The absorption spectra of Ce3+, Nd3+, Eu3+, Tb3+, Er3+, Tm3+ and Yb3+ rare earth (RE) impurity ions incorporated in SrxBa1-xNb2O6 (SBN) crystals were studied in a wide spectral range from the visible to the mid-infrared. On the basis of the analysis of the Cd3+ absorption line shapes and their polarization properties the conclusion was drawn about the incorporation of multiple Ce3+ centres mostly in the position Al of the crystal lattice of SBN. From the observed temperature dependence of the Ce3+ absorption line broadening the electron coupling strength (160 cm(-1)) and effective phonon energy (300 cm(-1)) were determined. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The optical spectra of europium-doped stoichiometric lithium niobate (LiNbO3:Eu3+) crystals have been studied using combined excitation-luminescence spectroscopy in the range of 5D0 → 7F1, 7F0 optical transitions. Analysis of the results shows that Eu3+ ions can occupy 14 energetically nonequivalent positions in the LiNbO3 crystal lattice. This multiplicity of impurity centers is related to possible variants of the incorporation of Eu3+ ions into the LiNbO3 crystal lattice and the compensation of excess charge. Energy positions of the 5D0 level and the lowest sublevel of the 7F1 Stark multiplet are determined for Eu3+ centers of all 14 types.

The EPR spectra of rare-earth Er3+ ions in a polycrystalline corundum α-Al2O3 synthesized by the sol-gel technology were revealed. It is shown that the EPR spectra belong to the Er3+ ions in the ground state corresponding to the lower Stark sublevel of the 4 I 15/2 term and can be described by the spin Hamiltonian of axial symmetry with an effective spin S = 1/2 and the g tensor with components g ‖ = 12.176 and g ⊥ = 4.14. The average value of the g tensor (〈g〉 = 6.82) corresponds to the Γ7 state in a cubic field. Erbium is assumed to substitute for aluminum in the Al2O3 corundum crystal. The local symmetry C 3 of the Al3+ ion remains despite the pronounced expansion of the lattice around the Er3+ ion.

Hydrostatic pressure induced splitting of luminescence transitions under UV photoexcitation was observed in KTaO3 crystal. Two additional satellite bands of recombination luminescence arise at lower and higher energies with approximately the same shift with respect to the initial central band position. After the appearance of satellites at elevated hydrostatic pressures (1.9–6.1 GPa), the luminescence band shape does not exhibit any significant change with further increasing of the applied pressure up to 12.7 GPa. This effect is explained by light-induced occupation of Charge Transfer Vibronic Exciton (CTVE) ferroelectric phase. CTVE-phase occupation increases with hydrostatic pressure up to its proximity to the free CTVE level. Saturated ferroelectric order parameter of this phase splits the recombination luminescence band with two satellites’ appearance. Hydrostatic pressure induced satellites in recombination luminescence can be considered as the CTVE-phase manifestation for KTaO3 crystal.

Simulation of the chemical fluctuation regions in PMN-like relaxors through growth of the Pb x Nb y O z eramics was performed. Different Pb x Nb y O z clusters (chemically and structurally) coexist in such ceramics. Hole polaron and bipolaron (Cr 3+ -two polaronic-hole) paramagnetic complexes were considered for explanation of the EPR spectra in Pb x Nb y O z ceramics. Dynamical averaging, light-induced effects, and significant effects of reduction treatment giving the coexistence of Nb 5+ and Nb 3+ ions, as well as of a strong internal magnetic field, were discovered in this ceramics. The latter could be related to antiferromagnetic phase realization in Pb x Nb y O z clusters containing a sufficiently high concentration of magnetic Nb 3+ host lattice ions. Such a situation leads to antiferromagnetic resonance on Nb 3+ ions, as well as to EPR of Cr 3+ -related paramagnetic complexes in a Nb 3+ -induced internal magnetic field. Charge transfer vibronic excitons (CTVE) in free and in CTVE phase states were detected in Pb x Nb y O z ceramics by photoluminescence studies.

