Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

AgI-type solid electrolytes

SINCE the discovery1 that the high-temperature phase of silver iodide (α-AgI) has an ionic conductivity comparable to that of the best liquid electrolytes, solid electrolytes have attracted wide interest. Possible applications of these materials range from solid-state batteries to electrochromic displays and sensors2. Although α-AgI displays conductivities of more than 10 S cm−1 (ref. 3), owing to the almost liquid-like mobility of Ag+ ions, the crystal transforms below 147 °C to the β-phase with a conductivity of only ∼10−5 S cm−1 at room temperature. Efforts to achieve good conductivities at lower temperatures have focused on the addition of a second component to AgI to form solid solutions or new compounds such as RbAg4I5 and Ag2HgI4 (refs 4–7). Here we report our success in depressing the α→β transformation temperature so as to stabilize α-AgI itself at room temperature. We use a melt-quenching technique to prepare crystallites of α-AgI frozen into a silver borate glass matrix. The quenched material showed diffraction peaks characteristic of α-AgI and displayed ionic conductivities of about 10−1S cm−1. Further development of these glass/crystal composites may make the high ionic conductivity of α-AgI available for room-temperature solid-state applications.

Stabilization of superionic α -Agl at room temperature in a glass matrix

Publisher Summary The chapter presents a study on the relationship between structure, vibrational dispersion relations, and band structure for chalcopyrite and related compounds. The essential step in this procedure is the “folding” of the dispersion relations [ ω (k) or E (k)] for the simplest structure into the smaller Brillouin zone of the more complicated one. In one-dimensional models, such folding is trivial enough and is discussed in relation to the nearly free electron approximation, Peierls instability, and so on. In three dimensions, it is more complex and therefore merits the detailed and concentrated description that is presented in this chapter. In addition to the chalcopyrites, the chapter includes certain quaternary and “defect” or “vacancy” compounds, such as HgIn 2 Te 4 . The vacancy compounds may be considered to have tetrahedral structures with a sublattice of unfilled sites. In this case, it is questionable whether the compound is to be regarded as derived from its parent structure by a small perturbation, as is often assumed for chalcopyrites. Compounds that have tetrahedrally coordinated structures generally have an average number of four valance electrons per atom. This is the Grimm–Sommerfeld rule.

Beyond the Binaries—The Chalcopyrite and Related Semiconducting Compounds

The oxide of chromium (IV), Cr02, is a highly interesting compound for materials investigation in that it is ferromagnetic at room temperature and a good electrical conductor. The Curie temperature is 118°C, and the material can readily be demagnetized with little energy input. The electrical conductivity of this black compound is quite high for an oxide; it even rivals some of the metal conductors such as Mn and Gd at roomtemperature. Thls review describes procedures for preparing Cr02, presents its chemical and physical properties including the modifications observed with metal-ion substitutions, and discusses the reactions of Cr02 with other metal oxides together with the physical properties of the ternary products containing tetravalent chromium.

Lawrence Suchow

Papers

Electrical Conductivity of Ag2HgI4, Cu2HgI4 and their Eutectoid

Solid Phases in the System Ag2HgI4—Cu2HgI4

Compounds of the Alkali Metals and Chromium of the Type MCr3O81,2,3

Photosensitive and Phototropic Products of Solid State Reaction between Silver Sulfide and Mercuric Iodide