V. Skvortsova

Displacement defect formation in complex oxide crystals under irradiation

The work is devoted to an analysis of formation processes of the radiation displacement defects (RDDs) and colour centres (CCs) in complex oxide crystals under irradiation. The calculation results on: the displacement process simulation as well as an analysis of the RDD and CC accumulation kinetics are presented. New experimental results on additional absorption spectra induced by neutron irradiation of LiNbO(3) (LNO) crystals doped with Fe and Cr and YAlO(3) (YAP) crystals doped with Nd and Er as well are presented. Dose dependencies of the additional absorption are compared and their peculiarities are discussed. The obtained results confirm that CCs causing the irradiation induced absorption in LNO and YAP crystals are formed in secondary processes which area affected by impurities. It is also found that the induced absorption dose dependence in LNO crystals has an unexpected slope.

Radiation displacement defect formation in some complex oxide crystals

The work is devoted to an analysis of the formation processes of radiation displacement defects (RDDs) and color centers (CCs) under irradiation of complex oxide crystals. The results of the displacement process simulation as well as the analysis of the RDDs and CCs accumulation kinetics in some complex oxide crystals are presented. New experimental results on additional absorption spectra induced by neutron irradiation of LiNbO3 (LNO) crystals doped with Fe and Cr and YAlO3 (YAP) crystals doped with Nd and Er as well are presented. Dose dependencies are compared and their peculiarities are discussed. ” 2000 Elsevier Science B.V. All rights reserved.

Magnetic ions exchange interactions in NiO{MgO solid solutions

In this work, a review of recent experimental data and their interpretation for NicMg1−cO solid solutions is given. In particular, the influence of exchange interactions between Ni2+ ions on the structural, optical, magnetic, and vibrational properties is discussed.

Optical properties of natural topaz

The results of investigation of infrared, Raman and UV-Visible absorption spectra of natural topaz crystals from Ukraine before and after fast neutron irradiation are presented. We assume that the ~ 620 nm band in topaz crystals is associated with the presence of Cr 3+ , Fe 2+ and Mn 2+ impurities. The broad band with maxima at 650 cm -1 observed in Raman spectra for topaz irradiated by fast neutrons may be connected with lattice disorder. Exchange interaction between radiation defect and impurity ions during neutron irradiation leads to appearance of additional absorption band in UV-VIS spectra and bands broadening in infrared and Raman spectra of investigated crystals.

One- and two-magnon contributions in optical spectra of KNiF3single crystal

Abstract The one-magnon (1M) and two-magnon (2M) contributions were studied in optical absorption spectra of KNiF3 single crystal, measured in the temperature range from 5 K to 300 K. The three absorption bands were considered in details. The first band is due to the magnetic-dipole transition3 A2g(F) → 3T2g(F), centred at 7700 cm−1, and contains the Brillouin zone-centre one-magnon contribution with the energy ω1M = 25 ± 5 cm−1. The other two bands are due to the electric-dipole transitions 3A2g(F) → 1Eg(D) and 3A2g(F) → 1Eg(G), centred at 16000 cm−1 and 31200 cm−1, respectively. Both bands contain the Brillouin zone-boundary two-magnon contribution with the energy ω2M = 813 ± 10 cm−1.

Low Temperature Optical Absorption by Magnons in KNiF3 and NiO Single-Crystals

Optical absorption spectra of KNiF3 and NiO stoichiometric single-crystals were measured at 5 K in the range from 4000 to 50000 cm-1. The observed bands are interpreted based on the energy levels diagram for Ni2+ (3d 8 ) ion in a cubic crystal field. The crystal-field parameter Dq is equal to 766 cm-1 in KNiF3 and 890 cm-1 in NiO. Particular attention is paid to the band due to the magnetic-dipole 3A2g(F)→3T2g(F) transition, located at 7700 cm-1 in KNiF3 and at 8900 cm-1 in NiO. We show that the energy difference between the two peaks, located at the low energy side of the band, is related to the zone-center (k=0) one-magnon energy, so that the low-energy peak is attributed to the pure exciton transition, whereas the high-energy peak to the excitors magnon excitation. The estimated onemagnon energies are 25 ± 5 cm-1 in KNiF3 and 39 ± 3 cm-1 in NiO.

