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In KTaO3:Er crystals, we observed a temperature shift, unusual for rare earth impurities, for narrow zero-phonon luminescence lines, due to the 4S3/2 → 4I13/2, 4F9/2 → 4I15/2, and 4S3/2 → 4I15/2 transitions in Er3+ impurity ions. The magnitude of the shifts turned out to be comparable with the large shifts of the R lines of the Cr3+ luminescence in SrTiO3:Cr and KTaO3:Cr crystals, that is, substantially larger than that for the three-charged rare earth ions usually observed in the f–f luminescence spectra in ionic crystals.

The f–f absorption and emission spectra of Er3+ impurities were observed and identified in KTaO3 and K1 – xLixTaO3 single crystals. It was shown that Li off-centers related random fields markedly influence intensity, width and position of zero-phonon lines. The detail analysis of the absorption spectra obtained allowed one to determine energies of the Stark sublevels of excited states for Er3+ dominant centers of noncubic symmetry.

In magnetic compounds with Jahn–Teller (JT) ions (such as Mn3+ or Cu2+), the ordering of the electron or hole orbitals is associated with cooperative lattice distortions. There the role of JT effect, although widely recognized, is still elusive in the ground state properties. Here we discovered that, in these materials, there exist excitations whose energy spectrum is described in terms of the total angular momentum eigenstates and is quantized as in quantum rotors found in JT centers. We observed features originating from these excitations in the optical spectra of a model compound LaMnO3 using ellipsometry technique. They appear clearly as narrow sidebands accompanying the electron transition between the JT split orbitals at neighboring Mn3+ ions, displaying anomalous temperature behavior around the Neel temperature T N ≈ 140 K. We present these results together with new experimental data on photoluminescence found in LaMnO3, which lend additional support to the ellipsometry implying the electronic-vibrational origin of the quantum rotor orbital excitations. We note that the discovered orbital excitations of quantum rotors may play an important role in many unusual properties observed in these materials upon doping, such as high-temperature superconductivity and colossal magnetoresistance.

Results of a detailed study of optical absorption and photoluminescence of slightly erbium-doped (~50 ppm) strontium titanate (SrTiO 3 ) single crystals are presented. This study allowed us to specify the structure of energy levels that determine the main internal optical transitions within the 4f 11 electron configuration of Er 3+ ions in SrTiO 3 :Er crystals at temperatures below the transition from cubic to tetragonal phase. The numbers of energy levels of the studied multiplets obtained at 2 K correspond exactly to the theoretically expected numbers of crystal field levels for multiplets of only one type of Er 3+ centers with non-cubic symmetry. Experimental results indicate that Er 3+ ions most probably substitute for Sr 2+ ions acting in SrTiO 3 as ionized donors. Such location of Er 3+ ions is consistent with both n-type conductivity and substantial free charge carrier absorption in the infrared spectral region observed in the studied SrTiO 3 :Er single crystals at room temperature.

Cubic perovskite SrTi1−xMnxO3 nanopowders with x = 0–0.5, which is much higher than the conventionally believed Mn incorporation limit, and particles sizes 10–80 nm have been successfully synthesized using the citrate sol–gel method. The crystalline structure, the morphology, the chemical composition and the lattice constant behavior versus the annealing temperature and the Mn concentration have been characterized using X-ray diffraction, scanning electron microscopy and proton-induced X-ray emission techniques. Studies of Raman light scattering and the magnetic properties revealed the activation of the TO2 polar mode and the magnetic ordering effects at low temperatures. Such features are treated as possible evidence of non-d0 Mn-driven transition to a polar phase with multiferroicity in heavily concentrated perovskite SrTi1−xMnxO3 nanopowders.

Optical absorption and photoluminescence of Cr3+ impurity ion probe were studied on nanocrystalline SrTiO3/Cr(0·1%) powders with an average particle size between 13 and 100 nm prepared by the Pechini type polymeric sol–gel method. Only Oh1 cubic perovskite phase was revealed in the powders at room temperature. The optical absorption edge and the position of the zero-phonon emission R-line of the octahedral Cr3+ centres shifted to higher energies with decreasing particle size. The temperature shift of the R-line position appeared for all powders nearly the same as the 'dielectric related' one in the bulk crystals, evidencing that SrTiO3 retains quantum paraelectric behaviour down to a particle size of about 10 nm. However, behaviour of the R-line position was unstable at low temperatures in powders with a particle size of about 13 and 20 nm. This instability was speculated as a manifestation of a possible low temperature ferroelectric phase transition in small enough SrTiO3 nanoparticles.

