Luminescence of yttrium aluminum borate single crystals doped with manganese

The optical absorption and luminescence spectra of single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are measured. It is established that the optical absorption spectra of yttrium aluminum borate single crystals doped with manganese ions are determined primarily by the contribution from the Mn⁴⁺ ions. The luminescence spectra of the YAl₃(BO₃)₄ single crystals doped with manganese exhibit narrow lines attributed to the Mn⁴⁺ ions and an extended multiband structure which is associated primarily with the contribution from the Mn²⁺ ions.     

Spectroscopy of radiation-induced and structural defects in garnet crystals with V and Cr impurity ions

The optical absorption spectra of crystals with garnet structure (A₃B₂C₃O₁₂), containing V or Cr impurity ions, before and after oxidation annealing, γ irradiation, and coactivation with Ca and Mg ions have been experimentally and theoretically investigated. It is shown that V³⁺ and Cr⁴⁺ ions are incorporated into both octahedral and tetrahedral sites of the garnet lattice. The absorption spectra and energy level diagrams of impurity ions are identified.     

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl₂O₄ and MgAl₂O₄:Mn²⁺ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl₂O₄:Mn²⁺ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F⁺ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring ²⁷Al nuclei with nuclear spin 5/2.     

Charge state of manganese ions and the nonstoichiometry of Ca1 ? xLayMnO3 ? δ single crystals

The charge states and effective spins of manganese ions in nonstoichiometric single crystals of Ca_{1 − x} La _y MnO_{3 − δ} (x = 0, 0.05, 1) manganites have been determined from the exchange splitting of Mn 3s X-ray photoelectron spectra. The chemical composition of the manganite samples under investigation, including the concentration of oxygen, has been determined using the data of the X-ray microanalysis of cations. It has been found that the manganite single crystals under investigation have large defectiveness in both the cation and oxygen sublattices. The efficiency of the complex use of 3s photoelectron spectroscopy and X-ray microanalysis in the determination of the charge state of manganese ions and violations of stoichiometry in doped and defected manganites has been demonstrated.     

Growth, optical and EPR properties of Li1.72Na0.28Ge4O9 single crystals pure and slightly doped with Cr

Single crystals of lithium-sodium-tetragermanate, a member of the solid solution series Li_2−xNa _x Ge₄O₉ with x=0.28, pure and slightly doped with Cr³⁺ ions (0.03 mol.% and 0.1 mol.%), were grown in ambient atmosphere by the Czochralski technique from stoichiometric melt. The crystals with dimensions up to 20 mm in diameter and 50 mm in length were obtained. The crystal structure has been determined by means of X-ray diffraction. Phase analysis and structural refinement of the Li_1.72Na_0.28Ge₄O₉ crystals were performed by X-ray powder diffraction using Ni-filtered Cu Kα radiation with a Siemens D5000 diffractometer. The absorption, excitation and photoluminescence spectra of the crystals were measured in the UV-VIS and IR range at low temperatures. EPR investigations were performed using a conventional X-band Bruker ELEXSYS E 500 CW-spectrometer operating at 9.5 GHz with 100 kHz magnetic field modulation. Temperature and angular dependences of the EPR spectra of the crystal samples were recorded in the 3–300 K temperature range.     

Spectroscopic properties of Pr<Superscript>3 + </Superscript>-doped erbium oxalate crystals

Spectroscopic properties of praseodymium ions-doped erbium oxalate (Er₂(C₂O₄)₃ · nH₂O) crystals have been investigated. The crystals were grown by hydro silica gel method under suitable pH conditions and by single diffusion method. The well-grown crystals are bright and transparent. The dark green colour of these crystals changes with the variation of the concentrations of the dopant ions. The absorption spectra have been measured in the region 200–800 nm at room temperature. Judd–Ofelt intensity parameters for f–f transitions of the Pr³⁺ ions have been determined as Ω₂ = 166.7, Ω₄ = 1.103 and Ω₆ = 2.898. Analyses of the absorption spectra also show a possible energy transfer from the host material to the dopant.     

EPR of Mn4+ in LiF:U,Mn

A new manganese center in LiF single crystals doped with U₃O₈ and MnO₂ has been observed at room and liquid nitrogen temperatures. The trigonal EPR spectrum consists of one very anisotropic fine structure transition with six well-resolved hyperfine components. An analysis of the spectra recorded at X and Q bands shows that the zero field splitting is larger than the electronic Zeeman interaction.By comparing a theoretical calculation of the g values and the hyperfine structure with the experimental X and Q band data it was concluded that the ion is Mn⁴⁺. The presence of Mn⁴⁺ in these crystals confirms an assumption of Runciman regarding the possibility of introducing high valency ions in alkali fluorides containing uranium with an excess of oxygen.     

