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Appendix E Classes of Materials Organics Also called molecular crystals, organics are composed of carbon-based mol- ecules. This group of materials also includes inorganic coordination compounds, which are sometimes referred to as metallo-organic or organometallic com- pounds. Organics exhibit many technologically important properties, such as magnetic ordering, high electrical conductivity, high- and low-dielectric con- stants, piezoelectricity, and superconductivity. Like atoms in inorganic materials, dissimilar molecules can be co-crystallized. Although generally thought of as being neutral species, when the electronegativities of co-crystallized molecules are sufficiently different, electron transfer occurs creating anions and cations, and examples can exhibit high direct current electrical conductivity or ferromagnetic ordering. When intermediate differences in the electronegativities occur, charge- transfer complexes form. Oxides Oxides are materials containing oxygen as a primary anion—examples of current interest are BaTiO3 (a ferroelectric), YBa2Cu3O7 (a superconductor), and La1-xSrxMnO3 (a magnetoresistive material). The wide range of behavior found in metal oxides is due in part to the presence of strong interactions between metal d electrons and oxygen p electrons and a delicate balance between nearly energeti- cally equivalent electronic configurations that involve coupling between electronic, 169

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170 Frontiers in C rys ta l l i n e M at t e r magnetic, and structural degrees of freedom. The presence of oxygen is particularly critical to the creation of exotic properties—the orbital energies of the oxygen ion are well matched to transition metal orbital energies, yielding the added com­plexity of a balance between covalent and ionic bonding that differs from compound to compound. Finally, the relatively straightforward methods generally used in the synthesis of oxides and their chemical stability make them widely available to the condensed-matter and solid-state chemistry communities, resulting in vigorous worldwide research on many different compound systems. Examples of chal- lenges that drive this field are the quest for room-temperature superconductivity and the production of a room-temperature ferromagnetic insulator. There is also a vigorous effort devoted to realizing qualitatively new magnetic phases such as two-­dimensional critical spin liquids. Intermetallics Intermetallics are materials made of combinations of two or more metallic elements. When one of the elements is a magnetic rare-earth or actinide metal (e.g., cerium and uranium), such crystalline systems embody the concentrated limit of the Kondo effect, in which the conduction electron forms a pair with the magnetic ion in a lattice periodic fashion. In this many-body limit, novel “heavy” electron states emerge. These materials constitute the frontier of exploration into the possibility of conducting states of matter that are qualitatively different from the conventional Landau Fermi-liquid, which describes metals such as copper and aluminum. Intermetallic compounds are widely used, though for different reasons, in today’s mechanical and electrical systems.