Synthesis and Luminescence of Ba2ScCl7

Physics of the Solid State - New results on luminescence in Ba2ScCl7 are reported. Lanthanides Eu2+ and Ce3+ were chosen as activators. The phosphors could be prepared by a simple wet chemical...     

The Systematics of Muonium Hyperfine Constants

The hyperfine constants for muonium in elemental and binary inorganic solids suggest formation of three different families of defect centre, with distinct electronic structures. The overall range of values, spanning nearly five orders of magnitude, and their correlation with host properties such as band gap and electron affinity, reveal a deep-to-shallow instability which has profound implications for the electrical properties of hydrogen impurity in electronic materials, both semiconducting and dielectric.     

Lattice dynamics and hydrostatic-pressure-induced phase transitions in ScF3

New phase transitions induced by hydrostatic pressure in a cubic (under standard conditions) ScF₃ crystal are discovered by the methods of polarization microscopy and Raman scattering. The space groups $R\bar 3c$ for Z=2 and Pnma for Z=4 are proposed for the high-pressure phases. A nonempirical computation of the lattice dynamics of the crystal is carried out. It is shown that, under normal pressure, the cubic phase is stable down to T=0 K, while the application of a hydrostatic pressure gives rise to a phonon branch in the vibrational spectrum (between points R and M of the Brillouin zone) with negative values of squares of frequencies. The condensation of soft mode R₅ at the boundary point of the Brillouin zone leads to rhombohedral distortion of the cubic structure with the unit cell volume doubling. The calculated frequencies at q=0 of the ScF₃ lattice in the distorted rhombohedral phase are real-valued; the number and position of frequencies active in Raman scattering are in accord with the experimental values.     

Hyperfine spectrum of NaF

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of ²³Na¹⁹F. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and their dependence on vibrational and rotational state have been determined.     

Hyperfine spectra of KBr and KI

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectra of ³⁹K⁷⁹Br, ³⁹K⁸¹Br and ³⁹K¹²⁷I. Coupling constants for the nuclear electric quadrupole interactions, the spin–rotation interactions, the tensor and scalar spin–spin interactions, as well as the electric dipole moment of KI, and their dependence on vibrational and rotational state have been determined. A few transitions observed for ⁴¹K¹²⁷I show a small shift in the iodine nuclear electric quadrupole interaction, and the fit improves somewhat with the inclusion of an iodine nuclear electric hexadecapole interaction term.     

Absorptionsspektren von Er3+ in InCl3 und in ScCl3

The optical absorption spectra of Er³⁺ in single crystals of InCl₃ and ScCl₃ have been obtained between 14,000 and 29,000 cm^?1. The observed crystal field splittings are interpreted in terms of crystal potential of symmetry 3m for ScCl₃ and of symmetry 2m for InCl₃. There is however found, that for both cases the splittings will be approximately explained by a crystal potential of cubic symmetry. The employed hostlattices are of special interest, because the ions of the iron series can occupy the same sites as the rare earth ions.     

Hyperfine spectrum of RbI

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of RbI. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and their dependence on vibrational and rotational state have been determined.     

The Hyperfine Transition for the Definition of the Second

The unit of time of the International System of Units (SI), the “atomic second” was defined through a constant of physics in 1967. It is derived from the frequency of the hyperfine transition of the atom of cesium 133. From the astronomical definition of the second until today, the accuracy of the realization of the second has improved by eight orders of magnitude, with a rate that has increased since the development of the cesium frequency standards, to reach parts in 10¹⁶ for the best clocks today. In 2018, when the new SI was adopted, the time metrology community proved that a new generation of frequency standards operating in optical wavelengths has uncertainties at the level of 10^–18, and challenge the implementation of high accurate frequency and time comparison techniques to decide on a revision of the definition of the second. Herein, the progress in the definition and realization of the second from astronomy until today is reviewed, an inventory of the present resources is assembled and a brief view for the future given.     

Structure and lattice dynamics of the high-pressure phase in the ScF3 crystal

The high-pressure phase of the ScF₃ crystal has been studied using synchrotron radiation diffraction and Raman scattering. This phase existing in the pressure range 0.6–3.2 GPa is optically anisotropic: its structure is described by space group R $ \bar 3 $ \bar 3 c, Z = 2, and the transition is associated with the rotation of ScF6 octahedra around the threefold axis. The pressure dependences of the lattice parameters and the rotation angle have been determined. The number of lines in the Raman spectrum corresponds to the expected number for this structure; the recovery of soft modes has been observed above the phase transition.     

