B.V. Padlyak

Radiation-induced centers in the Li2B4O7 single crystals

Abstract The X-band (ν≅9.4 GHz) electron spin resonance (ESR) spectra of the X-ray-, γ- and neutron-irradiated lithium tetraborate (LTB) single crystals were investigated at room and liquid nitrogen temperatures. Neutron irradiation (fluence Φn=1.7×1019 cm−2) of the LTB single crystals with natural isotope abundance leads to the generation of four types of paramagnetic centers, stable at room temperature. The observed ESR spectra were described by spin Hamiltonian formalism. Thermal stability of the neutron-induced centers was investigated in the 300–800 K range. The models and new mechanism of formation of the neutron-induced defects in the LTB crystals are suggested.

Synthesis and X-ray structural investigation of undoped borate glasses

A series of undoped borate glasses: Li2B4O7, LiKB4O7, LiB3O5, SrB4O7, and LiCaBO3 of high optical quality and chemical purity were obtained from the corresponding polycrystalline compounds using standard glass synthesis under technological conditions developed by the authors. The glasses were obtained by rapid cooling of molten crystalline material, which was heated to 100 K above the melting point to prevent crystallization and to exceed the glass transition point. The X-ray diffraction intensity profiles of the investigated glasses were typical of glassy compounds. The most typical intensity profile, consisting of almost symmetrical peaks, was observed in the case of Li2B4O7 and LiB3O5. Substitution and partial substitution of Li atoms by Sr and Ca atoms was accompanied by significant changes in the intensity profiles of the investigated glasses. Pair correlation functions and structural parameters (average interatomic distances and coordination number to oxygen) of the investigated glasses were evaluated and analyzed. Structural peculiarities of the investigated borate glasses are discussed in comparison with structural data available for their crystalline analogs.

Neutron-induced defects in the lithium tetraborate single crystals

The X-band ( x ; 9.4 v GHz) electron spin resonance (ESR) spectra of the un-doped isotopically enriched lithium tetraborate (LTB) Li 2 B 4 O 7 single crystals, irradiated by thermal neutrons (fluences | n =2.74 ‐ 10 15 1 1.79 v ‐ 10 18 v cm m 2 ) were investigated at 300 and 77 v K. The LTB crystals of high chemical purity and optical quality with different isotope compositions ( 6 Li 2 10 B 4 O 7 , 6 Li 2 11 B 4 O 7 , 7 Li 2 10 B 4 O 7 and 7 Li 2 11 B 4 O 7 ) were grown by Czochralski technique. The thermal neutrons (the total quantity >90%) with fluence near 10 18 v cm m 2 induce at least 4 different types of stable paramagnetic centers in the Li and B isotopically enriched LTB crystals. The ESR spectra, electron structure and efficiency of generation for centers, induced by thermal neutrons, essentially depend on neutron fluence and isotope composition of the LTB crystals. The local symmetry and the spin Hamiltonian parameters of the observed paramagnetic centers were determined and their electron structure were established. The possible models and formation mechanism of the radiation defects, induced by thermal neutrons in the LTB lattice, are proposed.

EPR study of the impurity paramagnetic centres in (CaO−Ga2O3−GeO2) glasses

The X-band EPR spectra of Cr3+, Mn2+, and Fe3+ impurity ions in glasses of (CaO−Ga2O3−GeO2) system are investigated in the 77÷300 K temperature range. The experimental data analysis yields the following results: (i) Impurity chromium ions are incorporated into the (CaO−Ga2O3−GeO2) glasses network in Cr3+ (3d3,4F3/2) paramagnetic valence state only and occupy the strong distorted oxygen coordinated octahedral sites. (ii) For all activated and non-activated (CaO−Ga2O3−GeO2) glasses the iron impurity is present at concentration roughly 0.01 wt.%. Isotropic EPR signals atgeff=4.29 andgeff=2.00 are assigned to Fe3+ (3d5,6S5/2) ions in the sites with strong rhombic distortion and in the sites with nearly cubic symmetry respectively. (iii) The manganese EPR spectrum in (CaO−Ga2O3−GeO2) glasses is weakly dependent on temperature, doping procedure as well as manganese concentration. EPR spectra of impurity manganese ions in glasses with Ca3Ga2Ge3O12 and Ca3Ga2Ge4O14 compositions are virtually identical and belong to Mn2+ (3d5,6S5/2) ions. Impurity manganese ions are incorporated into the (CaO−Ga2O3−GeO2) glass network as isolated Mn2+ centres and clusters of Mn2+ ions.