Technology aspects, structure and characteristic properties of the atmospheric barrier torch discharge technique grown nominally pure and Al (0.1-5%) and Mn (0.05-0.5%) doped ZnO nanocrystalline thin films deposited on fused quartz substrates are reported. The set of films with various thicknesses in the range from 70 nm to 1000 nm were prepared for each composition. XRD and AFM analysis show well formed [001] preferably oriented hexagonal high quality films witch could be operatively fabricated in air atmosphere with controlled grain size. All films under study revealed n-type conductivity except some Mn doped films, which revealed p-type. Spectroscopic optical ellipsometry and luminescence spectroscopy studies evidenced luminescence properties typical for well formed ZnO and appreciable influence of Al doping and grain size on the optical absorption edge. Al doping shifts strongly absorption edge to the higher energies. The grain size decreasing (from 100 to 15 nm) leads to distinct diffusing of the absorption edge accompanied by increase of absorption in the near band edge region. For the smallest grains the pronounced Urbach-type absorption tail can be observed manifesting films inhomogeneity development.

Photoluminescence (PL) spectra of Cr doped BaxSr 1 − x TiO 3 (x = 0–0.84) (BST) thin films were investigated for the first time. XRD patterns revealed typical peaks of BST microcrystalline perovskite phase. Chromium doping of BST thin films was accompanied by the appearance of PL consisting of rather pronounced lines centered at 1.570–1.600 eV (789.8–774.9 nm) at 12 K, for x = 0–0.84, respectively. These lines are attributed to the zero-phonon transitions 2 E → 4 A 2 of octahedral coordinated Cr 3+ ions in Ti sites of BST. At T ≥ 165 K, the intra ion d-d transitions in Cr 3 + are not significantly influenced by the neighborhood of CrO 6 complexes.

The emission and excitation spectra of luminescence of Sn2P2S6 crystals were studied within wide temperature (12–360 K) and spectral (260–1200 nm) regions. Analysis of the photoluminescence emission spectra revealed five overlapping bands of nearly Gaussian shape peaking near 1.98, 1.72, 1.48, 1.28, and 1.02 eV at 12 K. Annealing of the orange tinted crystal in vacuum at 300°C for 30 minutes caused a substantial increase of photoluminescence intensity in comparison to the as-grown crystal and their brown coloration. The origin of photoluminescence of the Sn2P2S6 crystals was connected with sulfur vacancies.

Thermoluminescence (TL) of five nominally pure Sn2P2S6 crystals was studied as a function of an exposure wavelength within wide temperature (12–360 K) and spectral (300–1200 nm) regions. After an exposure to monochromatic light with the wavelength shorter than about 710 nm at 12 K, weak TL was observed within 15–240 K. The glow curves of integral TL were fitted well by superposition of functions describing the shape of glow peak of general order kinetics. This analysis revealed 17 glow peaks with strongly sample-dependent intensities. The activation energies of thermal ionisation and frequency factors corresponding to charge carrier shallow traps related to the glow peaks were found as fitting parameters within 50–542 meV and 107–1011 s–1, respectively. TL was spectrally similar to broad-band photoluminescence in the red and near-infrared spectral regions that can be excited with monochromatic light with the wavelength shorter than about 710 nm at low temperatures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The emission and excitation spectra of luminescence of a Sn2P2S6 crystal were studied in detail within wide temperature (12–360 K) and spectral (260–1000 nm) regions. Illumination with monochromatic light with the wavelength shorter than about 710 nm at low temperatures enabled to observe broad-band photoluminescence in the red and near-infrared spectral regions. Analysis of the emission spectra revealed a superimposition of three overlapping bands of nearly Gaussian shape peaking near 1.75, 1.57, and 1.39 eV at 12 K. At steady-state excitation at 12 K, the changes of photoluminescence intensity were observed with time of crystal illumination. During subsequent heating of the crystal with a rate of 0.082 K/s weak thermoluminescence was observed within 15–130 K. The trap depths of 41 and 88 meV, corresponding to two pronounced glow peaks near 22 and 41 K, were determined by the “initial rise” method.

Decay kinetics of Mn4+ luminescence in SrTiO3 and thermal stimulation of this luminescence (TSL) after low-temperature irradiation with light from the 355—520 nm region, have been investigated in the temperature range of 4.2—150K and 12—110K, respectively, for the first time. It is concluded that TSL glow peaks are associated with thermal release of carriers from shallow traps followed by Mn5+→Mn4+(2E) and/or Mn3+→Mn4+ (2E) charge transfer decay kinetics has been explained by considering trap energy levels taking part in the TSL process.

