The influence of auxiliary codopants on persistent phosphor Sr2P2O7:Eu2+,R3+ (R = Y,La, Ce,Gd, Tb and Lu)

We investigate the persistent luminescence in europium-doped strontium pyrophosphate upon codoping with auxiliary rare earth ions. The persistent phosphors are synthesized via solid-state reaction method under flowing N₂ + H₂. Under UV irradiation, broadband emission persistent luminescence located at 420 nm is observed in all of these phosphors at room temperature. The effects of auxiliary rare earth ions on Sr₂P₂O₇:Eu²⁺ are discussed according to the decay curves and thermoluminescence spectra. Sr₂P₂O₇:Eu²⁺,Lu³⁺ shows the best performance, while and La³⁺ and Ce³⁺ codoped samples are the weakest. The influence of auxiliary codopants is discussed in terms of ionic potential and ionic radius. We derive an empirical formula based on the experimental results.     

Pressure-induced order–disorder transitions in pyrochlore RE2Ti2O7 (RE = Y,Gd)

Structure evolution of pyrochlore titanates Y₂Ti₂O₇ and Gd₂Ti₂O₇ at high pressure was investigated with Raman and X-ray diffraction methods. Pressure-induced order–disorder transitions are found in titanate pyrochlores. The cations and anions in Y₂Ti₂O₇ began to be disordered at pressures above 50 GPa and the structure changes from pyrochlore (Fd-3m) to defect fluorite structure (Fm-3m) A₄O₇. This transition is not stable in Gd₂Ti₂O₇, and amorphization proceeds at pressure above 32 GPa.     

Influence of short time milling in R5(Si,Ge)4, R = Gd and Tb,magnetocaloric materials

The effect of the short milling times on R₅(Si,Ge)₄ R = Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5 h), a reduction of the Gd₅Si_1.3Ge_2.7 and Tb₅Si₂Ge₂ particles size was achieved down to approximately 3.5 μm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd₅Si_1.3Ge_2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 min of milling. Interestingly, an opposite effect is observed in Tb₅Si₂Ge₂ where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process.     

Crystal structure and non-linear properties of A2(MoO4)3 (A = Eu,Gd, Tb,Dy and Ho)

Ferroelectric–ferroelastic rare earth (RE) molybdates RE₂(MoO₄)₃ have been prepared by the conventional solid-state synthesis and X-ray powder diffraction has been collected using two diffractometers PANalytical X'Pert Pro at room temperature. The crystal structures have been calculated by Rietveld refinement, using a new symmetry modes procedure in order to evaluate the correlation between the distortions, with respect to the paraelectric–paraelastic structure, with the ionic radii. Although the coordinates of the oxygen atoms are not very precise and accurate, we have observed different behavior of the ferroelastic–ferroelectric order parameter.     

Synthesis and luminescent properties of SrZnO2:Eu3+,M+ (M = Li,Na, K) phosphor

In this paper, a series of novel luminescent materials, SrZnO₂:Eu³⁺,M⁺ (M = Li, Na, K) have been synthesized by conventional solid-state reaction. X-ray diffraction (XRD) patterns and photoluminescence (PL) spectra were carried out to characterize their structural and luminescent properties. It was found that under ultraviolet excitation with a wavelength of 301 nm, SrZnO₂:Eu³⁺ gives a red luminescence that was attributed to the transitions from ⁵D₀ excited states to ⁷F_J (J = 0–4) ground states of Eu³⁺ ions. The feature and the high intensity of hypersensitive transition ⁵D₀ → ⁷F₂ indicate that Eu³⁺ prefers to occupy a low symmetry site. The incorporation of alkali metal ions greatly enhanced the luminescence intensity and slightly changed the excitation and emission peak position, probably due to the influence of the coordination conditions for Eu³⁺ ions.     

