Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

Spectroscopic and crystal field studies of (Ce,Gd)Sc3 (BO3)4:Cr crystals

Complex spectroscopic studies of (Ce,Gd)Sc₃(BO₃)₄:Cr³⁺ (CSB:Cr³⁺) crystals (crystal growth, absorption and luminescence spectroscopy, crystal field calculations, analysis of the radiative and non-radiative decays) are presented. The main results of the paper include calculations of crystal field parameters and energy level scheme for Cr³⁺ at distorted octahedral Sc³⁺ sites, evaluation of the Huang-Rhys factor, effective phonon frequency, zero-phonon line energy, and parameters of radiative and non-radiative decays. Comparison with experimental results and other literature data is discussed. A very unusual value of the frequency factor (related to the non-radiative processes) is explained as being due to heterodesmic nature of chemical bonds in the CSB crystal. Cr³⁺-doped CSB crystals (with Cr³⁺ concentration 5.1×10¹⁹ cm⁻³ or 1%) are suggested as promising candidates for potential applications as active media for solid state lasers.     

Optical properties of Er:SrMoO4 single crystal

Er³⁺:SrMoO₄ crystal of high optical quality was grown by the Czochralski method. The room temperature polarized absorption and emission spectra together with the lifetime decay curve were measured. Based on the Judd-Ofelt theory, three intensity parameters, radiative transition rates, radiative lifetimes and fluorescent branching ratios, were obtained. Emission cross-section and gain cross-section around 1.54 μm were also obtained.     

Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3

The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu.     

EPR and optical investigation of VO(II) in Zn(C3H3O4)2(H2O)2 single crystals: An interstitial site

EPR spectroscopic investigations on single crystals of diaquabis[malonato(1-)-κ²O,O′] zinc(II) doped with VO(II) ion have been carried out at X-band frequencies and at 300 K. The single crystal, rotated along the three mutually orthogonally axes, has yielded spin-Hamiltonian parameters g and A as: g_xx=1.980, g_yy=1.972, g_zz=1.937 and A_xx=8.4, A_yy=6.1, A_zz=18.1 mT, respectively. These spin-Hamiltonian parameters reflect a slight deviation from axial symmetry to rhombic, which is elucidated by the interstitial occupation of vanadyl ions. The isofrequency plots and powder EPR spectrum have been simulated. The percentage of metal-oxygen bond has been estimated. The optical absorption spectrum exhibits four bands at 257, 592, 720 and 764 nm suggesting a C_4v symmetry. The admixture coefficients and bonding parameters have also been calculated by collaborating EPR data with optical data.     

Effect of Ce co-doping on KY3F10:Pr crystals

A spectroscopic investigation on the effect of Ce³⁺ co-doping in fluoride KY₃F₁₀:Pr³⁺ crystals is presented. In particular spectroscopic measurements of three different samples of KY₃F₁₀ crystal doped with 0.3at% Pr³⁺ and co-doped with 0at%, 0.17at% and 0.3at% Ce³⁺ are discussed. Details on the growth of the crystals are also reported. Measurements were performed in the temperature range 10-300 K. Fluorescence and lifetime measurements have shown a cross relaxation between ³P₀-¹D₂ levels of Pr³⁺ and ²F_7/2-²F_5/2 of Ce³⁺. Data exhibit that this effect is strictly related to the Cerium concentration.     

Preparation and properties of disordered NaBi(XO4)2, X=W or Mo, crystals doped with rare earths

NaBi_1−xRE_x(XO₄)₂, X=W or Mo and RE=Pr, Nd, Ho, Er and Yb single crystals have been grown by the Czochralski technique. Rare earth concentrations about 3.5×10²⁰ cm⁻³ have been achieved in crystals with good optical quality. Melt stability is obtained by synthesising NaBi(XO₄)₂ from the precursor Na₂X₄O₁₃ phase and minimising Mo volatility. The strength of W and Mo compounds to chemical attack and thermal annealing in several atmospheres is reported. Mo compound is etched by inorganic acids and becomes coloured after vacuum annealing. The optical absorption, photoluminescence and refractive indices of the hosts are characterised and show a dichroic character. The lattice disorder induces broadening of the 10 K optical absorption of the rare earth impurities.     

