S. V. Nistor

Electron and hole trapping in PbCl2 and PbCl2:Tl crystals

Abstract Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl 2 single crystals. Besides Pb 2 3+ self-trapped electron (STEL) centers the PbCl 2 :Tl crystals exhibit trapped-electron (PbTl) + -type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl 2+ centers the holes are self trapped at pairs of neighbouring Cl − anions resulting in V k type centers with various orientation and length of the Cl–Cl axis.

Off‐Center Displacement of Fe+ Ions in Irradiated SrCl2: Fe Crystals Grown in Chlorine

Chlorinated SrCl 2 :Fe 2+ crystals exhibit, after X-ray irradiation, two trapped-electron Fe + (I) and Fe + (II) centers both with axial symmetry, but different EPR spectrum parameters and production properties. The analysis of the experimental data strongly suggests a substitutional eight-fold coordination of the Fe + ion in the Fe + (I) center and a strong off-center displacement to a fourfold coordinated site in the Fe + (II) center resulting in S = 3/2 and S = 1/2 ground states, respectively.

Point defects in cubic boron nitride crystals

Abstract The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S =1/2, local symmetry axis along one of the crystal 〈111〉 axes and principal g values g ∥ =2.0032±0.0009 and g ⊥ =2.0094±0.0005 at T =10 K.

Electron self-trapping and photolysis in Pbcl2 crystals

Abstract Strongly anisotropic paramagnetic centers produced by X-ray or UV-irradiation of PbCl2 crystals at T = 80 K have been attributed to Pb3+ 2 self-trapped electron (STEL) centers. The self-trapping of free (conduction) electrons at pure cation sites is further supported by production experiments using selective optical excitation in the fundamental absorption band. Optical bleaching experiments suggest the STEL center to be the primary defect responsible for the photolysis in PbCl2, with the g ≈ 2 centers representing hole-trapped centers. P.A.C.S. numbers: 61.72.Ji; 63.20.Kr; 76.30.Mi; 78.50.Ec; 61.80.Ba, Cb.

Localization and Charge Conversion of Copper in Rb2ZnCl4:Cu Crystals: An ESR and Optical Absorption Study

Electron spin resonance and optical studies reveal the presence of both Cu 2+ and Cu + centers in Rb 2 ZnCl 4 :Cu single crystals grown from melt and their conversion by X- or γ-irradiation. Two types of paramagnetic Cu 2+ centers with different concentrations and production properties have been identified. The more abundant Cu 2+ (I) center consists of a Cu 2+ (II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb 2 ZnCl 4 lattice as Cu 2+ , mainly at Zn 2+ sites, part of it being afterwards converted to Cu + precursor centers. The presence of a neighboring charge compensating anion vacancy and its departure during the radiolytic Cu + (I)→Cu 2+ (I) conversion seems to play an essential role in the stabilization of the Cu + (I) and Cu 2+ (I) centers, respectively.

ESR of paramagnetic Tl2+-type centres in Rb2ZnCl4 crystals

Abstract Tl2+(6s 1)-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4: TlCl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

EPR vs. temperature of Fe3+ ions produced by radiolysis in CdCl2: Fe crystals

Abstract Fe3+ ions are produced by hole trapping at substitutional Fe2+ ions in the layered CdCl2 lattice, after brief X-ray irradiation at 78 K or 296 K. The resulting EPR spectra have been analysed with an improved fitting procedure, resulting in more accurate zero-field-splitting (ZFS) parameters b 0, b 0 and b 3 4. It is shown that the superposition model of Newman describes in a satisfactory manner the resulting ZFS parameters, predicting the lattice contraction around the Fe3+ ion and the thermal expansion coefficient normal to the lattice layers of CdCl2

Observation of hole trapped Tl2+ centers in PbCl2:TlCl crystals

Abstract Paramagnetic hole trapped Tl2+ centers with orthorhombic symmetry have been identified by ESR spectroscopy in PbCl2: TlCl crystals X-ray irradiated at 77 K. Changes in both ESR and optical absorption spectra were observed after pulse-annealing at room temperature. The resulting ESR spectrum and the absorption band at 335 nm are considered to belong to substitutional Tl2+ ions at cation sites of the PbCl2 lattice.

Electron-hole recombination in PbCl2 : Tl crystals

A correlated ESR and optical emission study on samples doped with different concentrations of Tl + impurity ions shows the involvement of paramagnetic Pb 3+ 2 self-trapped electron centers (STEL) and trapped hole A centers in the electron-hole recombination responsible for the 2.6 eV blue-green luminescence.

EPR detection of the presence and movement of anion vacancies in X-ray irradiated PbCl2:Tl+ crystals

Abstract An EPR study of Tl2+ centers in the PbCl2 : Tl+ layered compound reveals the presence of neighboring charge compensating anion vacancies, next to the Tl+ impurity ions, in the same lattice layer. In the low temperature X-ray irradiated samples the vacancies are thermally activated and move away from the hole-trapped Tl2+ ion at T ≥ 120 K. This value is in good agreement with the activation energy for vacancy migration of 0.35 eV, previously determined from ionic conductivity measurements.