Ultra-fast yttrium hydride chemistry at high pressures via non-equilibrium states induced by x-ray free electron laser

Controlling the formation and stoichiometric content of desired phases of materials has become a central interest for the study of a variety of fields, notably high temperature superconductivity under extreme pressures. The further possibility of accessing metastable states by initiating reactions by x-ray triggered mechanisms over ultra-short timescales is enabled with the development of x-ray free electron lasers (XFEL). Utilizing the exceptionally high brilliance x-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with non-stoichiometric changes in hydrogen content as probed at a repetition rate of 4.5\,MHz using time-resolved x-ray diffraction. Exploiting non-equilibrium pathways we synthesize and characterize a hydride with yttrium cations in an \textit{A}15 structure type at 125\,GPa, predicted using crystal structure searches, with a hydrogen content between 4.0--5.75 hydrogens per cation, that is enthalpically metastable on the convex hull. We demonstrate a tailored approach to changing hydrogen content using changes in x-ray fluence that is not accessible using conventional synthesis methods, and reveals a new paradigm in metastable chemical physics

Magnetotransversaler Phononentransport

In analogy with Ohm s law for the electrical conductivity, diffusive transport of heat can be described by a linear relation between the resulting heat current and the original gradient. In 1931 Onsager showed, that the thermal conductivity tensor may contain an antisymmetric contribution, induced by a magnetic field. This leads to a heat current perpendicular to both, the original gradient and the applied magnetic field. In a confined sample geometry, this transverse current is balanced by a transverse temperature difference. in metals the corresponding effect has been known for a long time: The Righi-Leduc-effect is caused by the electronic contribution to the thermal conductivity. Later, a magneto-transverse thermal conductivity of paramagnetic molecular gases, the so called Senftleben-Beenakker-effect was observed. The effect is due to an anisotropic scattering cross-section of the diffusing gas molecules responsible for the conduction of heat.As there is no net charge associated with phonons no such effect has been expected for the phonon thermal conductivity. But phonons describe the collective motion of particles carrying charge and spin and are thus influenced by a magnetic field. The observation of the magneto-transverse diffusion of light gives a clue on how such an effect might be realized in the diffusion of classical waves.To our knowledge there is no investigation of the magneto-transverse thermal conductivity reported in the scientific literature. In the diploma-thesis of the author, weinvestigated this new effect in diamagnetic samples, however without an unambiguousobservation of the effect.In this thesis we investigated the effect in paramagnetic materials in which largeeffects are expected due to the resonant scattering of phonons.In the theoretical introduction we argue, that an antisymmetric contribution to the thermal conductivity tensor is thermodynamically allowed, using Onsagers relations for diffusive transport. We show that this contribution will lead to a transverse temperature difference in confined sample geometries. Today there is no theory on a microscopic realization of the effect. We therefore recur on a comparison of phonon sand photons. After a review of the results of Rikken and Tiggelen for the optical case, we therefore worked out the analogies between photons and phonons and presented useful relations for the choice of samples and the understanding of the experiments.In the experimental part we presented the techniques for the measurement of the thermal conductivity as a function of temperature and magnetic field. We specially focused on the setup for the measurement of the magneto-transverse thermal conductivity with ultra high resolution. we analyzed the setup and the protocols to identify possible artifacts.All measurements presented in this thesis have been performed on samples of Terbium-Gallium-Garnet. The sample has been specified by measuring the longitudinal thermal conductivity as function of temperature and magnetic field. The analysis of the low temperature part of the thermal conductivity shows that scattering mean free path is dominated by scattering at point defects and by resonant scattering. From the magnetic field dependence of the thermal conductivity on can conclude on a strong spin phonon coupling which makes the material suitable for an investigation of the magneto-transverse thermal conductivity.We present a complete data set fulfilling all criteria for a phenomenological observationof the effect. A detailed analysis of possible artifacts supports the significanceof the data