We studied the dynamics of energy transfer processes involving lanthanide ions in the nanocrystalline matrix of Y2O3. A set of Yb3+ and Er3+ co-doped Y2O3 powder samples consisting of crystalline particles of nanometer size in a range of 15–90 nm with low size dispersion were prepared by multistage sol-gel technology (J. Sol-Gel Sci. Technol. 21, 135 (2001). [1]). The energy transfer phenomena were studied by means of time-resolved spectroscopy, which allows to measure fluorescence response selectively for donors (Yb3+) and acceptor (Er3+) ions after flash optical excitation. The experimental results can be fitted well by the diffusion-limited energy transfer model. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Non-volatile holographic storage, based on two-centre holographic recording (TCHR) has been tested in analogy to successful attempts in LiNbO3:Fe:Mn, in doubly doped strontium barium Niobate (SBN) (SrxBa1–xNb2O6). Several tests with differently co-doped (Ce+Cr, Fe+Mn) crystals are carried out after various oxidation/reduction treatments and with several wavelengths for sensitising and recording. Up till now no non-volatile hologram storage in SBN could be observed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We performed a simulation of the chemical fluctuation regions in the lead-magnesium-niobate-like relaxors by growing Pb x Nb y O z ceramics. The coexistence of Pb x Nb y O z clusters with different chemical composition that is inclusive of Nb5+ as well as Nb3+ ions was assumed. The existence of mobile polaronic pseudo-Jahn-Teller holes on the oxygen ions in different Pb x Nb y O z clusters has been suggested by the electron paramagnetic resonance (EPR) experiment. Here light- induced EPR and EPR motional averaging effects related with such polaronic holes have been manifested. The appearance of new paramagnetic centers (S=5/2, 3/2, 1/2) related with Cr3+ impurities at the Nb5+ sites with trapped hole pairs (hole bipolarons), the charge compensation and the exchange effect have been proposed as explanation for the results of the EPR experiment. The charge-transfer vibronic excitons with O → Nb5+ charge transfer were directly detected by photoluminescence studies. The R-line in the Cr3+ impurity photoluminescence evidences the appearance of the Cr3+ ion in such matrices.

Optical absorption in the IR region has been recorded and first-principles computations have been done for some Fe contining perovskites. The IR absorption reveals a broad peak at about 11000 cm−1. First principles computations established that BaFe1/2 Nb1/2O3 is not ferroelectric, but PbFe1/2Nb1/2O3 does have a ferroelectric instability. These data confirm that the large dielectric permittivity found in BaFe1/2Nb1/2O3 is not due to a ferroelectric phase transition but rather because of extrinsic effects.

The simulation of chemical fluctuation regions in the PMN-like relaxors by growth of the Pb x Nb y O z —ceramics was performed. The coexistence of Pb x Nb y O z —clusters with different chemical composition inclusive Nb 5+ as well as Nb 3+ ions was assumed. An existence of paramagnetic Cr 3+ ions on Nb 5+ sites with charge compensation by two-oxygen hole bi-polarons with intermediate radius and in singlet states was suggested on the basis of EPR-experiment. A dissociation of such a bi-polaron with temperature via crossover with large radius bi-polaron produces finally the EPR-active free holes. They take part in spectrum motional averaging due to hopping transitions between different PbxNbyOz—clusters. New light induced EPR-lines were detected. This new lines are connected with two origins. First, it is effect of light-induced complexes which are appeared due to trapping of a space charge on the core-like Fe 2+ ions located on Nb 3+ sites in Pb x Nb 3+ y O z —clusters. Second, it is effect of Ti 2+ ions on Pb 2+ sites which are appeared under successive photo-recharging of non-paramagnetic Ti 4+ Pb 2+ and paramagnetic Ti 3+ Pb 2+ ions in accord with photo-chemical recharging transformation: Ti 2+ Pb 2+ ↞ Ti 3+ Pb 2+ ↞ Ti 4+ Pb 2+. Charge transfer vibronic excitons with O → Nb 5+ charge transfer was directly detected by photoluminescence studies. R-line Cr 3+ impurity photoluminescence manifestation independently confirms the Cr 3+ ion appearance on Nb ion sites in such matrices.

