E. Tomaszewicz

Eu3+ luminescence from different sites in a scheelite-type cadmium molybdate red phosphor with vacancies

Eu3+-doped scheelite-type cadmium molybdate with the chemical formula Cd1−3xEu2x□xMoO4 (vacancy is denoted by □) was investigated as a red-emitting conversion phosphor for white light emitting diodes (WLEDs) by taking advantage of the Eu3+ spectroscopic probe to analyze in detail the structural properties as a continuation of our previous analysis on both, Cd1−3xNd2x□xMoO4 and Cd1−3xYb2x□xMoO4. A series of microcrystalline samples were synthesized using a high-temperature solid state reaction. X-ray diffraction patterns indicate that the samples show only one phase and crystallize in the tetragonal scheelite-type structure (space group I41/a with point symmetry S4), when the concentration of Eu3+ ions is in the range from 0.05 mol% to 66.67 mol% with respect to Cd2+ ions. Substitution of divalent Cd2+ by trivalent Eu3+ cations leads to the formation of cationic vacancies in the framework due to the charge compensation: 3Cd2+ → 2Eu3+ + □ vacancy. The □ vacancy concentration dependence brings some originality to this research program in optical materials. High-resolution photoluminescence excitation and emission spectra were investigated. Time-resolved laser spectroscopy performed at room temperature and at 77 K allowed the separation of lines corresponding to transitions from the 5D1 and 5D0 levels, respectively, and to define the contributions of the individual emission lines in the steady state spectrum. Low temperature fluorescence spectra obtained by applying laser site-selective excitation confirmed the presence of multisite characteristics of Eu3+ ions in this host lattice, similar to that found for Nd3+ and Yb3+ dopants. Three types of Eu3+ sites have been identified. These phosphors are efficiently excited by UV light, and exhibit a very strong red luminescence corresponding mainly to the electric dipole 5D0 → 7F2 transition at 616 nm, and thus they are considered as promising phosphors for WLEDs. Concentration quenching of Eu3+ luminescence as well as the optimum doping of optically active ions were investigated.

Structural and spectroscopic characterizations of two promising Nd-doped monoclinic or tetragonal laser tungstates

The tetragonal Cd0.25Nd0.50□0.25WO4 compound (□ – statistically distributed vacancies in cation sublattice) and its gadolinium diluted Cd0.25Gd0.50□0.25WO4:Nd3+ analogue as well as the monoclinic CdNd2W2O10 phase and its yttrium diluted CdY2W2O10:Nd3+ analogue were synthesized by the high temperature solid-state reaction. The gadolinium or yttrium diluted samples were doped with different concentrations of the active Nd3+ ion: 0.25, 0.5, 2.5 and 5 mol%. X-ray powder diffraction, IR spectra as well as electron microscopy (SEM and HRTEM) were used to identify their structure and morphology. All micromaterials were obtained as spherical particles with average diameters between 1 and 10 μm. In order to study their spectroscopic properties, high resolution absorption and emission spectra in the visible and IR regions were measured at room temperature, 77 K and 4 K. The kinetics of luminescence from the lowest 4F3/2 energy level as a function of the Nd3+ ion concentration and temperature is also reported. Both the values of the absorption cross-section calculated for 5 mol% of Nd3+ ion, and the very strong emission of the 4F3/2 → 4I11/2 laser channel of Nd3+ recorded under Xe lamp excitation or OPO laser pumping, indicate that the scheelite-type Cd0.25Gd0.50□0.25WO4:Nd3+ phase can be considered as a promising laser material. The radiative lifetimes of IR luminescence are also appropriate for potential applications of this phosphor as a solid-state laser.

Structural and spectroscopic characterizations of new Cd1−3xNd2x□xMoO4 scheelite-type molybdates with vacancies as potential optical materials

The necessity to develop new laser materials of tungstates and molybdates is our motivation to study a new family of molybdates activated by Nd3+ ions, which crystallizes in the scheelite-type structure (the tetragonal symmetry, the space group I41/a). A series of Cd1−3xNd2x□xMoO4 stable solid solutions with concentration of Nd3+ ions from 0.1 mol% to 66.67 mol% with respect to Cd2+ ions were successfully synthesized by a high-temperature annealing, using CdMoO4 and Nd2(MoO4)3 as the starting materials. Structural and spectroscopic characterizations of Nd3+ substituting Cd2+ ions were carried out. The substitution of divalent Cd2+ by trivalent Nd3+ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □). The Nd3+ ions in this matrix occupy two non-equivalent symmetry sites depending on the location of these vacancies. Concentration of vacancies depends essentially on the composition of initial CdMoO4–Nd2(MoO4)3 mixtures. The SEM analysis of crystal morphology reveals high homogeneity of the nearly-spherical shaped products with an average grain size of about 1–10 μm. Optical analysis and the laser parameters suggest that Cd1−3xNd2x□xMoO4 can be considered as a potential laser material. Both the values of the absorption cross-section and the strong emission of the 4F3/2 → 4I11/2 laser channel of Nd3+ recorded under LED source excitation or OPO laser pumping, as well as the radiative lifetimes of NIR luminescence, are appropriate for potential applications of this optical material as a solid state laser.

