Spin lattice relaxation of F+ centers in neutron irradiated cao crystals

Abstract

We have measured the electron spin-lattice relaxation rates of F⁺ centers in CaO crystals at different F⁺ centers concentrations (from 5.8·10¹⁵ to 1.4·10¹⁷ cm^?3) in magnetic fields from 10^?3 to 4.8 T and temperatures from 0.4 to 4.2 K. At temperatures above 1 K the relaxation rate is nearly proportional to the F⁺ centers concentration. Spin diffusion to fast relaxing centers is essential in determining the F⁺ centers spin polarization decay.     

EPR investigation of local paramagnetic centers in perovskite-like crystals

In ScF₃ single crystals (pure and doped) as well as in Rb₂KScF₆ and Rb₂KDyF₆ crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F^? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF₃ crystal under uniaxial pressure.     

The dependence of EPR linewidth on concentration in Mn/MgO single crystals

The EPR spectra of Mn/MgO single crystals grown by electrofusion have been examined at 9 GHz for manganese concentration of between 840 and 2900 ppm. The values of the spin Hamiltonian parameters deduced agreed well with those previously reported by Low (1957, 1958) and by Matarrese and Kikuchi (1956). The variation of linewidth with manganese concentration was also examined and compared with the predictions of the de Biasi and Fernandes theory of dipolar broadening in magnetically dilute systems; the peak-to-peak linewidth is concentration dependent and is of the right order of magnitude to be explained adequately in terms of this model. The results indicate that clustering, effects are absent in these Mn/MgO single crystals.     

Trapped-hole centers in MgO single crystals

The properties of the majority trapped-hole centers in MgO, such as g-factors, positions of absorption and luminescence bands, and temperatures of thermal destruction, have been analyzed with the emphasis on the observed regular trends and interrelations between the properties of these centers. Particular emphasis has been placed on the positively charged [Be]⁺ and [Ca]⁺ trapped-hole centers, which have a large cross section for recombination with conduction electrons. In these centers, a hole is localized at an oxygen ion near the impurity Be²⁺ or Ca²⁺ ion located at a regular cation site. The generation and transformation of defects due to the recombination of either relaxed conduction electrons with OH⁻-containing hole centers or cold and hot electrons with [Be]⁺ and [Ca]⁺ centers have been considered. Using the interrelation of the characteristics of hole centers and taking into account that the recombination emission band revealed at ∼6.8 eV is due to the Ca²⁺-containing centers that are stable below 50 K, the prospects for the EPR detection of the [Ca]⁺ center at T < 4.2 K have been discussed.     

The optical and paramagnetic absorption spectra of MgO:Mn single crystals irradiated in a reactor

The optical and paramagnetic absorption spectra of MgO:Mn crystals of different origin, irradiated in a reactor, have been investigated. It is found that, in spite of only a small amount of Mn²⁺ impurity ions in the investigated crystals, their influence on the efficiency of formation and accumulation of F⁺-centers is dominant. The dependence of the number of F⁺-centers both on irradiation dose and on manganese concentration is studied.     

EPR of ultraviolet irradiated lithium hydride crystals

Results are presented of EPR measurements on lithium hydride single crystals at low temperatures. Loose aggregates of F-centers are formed during u.v. irradiation. The F-center production is interpreted as an excitonic process where the interstitials are stabilized as H₂-molecules.     

Ion bombardment-induced charge state effects CaO single crystals: F+ and F centers

The effects of heavy-and light-ion bombardment on defect formation in CaO have been investigated by UV-absorption spectroscopy and volume measurements. While 500 keV Ar or Ca implantation produces only F⁺ centers, 240 keVH produces both F⁺ and F centers at a F⁺ to F ratio of 5.6 to 1. On the other hand, when an argon implanted sample is subsequently bombarded with hydrogen, about 30% of the F⁺ centers anneal during 1 ×10¹⁴ H/cm²; at higher H fluences, new F⁺ and F centers are produced. An effect of energy partition between ionization and nuclear/atomic collision processes for the incident ions on the charge state of the resulting defect is thus clearly demonstrated.

The formation and annealing of these defects are accompanied by volume changes in the ion implanted surface layer which can be monitored in sltu with a cantilever beam technique. The measurements show volume expansion of the order of 1.5% following 10¹⁶ 500 keV Ar implantation; subsequent implantation of 10¹⁸ 240 keV H compacts the previously expanded material by 25 %. These results are in qualitative agreement with the optical data and seem to indicate that volume changes are associated with the formation and annealing of F⁺ centers.     

Effect of uniaxial pressure of forbidden transitions in the EPR spectrum of Mn2+ in MgO

An analysis is made of forbidden transitions in the EPR spectrum of Mn²⁺ in MgO when a crystal is subjected to uniaxial impression. The increase or decrease in the rate of forbidden transitions is shown to depend on the angle between the compression axis and the magnetic field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 12–15, June, 1970.     

Multi-step damage accumulation in irradiated crystals

This article presents a model of damage accumulation in irradiated crystals. This model is based on the assumption that the damage accumulation occurs through a series of structural transformations triggered by the destabilization of the current structure of crystals. Formal equations describing the damage accumulation build-up and experimental assessment of the model are presented and discussed in the framework of the actual knowledge of radiation effects in oxide crystals (yttria-stabilized zirconia (YSZ) and magnesium-aluminate spinel (MAS)), silicon carbide crystals and zirconia implanted nickel crystals.     

Spin quenching and EPR of Mn2+ in single crystals of nickel salts

An EPR study of Mn²⁺ in (NH₄)₂Ni(SeO₄)₂·6H₂O crystals has been reported for the first time at room temperature (≈300 K) and at the X-band. The observations regarding the anisotropic and field dependent linewidths of Mn²⁺ in nickel Tutton salts at room temperature are discussed in terms of the spin quenching ideas. Also, the g_z-shift in Mn²⁺-doped Ni-Tutton salts is attributed to the internal magnetic field at the site of the Mn²⁺ ion, caused by the moments induced on Ni²⁺ ions.     

External-field-induced formation of stabilized F 2 + centers in colored LiF crystals

The production of complexes, which are thermally stable at room temperature, with F ₂ ⁺ centers in radiation-colored LiF crystals by the combined action of different fields is investigated. The half-life of these laser centers produced by, specifically, hard UV radiation and a shock wave increased by almost two orders of magnitude. Fiz. Tverd. Tela (St. Petersburg) 40, 2044–2050 (November 1998)     

EPR of hole centers in nonlinear LiBO3 crystals

Results are presented of an EPR study of theO⁻ hole center in LiB₃O₅ (lithium triborate) crystals. Analysis of angular dependences of EPR spectra is used to determine the principal values of the g tensor (g _XX=2.032, g _YY=2.020, g _ZZ=2.007), along with the modulus of the isotropic part of the A ² tensor (|A|=12.2 G) and the orientations of the principal axes of the paramagnetic O⁻ center. The most probable model for the O⁻ center in lithium borate crystals is the following: a hole trapped by the center is localized on a p-orbital of an oxygen ion linking two boron atoms with coordination numbers three and four near a negatively charged stabilizing structural defect. In this case the characteristic hyperfine splitting is due primarily to the interaction of the unpaired electron spin with the ¹¹B nucleus. Fiz. Tverd. Tela (St. Petersburg) 39, 1380–1383 (August 1996)