Study on luminescent properties of Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors prepared by co-precipitation

Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors with different concentrations have been prepared by co-precipitation. XRD (X-ray diffraction) and SEM (scanning electron microscopy) were used to investigate the structure and morphology. The emission spectra, excitation spectra and fluorescence decay curves were measured, and partial J-O parameters and quantum efficiencies of Eu3+ 5D0 energy level were calculated. Furthermore, concentration quenching curves of Eu3+ in different hosts were drawn. The photoluminescent properties of Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors have been studied. The results indicate that Eu3+ 5D0-7F2 red luminescence can be effectively excited by 395 nm and 465 nm in Gd2WO6 and Gd2W2O9 hosts, similar to the familiar Gd2(WO4)3:Eu. Especially Gd2W2O9:Eu has strong red emission and high quenching concentration, so it has potential applications for trichromatic white LED as red fluorescent materials.     

The VUV–vis spectroscopic properties of phosphors Ca3Gd2(1−x)Ln2x(BO3)4 (Ln3+ = Ce,Sm, Eu,Tb)

The spectroscopic properties in VUV–vis range for phosphors calcium and gadolinium double borate Ca₃Gd₂(BO₃)₄ doped with rare-earth ions Ce³⁺, Sm³⁺, Eu³⁺ and Tb³⁺ were investigated. The host-related absorption, the f–d transitions of Ce³⁺ and Tb³⁺, as well as the charge transfer transitions of Sm³⁺ and Eu³⁺ in the host lattice are assigned and discussed. The CIE chromaticity coordinates for Eu³⁺- and Tb³⁺-activated phosphors are calculated.     

Absorption spectra of Er3+ ions in Li6Y(BO3)3 crystals

The optical absorption spectra of Li₆Y(BO₃)₃:Er³⁺ crystals have been studied. The absorption lines corresponding to intraconfiguration electronic transitions from the ⁴I_15/2 ground state to the levels of excited states of Er³⁺ ions have been found in the spectral range of 370–700 nm. The transitions to the ⁴F_9/2, ⁴S_3/2, ²H_11/2, ⁴F_7/2, ⁴F_5/2, ⁴F_3/2, ²H_9/2, and ⁴G_11/2 levels have been investigated in detail at 2 K. The number of lines observed for these transitions corresponds to the theoretical maximum for the f-f electronic transitions in Er³⁺ ions located in the noncubic crystal field. The narrowness of the lines observed and their number indicate that erbium occupies one regular position (specifically, the yttrium position). The energy levels of the excited states have been determined for the transitions under study.     

Photoluminescence properties of Y0.95 − xMxBO3:Eu3+ (M = Ca,Sr, Ba,Zn, Al, 0 ≤ x ≤ 0.1) in 100–450 nm regions

The single phases of Y_0.95 − xM_xBO₃:5%Eu³⁺ (M = Ca, Sr, Ba, Zn, Al, 0 ≤ x ≤ 0.1) were synthesized successfully by solid-state reaction. Their luminescent properties were studied under UV and VUV excitation. The results indicated that with the incorporation of Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺ or Al³⁺ into the host lattice of YBO₃:Eu³⁺, the high symmetry around Eu³⁺ was destroyed and the ratio of red emission(⁵D₀–⁷F₂) to orange one (⁵D₀–⁷F₁) increased, leading to a better chromaticity. Furthermore, the co-doping ions such as Ca²⁺, Zn²⁺ and Al³⁺ were beneficial to enhance the luminescent intensity of Eu³⁺. These phenomena were evaluated, and possible explanations were proposed.     

VUV–UV spectroscopic properties of RE (RE = Ce, Eu and Tb)-doped KMLn(PO4)2 (M = Ca, Sr; Ln = Y, La, Lu)

The VUV excited luminescent properties of Ce³⁺, Eu³⁺ and Tb³⁺ in the matrices of KMLn(PO₄)₂ (M²⁺ = Ca, Sr; Ln³⁺ = Y, La, Lu) were investigated. The bands at about 155 nm in the VUV–UV excitation spectra are attributed to the host lattice absorption, which indicates that the optical band gap of KMLn(PO₄)₂ is about 8.0 eV. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 300–450 nm, and the energy transfer from host lattice to Ce³⁺ is efficient. For Eu³⁺-doped samples, the O²⁻–Eu³⁺ CTBs are observed to be at about 228 nm except KSrLu(PO₄)₂:Eu³⁺ (247 nm). As for Tb³⁺-doped samples, typical 4f → 5d absorption bands in the region of 175–250 nm were observed.     