EPR spectroscopy of the Cr3+ centers in LLGG:Cr single crystals

Abstract The X-band ( ν ≅9.4 GHz) electron paramagnetic resonance (EPR) of series of Cr-doped lanthanum lutetium gallium garnet (LLGG) single crystals with various compositions and Cr concentrations were investigated at room and liquid nitrogen temperatures. All observed EPR spectra were assigned to Cr 3+ ions (3d 3 configuration, 4 A 2g ground state). Angular dependencies of the observed EPR spectra show three different types of Cr 3+ centers in the octahedral sites of LLGG lattice: isolated Cr 3+ centers of axial symmetry, isolated Cr 3+ centers of rhombic symmetry, and magnetically coupled Cr 3+ clusters (or clumps). The parameters of the Cr 3+ centers of axial and rhombic symmetry were evaluated. The possible crystallographic models of the isolated Cr 3+ centers and the influence of the substitutional disorder on the formation process of the different Cr 3+ centers in the LLGG lattice are discussed.

-interaction between components in the Ti-Mn-Sb system at 870 K and the physical properties of Ti5Mn0.45Sb2.55

Abstract The isothermal section of the phase diagram at 870 K of the ternary Ti–Mn–Sb system was investigated. The existence of a new ternary compound, Ti 5 Mn 0.45 Sb 2.45 , was revealed crystallizing in the W 5 Si 3 -type tetragonal structure (space group I4/mcm ) with lattice parameters a =1.0462(1) nm and c =0.52776(3) nm. This compound has been characterized by measurements of the electrical resistivity, thermopower, and magnetic susceptibility over a wide temperature range and the EPR spectrum at room temperature.

Synthesis, crystal structure and physical properties of ZrNiSn semiconductor doped with Mn

The resistivity, thermopower, dc magnetic susceptibility and electron paramagnetic resonance (EPR) spectra of ZrNi, x Mn x Sn solid solution with low (0.001≤x≤0.05) Mn concentration were investigated in the 78-400 K temperature range. Polycrystalline samples of the ZrNi x Mn x Sn compounds were prepared by arc-melting of the pure metals. Quantitative atomic emission spectroscopy confirmed the appropriate Mn contents in all alloys. The lattice parameters of the ZrNi 1-x Mn x Sn compounds with different Mn concentration are almost unchanged. Magnetic susceptibility shows the linear dependence versus inverse magnetic field for all samples, what is evidence for the magnetic interactions in the ZrNi 1-x Mn x Sn compounds The temperature dependence of magnetic susceptibility for the compounds with 0.005≤x≤0.05 can be described approximately by a Curie-Weiss law. The results of static magnetic measurements were confirmed by EPR technique. The broad line of I type belongs to Mn 2+ ions in the Ni-sites in crystal lattice coupled by magnetic dipolar interaction. The narrow line of II type is interpreted as an EPR spectrum of exchange-coupled pairs or clusters of more than two Mn 2+ ions. The unusual magnetic properties and the features of dipolar and exchange interaction of the Mn 2 ions in the lattice of ZrNi 1-x Mn x Sn solid solution are discussed.