In Hg 2 Cl 2 crystals broad luminescence bands excited with UV light have been studied in the spectral range 0.8–2.25 eV. The measurements have been performed on as-grown crystals and on crystals previously exposed to UV light at RT. The emission spectra consist of six bands peaking at 0.82, 1.06, 1.23, 1.39, 1.51, and 1.72 eV with FWHM of 0.21–0.33 eV. Their intensities depend conspicuously on the concentration of the photochemical products induced by irradiation of the crystals at RT. It is concluded that the infra-red luminescence of Hg 2 Cl 2 originates from crystal defects, especially that complexes (HgCl x Br 3− x ) − and (HgCl x Br 4− x ) 2− are responsible for these emissions. Excitation of the luminescence can occur: (i) via excitons in the Hg 2 Cl 2 matrix, (ii) via excited states of Hg 2 Br 2 molecules, and (iii) resonantly through the excitation bands of defect centres. A configurational coordinate diagram for the emission observed at 1.51 eV is proposed. A model for the temperature quenching of luminescence and energy transfer between two different kinds of centres is used to explain the more complicated temperature dependence of this luminescence.

The dielectric permittivity and photoluminescence of the title films were studied in detail. SrTiO3:Cr films show a temperature maximum of e′ (T) and a saturation of the TO-mode softening near 100 K. In (SrCa)TiO3:Cr films a temperature maximum of e′ occurs at 28 K; we suggest this is due to an onset of XY, n = 2 quantum ferroelectricity. A ‘blue shift’ of the zerophonon R-luminescence line of octahedral Cr3+ in SrTiO3 films, and an increase of vibronic sideband intensities in comparison with Cr3+ luminescence in the same crystals are reported and discussed. Attention was focussed on SrTiO3:Cr films in order to explain a large and unusual temperature shift of the R-luminescence line position. It is shown that this shift is a consequence of local configurational instability and the off-center position of Cr3+ in the ground 4A2-state and the excited 2E-state, which occurs at least for T < 77 K.

Manganese doping of SrTiO3 results in structured luminescence consisting of narrow zero-phonon (ZPL) R-line of Mn4+ ( 2 E → 4 A 2 ) peaking at 13 826.8 cm−1 (T=15 K) and vibrational sidebands. ZPL of Mn4+ exhibits an unusually large shift with temperature, which follows the reciprocal dielectric constant. The excitation spectrum of Mn4+ consists of two broad bands at 18 182 and 23 585 cm−1, which are assigned to 4 A 2 → 4 T 2 and 4 A 2 → 4 T 1 transitions, respectively. The Mn4+ luminescence intensity decreases with time at steady-state excitation with photon energy higher than 1.9 eV at temperatures lower than 100 K. The behavior of Mn4+ luminescence is discussed taking into account the specificity of impurity–lattice interaction and the possibility of off-center behavior of d3 ions in ABO3 perovskites with soft TO modes.

In addition to the blue and green luminescence usually observed, our CaWO4 crystals show a structured low energy emission which consists of two groups of bands in the red (A) and infra-red (B) spectral region. The B emission is attributed to 4 F 3 2 → 4 I 9 2 transitions in Nd3+ ions by comparison with their absorption properties in CaWO4. We ascribe the A emission to the broadband luminescence of a Ca-rich tungstate phase, where the observed structure is due to reabsorption caused by Nd3+ ions.

The crystal structure, optical absorption, and photoluminescence of chromium impurity centers were studied in nanocrystalline SrTiO(3):Cr (0.1 mol%) powders with average particle size within the range 13-100 nm prepared by the Pechini-type polymeric sol-gel method. Only the presence of a cubic perovskite phase of O(h)(1) symmetry was proved for the powders at room temperature, by means of x-ray diffraction. The lattice constant a = 3.910 Å, larger than that of bulk SrTiO(3) crystals (a = 3.905 Å), was found for nanoparticles with the size about 20 nm. The optical absorption edge and the zero-phonon R-line ([Formula: see text]) of luminescence of the octahedral Cr(3+) centers shifted to higher energies with decreasing nanoparticle size. These size effects were regarded as intrinsic to SrTiO(3). An unusual and large temperature shift of the R-line position very similar to the 'dielectric related' one of the bulk crystals was observed for all powders, evidencing their quantum paraelectric behavior. However, the powders with the average particle size about 13 and 20 nm did not reveal completely reproducible behavior of the R-line position at low temperatures. This instability was considered a possible manifestation of a low-temperature phase transition in small enough SrTiO(3) nanoparticles.