Double Tungstate Research Articles

Calcination-free crystallization of rare earth double tungstate Na(La1 − xEux)(WO4)2 by sacrificial-template hydrothermal reaction, restrained concentration quenching, and optical thermometry

Calcination-free crystallization of rare earth double tungstate Na(La1 − xEux)(WO4)2 by sacrificial-template hydrothermal reaction, restrained concentration quenching, and optical thermometry

Polarized spectroscopy of Sm3+ ions in monoclinic KGd(WO4)2 crystals for lasers emitting in the red

Single-crystals of potassium gadolinium double tungstate KGd(WO4)2 doped with Sm3+ ions up to 20 at.% were grown from the flux using K2W2O7 as a solvent and a detailed polarization-resolved spectroscopic study of Sm3+ ions was performed with the goal of developing novel materials emitting in the visible spectral range. Polarized absorption spectra were measured and 4f-4f transition intensities of Sm3+ ions were calculated using a modified Judd-Ofelt theory yielding the intensity parameters of Ω2 = 8.027, Ω4 = 7.210 and Ω6 = 2.322 [10−20 cm2] and α = −0.017 [10−4 cm]. Polarized stimulated-emission properties of Sm3+ ions were studied. For the 4G5/2 → 6H9/2 transition in the red, the maximum σSE is 0.55 × 10−20 cm2 at 649.0 nm for light polarization E || Np (luminescence branching ratio: 40.2 %). The radiative lifetime of the 4G5/2 state is 734 μs. The luminescence dynamics from this manifold was analyzed using the Inokuti-Hirayama model. The possible role of 4f – 4f excited-state absorption from the 4G5/2 level in preventing laser operation in the orange and deep red is analyzed. A 0.8 at.% Sm:KGd(WO4)2 laser generated an output power of 37.8 mW at 649 nm, with a laser threshold of 87 mW and a slope efficiency of 17.6 %.

Extendible synthesis of NaLnW2O8∙2H2O (Ln=Sm–Lu, and Y) via one-step hydrothermal reaction: Phase, morphology and luminescence

The Hydrous lanthanide (Ln) double tungstates of NaLnW2O8·2H2O in orthorhombic structure have been directly produced by hydrothermal reaction of Na2WO4 and Ln(NO3)3 at pH=8 for 24 h under 200 °C for the small ions of Ln = Tm–Lu, under 150 °C for the slightly larger lanthanides of Ln = Ho, Y and Er, and under 100 °C for the even larger lanthanides of Sm–Dy. Moreover, NaLnW2O8·2H2O for Ln = Eu–Dy can also be synthesized at a slightly higher reaction pH of 9 at 150 °C, while it was failed for the other members in the lanthanide series of La–Nd even though various efforts were attempted in hydrothermal conditions. On the basis of the crystal structure of NaLnW2O8·2H2O and lanthanide contraction, this outcome conformed well to Pauling rules. Taking Ln = Lu for example, the phase evolution and morphology were studied by the combined techniques of XRD, FE-SEM, IR and TG/DSC. The photoluminescence properties of NaEuW2O8∙2H2O and NaTbW2O8∙2H2O, including excitation/emission, CIE chromaticity, quantum yields, and fluorescence decay were characterized.

Red Sm:KGd(WO4)2 laser at 649 nm.

We report on the first laser operation of a Sm3+-doped monoclinic KGd(WO4)2 double tungstate crystal in the red spectral range. Pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) at 479.1 nm, the 0.8 at.% Sm:KGd(WO4)2 laser generated an output power up to 17.6 mW at 649.1 nm (the 4G5/2 → 6H9/2 transition) with a slope efficiency of 16.9%, a laser threshold down to 29 mW and a linear polarization. The laser exhibited a self-pulsing behavior, delivering µs-long pulses with a repetition rate of a few kHz. The polarized spectroscopic properties of Sm3+ ions were determined as well.

Excited-state absorption and upconversion pumping of Tm3+-doped potassium lutetium double tungstate.

