Mechanism of Th, Zr doping in PbWO4 crystals

To check the doping mechanism of trivalent ion doping in PbWO₄ (PWO) and verify the formation of dipoles [2M_Pb³⁺-V_Pb], the researches on an investigation of tetravalent ion (Th⁴⁺, Zr⁴⁺) doped PWO was conducted by dielectric loss spectroscopy, thermoluminescence and optical absorption spectroscopy. The doping mechanism of tetravalent ion doping is similar to that of trivalent ion doping, while the dipole defects should be formed more easily due to the convenience for the two defects V_Pb and M_Pb⁴⁺ to get close to each other. However, in the case of Zr⁴⁺ doping, Zr was found to enter interstitial sites because of its small ionic radius, with the introduced charges compensated by V_Pb far away and hence does not exert an obvious influence on the performance of this material. Thus, no dielectric relaxation was found in the DS (dielectric spectra) experiment.     

Growth and optical properties of Eu/Li-co-doped SrB4O7 single crystals

This paper reports the growth and optical properties of Eu²⁺/Li⁺-co-doped SrB₄O₇ single crystals. High-quality Eu,Li:SrB₄O₇ crystals without macro-defects or cracks were grown using the top-seeded solution growth (TSSG) method. The absorption and luminescent properties were measured and different spectra were observed in the as-grown crystals. As the doping amount of lithium increases, the absorption peak at 300 nm becomes stronger and the emission peak shifts to a longer wavelength. This phenomenon could be attributed to the doping lithium ions, which might affect the electric field distribution in the lattice structure.     

Low‐dose ion implanted active waveguides in Nd3+ doped near‐stoichiometric lithium niobate: promising candidates for near infrared integrated laser

By applying low‐dose oxygen ion implantation, active planar waveguides in Nd³⁺ doped near‐stoichiometric lithium niobate laser crystals have been, for the first time to our knowledge, successfully produced. The waveguide exhibits good transmission properties with relatively low propagation loss of ～2 dB/cm. The confocal micro‐luminescence investigations indicate that the emission properties of Nd³⁺ ions in the waveguide are well preserved with respect to the bulk, thus showing promising potentials for efficient waveguide laser action operating at the Nd³⁺ near‐infrared bands. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Propagation of high-frequency acoustic waves through the structure of Jahn-Teller ions in lithium niobate with iron

This paper reports on the results of an investigation into the specific features of the propagation of acoustic waves in lithium niobate LiNbO₃ with iron ions under laser irradiation of regions with increased and decreased concentrations of Fe²⁺ ions. It is shown that the observed effects of local variations in the damping and velocity of acoustic waves are associated with the strong electron-phonon interaction of Jahn-Teller Fe²⁺ ions.     

Dynamics of the Yb3+ to Er3+ energy transfer in LiNbO3

The energy transfer dynamics between Yb³⁺ and Er³⁺ ions in lithium niobate is investigated after ytterbium-pulsed excitation at 920 nm. The sensitisation of the LiNbO₃:Er³⁺ system with Yb³⁺ ions does not modify the lifetime of the ⁴I_13/2 erbium level (1.5-μm emission), whereas it induces a marked, concentration-dependent change in the lifetime of the ²F_5/2 (Yb³⁺) and ⁴S_3/2 (Er³⁺) multiplets (1060-nm and 550-nm emissions, respectively). The results are analysed by using the rate-equation formalism and cross-relaxation model for the energy transfer. Received: 15 October 1998 / Revised version: 24 November 1998 / Published online: 24 February 1999     

Spectral separation of Cr<Superscript>3+</Superscript> optical centers in stoichiometric magnesium-doped lithium niobate crystals

The broadband luminescence of chromium optical centers with strongly overlapping spectral lines and similar emission probabilities from excited ⁴ T ₂ states of red and green Cr³⁺ centers in stoichiometric magnesium-doped lithium niobate crystals has been separated for the first time. The spectral-luminescence characteristics and parameters of intracenter interaction between red and green optical Cr³⁺ centers in stoichiometric lithium niobate have been calculated. The luminescence quantum efficiencies of red and green chromium centers are determined.     

Tomographic study of the three-dimensional distribution of a high-fluence implant in a polymer

6 Li⁺ ions were implanted into PMMA at high flux up to fluences of 1×10¹⁵ cm^-2 under angles of 0° to 70° towards the surface normal. The Li depth distributions were determined by means of neutron depth profiling, and compared with theoretical simulations. The three-dimensional Li distribution was reconstructed from the one-dimensional depth profiles by means of a tomographic technique. It turned out that the measured Li depth distributions can be described by a superposition of Gaussian and exponential functions. This points at considerable Li mobility during or after the ion implantation, with trapping in unsaturable traps in the ion-irradiated region which roughly follow the electronic energy transfer distribution. The Li redistribution is more pronounced along the track direction than transversely to it. The normalized Li distributions in various implantation directions were fed into our tomographic program to reconstruct the three-dimensional distribution of the deposited lithium. As expected, the lithium preferentially distributes along the ion tracks. This work is another hint that mobility of implanted ions in solids does not proceed isotropically, but is strongly influenced by the radiation-damage distributions. Received: 11 May 1998 / Accepted: 9 September 1998 / Published online: 24 February 1999     

Monoclinic niobate LaNbO<Subscript>4</Subscript> as a new SRS-active crystal

The effect of the generation of stimulated Raman scattering in monoclinic niobate LaNbO₄, which is known as a base crystal for lasant ions Ln³⁺, has been discovered. The χ⁽³⁾ nonlinear Stokes and anti-Stokes biphonon lasing of this niobate has been studied under picosecond excitation.     

