Paramagnetic Centres in Be-Doped MgO Single Crystals

Using a variation of arc-fusion technique, Be-doped MgO single crystals were grown, in which about 0.01% of the cation sites are occupied by Be^2?. This gives rise to a variety of Be-containing paramagnetic centres, easily detectable by EPR. The models of the centres are proposed and the values of their spin-Hamiltonian parameters are determined and discussed. Two of them—V_OH-Be and H-Be centres—stem from well-known paramagnetic centres such as V_OH, and interstitial H atom. In addition, because of the non-central position of the ion, an isolated Be^2? can trap a hole forming a Be^2?O^? centre. The symmetry of the Be^2?O^? centre at T<30K is rhombic, at a higher temperature a motional averaging of the spectrum takes place. It is shown that unusually for V centres physical properties of the V_OH-Be centre (a relatively small g-factor anisotropy and high thermal stability, optical absorption energy and spin-lattice relaxation time) arc caused by the non-central position of the Be^2? ion.     

Behaviour of F and Fz Centres Under Thermal Stimulation in KCl:Eu2☎ Irradiated with Ionizing and UV Radiation

The optical absorption (OA) of F and F_z, centres in beta-and UV-irradiated KCl: Eu^2? have been studied. The spectra measured at room temperature and during the subsequent heatings up to 600 K, were analyzed by a deconvolution procedure. The OA bands of the F and F_z centres are well described respectively by asymmetric and symmetric gaussiane with parameters that are linearly dependent on the temperature. The thermal stability of the F_z centres correlates with the low-temperature thermostimulated luminescence (TL) peaks. The stability of the F centres is determined at least by two processes. Part of the F centres is destroyed together with the F_z centres. The remaining ones are destroyed during further heating up to 470 K, correlating with the TL peak at 425 K (at heating rate of 0.1 K/s).     

Investigations on the g factors of the ground and excited states for Cu2+ ions in ZnO crystal

The g factors g _// and g _⊥ of the ground Γ₆(² T ₂) and excited Γ_4,5(² E), Γ₆(² E) states for trigonal Cu²⁺ centres in ZnO crystals are calculated from three theoretical methods, the complete diagonalization (of the energy matrix) method, the second-order perturbation method (PTM-I) and the simplified second-order perturbation method (PTM-II, this method was described in an earlier paper). These methods are based on the cluster approach in which the spin-orbit coupling parameters ζ, ζ′ and the orbital reduction factors k, k′ are calculated from a semi-empirical molecular orbital method. The crystal-field parameters used in the calculations are obtained from the superposition model and so the defect structure of Cu²⁺ centres in ZnO can be acquired. The calculated g factors from the three methods are in reasonable agreement with the experimental values and the defect structure of Cu²⁺ centres in ZnO is acquired. It appears that in some cases the approximate PTM can be applied in the studies of g factors of various states. The conditions that the PTM are ineffective are discussed.     

EPR Study of Nd3☎ Ions in β-BaB2O4 Single Crystals

An Electron Paramagnetic Resonance (EPR) study of Nd^3? ions in single crystals of the low-temperature phase of barium borate (β-BaB₂O₄ or BBO) is presented. The EPR spectra show the existence of a minimum of three different Nd^3? centres. The g-matrices for three of the centres have been determined from the analysis of the angular dependencies of the spectrum in two planes of the crystal. This study allows us to conclude that neodymium is incorporated in the low-symmetry Ba^2? site. Possible reasons for the appearance of various Nd^3? centres are discussed.     

Magnetic circular dichroism in the absorption of theF A centre in KCl doped with Li and Na

Summary The spin-orbit structure ofF _A centres in KCl:Li and KCl:Na have been studied by means of the magnetic circular dichroism. Due to theirC _4V symmetry theF _A centres have two different spin-orbit parameters, Δ^‖ and Δ^⊥, which only in the KCl:Li case follow the relation: Δ^‖ < Δ_F < Δ^⊥ as expected from the theory. For a close comparison we have also reported our measurement of the spin-orbit coupling ofF centre in KCl. The spin-orbit parameters of theF andF _A centres have been determined using the method of moment.     