We report on a bulk thulium laser operating on the 3H4 → 3H5 transition with pure upconversion pumping at 1064 nm by an ytterbium fiber laser (addressing the 3F4 → 3F2,3 excited-state absorption (ESA) transition of Tm3+ ions) generating 433 mW at 2291 nm with a slope efficiency of 7.4% / 33.2% vs. the incident / absorbed pump power, respectively, and linear laser polarization representing the highest output power ever extracted from any bulk 2.3 µm thulium laser with upconversion pumping. As a gain material, a Tm3+-doped potassium lutetium double tungstate crystal is employed. The polarized ESA spectra of this material in the near-infrared are measured by the pump-probe method. The possible benefits of dual-wavelength pumping at 0.79 and 1.06 µm are also explored, indicating a positive effect of co-pumping at 0.79 µm on reducing the threshold pump power for upconversion pumping.

A thermal-stable Mn4+-doped far-red-emitting phosphor-converted LED for indoor plant cultivation

Artificial light-emitting diode (LED) light source, regarded as a front-runner technology for the promoting of plant growth, has attracted much attention toward the burgeoning fertilizer-free indoor plant cultivation. It still remains a challenge that explore a far-red-emitting phosphor of high-thermal stability. Overall, we report a Mn4+ ion-doped ordered double perovskite tungstates prepared by high-temperature solid-state reaction method in air. The crystal structure, morphology, and fluorescence properties were investigated and discussed in detail. The far-red emission of the optimized CaSrMgWO6:1%Mn4+, covering the wavelength region of 630–780 nm, matches the absorption ranges of the chlorophyll a and phytochrome (Pfr). The value of Dq/B is estimated to be 2.81. Most notable is the excellent thermal quenching resistance of the Mn4+ emission, remaining 86.7% at 150 °C, thereby suitably fabricating a far-red LED device. The LED light-driven regulation of the physiological and biochemical changes enables a proof-of-concept garlic cultivation. The growth of weight, stem length, and root length of the garlic was significantly promoted with the extra supply of the far-red light. This work demonstrates that the as-obtained far-red-emitting phosphor holds a promising perspective for the phosphor-converted far-red LED light sources toward the flourishing fertilizer-free indoor plant cultivation.

Synthesis and luminescent properties of new tungstates Ln2Zr(WO4)5 (Ln = Tb, Dy)

New polycrystalline powder samples of double Ln2Zr(WO4)5 (Ln = Dy, Tb) tungstates were synthesized using high-temperature solid-phase and sol-gel methods. The conditions of the sol-gel synthesis of tungstates were optimized. The obtained phases were characterized by the X-ray powder diffraction on the basis of the crystallographic data of similar Ln–Zr molybdates. It is found that Ln2Zr(WO4)5 (Ln = Dy, Tb) double tungstates crystallize in the orthorhombic crystal system, space group Cmc21 (Z = 4). The intensive luminescence in the green spectral region for Tb2Zr(WO4)5 and yellow spectral region for Dy2Zr(WO4)5 was shown.

First-principles study of crystal and electronic structures and magnetic property for NaFeCr(WO4)2 compounds

We have first studied the crystal and electronic structures, and magnetic property for sodium double tungstate NaFexCr 1-x(WO 4)2 (x =0, 0.50 and 1.0) compounds using the plane wave self-consistent method within the framework of density functional theory (DFT). We show that the ground state of sodium double tungstate compounds is the antiferromagnetic (AFM) state. We emphasize that the AFM coupling on the bc plane is generated by the 90 o superexchange mechanism. While the AFM coupling of the long-range magnetic interaction between the layers along the a - axis is generated by the Na mediated super - superexchange mechanism.

Temperature dependent bandgap in NaBi(WO4)2 single crystals

The double tungstates have been an attractive research interest due to their optoelectronic applications. In the present work, NaBi(WO4)2, one of the members of double tungstates family, was grown by Czochralski method as single crystal form and optically investigated. X-ray diffraction pattern presented three peaks associated with tetragonal scheelite crystalline structure. Optical properties of the crystal were studied by performing temperature-dependent transmission measurements between 10 and 300 K. The shift of the absorption edge to higher energies was observed with decrease of temperature. The analyses indicated that direct band gap energy increases from 3.50 to 3.60 eV when the temperature was decreased from room temperature to 10 K. The temperature dependency of bandgap was studied considering the Varshni model and fitting of the experimental data under the light of model presented the optical parameters of band gap energy at 0 K, rate of band gap change with temperature and Debye temperature as Eg(0) = 3.61 eV, γ = − 8.83 × 10−4 eV/K and β = 456 K, respectively. Urbach energies were also determined from the analyses as 122 and 113 meV for 10 and 300 K experimental data, respectively.