The roles of Li and F ions in Li-F-codoped TiO2 system

An overall comparative study was carried out on Li-doped, F-doped, and Li-F-codoped TiO₂ powders in order to elucidate the roles of Li⁺ and F⁻ ions in photocatalyst. The characteristic data were based on the analysis of XRD, XPS, and PL spectra. The effects of atomic ratio of Li/Ti and F/Ti on the photocatalytic activity were also investigated. As the results, Li doping accelerated the phase formation of rutile in lower temperature while F doping prevented the phase transition from anatase to rutile. Li doping inducted a large amount of O_OH on the surface of TiO₂, while F doping consumed much of O_OH. Li⁺ ions acted as the roles of recombination center of electron-hole pairs while F doping could restrain the recombination of electron-hole pairs on the center of Li⁺ ions. The roles of Li⁺ and F⁻ ions were also confirmed in the experimental section, where the photocatalytic activity of TiO₂ was improved greatly by synergistic reaction of Li⁺ and F⁻ ions.     

Magnetic properties of transition metal substituted La0.85Ag0.15Mn1−yMyO3 compounds (M=Co, Cr and Al)

In this paper we report a systematic study of Mn-site substitution by M=Co, Cr and Al in La_0.85Ag_0.15MnO₃ series to understand the magnetic interactions between Mn and other transition metals. The long-range ferromagnetic (FM) ordering of the parent compound was significantly affected by Mn-site substitution. The measured magnetic properties of Co-doped samples have been explained on the basis of FM interactions in Mn³⁺-O-Mn⁴⁺, Co²⁺-O-Mn⁴⁺, Co³⁺-O-Mn⁴⁺ networks and simultaneous antiferromagnetic (AFM) interactions in Mn⁴⁺-O-Mn⁴⁺, Co²⁺-O-Mn³⁺ networks. The magnetic properties of Cr-doped compounds could be understood on the basis of double exchange FM interactions in Mn³⁺-O²⁻-Mn⁴⁺ networks and competing AFM in Cr³⁺-O-Mn⁴⁺, Mn⁴⁺-O-Mn⁴⁺, Cr³⁺-O-Mn³⁺ networks. However, it is found that the doping of Al ions play a role of magnetic dilution, without contributing any other competing magnetic interaction. The field variations of magnetization of all the above three series could be analysed by fitting to Brillouin function model and the effective spin contribution for FM has been determined. The measured saturation magnetization has been explained quantitatively.     

Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals

The ultraviolet absorption edges of LiNbO₃ crystals with different Li₂O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α=15 or 20 cm⁻¹ of all these crystals fit the Urbach rule perfectly. The origin of this absorption edges in non-stoichiometric LiNbO₃ crystals is attributed to the presence of Li vacancies.     

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr²⁺ and Cr³⁺ replacing magnesium ions at octahedral positions in Mg₂SiO₄: Cr and Mg₂SiO₄: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \(P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr²⁺ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \(P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \(P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr²⁺ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr³⁺ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr³⁺ and Li⁺ ions. Doping with lithium increases the concentration of Cr³⁺ ions and, thus, decreases the fraction of Cr²⁺ and Cr⁴⁺ ions in the total content of chromium centers.     

Multi-range high-frequency EPR spectroscopy of LiYF<Subscript>4</Subscript> and LiLuF<Subscript>4</Subscript> crystals doped by rare-earth ions

EPR spectra of isostructural LiYF₄ and LiLuF₄ crystals doped by Dy³⁺, Er³⁺, and Ho³⁺ ions are measured at 4.2 K in the frequency range 40–800 GHz. The effects caused by isotopic disorder in the lithium sublattice, the random crystal field, and the interaction between paramagnetic impurity ions are detected and studied. The results of the measurements are used to determine the spectral characteristics of the compounds and the crystal field parameters. It is demonstrated that the formation of the isotope structure of the EPR signal is dominated by local deformations of the crystal lattice induced by mass defects.     

Manifestation of a Photorefractive Effect in the Raman Scattering Spectra of Lithium Niobate Crystals of Variable Composition

Using the Raman scattering spectra, we investigated the ordering of the structural units in the cation sublattice and the photorefractive properties of lithium niobate single crystals of variable composition, i.e., nominally pure ones with different Li/Nb ratios and those doped with the nonphotorefractive cations Mg²⁺, Gd³⁺, and Y³⁺. It is shown that at low concentrations of Mg²⁺, Gd³⁺, and Y³⁺ the magnitude of the photorefractive effect is substantially determined by the ordering of the structural units of the cation sublattice. It has been found for the first time that the intensity of the line corresponding to the bridge valence vibrations of oxygen atoms in the octahedrons of NbO₆ is sensitive to the dipole ordering of the cation sublattice of the crystal. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 611–614, September–October, 2005.     