Three relatively sharp FIR absorption bands of Ce3+ in Strontium Barium Niobate (SBN) are observed in the 2000–3000 cm−1 region. A subband structure of these bands indicates three slightly inequivalent positions of the Ce3+ in the SBN crystal lattice. The concentration and temperature dependence of these bands is investigated.

Congruent SBN and BCT crystals doped with Fe, Cr, Ce to enhance photorefractive properties, are investigated at low temperature (T>1 K) under illumination with Ar+- and Kr+-laser light. Light-induced absorption changes in a wide spectral range from UV to IR indicate photoinduced charge transfer processes from impurities to polaronic centers. Broad NIR absorptions (at about 0,7 eV) associated with Ti3+ polarons in BCT or with Nb4+ polarons in SBN are observed and their non-linear behaviour with illumination intensity, polarization and temperature is described on the basis of the simple model for the photo charge transport (in SBN: Ce3+ +Nb5+ <----> Ce4++ Nb4+). A broad visible absorption VIS (at about 2 eV) appearing together with the NIR polarons, is shown by photodissociation (with a Kr+-laser) to consist at least partly of small polarons (in BCT and SBN).

Low-frequency (100 Hz–1 MHz) dielectric permittivity and infrared reflectivity studies were performed for barium-calcium-titanate single crystals of the congruently melting composition Ba0.77Ca0.23TiO3, nominally pure (BCT77/23) and doped by Cr (70 ppm, BCT77/23:Cr). Both the materials reveal a sharp ϵ′(T) maximum at 375 K (cooling run) with a pronounced temperature hysteresis, evidencing a first-order ferroelectric phase transition (PT) O h → C 4v. The high-temperature wing of the ϵ′(T) maximum reveals a weak dispersion of a non-relaxation nature. The low-temperature wing contains a reproducible step in the ϵ′(T) dependence. Another low-temperature PT was found at ∼41 K. Cr doping strongly influences this PT shifting the position of the ϵ′(T) maximum to ∼20 K. Polarised IR reflectivity shows a huge A 1 − E splitting of TO 1 polar phonons below the ferroelectric PT as in the pure BaTiO3. IR spectra indicate a displacive and diffuse character for the low temperature PT, but leave open the question about the predominance of displacive or order-disorder character of the high temperature PT.

Photo-refractive properties of SrxBa1-xNb2O6 (SBN, x = 0.61 congruent) are enhanced by doping with Cerium and Chromium ions. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers from these defect centers. The well separated thermo-luminescence peaks detected and their spectral line shape point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb4+ polarons and of VIS-centers are estimated to be 0.18 +/- 0.02 eV and 0.30 +/- 0.05 eV respectively. Possible relaxation mechanisms of SBN:Ce,Cr are discussed.

Congruent Sr x Ba 1 m x Nb 2 O 6 (SBN, x=0.61) doped with Ce or Cr ions exhibits enhanced photorefractive properties and new spectral features like increased red sensitivity. Here special emphasis is placed on the luminescence features of doubly doped Ce+Cr SBN crystals. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers und their recombination. The well separated thermo-luminescence peaks detected and their spectral line shape in emission point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb 4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb 4+ polarons and of VIS-centers are estimated to be 0.18 - 0.02 v eV and 0.30 - 0.05 v eV respectively. Possible excitation and recombination mechanisms in SBN:Ce+Cr are discussed.

