Transition Metal Oxides: Geometric and Electronic Structures Introducing Solid State Topics in lnorganic Chemistry Courses Adela Muiioz-Paez Departamento de Quimica Inorganica, Facultad de Quimica, Universidad de Sevilla, Apdo, 553,41012 Sevilla, Spain The Study of lnorganic Solids Although transition metal oxides have been known for a long time, in the last decade there has been an increasing interest in them due to their important role in heteroge- neous catalysis (1) and more recently in material science, where high temperature superconductors have attracted much attention among research groups from all over the world (2). When studying any group of compounds the first questions asked concern the geometric and electronic structures because these determine their physical proper- ties and reactivity. For transition metal oxides much infor- mation is available on geometric structure. Diversity in Structure Upon examining this information, a wide variety of geo- metric arrangements are found, including three-dimensional layered chain molecular This variety produces a wide range in the values of cer- tain physical properties, such as melting points from 2800 'C for HfOz to 6 'C for MnzO7 color, including the entire spectrum electrical pmperties, fmm insulators (CrO) to metallic conductors (TiO) Nevertheless, most of the oxides fall in the first group of structures: the infinite three-dimensional. Thus, they are usually high melting point solids. The last three classes form a negligible fraction of all oxides. Information about the electronic structure of is scarce and can lead to different and often contradictory conclu- sions, depending on the source. The reason for this lack of information is the special difficulty encoutered in studying the bonds in these compounds. Although most are solids a t room temperature, they do not fit the ionic model well, nor can they be included among the compounds that form infi- nite covalent networks. Moreover, some properties of these compounds show values close to those of metals, for exam- ple, the electrical conductivities of Ti0 and VO. Increased Interest and Use My main purpose for this presentation is the growing value of studying the solid state in inorganic chemistry. The importance of solid state materials in everyday life is reflected in the increasing use of such compounds in diode lasers, semiconductors, or liquid crystals. The preparation and characterization of many of these compounds has been carried out by chemists. Consequently, the ACS Division of Inorganic Chemistry has developed an integrated set ofin- structional materials in solid state chemistry. Additionally, segments of this topic have begun appear- ing in inorganic chemistry textbooks (351. In the same trend, solid state chemistry is the subject of several papers that have recently appeared in this Journal (6-8). Even a movement to introduce topics in solid state chemistry in the laboratories has been reported (9). Pedagogical Value of Focusing on These Metal Oxides Transition metal oxides have been selected to achieve this goal for several reasons. ' Agood organizingprinciple is involved: They are binaryeom- binations of oxygen and elements fmm the transition series. This emu0 of com~ounds eonvenientlvillustrates the varietv and &m&xity df crystalline struckres possible for inor- ganic solids. 'These compounds provide a good way of introducing bond theories in solid state chemistry. Unfortunately, chemistry curricula are oRen restricted to the study of bonding in molecular compounds. A serious approach to the study of the electronic structure of inor- ganic solids thus challenges the student withuew theories. I have divided this paper into two parts. First I deal with geometric structures and offer a summarized description of the crystal structures that appear most otten in these compounds. Because this is intended as only an introduc- tion to the studv of transition metal oxides. onlv bulk , " structures of stoichiometric compounds will be examined, thus excluding surface and defective structures. which are by themselvesanother wide topic. The main s o k e s of in- formation for this part are the textbooks from Wells (101, Kung (I), Gutierrez Rios (Il), and Shriver et al. (3). The second half of this uauer is devoted to the studv of the electronic structure ofthi oxides and includes a reAew of bond theories currently amlied in the field. This section is thus more subjective andopen to discussion. The books used to prepare it are by Phillips and Williams (121, Cox (131, Duffy (141, Ziman (151, and West (16). A similar version of this presentation (except for the part dealing with the Hubbard model) has been used with un- dergraduate students who had previously completed coursework on bonding and crystallography. As a comple- mentary work in the laboratory, the preparation of Y-Ba- Cu superconductors is proposed as well as the study of the Meissner effect. This complex oxide has been chosen be- cause it is one of the most famous compounds involving transition metal oxides. Also, its preparation involves solid state techniques easily available in inorganic chemistry laboratories (171, and the degree of success is easily checked using the Meisnner effect. Determinants of Geometric Structure An examination of the known structures of binary ox- ides. summarized in Table 1 (1). shows that transition . .. metal oxides exist in many different crystal forms. The classification used in Table 1 was chosen to show schemat- ically all the types of structures that appear in these ox- ides. The oxides are arranged horizontally, as in the peri- Volume 71 Number 5 May 1994 381