Study on the color tunability and energy transfer mechanism in Tb3+, Sm3+ co-doped LiLaSiO4 phosphors

A several of LiLaSiO₄: xTb³⁺, ySm³⁺ (LLSO) phosphors were synthesized by high-temperature solid-phase reaction. Through SEM, XRD and fluorescence spectrometer, the phase, morphology, luminescence properties and energy transfer of the samples were systematically analyzed and discussed. Under an excitation of 378 nm wavelength, LLSO: xTb³⁺ phosphors emit green light, and the concentration quenching point of Tb³⁺ ions was x = 0.08. In LLSO: xTb³⁺, ySm³⁺ phosphors, When Sm³⁺ ions doping molar mass fraction increases, the fluorescence intensity of Tb³⁺ ion decreases while the fluorescence intensity of Sm³⁺ ions first strengthen and then weaken. The concentration quenching point of Sm³⁺ ions was y = 0.04. By changing the proportion of Sm³⁺ and Tb³⁺ ions, the luminous color can be adjusted from green to red. There is effective energy transfer between Tb³⁺→Sm³⁺. The molar mass fraction of doping Sm³⁺ ions is y = 0.10, the energy transfer efficiency reaches 96.67%. The energy transfer mechanism is the quadrupole-quadrupole interaction. The quantum yield is 22.34%. Therefore, LLSO: xTb³⁺, ySm³⁺ phosphors have certain potential application value in the field of ultraviolet-near ultraviolet white LEDs.     

Raman study of LiTaO3-related non-stoichiometric solid solutions isolated inside the ternary systems Li2O–Ta2O5–(M′O)2 with M′=Mn,Co

We have characterized by Raman spectroscopy the disorder and the local modifications of cation environment in the lithium tantalate structure resulting from the incorporation of bivalent cations Co²⁺ and Mn²⁺ as potential substitutes for Li and/or Ta ions. Frequency and damping of the E(TO1), E(TO6) and E(TO8) phonon modes of ceramic powders are studied along seven lines in the ternary phase diagrams Li₂O–Ta₂O₅–(M′O)₂ with M′=Mn and Co, and compared to those of the pure stoichiometric LiTaO₃. Raman spectroscopy is found to be very sensitive to the substitution ions and defects generated in the lattice vibration. Dopants occupy primarily the Li site in the region of lithium oxide excess. The site of Ta becomes progressively implicated in the substitution process when the concentration of dopant increases. In the Li-poor region of the ternary-phase diagram, corresponding to under-stoichiometric compositions, we retain the charge compensation mechanism involving both Li and Ta site according to: 3Li⁺+Ta⁵⁺→4M′²⁺ with M′²⁺=Mn²⁺ or Co²⁺.     

Manifestation of the specific structural features of lithium niobate single crystals of different composition in their Raman spectra

The Raman spectra of nominally pure lithium niobate single crystals of congruent, close to stoichiometric, and stoichiometric compositions and congruent lithium niobate single crystals doped with Gd³⁺, Y³⁺, and Mg²⁺ ions have been investigated. Weak lines whose widths anomalously decrease with an increase in cation sublattice disordering at a change in the single crystal composition were found for the first time. These lines may be indicative of fine ordering processes involving structural units of the cation sublattice, as a result of which this sublattice is disordered as a whole.     

Polaron luminescence in iron-doped lithium niobate

Photoluminescence related to the bound polaron Nb_Li⁴⁺ is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe²⁺ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.     

High-accuracy contactless method for determination of chemical composition of lithium niobate crystals by their birefringence

A high-accuracy method for determining the chemical composition of non-stoichiometric lithium niobate crystals is proposed based on precise measurements of birefringence of thin substrates using the laser ellipsometry in transmission. It is shown that, for compensators made of LiNbO₃, this method allows one to monitor the ratio Li/(Li + Nb) with an accuracy of up to ∼10⁻⁸ at λ = 0.6328 μm. The accuracy is limited by temperature fluctuations of the crystal.     

Preparation and characterization of lithium niobate as a novel photocatalyst in hydrogen generation

Several chemical compounds based lithium niobate have been tested in the reaction for the photocatalytic hydrogen generation. The photocatalysts have been prepared by impregnation of Nb₂O₅ in the aqueous solution of lithium hydroxide and then the calcination at the temperature range of 400-650 °C. In this report, we present the interesting study showing that the most active catalyst for the photocatalytic generation of hydrogen is the one containing two lithium niobate phases such as LiNbO₃ and LiNb₃O₈. It means that the lithium niobates based catalyst without any further modification or doping can be applied as a novel material for this process.