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, ‘green luminescence’ (GL-band), and ‘red luminescence’ (RL-band) is proposed on the basis of the extended photoluminescence experiments in SBN/Cr, and also in SBN/Ce and in nominally pure SBN systems under different conditions. While the RL-band is suggested to be connected with charge transfer vibronic exciton (CTVE) clusters induced by Cr3+ impurities in the Nb-sites, the GL-band is connected with Nb4+ electronic polarons in a new excited state. Here Nb4+ centers are the cores of the CTVE clusters induced by these charged cores. The PL mechanism is the in-cluster CTVE recombination for both bands under discussion. But the CTVE states are quasi-resonantly mixed here with 4T2 states of the Cr3+ core in the RL-band case, and with 5s-states of the Nb4+ core in the G-band case. The CTVE-clusters induced by Cr3+ ions produce strong lattice distortion which can be the origin of the uncommonly high red shift of the fundamental absorption band edge detected for the SBN/Cr case.

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, 'green luminescence' and 'red luminescence' is proposed on the basis of the extended photoluminescence experiments in SBN:Cr, and also in SBN:Ce and in nominally pure SBN systems under different conditions. While the RL-band is suggested to be connected with charge transfer vibronic exciton (CTVE) clusters induced by Cr3+ impurities in the Nb-sites, the GL- band is connected with Nb4+ electronic polarons in a new, charge transfer excited states. Here Nb4+ centers are the cores of the CTVE clusters induced by these charged scores. The PL mechanism is the in-cluster CTVE recombination for both bands under discussion. But the CTVE states are quasi-resonantly mixed here with 4T2 states of the Cr3+ core in the RL-band case, and with 5s-states of the Nb4+ core in the GL-band case. The role of excitonic polarons of CTVE nature is also discussed in connection with 'green' luminescence origin in KTaO3 and KNbO3 crystals.

Starting from previous investigations in LiNbO3 bulk crystals, we studied the optical properties of Er3+ ions in Ti:LiNbO3 channel waveguides and investigated the waveguide-specific lattice environment of the Er3+ ions (“sites”) caused by the doping method used and the presence of a large number of Ti4+ ions. For that purpose the method of combined excitation–emission spectroscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-IR luminescence (λ≈550, ≈650 and ≈980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 μm) laser excitation, we found several distinguishable Er3+ sites which in terms of energy levels and relative numbers are similar to those in bulk material, but exhibit significantly different up-conversion efficiencies and strongly inhomogeneously broadened transitions. Moreover, we were able to distinguish isolated and cluster Er3+ sites by their characteristic excitation and emission transition energies and studied the respective excitation/relaxation channels. The cluster sites are most efficient in the up-conversion process, especially under 3-step excitation. Using accepted microscopic models for Er3+ and Ti4+ incorporation into the LiNbO3 crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequences for laser applications in different spectral regions are discussed.

A new type of self-consistent charge transfer–lattice states induced by charged impurities in their vicinity in ionic–covalent matrices are proposed and considered. A theory of these states is based on a charge transfer vibronic exciton (CTVE) model. The CTVE model explains the laser excitation-induced conversion of the sites accompanied by memory effect both caused by Eu3+ impurities in BaFCl crystal. The origin of this phenomenon connects with cross-relaxation between CTVE trapped by impurity, and excited states of this impurity. The CTVE model allows to propose a general mechanism of multi-site structure appearance induced by charged impurities in ionic–covalent solids. This mechanism is considered as an example of multi-site structure caused by RE3+ impurities in LiNbO3 crystal.

The paper reviews the studies of sol-gel produced nanocrystalline γ-Al 2 O 3 and cubic Y 2 O 3 doped with RE 3+ and TM 3+ ions. The effects of spatial confinement (on nanometer scale) in optical properties of sol-gel produced insulating nanocrystalline oxide materials doped with rare earth and transition metal ions are discussed. The experimental studies showed that in insulators these effects are mostly connected with the changes in the vibrational spectrum and the increased role of the surface in nanocrystals. The transformations between the crystalline forms of Al 2 O 3 and the properties of doped corundum ceramics are studied. The methods of sol-gel synthesis of highly dispersed nanocrystalline γ-Al 2 O 3 and cubic Y 2 O 3 doped with RE 3+ and TM 3+ ions in wide range of concentrations are described. It is shown that optical techniques are useful for characterization of sol-gel produced materials.