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xvii PART I PROPERTIES OF CRYSTALLINE MATERIALS The Effective U in Oxides and in Sulfides: Conceptual Phase Diagrams and their Applications...............;....... •........ 1 John B. Goodenough 1. Definitions 1 2. Interatomic interactions 23 References 44 Electron Correlations in Elementary Structures. The Case of Weak Correlations: Metallic and Covalent Bondings................. 45 J. Friedel Introduct i on 45 1. The H2 molecule 46 2. Other elementary diatomic molecules 52 3. Metallic aggregates 53 4. Macroscopic phases of transition metals 57 5. Aggregates and solids of tetravalent (sp) elements 64 6. Aggregates of covalent dimers 67 7. Weak distortions of metallic closepacked or covalent structures 69 Appendixes 74 References 82 Statistical Thermodynamics of Mixed Valence at Low Temperatures. •................................... •............ 85 Dieter Wohlleben 1. Introduction 85 2. The free enthalpy without mixing 86 3. The valence fluctuation temperature 89 4. 'The free enthalpy with interconfigurational mlxlng 90 5. Equation of state for the valence fluctuation ~~n~e ~ 6. The conduction electron density of states near T=O 98 7. Application to Ce metal 101 8. Acknowledgements 104 References 104 Mixed Valency in Inorganic Chemistry •... •.... ~... •.. •........ 109 P. Day CONTENTS vi Important Discussion of Part I Intermedi ate Val ency (IV) Compounds.......................... 111 N. F. Mott Thermal Expansion and Specific Heat of Mixed Valence Compounds................................................... 117 R. Pott 1. Introduction 117 2. Experiments and experimental results 117 3. Discussion 120 4. Acknowledgements 122 5.