Investigation on the luminescence of RE3+ (RE = Ce,Tb, Eu and Tm) in KMGd(PO4)2 (M = Ca,Sr) phosphates

The VUV excited luminescent properties of Ce³⁺, Tb³⁺, Eu³⁺ and Tm³⁺ in the matrices of KMGd(PO₄)₂ (M = Ca, Sr) were investigated. The bands at about 165 nm and 155 nm in the VUV excitation spectra are attributed to host lattice absorptions of the two matrices. For Ce³⁺-doped samples, the Ce³⁺ 5d levels can be identified. As for Tb³⁺-doped samples, typical 4f–5d absorption bands in the region of 175–250 nm were observed. For Eu³⁺ and Tm³⁺-doped samples, the O²⁻–Eu³⁺ and O²–Tm³⁺ CTBs are observed to be at about 229 nm and 177 nm, respectively. From the standpoints of color purity and luminescent efficiency, KCaGd(PO₄)₂:Tb³⁺ is an attractive candidate of green light PDP phosphor.     

New powellite type oxides in Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi)—Their synthesis, structure and dielectric properties

New complex oxides having powellite (CaMoO₄) type structure in the Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi) were prepared employing the method of solid state reaction between the component oxides at high temperature (1000–1100 °C). The new compounds, CaRNbMoO₈ (R = Y, La, Nd, Sm, Bi) are colorless and electrical insulators. The dielectric constants (K at 1 MHz) of these compounds are in the range 14–33 and K shows very little variation in the temperature range 30–100 °C. Their temperature coefficient of dielectric constant (TCK) is negative, which varies from − 21 to − 220 ppm/°C.     

Fracture of Gd2 (MoO4)3 single crystals

Fracture behaviour of Gd2 (MoO4)3 single crystal, which is improper ferroelectric–ferroelastic, has been examined by means of point loading. There are three fracture planes in gadolinium molybdat (GMO): ;100} – main cleavage plane, {210} and {110} – secondary cleavage ones. It is shown that cracks have a tendency to transit from secondary planes up to cubic plane and vice versa. This would be considered as the main cause of river pattern appearance on fracture surfaces of samples. Mechanical twinning and crack growth are independent channels for relaxation of elastic energy in GMO, which do not connected between themselves, so excluding of twinning leads to increase of crack length.     

Electronic structure of the LiAA′O6 (A = Nb,Ta, and A′ = W,Mo) ceramics by modified Becke-Johnson potential

DFT is used to study various transition metal based ceramics LiAA′O₆ (A = Nb, Ta, and A′ = W, Mo) in tetragonal phase with space group 421 m (No. 113). The calculated structural and geometrical parameters are found in closed agreement with the experiments. Electronic clouds explain the chemical bonding and reveal that Li atom occupy central position and form ionic bond. Other bonds in these compounds are significantly covalent due to the sharing of electrons between O and A/A′. The electronic properties demonstrate that these compounds are wide bandgap semiconductors in the energy range of 2.18–2.60 eV. These bandgap energies confirm the suitability of these oxides in optoelectronic devices operating in the visible range of the electromagnetic spectrum.     

Study of the R-(Zr,W)-(O,N) (R = Y, Nd, Sm, Gd, Yb) oxynitride system

The replacement of tantalum by the couple Zr/W within the RTa-O-N systems (R = Y, Nd, Sm, Gd, Yb), enables the preparation of novel oxide and oxynitride phases in the R-Zr-W-O-N system. R₂Zr_2−xW_xO_7+x oxides exhibit the fluorite-type (x < 0.9) and scheelite (x ∼ 1) structures. Corresponding oxynitride compositions are of the fluorite-type and show different colors, for example in the case of ytterbium: pale yellow (x = 0.2 or 0.25), green (x = 0.5-0.8) and brown for the tungsten-rich samples (x = 0.9, 1). Photocatalytic activity measurements have been performed to investigate the overall water splitting behavior of these colored phases.     