Optical spectroscopy of Li2B4O7, CaB4O7 and LiCaBO3 borate glasses doped with europium

We have investigated the optical properties (optical absorption, photoluminescence emission and excitation spectra, and luminescence kinetics) of a series of Eu-doped glasses with the compositions Li2B4O7, CaB4O7 and LiCaBO3. Li2B4O7:Eu, CaB4O7:Eu and LiCaBO3:Eu glasses of high chemical purity and optical quality have been obtained from the corresponding polycrystalline compounds in the air atmosphere, using a standard glass technology. The Eu impurity has been introduced into the borate compounds in the form of Eu2O3 oxide in the amounts of 0.5 and 1.0 mol. %. On the basis of electron paramagnetic resonance and optical spectroscopy data we have shown that the impurity is incorporated into the glass network exclusively as Eu (4f, F0) ions. We have identified all 4f–4f transitions of the Eu centres observed in the optical absorption and luminescence spectra. The most intense emission band of Eu peaked about at 611 nm (the D0  F2 transition) is characterized by a single exponential decay, with typical lifetimes that depend on the basic glass composition and the local structure of the luminescence centres Eu. The peculiarities of the electron and local structures of the Eu centres in the Li2B4O7, CaB4O7 and LiCaBO3 glasses have been discussed and compared with the reference data for Eu-doped borate single crystals and polycrystalline compounds with the similar chemical compositions, as well as with other borate glasses.

Rare Earth Dopant (Nd, Gd, Dy, and Er) Hybridization in Lithium Tetraborate

The four dopants (Nd, Gd, Dy, and Er) substitutionally occupy the Li+ sites in lithium tetraborate (Li2B4O7: RE) glasses as determined by analysis of the extended X-ray absorption fine structure. The dopants are coordinated by 6-8 oxygen at a distance of 2.3 to 2.5 A, depending on the rare earth. The inverse relationship between the RE¬ O coordination distance and rare earth (RE) atomic number is consistent with the expected lanthanide atomic radial contraction with increased atomic number. Through analysis of the X-ray absorption near edge structure, the rare earth dopants adopt the RE3+ valence state. There are indications of strong rare earth 5d hybridization with the trigonal and tetrahedral formations of BO3 and BO4 based on the determination of the rare earth substitutional Li+ site occupancy from the X-ray absorption near edge structure data. The local oxygen disorder around the RE3+ luminescence centers evident in the structural determination of the various glasses, and the hybridization of the RE3+ dopants with the host may contribute to the asymmetry evident in the luminescence emission spectral lines. The luminescence emission spectra are indeed characteristic of the expected f-to-f transitions; however, there is an observed asymmetry in some emission lines.

Radiation-induced paramagnetic centers in the glasses of CaO-Ga2O3-GeO2 system

The X-band (v ≅ 9.4 GHz) ESR spectra of the UV-, X-, and γ-irradiated glasses of CaO-Ga2O3-GeO2 system with compositions similar to Ca3Ga2Ge3O12, Ca3Ga2Ge4O14, and Ca3Ga2O6 crystals have been investigated at 300 and 77 K. It was shown that X- and γ-irradiation of the Ge-contained glasses induce simultaneously electron and hole paramagnetic centers, stable at room temperature. The UV-irradiation of Ge-contained glasses leads to the generation of stable centers of electron type, whereas the X- and γ-irradiation of glasses with Ca3Ga2O6 composition induces stable hole centers, exclusively. The electron centers are assigned to E′ (Ge) centers with different local environments. ESR spectrum of hole centers is ascribed to O− centers, localized on non-bridging oxygen of the glass network. The E′ (Ge) and O− centers show high thermal stability in the CaO-Ga2O3-GeO2 glass network. The obtained ESR spectra are described by spin Hamiltonian with g-factor of axial and rhombic symmetry for the E′ (Ge) and O− centers, respectively. The nature, electron structure and some formation peculiarities of the radiation defects in CaO-Ga2O3-GeO2 glasses are discussed in comparison with other glasses as well as Ca3Ga2Ge3O12 and Ca3Ga2Ge4O14 crystals.