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.     

Preparation of Ca(1−x)EuxGa2S4 crystals and their photoluminescence, absorption and excitation spectra

The single crystal of CaGa₂S₄:Eu is expected as a useful laser material with a high quantum efficiency of light emission. However, as far as our knowledge is concerned, the systematic study of the mixed compounds of Ca_(1−x)Eu_xGa₂S₄ as a function of x has not been reported up to now. Here, we have first constructed the phase diagram of the CaGa₂S₄ and EuGa₂S₄ pseudo binary system, and show that it forms the solid solution. Then we have grown single crystals of these compounds. The maximum photoluminescence efficiency is achieved at x=0.25. From the three peak energies observed in the photoluminescence excitation (PLE) and absorption spectra, the 5d excited states are suggested to consist of three levels arising from the multiplets of Eu²⁺ ions.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH2CH2CH2)3N)]Sb4S7

The chromium(II) antimony(III) sulphide, [Cr((NH₂CH₂CH₂)₃N)]Sb₄S₇, was synthesised under solvothermal conditions from the reaction of Sb₂S₃, Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction, elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2₁/n with a=7.9756(7), b=10.5191(9), c=25.880(2) Å and β=90.864(5)°. Alternating SbS₃³⁻ trigonal pyramids and Sb₃S₆³⁻ semi-cubes generate Sb₄S₇²⁻ chains which are directly bonded to Cr(tren)²⁺ pendant units. The effective magnetic moment of 4.94(6)μ_B shows a negligible orbital contribution, in agreement with expectations for Cr(II):d⁴ in a ⁵A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides.     

Optical and electric properties of Nb-doped anatase TiO2 single crystal

Nb-doped anatase TiO₂ single crystal has been grown by chemical vapour transport method. Raman spectra shows that the obtained crystal with Nb of 0.08 wt% has typical anatase structure. An absorption band was observed at around 2.2 eV, which seems to be due to the d-d transition in the conduction band. The electron paramagnetic resonance and electric resistivity measurements show that the doped niobium makes quite shallow donor level whose orbital is d_xy-like centered at the titanium position of anatase.     

Synthesis, crystal structure and photoconductivity of new molecular complex of C60 with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene): Bz4BTPE C60

A new molecular complex of C₆₀ with tetrabenzo(1,2-bis[4H-thiopyran-4-ylidene]ethene), Bz₄BTPE C₆₀ (1) has been obtained. The complex has a layered structure in which closely packed hexagonal layers of C₆₀ alternate with the layers composed of Bz₄BTPE molecules. The complex has a neutral ground state according to UV-vis-NIR spectrum. It has been found that single crystals of 1 show low ‘dark’ conductivity of σ∼10⁻¹⁰ (Ω cm)⁻¹. A 10² increase in photocurrent has been observed upon illuminating the crystal with white light. Photoconductivity of 1 is sensitive to magnetic field with B₀<1 T and increases up to 5% in magnetic field. The photoconductivity spectra of the complex indicate that free charge carriers are generated in the UV-visible range mainly by the Bz₄BTPE excitation (the peaks at 622, 562, 472 and 348 nm) with a possible contribution of charge transfer excitations between neighboring C₆₀ molecules (the peak at 472 nm).     

Optical studies of (NH4)2SO4 single crystal in the paraelectric phase

Transmittance and absorbance spectra of (NH₄)₂SO₄ single crystals along [010] direction were measured at different temperatures (296, 308, 318, 328 and 348 K) in the paraelectric phase. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transitions in (NH₄)₂SO₄ single crystals. The direct and indirect band gaps were shifted towards the longer wavelength with increasing temperature. The data on the allowed indirect transition was analyzed and interpreted in terms of two valence bands originated by spin orbit interaction and crystal field splitting. The momenta E_p were calculated as the difference between E_g1, the first valence to conduction band, and E_g2 for the second valence band at different temperatures. The results of extinction coefficient (k), the refractive index (n), and dielectric constants (ε) were also discussed and calculated as a function of wave length (λ). The heat treatment of the crystals proved that the variations of these optical parameters can be consequence of the internal microstructure changes caused by annealing.     

Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet

The influence of cation substitution on the magnetic properties of single and polycrystals of FeCr₂S₄ spinel has been studied. The tetrahedral A-site substitution of the Fe by Cu in Fe_1−xCu_xCr₂S₄ was found to increase significantly the value of temperature T_m of the spin-glass like magnetization anomaly, whereas the octahedral B-site substitution of the Cr by In decreases T_m. This effect is suggested to result from a structural transformation influenced by variation of internal (chemical) pressure due to lattice contraction (Cu) or expansion (In). The observed reduced values of the Curie temperature for Cu-substituted single crystals compared to that of the polycrystalline samples are attributed to presence of Cl ions in samples detected by electron-probe microanalysis. The observed reduced value of saturation magnetization in the polycrystals compared to the single crystals is ascribed to the effect of surface anisotropy. Based on the experimental data the superexchange is concluded to be the dominant exchange interaction for 0≤x≤0.5 in Fe_1−xCu_xCr₂S₄, whereas the indirect exchange through the charge carriers is considered of minor importance.     

Synthesis of goethite from Fe(OH)2 precipitates: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration <∼1 μg kg⁻¹) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)₂ precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO₄·7H₂O and NaOH as reactants with the following experimental conditions: temperature=45 °C, [FeSO₄·7H₂O]=0.50 mol L⁻¹, [NaOH]=0.20 mol L⁻¹, stirring speed=760 rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe₃O₄ when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of δ-FeOOH or to domains structurally similar to δ-FeOOH inside the multidomainic crystals of δ-FeOOH.     

Luminescent properties of R2(MoO4)3:Eu (R=La, Y, Gd) phosphors prepared by sol-gel process

A series of red phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) have been synthesized by sol-gel method. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA), and the properties of these resulting phosphors have also been characterized by photoluminescence (PL) spectra and reflectance spectra. Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and the size of the samples. The results of TG-DTA and XRD indicated that all of the R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors crystallized completely at 650 °C. Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ have two structures, monoclinic and orthorhombic, while La_0.8Eu_1.2(MoO₄)₃ only adopts monoclinic structure. The luminescent properties of phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) are dependent on their structures to some extent. The orthorhombic Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ phosphors show very similar luminescent properties, which differ from those of phosphors with monoclinic structure. For all of R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors, intense red emission is obtained by exciting at ∼394 and ∼465 nm which are owing to the sharp ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ lines of Eu³⁺. Two strongest lines at 394 and 465 nm in excitation spectra of these phosphors match well with the two popular emissions from near-UV and blue GaN-based LEDs, so they could be used as red components for white light-emitting diodes.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

Conical parametric scattering of a single extraordinary beam in BaTiO3 crystal

In this paper, the parametric scattering of a single extraordinary polarized beam of laser in BaTiO₃ photorefractive crystal has been investigated experimentally and theoretically. The resulting pattern consists of beam fanning, isotropic ring, and anisotropic one. Among all parts of scattering pattern, isotropic ring has not been studied as much as beam fanning and anisotropic ring, and there still are some differences in reports about it. Therefore, the study has mainly focused on this part. In this experimental configuration, isotropic ring is just visible in positive angles although the other parts of parametric scattering pattern can be visible from behind and in front of the crystal. In addition to steady state pattern in forward and backward directions, its transient behavior with the rotation of crystal has been studied. The results of experiments have been analyzed carefully, and their theoretical explanations have been presented based on the standard theory of parametric scattering in photorefractive crystals. It has been shown that this configuration corresponds to the so called parametric B-process scattering.     

Optical spectra and energy levels of the Cr ions in MWO4 (M=Mg, Zn, Cd) and MgMoO4 crystals

Single crystals of MWO₄ (M=Mg, Zn, Cd) and MgMoO₄ doped with Cr³⁺ have been grown by the flux growth method. Their optical spectra have been systematically measured and assigned on the basis of the classical Ligand Field Theory. The exchange charge model of the crystal field has then been applied to calculate the crystal field parameters (CFPs) and the energy levels of the Cr³⁺ ion in all studied crystals. These are in reasonable agreement with the experimental data. Systematic trends in the CFPs values, crystal field splittings and Racah parameters have been evidenced and their relation with sizes and symmetry properties of the host cavities occupied by Cr³⁺ has been pointed out.