Dielectric and magnetic permittivities of three new ceramic tungstates MPr2W2O10 (M = Cd, Co, Mn)

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε r = 884) for MnPr2W2O10, a smaller value (ε r = 156) for CoPr2W2O10 and the smallest value (ε r = 22) for CdPr2W2O10 at low frequency (ν = 0.1 Hz) and above room temperature in the insulating and paramagnetic state. Below 273 K, the relative permittivity (ε r ∼ 24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr2W2O10 was a ferrimagnetic order below 45 K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism.

Development of Nd3+-doped Monoclinic Dimolybdates La2Mo2O9 as Optical Materials

The work presented is mainly focused on synthesis and study of structural and optical properties of microcrystalline Nd3+-doped monoclinic dilanthanum dimolybdate at both room and cyrogenic temperatures (4K and 77 K). These compounds might be useful for application in the future as optical materials and also as transparent ceramics when the structure is cubic. The Nd3+-doped phases with monoclinic structure (α-form, space group P21, unit cell parameters a = 7:1426, b = 7:1544, c = 7:1618 Å and β = 89:538°) were observed for a concentration of the optically active ions equal to 5%. When the concentration of the Nd3+ ions is higher than 15%, a cubic structure is formed (β-form, space group P213, with the lattice parameter a = 7:155±0:005 Å). A series of Nd3+- doped La2Mo2O9 phases with different concentration of Nd3+ were prepared using conventional solid-state reactions. The formation of phase-pure Nd3+-doped La2Mo2O9 has been monitored by powder X-ray diffraction, DSC, SEM, Raman, and FT-IR absorption techniques. High-resolution absorption and emission spectra, as well as the dynamics of the Nd3+ excited states characterized by decay time measurements were recorded from room temperature to 4 K. At least two slightly different crystallographic sites are available for the Nd3+ ions. First results show that this new Nd3+-doped monoclinic La2Mo2O9 molybdate phosphor is promising for applications of ultra-short pulse lasers. Graphical Abstract Development of Nd3+-doped Monoclinic Dimolybdates La2Mo2O9 as Optical Materials

Paramagnetic Behaviour in RE2W2O9 Tungstates (RE = Pr, Nd, Sm – Gd)

Magnetic susceptibility measurements revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to the magnetic field and the temperature for Sm2W2O9 and Eu2W2O9 tungstates. The temperature independent component of magnetic susceptibility has a positive value for RE2W2O9 (RE = Pr, Nd and Gd) indicating a domination of van Vleck contribution. Only for Gd2W2O9 the magnetization is a universal function of µ0H/T, characteristic for the superparamagnetism.

Polarized Raman and IR spectra of oriented Cd0.9577Gd0.0282□0.0141MoO4 and Cd0.9346Dy0.0436□0.0218MoO4 single crystals where □ denotes the cationic vacancies

Polarized Fourier Transform IR and Raman spectra of Cd0.9577Gd0.0282□0.0141MoO4 and Cd0.9346Dy0.0436□0.0218MoO4 oriented single crystals have been recorded and analyzed using the factor group approach (□ denotes the cationic vacancies). The tetragonal I41/a (C4h(6)) space group with Z=2 has been applied in the discussion. The influence of the structural changes induced by the defects in the CdMoO4 host lattice on the vibrational symmetry rules has been analyzed. The assignment of the observed bands to the internal and external modes has been proposed.

Electrical and Magnetic Properties of CuEu2W2O10 and Cu3Eu2W4O18

Magnetic susceptibility measurements showed both a weak response to magnetic field and a lack of the Curie-Weiss region for CuEu2W2O10 and Cu3Eu2W4O18 tungstates characteristic for the multiplet widths comparable to thermal energy. Magnetization measurements displayed the linear temperature dependence with the lower magnetic moment for Cu3Eu2W4O18 in comparison with CuEu2W2O10, indicating that the effect of the electric charges associated with the surrounding ligands can change the multiplet width of individual states. It is affecting the electrical properties of examined tungstates which reveal the insulating state and low relative permittivity εr ~ 29 in case of CuEu2W2O10 and the thermally activated p-type electrical conduction for Cu3Eu2W4O18 with the activation energy of 1.11 eV and the large value of εr ~ 217 above the room temperature.

Magnetic Properties of CdMoO4:Dy3+ Single Crystal

X-ray diffraction measurement at 298 K of CdMoO4:Dy3+ showed that the molybdenum ions are tetrahedral coordinated and Cd/Dy – dodecahedral coordinated. The Dy3+ ions are randomly distributed in the unit cell, substituting the Cd2+ ones. The temperature dependence of ac and dc magnetic susceptibility showed a lack of the Curie-Weiss behaviour and a weak response to the magnetic field. The magnetization isotherms, M(H), showed a paramagnetic-diamagnetic transition at 17 K for 〈100〉 direction and at 35 K for 〈001〉 one in the magnetic field of 70 kOe. As the temperature increased this transition was moving toward smaller magnetic fields.

Influence of Co Moment on Magnetic Properties of Co2Sm2W3O14 Tungstate

The Co2Sm2W3O14 compound crystallizes in the orthorhombic system, and melts congruently at 1443 K. The magnetic measurements showed that Co2Sm2W3O14 is a paramagnet in the temperature range 4.2-225 K showing both the residual magnetic interactions since the paramagnetic Curie-Weiss temperature   0 and the uncompensated temperature independent contributions of magnetic susceptibility since 0  0. The Brillouin fit of the Landé factor revealed an increase of the orbital contribution to the total magnetic moment of the compound what seems to be responsible for its hard and spontaneous magnetization at low temperatures.