Incommensurate–commensurate magnetic phase transition in double tungstate Li2Co(WO4)2

Magnetic susceptibility, specific heat, and neutron powder diffraction measurements have been performed on polycrystalline Li2Co(WO4)2 samples. Under zero magnetic field, two successive magnetic transitions at T N1 ∼ 9.4 K and T N2 ∼ 7.4 K are observed. The magnetic ordering temperatures gradually decrease as the magnetic field increases. Neutron diffraction reveals that Li2Co(WO4)2 enters an incommensurate magnetic state with a temperature dependent k between T N1 and T N2. The magnetic propagation vector locks-in to a commensurate value k = (1/2, 1/4, 1/4) below T N2. The antiferromagnetic structure is refined at 1.7 K with Co2+ magnetic moment 2.8(1) μ B, consistent with our first-principles calculations.

Optical spectroscopy study of KDy(WO4)2: Crystal-field levels of Dy3+ and the Jahn–Teller transition

Transmission spectra of a double potassium-dysprosium tungstate KDy(WO4)2 single crystal in the region of intermultiplet transitions of the Dy3+ ion and the Raman spectra of light scattering on the electronic levels of the 6H15/2 ground multiplet of Dy3+ are studied in a wide temperature range. The energy scheme for the crystal-field (CF) levels of Dy3+ in KDy(WO4)2 is created. The phase transition at the temperature 6.3 K is accompanied by the lowering of the energy of the ground Dy3+ CF state, which is evidenced by both the transmission and Raman spectroscopies. Splitting of the low-lying (∼13 cm–1) first excited CF level of the Dy3+ ion indicates the simultaneous structural phase transition. The role of Davydov interaction is discussed.

High-Efficiency Mini-Slab Laser Based on Tm-doped Double Tungstate Crystal

We report on highly-efficient room-temperature lasing in 5at.%Tm:KLu(WO4)2 mini-slabs side-pumped by a 35W diode bar. QCW (duty cycle ∼ 14%) output power of 1.47 W at 1908 nm has been demonstrated with optical and slope efficiencies being of 33 and 43% respectively. In our experiments, we used samples of active elements produced in the slabs form with Brewster’s angle cut faces and original laser cavity design.

Structure, vibrational and magnetic characteristics of LiYbX2O8 (X = W, Mo) single-crystals

Crystals of the Yb-containing double tungstate and double molybdate LiYbX2O8 (X = W, Mo) have been spontaneously grown by a high-temperature flux technique. Their structure has been determined from single-crystal X-ray diffraction at room temperature. LiYbW2O8 crystallizes in the monoclinic P2/n space group (No. 13), with unit cell parameters a = 4.9910(2), b = 5.7964(2), c = 9.8886(4) Å, β = 93.353(1) °. LiYbMo2O8 displays the tetragonal symmetry and is described in the noncentrosymmetric I4̅ space group (No. 82) with a = b = 5.1149(1), c = 11.0919(4) Å. Structural features such as coordination numbers around the atoms and ordered/disordered arrangement of the Yb3+ and Li+ cations explain the differences between these compounds whose vibrational and magnetic properties have also been studied.

Na(Gd,RE)(WO4)2 double tungstates (RE=Eu/Yb-Er): crystallization from a novel layered hydroxide precursor and favorable luminescence for optical thermometry

ABSTRACT Rare earth double tungstates Na(Gd,RE)(WO4)2 (RE = Eu/Yb-Er) were successfully obtained from layered rare earth hydroxide precursors via sacrificial reaction at 200°C with molar ratio WO4 2-/RE3+ = 2.5 for 24 h reaction without calcination and the employment of any organic reagents. The temperature sensing performances of Na(Gd,RE)(WO4)2 (RE = Eu/Yb-Er) were comparatively investigated by analyzing the temperature-dependent down-conversion (DC) and up-conversion (UC) luminescence spectra. The DC phosphors Na(Gd,Eu)(WO4)2 were found to be well capable of sensing temperature via fluorescence lifetime (FL) mode. The UC phosphors Na(Gd,Yb,Er)(WO4)2 were capable of sensing temperature via both FL mode and fluorescence intensity ratio (FIR) mode through thermally coupled 2H11/2,4S3/2 levels (I 520/I 552), and non thermally coupled 2H11/2,4F9/2 energy levels (I 520/I 670). The UC phosphors Na(Gd,Yb,Er)(WO4)2 have maximum absolute sensitivity (SA ) and maximum relative sensitivity (SR ) of 174 × 10−4 K−1 (548 K) and 80 × 10−4 K−1 (298 K) for the thermally coupled 2H11/2,4S3/2 levels and maximum SA and SR of ~260 × 10−4 K−1 (300–550 K) and 96 × 10−4 K−1 (298 K) for the non-thermally coupled 2H11/2,4F9/2 energy levels.