Hyperfine splitting in positronium and muonium

We calculate corrections to the ground state hyperfine structure of relative order α² ln α^?1 in positronium and relative order $\text{(}\text{m}{}_{\mathrm{<mtext>e</mtext>}}\text{m}{}_{\mathrm{\mu }}\text{)α}{}^2\text{lnα}{}^{\mathrm{?1}}$ in muonium due to the exchange of virtual photons. Our results are in agreement with those of Lepage. Contributions arising from the Coulomb potential and from the exchange of one transverse photon along with any number of ladder Coulomb photons are discussed in detail. In treating the single transverse photon-multiple Coulomb photon exchanges, we sum the contributions involving different numbers of Coulomb photons and reexpand the resulting expression in terms of a quantity that is inherently smaller than the Coulomb potential in the non-relativistic region. The procedure enables us to take into account from the beginning important cancellations that occur between the various terms in an expansion in powers of the Coulomb potential. The techniques developed here may be useful in calculating higher order corrections.     

Structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3

Cubic scandium trifluoride (ScF3) has a large negative thermal expansion over a wide range of temperatures. Inelastic neutron scattering experiments were performed to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25 meV. Phonon calculations with first-principles methods identified the individual modes in the densities of states, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO9 structure of ScF3, and accounts for phonon stiffening with the temperature and a significant part of the negative thermal expansion.     

Hyperfine fields at surfaces and interfaces

Magnetism and hyperfine fields at transition metal surfaces are discussed using state-of-the-art local spin density methods. Emphasis is placed on recent results obtained for the Fe(001), Ni(001), Cr(001) and Ag/Fe(001) ferromagnetic surfaces, and for the Knight shift in Pt(001).     

Hyperfine interactions and nuclear probes in chemistry: The active interface

A symposium entitled “Hyperfine Interaction and Nuclear Probes in Chemistry” was held in conjunction with the 198 th. National Meeting of the American Chemical Society in Miami Beach, Florida, 12 and 13 September 1989. The four half-day sessions consisted of 15 invited and 4 contributed papers, and allowed numerous opportunities for spirited discussion and information exchange, especially at the informal luncheons and pre-dinner periods, and Miami Beach proved to be a most effective venue for these activities. In the pages to follow are collected a number of the scientific reports presented at this symposium; other contributions will be published elsewhere at the discretion of the author(s).     

Hyperfine interactions-un peu d'histoire

The history of hyperfine interactions is outlined from the first observations by optical spectroscopy, through magnetic resonance in the solid state and in atomic beams, to the use of lasers. For lanthanide atoms and ions, a number of hyperfine structure corrections are discussed, with particular reference to the problem of <r ⁻³>. The various contributions to the nuclear electric quadrupole interaction are reviewed, including shielding effects. Results deduced from electronic spectra are compared with those involving the use of negative muons.     

Hyperfine structure of lithium-like scandium

The hyperfine structure of the ground state of the lithium-like scandium ion (Z = 21) is considered. The effects of electron-electron interaction are taken into account in the first order in 1/Z using the quantum electrodynamic perturbation theory and in higher orders using the method of configuration interaction in the Dirac-Fock-Sturm basis. One-loop radiative corrections are calculated in all orders in αZ with an effective local screening potential. The corrections for the finite nuclear size and the distribution of the magnetic moment over the nucleus have been refined.     

Franck-Condon Factors and R-Centroids for ScF,YF, and LaF Including Rotation-Vibration Effect

We have calculated Franck-Condon factors and R-centroids for several singlet systems of ScF, YF, and LaF, taking into account the effect of rotation-vibration interaction. It has been found that this effect is far from being negligible for several cases.     

Hyperfine fields and magnetic properties of barium monoferrite

First we studied the kinetics of the formation of Ba monoferrite by means of Mössbauer spectroscopy. The most important result is the considerable portion of hexaferrite existing after treatment of the starting materials between 800 and 900°C, caused by a higher rate of diffusion of the Ba²⁺ ions in comparison to the Fe³⁺ ions. Second we report susceptibility measurements recorded in a temperature range of 4.3 K to 280 K and Mössbauer spectra at room temperature and at 4.2 K without and in the presence of an applied magnetic field of 6 T. The experimental data demonstrate that the magnetic behaviour of Ba monoferrite deviates to some extent from a simple antiferromagnetic.