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

Recent developments in density functional theory (DFT) methods applicable to studies of large periodic systems are outlined. During the past three decades, DFT has become an essential part of computational materials science, addressing problems in materials design and processing. The theory allows us to interpret experimental data and to generate property data (such as binding energies of molecules on surfaces) for known materials, and also serves as an aid in the search for and design of novel materials and processes. A number of algorithmic implementations are currently being used, including ultrasoft pseudopotentials, efficient iterative schemes for solving the one-electron DFT equations, and computationally efficient codes for massively parallel computers. The first part of this article provides an overview of plane-wave pseudopotential DFT methods. Their capabilities are subsequently illustrated by examples including the prediction of crystal structures, the study of the compressibility of minerals, and applications to pressure-induced phase transitions. Future theoretical and computational developments are expected to lead to improved accuracy and to treatment of larger systems with a higher computational efficiency. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 895-910, 2000

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Electronic structure, properties, and phase stability of inorganic crystals: A pseudopotential plane-wave study

Hydrogen bonding in solids correlation of spectroscopic and crystallographic data

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

https://iopscience.iop.org/article/10.1088/0034-4885/67/11/R01/pdf

Electronic and ionic transport properties and other physical aspects of perovskites

XPS characterisation of in situ treated lanthanum oxide and hydroxide using tailored charge referencing and peak fitting procedures

The complete‐matrix least‐squares analysis on the neutron diffraction intensities of CaC2, YC2, LaC2, CeC2, TbC2, YbC2, LuC2, and UC2 yielded the C–C distances in the C2 groups, 1.191±0.009, 1.275±0.002, 1.303±0.012, 1.283±0.004, 1.293±0.009, 1.287±0.010, 1.276±0.012, and 1.340±0.007 A, respectively. The statistical averages of the neutron scattering amplitudes obtained from the dicarbide data and those reported previously are 0.488±0.007, 0.796±0.009, 0.832±0.014, 0.482±0.006, 0.756±0.020, 1.262±0.012, 0.73±0.02, and 0.851±0.022, all in 10—12 cm with positive sign, for Ca, Y, La, Ce, Tb, Yb, Lu, and U, respectively.The paramagnetic scattering analyses revealed that all metal atoms in these dicarbides are in their trivalent Hund ground state, except Ca (divalent), Yb (possible valency of 2.8), and U (possible tetravalent). The scattering data also deduced the screening constants for the 4f electrons in Ce, Tb, and Yb as 39, 43, and 43, respectively, which are compared with those obtained by other methods....

Neutron Diffraction Studies of CaC2, YC2, LaC2, CeC2, TbC2, YbC2, LuC2, and UC2

Using neutron diffraction data, the crystal structure of gypsum has been refined by Fourier and least squares methods. The hydrogen bond distance was found to be 2.82 A rather than 2.71 A, reported previously. The hydrogen positions are consistent with those obtained from nuclear magnetic resonance. The O–H distance is 0.99±0.03 A, and it appears that the O, H, O atoms are collinear. Infrared and Raman results are discussed in terms of the refined structure.

Neutron Diffraction Study of Gypsum, CaSO42H2O

The C–C distances of the C2 groups in calcium dicarbide and uranium dicarbide have been determined by the powder neutron diffraction method. The C–C distance in calcium dicarbide is 1.20±0.01 A, equal to the triple bond C–C distance, but not 1.4 A as reported previously. Uranium dicarbide is isostructural with calcium dicarbide and has the C–C distance of 1.34±0.01 A essentially that of a C–C double bond. The interatomic distances in calcium dicarbide, uranium dicarbide, lanthanum dicarbide, and lanthanum sesquicarbide are discussed in connection with the electronic configuration of the C2 group.

Structures of Calcium Dicarbide and Uranium Dicarbide by Neutron Diffraction

X‐ray diffraction study of a single crystal of B2F4 indicates a planar, centrosymmetric molecule with B–B=1.67±0.045 A, B–F=1.32±0.035 A and angle F–B–F=120°. There are two molecules in a monoclinic unit cell of symmetry P21/n and parameters of a=5.49, b=6.53, c=4.83, and β=102.5°.

Crystal and Molecular Structure of Diboron Tetrachloride, B2Cl4

X‐ray structure determinations on the cubic YCx (x ≈ 0.3 − 0.7, stable above about 900°C) and the trigonal Y2C (stable below 900°C) have been carried out using a quenched “single crystal” in which a transient state in the midst of the cubic‐to‐trigonal transition has been arrested. Our “single crystal” is composed of one cubic and four trigonal single crystals, the trigonal c axis being parallel to one of four cubic body‐diagonal axes. The cubic structure is of NaCl type with random holes in the carbon‐atom sites and with a = 5.115 ± (2) A for YC0.44. The ordering of the carbon atoms creates the anti‐CdCl2‐type trigonal Y2C with the yttrium positional parameter z = 0.2585 ± (3) in the hexagonal‐based unit cell, a = 3.617 ± (2) and c = 17.96 ± (1) A, containing three Y2C formula units. Mo Kα x‐ray was employed, and consequently the dispersion corrections on the atomic scattering factor of Y amount to 10%–20% of the nondispersion value and are considerably larger than the carbon‐scattering factor. The struc...

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Masao Atoji

Papers

Neutron Diffraction Studies of CaC2, YC2, LaC2, CeC2, TbC2, YbC2, LuC2, and UC2

Neutron Diffraction Study of Gypsum, CaSO42H2O

Structures of Calcium Dicarbide and Uranium Dicarbide by Neutron Diffraction

Crystal and Molecular Structure of Diboron Tetrachloride, B2Cl4

Crystal Structures of Cubic and Trigonal Yttrium Hypocarbides; A Dimorphically Interphased Single‐Crystal Study