Optical Properties of Natural and Synthetic Beryl Crystals

The results of investigation of photoluminescence and UV-Visible absorption spectra of natural beryl crystals from Ural Mountains before and after fast neutron irradiation and synthetic crystal grown in Belarus and Russia are presented. Photoluminescence (PL) spectra of synthetic beryl crystals contain a broad band with maxima 740 nm excited both by UV light (λex = 260 nm, 271 nm) and laser excitation (λex =263 nm). This band is connected with Fe 2+ ions. The temperature lowering down to 8 K leads to appearance of narrow lines in the 680 - 720 nm regions. Emission lines observed in the luminescence spectra are connected with electron transition 2 Eg→ 4 A2g of the Cr 3+ ions: R-lines (682.5 nm) arise from isolated Cr 3+ ions occupying AI 3+ sites; N-lines (691, 698, 703, 706 and 711 nm) arise from several types of exchange-coupled pairs of Cr 3+ ions occupying first, second and third nearest and related neighbour AI 3 + sites. It is shown that the absorption bands in the 690-580 nm region of natural pale blue beryl crystals caused by neutron irradiation belong to a complex center, which consists of Cr 3+ ions and radiation defect - F or F + - center. Presence of Fe 2+ ions contributes to the persistence of the complex defect.

Optical Properties of Irradiated Topaz Crystals

The results of an investigation of UV-Visible absorption and photoluminescence spectra of colorless topaz before and after neutron irradiation, natural blue topaz from Ukraine, and yellow topaz are presented. We assume that the absorption band ~ 620 nm and broad emission band 300-700 nm in topaz crystals are associated with exchange interaction between a radiation defect (anion vacancies, which capture one or two electrons) and impurity ions Cr 3+ , Fe 3+ and Mn 2+ .

Optical Properties of Hydrogen-Containing MgO Crystal

The photoluminescence (PL), its excitation (PLE) and absorption spectra in ultraviolet, visible and infrared (UV-VIS-IR) regions were used to investigate the MgO single crystals irradiated by fast neutrons. It is shown that the photoluminescence band of the MgO crystals at 730 nm belongs to the hydrogen-containing complex centers V-OH-Fe3+, which are transformed during the irradiation with fast neutrons. The behavior of the PL band 730 nm after fast neutron irradiation depends on the iron-chromium concentration. It is found that the fast neutron irradiation produces the interstitial proton H+i and the Mg(OH)2 microphase.

Exciton-magnon interactions in NicMg1−cO single crystals

The effect of chemical composition and temperature on exciton-magnon interactions in NicMg1−cO single crystals was studied using optical absorption spectra in the region of the magnetic-dipole 3A2g(G)→3T2g(F) and electric-dipole 3A2g(F)→1Eg(D) transitions. The two zero-phonon lines at ∼7800 and ∼7840 cm−1 on the low-energy side of the magnetic-dipole transition band were assigned to a pure exciton transition and an exciton-one-magnon transition at the center of the Brillouin zone. The intensity of the exciton-one-magnon peak decreases rapidly with increasing magnesium ion concentration and/or temperature, to vanish altogether at T=6 K for c<0.95 and at T=130 K for c≥0.99. Thus, the contribution of long-wavelength magnons in optical absorption spectra of NicMg1−cO becomes negligible at temperatures substantially lower than the Néel point TN (the antiferromagnetic ordering temperature). This observation can be explained as being due to a substantial decrease in the characteristic spin-spin interaction length with increasing concentration of the diamagnetic magnesium impurity ions (static disorder) and/or with increasing amplitude of thermal atomic vibrations (dynamic disorder). At the same time, the peak at ∼14500 cm−1, which lies in the electric-dipole transition region and corresponds to excitation of an exciton and two magnons at the Brillouin zone edge, remains visible up to the Néel temperature. This is accounted for by the short-wavelength magnons being sensitive to short-range magnetic order, which persists up to TN.