Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged

Much of our recent experimental work has focused on the use of X-ray and neutron diffraction to study the structural evolution of hydrogen bonded molecular systems, including polymorphic materials, molecular complexes, tautomeric molecular materials, systems exhibiting hydrogen atom transfer and disorder, and magnetic systems coupled through hydrogen bond motifs. In particular we have focused on multi-temperature and pressure approaches to these studies, revealing often subtle behaviour of the hydrogen bonding, the structural evolution and on some cases of evolution of physical properties. Some of these effects are sufficiently subtle as to challenge the limits of current experimental diffraction, and also to challenge our theories of hydrogen bond formation. As a complementary approach to understanding these systems, we have for some years been applying developing plane-wave (periodic) density functional theory calculations for studying hydrogen bonds in the solid state. These are shown to have real potential in the study of a variety of hydrogen bonding systems. In addition MD approaches have been developed for these calculations, which allow us to examine the temperature evolution of molecular structures in the solid state and to quantify proton transfer effects. This leads to a fuller understanding of hydrogen bond formation and offers an improved description of the structural evolution observed in experiments. These approaches will be illustrated by results from a range of studies including: proton transfer systems, including examples in which the rational design and control of the degree of proton transfer is achieved, with effect on optical and non-linear optical properties; materials with potentially cooperative hydrogen bonding; tautomeric hydrogen bonded systems in which very small experimental energy differences can be reproduced and understood; prediction of energy scales for polymorphism in hydrogen-bonded molecular complexes, and optimisation of magnetic coupling in inorganic materials by design of simple hydrogen-bonded linkages. This contribution is focused on quantitative relationships between interacting atoms in molecular crystals resulting from experimental charge density studies. The role of factors influencing the quality of such relationships-in particular, treatment of H-atoms and assumptions blindly applied during refinement process-will be discussed. Details of refinement of single crystal X-ray data in order to achieve the final structural results of the same quality as those resulting from neutron data will be presented. Will demonstrate how to refine high resolution single crystal X-ray diffraction data only to obtain electron densities as reliable as in the case of joint neutron/X-ray multipole refinement. Improvement of data collection and processing and extension of electron density models used allow to detect even the finest interactions in the solid state. These are key steps toward quantitative relationships in the field of crystal engineering. The fascinating spin-crossover ph...

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The behaviour of spin crossover compounds is among the most striking and fascinating shown by relatively simple molecular species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in iron(II) complexes, to offer some rationalisation for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones in order to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.

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SummaryThe article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices.The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligo- and polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

The Mössbauer spectrometer on Spirit measured the oxidation state of Fe, identified Fe‐bearing phases, and measured relative abundances of Fe among those phases for surface materials on the plains and in the Columbia Hills of Gusev crater. Eight Fe‐bearing phases were identified: olivine, pyroxene, ilmenite, magnetite, nanophase ferric oxide (npOx), hematite, goethite, and a Fe3+‐sulfate. Adirondack basaltic rocks on the plains are nearly unaltered (Fe3+/FeT < 0.2) with Fe from olivine, pyroxene (Ol > Px), and minor npOx and magnetite. Columbia Hills basaltic rocks are nearly unaltered (Peace and Backstay), moderately altered (WoolyPatch, Wishstone, and Keystone), and pervasively altered (e.g., Clovis, Uchben, Watchtower, Keel, and Paros with Fe3+/FeT ∼ 0.6–0.9). Fe from pyroxene is greater than Fe from olivine (Ol sometimes absent), and Fe2+ from Ol + Px is 40–49% and 9–24% for moderately and pervasively altered materials, respectively. Ilmenite (Fe from Ilm ∼3–6%) is present in Backstay, Wishstone, Keystone, and related rocks along with magnetite (Fe from Mt ∼10–15%). Remaining Fe is present as npOx, hematite, and goethite in variable proportions. Clovis has the highest goethite content (Fe from Gt = 40%). Goethite (α‐FeOOH) is mineralogical evidence for aqueous processes because it has structural hydroxide and is formed under aqueous conditions. Relatively unaltered basaltic soils (Fe3+/FeT ∼ 0.3) occur throughout Gusev crater (∼60–80% Fe from Ol + Px, ∼10–30% from npOx, and ∼10% from Mt). PasoRobles soil in the Columbia Hills has a unique occurrence of high concentrations of Fe3+‐sulfate (∼65% of Fe). Magnetite is identified as a strongly magnetic phase in Martian soil and dust.

Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.