Using x-ray absorption spectroscopy recent progress is achieved all over in solid state physics. This review focuses on these advances, with particular emphasis on applications to surface physics and to magnetism of ultrathin 3d and 5d films that are made possible by the use of undulators in third generation synchrotron radiation sources: the unambiguous appearance of an atomic extended x-ray absorption fine structure for atomic adsorbates and of σ* resonances in near-edge x-ray absorption fine structure spectra of oriented molecules is demonstrated. The induced magnetism at the interfaces of 3d and 5d layers is studied by x-ray magnetic circular dichroism. Fundamental aspects of the spectroscopy are clarified for rare earth crystals. The determination of the ground state properties and the detailed understanding of the underlying mechanisms was obtained by comparison of the experimental data to state-of-the-art ab initio calculations.

https://qmlab.ubc.ca/ARPES/PEOPLE/Andrea/Teaching/PHYS555_2007/XASreview.pdf

Recent advances in x-ray absorption spectroscopy

Eight crystalline dipeptides were studied:  AV (Ala-Val), VA (Val-Ala), AI (Ala-Ile), VV (Val-Val), IA (Ile-Ala), IV (Ile-Val), VI (Val-Ile), and LS (Leu-Ser) (all LL isomers). The first seven form an isostructural series (space group P61), whereas LS has a different structure (P65). All structures display H-bonded tubular assemblies of the dipeptide molecules resulting in open ultramicropores in the form of isolated one-dimensional (1D) channels. The total porosity of the materials ranges from 4 to 12% (micropore volume from 0.04 to 0.12 cm3/g). Calculations based on the crystal structures, He pycnometry, and solid-state 129Xe NMR methods were used to obtain a comprehensive description of the geometry and properties of the micropores. The following order was established for the channel diameter:  AV > VA > AI > VV > IA > IV > VI, with >5 A for AV and <4 A for VI; LS is close to AI. The observed sorption behavior cannot be described adequately based on the crystal structure and can only be understood if o...

Micropores in Crystalline Dipeptides as Seen from the Crystal Structure, He Pycnometry, and 129Xe NMR Spectroscopy

Variable-temperature single-crystal X-ray experiments have been performed on the thiourea pyridinium chloride, bromide, and iodide inclusion compounds. Two phases were found for the chloride, three for the bromide, and two for the iodide. As previously reported for the bromide, at room-temperature all three halides were found to form orthorhombic crystals in space group Cmcm. For each compound, the pyridinium cation is disordered in the square cross section thiourea channels. On cooling, the chloride undergoes a phase change between 200 and 230 K to give an orthorhombic form in Pbca with the a axis doubled and the pyridinium ions partially ordered. The bromide undergoes a phase change between 170 and 165 K to a partially ordered form in space group Cmc21 with the a axis tripled. A second phase change occurs between 140 and 150 K to a fully ordered phase in space group P21cn with cell dimensions approximately the same as the room-temperature form. The iodide undergoes a single-phase transition at 120−130 K...

%Variable-Temperature Studies of Order/Disorder Transitions in the Thiourea Pyridinium Halide Crystals by XRD and Solid-State 2H NMR

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately. The dynamic picture emerging from these techniques is that the secondary relaxation process is related to the different molecular surroundings around each nonequivalent atom of the molecule. Dynamical heterogeneities thus seem to be the cause of the secondary relaxation in this simple model of glass.

Dynamic heterogeneity in the glass-like monoclinic phases of CBr(n)Cl(4-n), n = 0,1,2.

Orders of magnitude decrease of (207)Pb and (199)Hg NMR longitudinal relaxation times T1 upon magic-angle-spinning (MAS) are observed and systematically investigated in solid lead and mercury halides MeX2 (Me = Pb, Hg and X = Cl, Br, I). In lead(ii) halides, the most dramatic decrease of T1 relative to that in a static sample is in PbI2, while it is smaller but still significant in PbBr2, and not detectable in PbCl2. The effect is magnetic-field dependent but independent of the spinning speed in the range 200-15 000 Hz. The observed relaxation enhancement is explained by laboratory-frame heteronuclear polarization exchange due to crossing between energy levels of spin-1/2 metal nuclei and adjacent quadrupolar-spin halogen nuclei. The enhancement effect is also present in lead-containing organometal halide perovskites. Our results demonstrate that in affected samples, it is the relaxation data recorded under non-spinning conditions that characterize the local properties at the metal sites. A practical advantage of fast relaxation at slow MAS is that spectral shapes with orientational chemical shift anisotropy information well retained can be acquired within a shorter experimental time.