Application of a europium complex,Eu(AA)3phen (AA = acrylic acid,phen = 1,10-phenanthroline) as a spectroscopic probe and cleaving reagent of DNA

A complex Eu(AA)₃phen was synthesized and characterized by elemental analysis, IR spectroscopy and ¹H NMR. Interaction between Eu(AA)₃phen and DNA was studied by UV/visible absorption spectroscopy, fluorescence spectrophotometer, circular-dichroism (CD) spectra and gel-electrophoresis measurements. Absorption spectral indicated that Eu(AA)₃phen binding to DNA was an electrostatic mode, which was authenticated by the effect of ionic strength, thermal denaturation and fluorescence quenching experiments. The intrinsic binding constant K_b of Eu(AA)₃phen was calculated to be 1.6 × 10⁴ L mol^? 1.     

Synthesis RMn2O5 (R = Gd and Sm) nano- and microstructures by a simple hydrothermal method

Single-phase RMn₂O₅ (R = Gd and Sm) nano- and microstructures have been successfully synthesized via a simple hydrothermal process at 250 °C for 24 h using NaOH as mineralizer. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selective area electron diffraction patterns (SAED) were used to characterize the as-synthesized GdMn₂O₅ and SmMn₂O₅ samples. The effect of NaOH concentration and the molar ratio of Mn²⁺/Mn⁷⁺ on the morphology and size of the final products was studied, and a possible formation mechanism of RMn₂O₅ (R = Gd and Sm) nanoplates and nanorods under hydrothermal conditions was proposed.     

Crystal structure and optical properties of oxynitride rare-earth tantalates RTa-(O, N) (R = Nd, Gd, Y)

X-ray absorption and photoluminescence were used to investigate the structure of rare-earth tantalum oxynitrides RTa-(O, N) (R = Nd, Gd, Y). Owing to the size of the rare-earth element, the crystal structure tends towards the pyrochlore or defect fluorite-type structure. EXAFS suggested neodymium and yttrium atoms are coordinated either by 6 + 2 or 6 oxygen/nitrogen atoms in the Nd or Y respective oxynitrides although the coordination number of tantalum is six in both compounds. Photoluminescence for 5 at.% doped europium showed the spectra compatible with the point symmetry C_3v lower than O_h in fluorite and D_3d in pyrochlore structure type for both the Gd and Y tantalum oxynitrides. These measurements supported that their structure types were basically pyrochlore for Nd and Gd tantalum oxynitrides and defect fluorite for Y tantalum oxynitride but they are highly defective.     

Phonon spectra of eulytite crystals Bi4M3O12 (M = Ge,Si): ab initio study

In this paper we present the results of ab initio DFT calculation of phonon spectra for bismuth ortho-germanate Bi₄Ge₃O₁₂ and bismuth ortho-silicate Bi₄Si₃O₁₂ crystals, in the center of the first Brillouin zone. First, the geometry optimization was performed using the analytical energy gradients, with respect to atomic coordinates and unit cell parameters. Vibrational frequencies and normal modes were calculated within the harmonic approximation by diagonalizing the mass-weighted Hessian matrix. The IR and Raman spectra of both crystals were simulated with the periodic ab initio Crystal 09 code and B3LYP hybrid functional and the two sets of Transverse-Optical and Longitudinal-Optical frequencies are generated, together with their intensities. Also, the influence of isotopic substitution for Bi, Ge and O in phonon modes and the picture with values of frequencies shift in each mode by isotopic substitution were calculated. The obtained results are discussed and the comparision between the computed spectra and experimental data is quite satisfactory, which justifies the model and simulation scheme used for the title systems.