Crystal structure and optical spectra of LiLa9(SiO4)6O2 crystals activated with Er3+

LiLa₉(SiO₄)₆O₂ (LLS) crystals activated with Er³⁺ have been grown by the flux growth method. Their apatite structure has been characterized by single-crystal X-ray diffraction. The absorption and emission spectra have been measured in the visible and NIR regions. They are affected by a significant inhomogeneous broadening making these crystals interesting active media for tunable solid-state laser operation.     

X-ray photoelectron spectroscopy of Co3O4, Fe3O4, Mn3O4, and related compounds

The metal 2p region spectra of the mixed valence spinels, Co₃O₄, Fe₃O₄, Mn₃O₄, and related compounds were studied. The satellite splittings of Co 2p_{$\text{3}\text{2}$} for the octahedrally coordinated cobaltous ions are 6.2 eV and those for the tetrahedrally coordinated ones are about 5.3 eV. The Co 2p spectrum for Co₃O₄ is considered to be the sum of spectra of magnetic cobaltous ions and low-spin cobaltic ions. In the cases of Fe₃O₄ and Mn₃O₄, the oxidation states were not clearly distinguished because both the divalent and trivalent ions of iron and manganese are high-spin.     

Comparative first-principles analysis of the absorption spectra of ZnAl2S4 and ZnGa2O4 crystals doped with Cr3+

Systematic first-principles analysis of the energy level schemes and ground state absorption spectra of trivalent chromium in ZnAl₂S₄ and ZnGa₂O₄ crystals has been performed in the present paper. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara et al., Phys. Rev. B 64, 115413 (2001)] was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy level schemes of the Cr³⁺ ion and its absorption spectra in both crystals were calculated, assigned and compared with experimental data. By performing analysis of the molecular orbital (MO) population, it was shown that the covalency of the chemical bonds between the Cr³⁺ and S^2- ions is more significant than that one between the Cr³⁺ and O^2- ions.     

Tin-doped spinel-related oxides of composition M3O4 (M = Mn, Fe, Co)

Tin-doped compounds of spinel-related M₃O₄ (M = Fe, Mn, Co) have been studied by ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectroscopy in the temperature range of 20–600 K. The ¹¹⁹Sn Mössbauer spectra recorded down to 20 K from the non-iron-containing compounds of Co₃O₄ and Mn₃O₄ contained only doublets showing no transfer of magnetic properties from cobalt or manganese to the dopant tin ions. In contrast, the tin-doped-(FeCo)₃O₄ and (FeMn)₃O₄ gave ¹¹⁹Sn and ⁵⁷Fe Mössbauer spectra, which showed magnetic hyperfine interactions. The Curie temperature has been estimated for the former sample.     

Electron paramagnetic resonance of Mn<Superscript>2+</Superscript> ions in single crystals of yttrium aluminum borate YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

Single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are studied using electron paramagnetic resonance spectroscopy. It is shown that manganese ions introduced at low concentrations into the sample predominantly occupy yttrium ion sites in the crystal structure. The shape of the electron paramagnetic resonance spectrum unambiguously indicates that the valence of manganese ions in this case is equal to 2+. The parameters of the spin Hamiltonian of Mn²⁺ ions in the YAl₃(BO₃)₄ matrix are determined at room temperature. The magnitude and sign of the fine structure parameter D allow the conclusion that the YAl₃(BO₃)₄ single crystals doped with manganese ions have a strong crystal field at the yttrium ion sites and easy-axis anisotropy.     

Structural transformations in LiGd9(SiO4)6O2 and Ca2Gd8(SiO4)6O2 crystals containing isolated [SiO4] complexes: Raman spectroscopic study

The vibrational spectra of single crystals of the LiGd₉(SiO₄)₆O₂ and Ca₂Gd₈(SiO₄)₆O₂ oxyapatites have been studied using Raman spectroscopy at room and high temperatures. The spectra of internal and external vibrations in these structures have been resolved. The structural transformation of the LiGd₉(SiO₄)₆O₂ and Ca₂Gd₈(SiO₄)₆O₂ oxyapatites in the processes of melting and crystallization, as well as during rapid quenching of the melt, has been investigated. It has been found that the melting of the LiGd₉(SiO₄)₆O₂ compound has an incongruent character and that new metastable disordered phases are formed during rapid quenching of the Ca₂Gd₈(SiO₄)₆O₂ melt.     