Theory studies of the local lattice distortions and the EPR parameters for Cu2+, Mn2+ and Fe3+ centres in ZnWO4

The local lattice distortions and the electron paramagnetic resonance (EPR) parameters (g factors, hyperfine structure constants and zero-field splittings) for Cu²⁺, Mn²⁺ and Fe³⁺ in ZnWO₄ are theoretically studied based on the perturbation calculations for rhombically elongated octahedral 3d⁹ and 3d⁵ complexes. The impurity centres on Zn²⁺ sites undergo the local elongations of 0.01, 0.002 and 0.013 Å along the C₂ axis and the planar bond angle variations of 8.1°, 8.0° and 8.6° for Cu²⁺, Mn²⁺ and Fe³⁺, respectively, due to the Jahn–Teller effect and size and charge mismatch. In contrast to the host Zn²⁺ site with obvious axial elongation (～0.31 Å) and perpendicular (angular) rhombic distortion, all the impurity centres demonstrate more regular octahedral due to the above local lattice distortions. The copper centre exhibits significant Jahn–Teller reductions for the spin-orbit coupling and orbital angular momentum interactions, characterised by the Jahn–Teller reduction factor J (≈0.29 ? 1). The calculated EPR parameters agree well with the experimental results. The local structures of the impurity centres are analysed in view of the corresponding lattice distortions.     

Magnetische Monopole und Quarks

We discuss the concept of Schwinger, which starts with the hypothesis of the existence of magnetical monopoles and results in a baryon model with magnetically charged constituents. Especially we analyse the mathematical consistency of such a theory. which admits a connection between some magnetically charged “quarks” and the homogeneous Maxwell-equations ?_v*F^μv(x^u) = 0, which, displaying a lack of symmetry with respect to the inhomogeneous one, ?_vF^μv(x^u) = 4?j^μ, are replaced by ?*j^μv. Here *j^μ(x^μ) means a conserved magnetic current which provides a monopole source for the magnetic field.     

The influence of different forms of surface oxide on the fast states of germanium

The alternating field effect was used to study the kinetics of the influence of water molecules on the fastN _t centres on the surface of germanium. It was observed that the time dependence in the adsorption of water and in the change in concentration of the fast centres caused by this adsorbed water differs, indicating that the water diffuses through the oxide to the fast centres with a diffusion constantD approximately equal to 10^–15-10^–14 cm²/sec. The magnitude of the diffusion constantD constantly increased after annealing the sample in a wet atmosphere, and temporarily decreased after annealing in a dry atmosphere. The influence of annealing on the concentration of the fast states and the penetrability of the water through the oxide layer of germanium was also studied.The author thanks S. Koc, C.Sc., for advice and the interest he showed in the work.     

Mössbauer effect study of the crystal chemistry of tourmaline

One of the main features of the tourmaline group of minerals is that the six coordinatedY andZ sites, possessing different point group symmetries and sizes, share edges. The ME parameters are, therefore, controlled by the mechanisms by which the tourmaline structure obtains congruency along the common edge between the two octahedra. For example, if the size of theY site in elbaite is increased by replacing Li and Al by Fe²⁺, then, for small values of Fe²⁺, size congruency is maintained by simultaneously replacing Al by Fe²⁺ at theZ site. The ME spectra of Fe-bearing elbaite are therefore characterised by two doublets assigned to Fe²⁺ in theY andZ sites. For higher values of Fe²⁺ at theY site, however, the tourmaline structure reduces the misfit further by oxidation of Fe²⁺ at theY site. The effect of the second mechanism is to increase Fe²⁺-Fe³⁺ interaction centres and thereby promote charge delocalisation. These Fe-Fe charge interactions introduce three more peaks with ME parameters intermediate between Fe²⁺ and Fe³⁺. On the other hand, the replacement of Fe²⁺ by Mg²⁺ reduces the density of Fe²⁺-Fe³⁺ centres, blocks Fe-Fe charge interactions and increases the intensity of Fe³⁺.     

