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[en] The formation of calcium, strontium and barium colloids both in heavily electron irradiated samples and in additively colored crystals of CaF₂, SrF₂ and BaF₂ has been investigated. Detailed data on the temperature dependence of the efficiency of colloid formation by irradiation have been obained. The growth of metallic particles in additively colored samples containing F and M centers has been studied for different color center concentrations and annealing temperatures. The optical absorption bands due to metallic colloids have been calculated using the theory of Mie. To take account of the pressure exerted by the matrix on the metallic particles several corrections to the optical constants of the metals have been introduced. A good agreement between theoretical calculations and experimental results has been obtained. The evolution of colloids along several thermal annealing experiments has also been investigated. A diffusion-limited model has been used which accounts for the dependence of the colloid radii with the annealing time

[en] The effect of the reorientation of V^- and [Li]⁰ defects in MgO on their EPR spectra has been studied at temperatures between 77 and 300 K, with the aid of the stochastic Liouville formalism. The EPR lineshapes are simulated for each temperature at different magnetic field orientations in order to obtain the temperature dependence of the jump rate. The effective activation energies and pre-exponential factors are obtained for temperatures within our measurement range. Using the small-polaron model for the hopping process, the high-temperature activation energies are calculated and compared with previous theoretical predictions. (author)

[en] The oxidation state and the localization of the chromium ions in the ceramic matrix of Cr-doped cassiterite (SnO₂) and Cr-doped malayaite (CaSnSiO₅) pigments have been investigated through the use of X-ray absorption near-edge (XANES), extended X-ray absorption fine structure (EXAFS), optical absorption and electron spin resonance (ESR) spectroscopies as well as by measurements of the unit cell parameters and the magnetic susceptibility of the pigments. We have found that three types of chromium species are present in the Cr-doped cassiterite pigments; a majority phase consisting of Cr(III) oxide clusters, a small amount of CrO₂ nanoparticles and Cr(IV) ions dissolved in the cassiterite lattice (∼5% of the total Cr amount), which must be responsible for the violet colour of this pigment. In the case of Cr-doped malayaite, most chromium cations are tetravalent forming a solid solution with the malayaite lattice by mainly substituting for Sn(IV) cations in isolated octahedral positions, although a very small amount of Cr(IV) also substitutes for tetrahedral Si(IV)

[en] The influence of 3d impurities on the production efficiency of Vsub(K) and F centers is studied in low temperature X-irradiated fluorite crystals. The optical absorption spectra of pure CaF₂ and CaF₂ doped with Mn, Fe, Co or Ni are compared before and after 15 K X-irradiation. An increase in the efficiency for Vsub(K)-centers production accompanied by the formation of M⁺ ions (M = Mn, Fe, Co, Ni) is observed at 15 K. A decrease of the F-centers production efficiency is also detected. (author)

[en] To a greater or lesser extent, all alkaline-earth oxide crystals contain hydrogen. Thermochemical reduction (TCR) of MgO and CaO crystals at high temperatures (∼2000K) and high pressures (4 to 7 atmospheres) of the metallic cation vapor (Mg and Ca respectively) results in a stoichiometric deficiency of oxygen ions, creating F centers (oxygen vacancies each with two electrons) and H_- ions (protons in the anion sublattice, each occupied by two electrons)> The positively-charged H^- ions serve as traps for electrons excited from the F centers. Steady-state photo-excitation of the F absorption band results in F luminescence which typically extends to several minutes. This photoluminescence is detrimental for certain applications, such as tunable lasers. The authors have been able to reduce the hydrogen concentration prior to TCR of MgO and during TCR of CaO. The result is that the oxygen vacancies are predominately in the one-electron F₊ charge state

[en] Thermoluminescence and EPR measurements in RT x-irradiated CaO are reported. Two glow peaks at 360 and 400 K are associated with the thermal destruction of two new donor-type paramagnetic defects of extrinsic character. The EPR spectrum of the defect that produces the first glow peak consists of an intense central line and six weak transitions which correspond to the hyperfine interaction with a ²⁵Mg nucleus. The spin-Hamiltonian parameters for this defect are g=2.0055+-0.0002 and Asup(Mg)=4.2+-0.1 MHz. The second defect shows the interaction with an ²⁷Al nucleus. The defect has trigonal symmetry and its spin-Hamiltonian parameters are gsub(parallel)=2.0075+-0.0002, gsub(perpendicular)=2.0042+-0.0002, Asub(parallel)sup(A1)= -11.7+-0.2 MHz, Asub(perpendicular)sup(A1)= -15.2+-0.2 MHz and Q=0.18+-0.05 MHz. The signs of the spin-Hamiltonian parameters are determined using the well resolved spectrum of the 'forbidden' lines. The activation energy for the thermal destruction of this centre is 1.1+-0.1 eV. Some possible models for these defects are also discussed. (author)

[en] The EPR spectrum of a hydrogen-related defect in MgO:Li is presented. This defect is created by LNT x-irradiation. The EPR signal measured at LNT shows trigonal symmetry and reveals a hyperfine interaction with hydrogen nuclei. The proposed model for the defect consists of a hydrogen atom in an interstitial position next to a Li⁺ ion in a Mg₂⁺ ion site giving a [Li⁺H]^- configuration. The observed EPR data have been explained using the following spin Hamiltonian: H = μsub(B)gS.H + aIsup(H).S + b[3Isub(z)sup(H)Ssub(z) - Isup(H).S] + Asub(perpendicular to)(Ssub(x)Isub(x)sup(Li) + Ssub(y)Isub(y)sup(Li)) + Asub(parallel to)Ssub(z)Isub(z)sup(Li) with the z axis along a <111> direction. The values obtained for the parameters are: g=2.002+-0.002, a=1240+-10 MHz, b=6+-1 MHz, Asub(parallel to)=33+-1 MHz and Asub(perpendicular to)=20+-1 MHz. The evolution of the EPR signal when measured at different temperatures is also given. A thermal averaging of the signal due to the motion of the hydrogen atom among the eight equivalent interstitial positions around the Li⁺ ion is observed. Destruction of the [Li⁺H]^- centres by thermal and optical treatment is also reported. (author)

[en] The optical absorption spectra of X-irradiated CaF₂:Cr and SrF₂:Cr are studied. X-irradiation induces the formation of a prominent band in the u.v. region (centred at 277 nm in CaF₂ and 285 nm in SrF₂) as well as a weak absorption in the visible region. Combined EPR and optical absorption measurements together with some optical bleaching experiments suggest that the u.v. absorption bands are related with either Cr⁺ or Cr³⁺ ions. Several arguments are presented in order to tentatively associate these bands with an interconfigurational transition of Cr⁺ in a cubal environment. (author)

[en] Thermoluminescence measurements have been performed on both MgO and CaO single crystals, as well as on single crystals of each material doped with Li, all of which were X irradiated at 80 K. The results obtained are summarized below