NMR longitudinal relaxation enhancement in metal halides by heteronuclear polarization exchange during magic-angle spinning

35 Cl NQR study of incommensurate state in thiourea-hexachloroethane inclusion compound

Atom-atomic potential calculation of the channel non-stoichiometric thiourea-hexachloroethane inclusion compound shows that the structure of the guest sublattice comprises two types of finite molecular chain, having different structure and separated by domain walls. In the present paper we present results of 35Cl NQR and 1H NMR measurements of thiourea-hexachloroethane, [2.95(NH2)2CS]C2Cl6, in the temperature range from 7.5 to 90 K, which confirm this model and show the existence of such a state at least below 60 K. Two resonances in the NQR spectra were assigned to the two nearly commensurate regions, while the third resonance, showing an anomalous behaviour, was attributed to the guest molecules in the domain wall. The observed structure results from the different periodicity of the guest and host substructures and shows a difference from conventional continuum models of the incommensurate state. Propagation motion of the domain wall over the channel is discussed.

Chlorine-35 NQR and 1H NMR study of guest structure and dynamics in the thiourea-hexachloroethane inclusion compound

The temperature dependence of the proton spin-lattice relaxation time (T
1) is investigated for channel urea clathrates with paraffin molecules. The results obtained are interpreted within the reorientational model of paraffin molecules and their fragments in clathrate channels. The specific features of the dynamics of normal paraffins in urea clathrates are associated with incommensurate regions in the structure of these compounds.

Investigation of the mobility of paraffin molecules in channel urea clathrates by nuclear magnetic resonance relaxation spectroscopy

We report on an NMR study of ultraslow motions in the channel thiourea-hexachloroethane inclusion compound, [295(NH 2 ) 2 CS]C 2 Cl 6  Temperature dependent 1 H NMR relaxation measurements of the powder thiourea-hexachloroethane at different resonance frequencies, from 23 to 555 MHz, have been carried out Significant reduction of the spin-lattice relaxation time at 38 and 47 MHz is caused by the cross-relaxation of protons via the quadrupole chlorine nuclei We show that the effective 1 H- 3 5 Cl cross-relaxation observed in a powder sample is due to a slow mode process, when the molecules of the domain wall exhibit a correlated translational and rotational motion over the channel Such propagation of the domain wall is confirmed by atom-atomic potential calculation

http://znaturforsch.com/aa/v57a/s57a0395.pdf

NMR Cross-Relaxation Study of Ultraslow Motion of the DomainWall in Channel Inclusion Compound

Specific heat and Cl nuclear quadrupole resonance (NQR) measurements of the channel thiourea-hexachloroethane inclusion compound are presented. Experimental NQR data confirm the results of atom-atom potential calculations that the guest sublattice comprises two types of nearly commensurate finite molecular chains, having different structure and separated by domain walls. Specific heat measurements show phase transition near 96 K.

/pdf/specific-heat-and-nuclear-quadrupole-resonance-study-of-2g8z0fkfg6.pdf

G. N. Chekhova

Papers

35 Cl NQR study of incommensurate state in thiourea-hexachloroethane inclusion compound

Chlorine-35 NQR and 1H NMR study of guest structure and dynamics in the thiourea-hexachloroethane inclusion compound

Investigation of the mobility of paraffin molecules in channel urea clathrates by nuclear magnetic resonance relaxation spectroscopy

NMR Cross-Relaxation Study of Ultraslow Motion of the DomainWall in Channel Inclusion Compound

Specific Heat and Nuclear Quadrupole Resonance Study of Thiourea-Hexachloroethane Inclusion Compound