Ramanspektren von ultramikroskopischen Streuzentren in LiF- und NaCl-Einkristallen

Stokes-Ramanspectra of unltramicroscopic scattering centres in LiF- and NaCl single crystals excited by the focussed light of a He-Ne-gaslaser of 21.4 mW at 632.8 nm are reported here for the first time. The spectra exhibit essentially three parts: 1. a continuum and the limiting frequency of transversal optical phonons at small frequency shifts as a first order Raman effect; 2. second order Raman effect not yet measured for LiF; 3. a strong continuum at larger wavenumbers that may be attributed to three phonon processes. This latter phenomenon becomes plausible when the high field strangth of 5×10⁵ volt/m in the closest constriction of the exciting light beam and the substantial lattice deformations in the scattering centres are considered. The spectra of LiF are by far more intensive than those of NaCl.     

Thermal decay of stable colour centres in Gd3Ga5O12 crystals

This work is devoted to an analysis of thermal decay processes of the stable colour centres (CC) created in gadolinium gallium garnet (GGG) Gd₃Ga₅O₁₂, crystals under irradiation by gamma-quanta (E=1.25 MeV, D=10⁶ Gy). An analysis of the model of CC accumulation under irradiation as well as an isochronous heating model is presented. The decay activation energy (Δ E) for O^? and F centres in GGG crystals is calculated in accordance with this model.     

Die EINSTEIN-Gruppe und die Symmetrie-Gruppen der Gravitationstheorie

The transformations x^?α = x^?α(x^β, g_μn?) in the function space of the g_μn?(x^λ) are corresponding to the coordinate transformations x^α = x^α(x^β) with some non-covariant conditions on the g_μn?(x^λ). Therefore, the transformations in the function space are corresponding to subgroups of the EINSTEIN group. The conditions for the g_μn? may be given in the space- time V₄ or on submanifolds (points, curves, surfaces and hypersurfaces) of the V₄. – A special case of the last problem is given by the CAUCHY conditions or by the DIRAC constraints for a special choice of the coordinates on a CAUCHY hypersurface x⁰ = 0. Then, the transformations x^?α = x^?α(x^l, g_rs, p^mn) in the phase space are EINSTEIN transformations preserving the synchronicity for x⁰ → 0.     

A study of lattice defects in alkali fluorides by thermoluminescence experiments

Summary Lattice defects in KF, NaF and LiF are investigated by recording the thermoluminescent emission of X-irradiated samplesvs. temperature and wavelength. Experimental results show the presence of cation vacancies and interstitials in KF and NaF and of F⁽⁻⁾ vacancies and interstitials in NaF and LiF. In NaF, the ionization energy of V_F centres is found to be 6.4 eV, that of H centres 5.6 eV.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Über Zusammenhänge zwischen Elektronenspinresonanz,elektrischer Leitfähigkeit und Photoleitung bei reinem und dotiertem Bor

The conductivity mechanism in pure and doped, β-rhombohedral, polycrystalline boron between 1.5 °K and 900 °K is clarified by measurements of electrical conductivity, photoconductivity, electron paramagnetic resonance and thermoelectric effect. The semiconductor behaviour of boron between 1.5 and 900 °K is similar to that of doped and compensated germanium and silicon at helium temperatures concerning the temperature-independent number of carriers and the thermally activated conduction process at low and high carrier concentrations. The paramagnetic centres are nearly localized electrons at 1.5 °K and nearly free electrons at 900 °K with a continuous transition between these two extreme kinds of behaviour. Mobilities of charge carriers in carbon doped boron over a range from 10¹⁶ cm^?3 to 10²⁰ cm^?3 and 77 °K to 900 °K were measured for the first time and were found to obey an exponential law.     

