The 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-, oligoand 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.

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Spin state switching in iron coordination compounds

The phonon density of states was measured by observing the nuclear resonant fluorescence of ${}^{57}$Fe versus the energy of incident x rays from a synchrotron radiation beam. An energy resolution of 6 meV was achieved by use of high-resolution crystal optics for the incident beam. Extremely low background levels were obtained via time discrimination of the nuclear fluorescent radiation.

http://csrri.iit.edu/~segre/phys570/15S/presentations/gao.pdf

Phonon density of states measured by inelastic nuclear resonant scattering

Evaluation methods for data obtained by nuclear resonant scattering techniques are discussed. The CONUSS software package for the interpretation of time or energy spectra from coherent elastic nuclear resonant scattering, i.e., forward scattering and Bragg/Laue scattering, is presented. The analysis of phonon spectra obtained by incoherent nuclear resonant scattering is demonstrated using the PHOENIX software.

CONUSS and PHOENIX: Evaluation of nuclear resonant scattering data

The Nuclear Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The sources of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the Mossbauer transition energies between 6 and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the “prompt” electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general lay-out and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results.

Nuclear Resonance Beamline at ESRF

The present synchrotron sources with brilliant x-ray beams, due to high photon fluxes, small source sizes and high collimation, have revolutionized x-ray physics. Enormous progress has been initiated in all established x-ray methods, with the aim of the development of new types of spectroscopy. This is particularly true for the spectroscopy of the dynamics in condensed matter. Meanwhile, there are two powerful x-ray methods with very high-energy resolution available for the study of low energetic excitations like phonons. This review summarizes the developments of these methods focusing on these instrumental developments of the spectrometers using either crystal optics in close-to-backscattering geometry or nuclear resonant techniques. Applications to measurements of phonon dispersion curves and of phonon density of states in ordered and disordered solids and in liquids are presented. It is shown how x-ray results are stimulating improvements in the theoretical approaches to the dynamics. New insights into the dynamics of liquids are discussed. The sensitivities of the spectroscopies allow the study of vibrational behaviour in very small amounts of material even in nanometre-sized thin films or particles. We can already analyse the phonon spectrum of a monolayered nuclear resonant isotope. Prospects of the techniques are also demonstrated.

https://iopscience.iop.org/article/10.1088/0034-4885/63/2/203/pdf

Phonon spectroscopy by inelastic x-ray scattering

Monochromatization of synchrotron radiation down to about ${10}^{\mathrm{\ensuremath{-}}8}$ eV at an energy of 14.4 keV has been achieved by double nuclear Bragg diffraction from $^{57}\mathrm{yttrium}$ iron garnet single-crystal films set for the electronically forbidden {200} reflection. The monochromatized \ensuremath{\gamma} quanta have been analyzed with respect to time delay and capabiltiy of resonance absorption. By setting of appropriate energy and time windows a pure beam of resonant \ensuremath{\gamma} quanta at a rate of about 1 Hz is available.

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Nuclear Bragg diffraction of synchrotron radiation in yttrium iron garnet.

We report the experimental observation of quantum beats in the coherently scattered radiation from nuclei excited by synchrotron-radiation pulses. The measurements were made with $^{57}\mathrm{Fe}$-enriched yttrium-iron-garnet crystals, by reflection from two parallel crystals using the pure nuclear (002) Bragg reflection. The resulting beat pattern gives a sensitive direct measure both of the hyperfine splittings of individual iron nuclei, and of the energy shifts between different nuclear sites.

/pdf/quantum-beats-from-nuclei-excited-by-synchrotron-radiation-188kh4qobi.pdf

Quantum beats from nuclei excited by synchrotron radiation.

We discuss the possibility of a new interference technique for x-ray and \ensuremath{\gamma}-ray optics\char22{}the simple idea of grazing-incidence antireflection films (GIAR films)\char22{}for creating high-efficiency antireflection coatings for near-grazing-incidence reflection of hard x rays and \ensuremath{\gamma} rays. The motivation is the possible application to producing ``ultranarrow'' bandpass filters for synchrotron radiation with frequency widths \ensuremath{\approxeq}${10}^{\mathrm{\ensuremath{-}}8}$\char21{}${10}^{\mathrm{\ensuremath{-}}6}$ eV, giving a unique high-resolution, long-coherence-length, x-ray source for probing soft inelastic excitations and opening up new possibilities in x-ray interferometry. In this first of two papers on nonresonant GIAR films, we develop the basic theory and discuss in detail the simplest ideas for achieving antireflection films\char22{}impedance-matched quarter-wave films and damping stabilized solutions\char22{}which can both be obtained by coating a high-density mirror with a single lower-density film.

/pdf/grazing-incidence-antireflection-films-i-basic-theory-2w87qy1b84.pdf

Grazing-incidence antireflection films. I. Basic theory

The time dependence of the coherent response of a $^{57}\mathrm{FeBO}_{3}$ single crystal excited by synchrotron-radiation (SR) pulses was studied in the pure nuclear (333) Bragg reflection near the N\'eel temperature, where the M\"ossbauer reflection spectrum consists of nearly a single line of almost natural linewidth. The time spectra revealed a delayed reemission of \ensuremath{\gamma} quanta into the Bragg direction starting at zero intensity. This behavior was determined by two factors: the coherent decay of the nuclear excited state and the interference of two nearly coinciding hyperfine components.

/pdf/time-spectra-of-a-nearly-single-line-pure-nuclear-reflection-3syfeqve2m.pdf

Time spectra of a nearly-single-line pure nuclear reflection excited by synchrotron radiation

The purpose of the Letter is to point out the possibility of a new interference technique in x-ray and $\ensuremath{\gamma}$-ray optics\char22{}the simple idea of impedance-matched grazing-incidence films\char22{}and to discuss applications to pure nuclear coherent reflections, resonant filtering of synchrotron radiation, and x-ray interferometry.

R. Rüffer

Papers

Nuclear Bragg diffraction of synchrotron radiation in yttrium iron garnet.

Quantum beats from nuclei excited by synchrotron radiation.

Grazing-incidence antireflection films. I. Basic theory

Time spectra of a nearly-single-line pure nuclear reflection excited by synchrotron radiation

Impedance-Matched Grazing-Incidence Films: Pure Nuclear Reflections, Resonant Filtering of Synchrotron Radiation, and X-Ray Interferometry