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mössbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe₂O₃ and FeBO₃ single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ～100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ～10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (H _eff) for the iron ions situated in the surface layers of thickness ～100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The H _eff values in the 2.4(9)-nm-thick surface layer of the α-Fe₂O₃ crystal and 4.9(9)-nm layer of FeBO₃ are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.     

Spin-wave resonances in Eu<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> and EuMn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Spin-wave resonances have been observed in superlattices arising due to the phase separation and self-organization of charge carriers in Eu_0.8Ce_0.2Mn₂O₅ single crystals. The resonances are found within the 5–80 K temperature range at frequencies close to 30 GHz. Similar resonances with intensities about an order of magnitude lower are also observed in EuMn₂O₅. The latter suggests the existence of charge transfer processes between the manganese ions of different valences in EuMn₂O₅.     

The influences of ZnO doping concentration on structure and photorefractive properties of Zn:Fe:LiNbO3 crystals grown by TSSG method

In this paper, LiNbO₃ crystals doped with same concentration of Fe₂O₃ and with different concentration of ZnO have been grown by the top-seeded-solution-growth (TSSG) method. Their IR transmittance spectra, UV-vis absorption spectra, lattice constants, and Curie temperature were measured. In two-wave coupling experiments, writing and erasure time constants, and maximum diffraction efficiency of the crystals were measured and compared with those of congruent LiNbO₃ (CLN) crystals doped with same ions. The crystals grown by TSSG method show shorter writing time, erasure time, and lower diffraction efficiency than the CLN crystals. The influences of ZnO doping concentration on photorefractive properties are studied.     

Absorption of light by exchange-coupled ions in a 2D antiferromagnet

The optical absorption spectra of Rb₂Mn_xCd_1?xCl₄ crystals are experimentally studied in the vicinity of a magnon sideband of the exciton band at a manganese content x ranging from 1.0 to 0.4. Additional absorption bands are observed with an increase in the magnetic structural disorder upon replacement of manganese ions by cadmium ions. An analysis of the evolution of the additional absorption bands in a magnetic field during the spin-flop phase transition and the change in the intensity with variations in the manganese content x demonstrates that these bands are associated with the excitation of the exchange-coupled pairs of manganese ions located in different environments in a plane square lattice. The phase boundary between the antiferromagnetic and spin-flop phases is constructed using the results of optical measurements. The manganese content corresponding to the magnetic percolation point is evaluated.     

A preliminary assessment of Lu2Y2Al2O9:Pr (LuYAM:Pr) as a potential scintillator

Lu₂Y₂Al₂O₉:Pr crystals have been grown by the micro-pulling-down method and their basic luminescence properties have been examined in order to estimate their utility for scintillator applications. The performed studies comprise the measurements of radioluminescence spectra as a function of temperature, photoluminescence emission and excitation spectra, and low temperature thermoluminescence glow curves. A comparison of the present results with those obtained recently for Y₄Al₂O₉:Pr indicates that in spite of a higher density Lu₂Y₂Al₂O₉:Pr is a much less promising material, which is mainly due to the absence of the fast Pr³⁺ d-f emission.     

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⁺ (3d³,⁴F_3/2) 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 atg _eff=4.29 andg _eff=2.00 are assigned to Fe³⁺ (3d⁵,⁶S_5/2) 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⁺ (3d⁵,⁶S_5/2) ions. Impurity manganese ions are incorporated into the (CaO?Ga₂O₃?GeO₂) glass network as isolated Mn²⁺ centres and clusters of Mn²⁺ ions.     

Kinetics of thermally stimulated recombination processes in lithium borate crystals

This paper reports on the results of the experimental and theoretical investigations of thermally stimulated recombination processes in crystals of the lithium borates Li₂B₄O₇ and LiB₃O₅. For both types of crystals, the measurements of thermally stimulated luminescence curves, spectra, and temperature dependences of the intensities of steady-state X-ray luminescence have been performed in a single experimental cycle. In the framework of a unified model for the Li₂B₄O₇ and LiB₃O₅ crystals, the thermally stimulated recombination processes have been calculated and the obtained results have been interpreted talking into account all available experimental data.