Thermal hopping ofμ + between “FμF” centres in NaF

Strongly hydrogen-bonded diamagnetic “FμF” centres are formed by theμ ⁺ in a wide variety of fluoride crystals. Hydrogen atoms are expected to form similar “FHF” complexes. Through the “motional narrowing” of the zero-field muon relaxation function in NaF, we have observed an Arrhenius temperature dependence of the dissociation rate of theFμF complex, yielding a binding energy of 1700 (200) K for theμ ⁺ in theFμF centre.     

Erhaltungssätze und Topologie

Conservation Laws and Topology It is shown, that for every conserved matter tensor T^ν_μ there exists a frame of reference h^a_μ, orthonormalized in a given gravitational field, such that the components h^a_νT^ν_μ are four conserved currents. The definition of global energy-momentum connected with these currents contains as special causes the definition of inertial frames in Minkowski space as well as the definition of energy in a comoving system in presence of Killing vectors in general relativity. Given on the other hand four closed 3-forms in a space-time with non-trivial topology, one can introduce an orthonormal frame of reference h^a_μ in such a way, that the space-time- components of the 1-forms, dual to the given 3-forms, define a symmetric matter tensor, which generates according to Einstein's equations the gravitional field. This means a partly topological definition of matter, since the non-trivially closed part of the 3-forms is determined by the topology of the manifold.     

New aspects of the luminescence of magnesiumtitanate part II: Activation with manganese

Measurements of the emission and excitation spectra of powdered Mg₂TiO₄:Mn⁴⁺ phosphors reveal that single Mn⁴⁺ ions are not responsible for the luminescence — as assumed till now — but so-called N centres. These centres are excited either by direct UV absorption or by radiationless energy transfer from the excited ²E state of the Mn⁴⁺ ions to the N centres. The Mn⁴⁺ ions absorb the energy corresponding to the transition ⁴A_2g→ ²E which is used for the excitation of the N centres, but do not emit the corresponding lines. The observed different luminescence spectra (sharp lines or broad bands) depend on the annealing conditions and indicate that the N centres consist of Mn⁴⁺ ions associated with unknown lattice defects. The annealing process does not only form the tetravalent state of the manganese, but creates above all the N₁ centres, which emit sharp lines. Two more lines observed at 697.8 nm and 699.4 nm, called R^m lines, are due to a certain amount of MgTiO₃:Mn⁴⁺ in the phosphor.     

Thermally stimulated luminescence and electron paramagnetic resonance studies of actinide doped calcium chloro phosphate

Electron paramagnetic resonance [EPR] and thermally stimulated luminescence [TSL] studies were conducted on self [α]-irradiated²³⁹Pu doped calcium chloro phosphate andγ-irradiated²³⁹Pu/²³⁸UO ₂ ²⁺ doped calcium chloro phosphate to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain trap parameters from both TSL and EPR data. TSL glow peaks around 135 K (# peak 1), 190 K (# peak 2), 435 K (# peak 5) and 490 K (# peak 7) were observed and their spectral characteristics have shown that Pu³⁺ and UO ₆ ⁶⁻ act as luminescent centres in calcium chloro phosphate with respective dopants. EPR studies have shown the formation of the radical ions H⁰, PO ₄ ²⁻ , O⁻, O ₂ ⁻ and [ClO]²⁻ under different conditions. Whereas the [ClO]²⁻ radical being stable up to 700 K, was not found to have any role in TSL processes, the thermal destruction of other centres was found to be primarily responsible for the TSL peaks observed. The trap depth values were determined both by using the TSL data and also the temperature variation of EPR spectra of these centres.     

EPR of radiation induced MoO 4 3- -centres in lead tungstate

Paramagnetic MoO ₄ ^3- impurity centres are created in undoped PbWO₄ single crystals by X-ray excitation at liquid nitrogen temperature. The Spin-Hamiltonian data of the axial electron-centre are discussed in a covalent model which takes into account contributions of cation states. Characteristic differences in EPR- and thermoluminescent features of PbWO₄ with respect to other scheelite type tungstates can be explained in terms of the lead ion 6s ²-configuration.Part of thesis, Giessen 1977 (D26)