inorganic compounds,covering Si3N4(characterized by triangular cluster),SiO2 and Al2MgO4(tetragonal),(NaCl,TiO2,Al2O3,CaTiO3,Mn2O3)(octahedral),and(CaF2,CsCl)(cubic).Via constructing the molecular units in these structures we determine the methods for phase composition formula,principal cluster,and structural unit.Further,we go through silica,the basic structure of conventional silicates and focus in particular glass-relevant oxides,including network formers,intermediates,and network externals.We point out that their contribution to glass formation is based on characteristic clusters.We also confirm that these structural units all satisfy octet rule and are molecule-like.(2)Limited by the missing model for short-range-ordering,the composition rule of eutectic liquids is for long unresolved.We extend the cluster-plus-glue-atom model in interpretation of conventional eutectic compositions in B-containing binary systems.Such an interpretation follows two reasonable assumptions:eutectic liquids is composed of two stable liquids in equal proportion;each liquid satisfies the composition formula of the ideal metallic glass,i.e.,one cluster is matched with one or three glue atoms.Therefore the structural unit of a eutectic liquid follows a dual-cluster model.We further propose the procedures towards the dual cluster structural units originated from the two eutectic phases.After summarizing the eutectic formulas of all B-containing systems,we confirm that a eutectic composition is indeed an equi-mole mixture of two structural units and the two clusters satisfy in general cluster matching rules.The present approach,describing the eutectic liquid structure and interpreting the eutectic compositions,also provides a new route towards designing multi-element eutectic alloys.(3)To develop composition tolerant amorphous borides for their superhard properties,we examine the cluster structures of some typical borides such as MgB12C2,and unveil the pentagonal prism as their common cluster type,characteristic for their short-range structures.Using this type of cluster we construct the cluster formula[B-B5C]Mg for high glass forming ability.Such a formula describes the molecule-like structural unit with its total electron number being 24.We finally verify the amorphous structure of the B-C-Mg films prepared using magnetron sputtering and prove again the universality of the cluster model in inorganic systems.In brief,the present work extends the cluster-plus-glue-atom model in the proposition of molecule-like structure units in inorganic compounds and build up the relevant method.Both the theoretical model and the relevant method are new and important for guiding composition design of inorganic materials.