On a hyperfine interaction in ε-Fe

We explore alternative ways to Mössbauer spectroscopy such as nuclear forward scattering of synchrotron radiation, and synchrotron radiation perturbed angular correlation spectroscopy to reveal the elusive and long-sought hyperfine interactions in ε-Fe. We indicate that synchrotron radiation perturbed angular correlation spectroscopy is the most viable method

Nuclear inelastic scattering and density functional theory studies of a one-dimensional spin crossover [Fe(1,2,4−triazole)2(1,2,4−triazolato)](BF4)\mathrm{[Fe(1,2,4-triazole)_{2}(1,2,4-triazolato)](BF_{4})} molecular chain

Nuclear inelastic scattering (NIS) experiments have been performed in order to study the vibrational dynamics of the low- and high-spin states of the polynuclear 1D spin crossover compound [Fe(1,2,4-triazole)$_2$(1,2,4-triazolato)](BF$_4$) (1). Density functional theory (DFT) calculations using the functional B3LYP* and the basis set CEP-31G for heptameric and nonameric models of the compound yielded the normal vibrations and electronic energies for high-spin and low-spin isomers of three models differing in the distribution of anionic trz$^−$ ligands and BF$_{4}^−$ anions. On the basis of the obtained energies a structural model with a centrosymmetric Fe(trzH)$_4$(trz$^−$)$_2$ coordination core of the mononuclear unit of the chain is proposed. The obtained distribution of the BF$_{4}-$ counteranions in the proposed structure is similar to that obtained on the basis of X-ray powder diffraction studies by Grossjean et al. (Eur. J. Inorg. Chem., 2013, 796). The NIS data of the system diluted to 10% Fe(II) content in a 90% Zn(II) matrix (compound (2)) show a characteristic change of the spectral pattern of the low-spin centres, compared to the low-spin phase of the parent Fe(II) complex (1). DFT calculations reveal that this is caused by a change of the structure of the neighbours of the low-spin centres. The spectral pattern of the high-spin centres in (2) is within a good approximation identical to that of the high-spin Fe(II) isomer of (1). The inspection of the molecular orbitals of the monomeric model systems of [Fe(trzH)$_4$(trz$^−$)$_2$] and [Fe(trzH)$_6$], together with calculations of spin transition energies, point towards the importance of an electrostatic effect caused by the negatively charged ligands. This results in the stabilisation of the low-spin state of the complex containing the anionic ligand and shortening of the Fe–N(trz$^−$) compared to the Fe–N(trzH) bond in high-spin, but not in low-spin [Fe(trzH)$_4$(trz$^−$)$_2$]

Coherent control of collective nuclear quantum states via transient magnons

Ultrafast and precise control of quantum systems at x-ray energies involves photons with oscillation periods below 1 as. Coherent dynamic control of quantum systems at these energies is one of the major challenges in hard x-ray quantum optics. Here, we demonstrate that the phase of a quantum system embedded in a solid can be coherently controlled via a quasi-particle with subattosecond accuracy. In particular, we tune the quantum phase of a collectively excited nuclear state via transient magnons with a precision of 1 zs and a timing stability below 50 ys. These small temporal shifts are monitored interferometrically via quantum beats between different hyperfine-split levels. The experiment demonstrates zeptosecond interferometry and shows that transient quasi-particles enable accurate control of quantum systems embedded in condensed matter environments

Revealing the Hidden Hyperfine Interactions in ϵ-Iron

Herein, evidence for the long-sought finite hyperfine interaction in the high-pressure hexagonal close-packed ε-iron is gained through synchrotron radiation perturbed angular correlation spectroscopy. This method yields an energy splitting of 3.5(5)neV between the mIe= ± 1/2 and mIe = ± 3/2 nuclear sublevels of the iron-57 14.412-keV nuclear excited state at 30(1)GPa and room temperature. This energy splitting is related to a nuclear quadrupole hyperfine interaction with an electric field gradient of eq=1.2(2) x 1016V/cm2. However, there is still a possibility that the splitting of the iron-57 nuclear levels is related to a modest magnetic hyperfine interaction of ca. 0.40(5) T