Highly efficient 2.3 µm thulium lasers based on a high-phonon-energy crystal: evidence of vibronic-assisted emissions

We report on highly efficient and power-scalable laser operation in a thulium-doped high-phonon-energy crystal [monoclinic double tungstate, K L u ( W O 4 ) 2 ] on the 3 H 4 → 3 H 5 T m 3 + transition giving rise to the short-wave infrared emission at ∼ 2.3 µ m . A 3 at. % Tm-doped crystal generated a maximum continuous-wave output power of 1.12 W at ∼ 2.22 and 2.29 µm with a record-high slope efficiency of 69.2% (versus the absorbed pump power), a slightly multimode beam ( M x , y 2 = 2.2 and 2.6), and a linear laser polarization. The ∼ 2.3 µ m laser outperformed the one operating on the conventional 3 F 4 → 3 H 6 transition (at ∼ 1.95 µ m ). The effect of the Tm concentration on the ∼ 2.3 µ m laser performance indicates a gradually increasing pump quantum efficiency for the 3 H 4 upper laser level with the Tm doping. For the 3 at. % Tm-doped crystal, it reached 1.8 ± 0.1 (almost two-for-one pump process), which is attributed to efficient energy-transfer upconversion. We discuss the physical nature of the laser emissions occurring at intermediate wavelengths between the electronic 3 H 4 → 3 H 5 and 3 F 4 → 3 H 6 transitions and highlight the role of electron-phonon coupling (vibronic processes) in the appearance of such laser lines. This allowed us to better understand the near- and mid-infrared emission from thulium ions, which can be used in broadly tunable and fs mode-locked 2–2.3 µm lasers.

Luminescence Properties of Rare Earth–Doped Cubic Double Perovskite Tungstate Ba2(1−x)(Na,RE)xZnWO6 (RE = Ce3+, Eu3+ and Dy3+) Phosphors

Herein, the photoluminescence (PL) and thermoluminescence (TL) properties of rare earth doped cubic double perovskite tungstate Ba2(1−x)(Na,RE)xZnWO6 (RE = Ce3+, Eu3+ and Dy3+) phosphors synthesized by the solid‐state reaction method are presented. The self‐luminescence of pure Ba2ZnWO6 material in the blue‐green region is enhanced by Ce3+ doping with its most effective concentration of 3 mol%. The dominant emission in orange (of Eu3+ at 597 nm) and blue (of Dy3+ at 485 nm) region in the emission spectra of Ba2(1−x)(Na,Eu)xZnWO6 and Ba2(1−x)(Na,Dy)xZnWO6 phosphor, respectively, shows highest intensity for 3 mol% of Eu3+ and 2 mol% of Dy3+ ion in the host lattice. The broad excitation band of Ce3+ and Dy3+ doped phosphor in the near UV region claims its candidature for solid‐state lighting. TL studies are also performed using γ‐radiation. Pure host material shows a simple glow curve while the rare earth ion doped phosphor shows a complex glow curve with enhanced intensity. Phosphor shows the fairly linear response up to 1 kGy radiation. Glow curves are deconvoluted and the trap parameters are calculated by Chen's peak shape method.

Synthesis of monoclinic Ho,Tm:KLu(WO4)2 microrods with high photothermal conversion efficiency via a thermal decomposition-assisted method

Monoclinic potassium lutetium double tungstate (KLu(WO4)2) microcrystals with precise rod shape were synthesized using a novel thermal decomposition-assisted method, exhibiting a high photothermal conversion efficiency (66%).

In-Band Pumped Continuous-Wave Lasers Based on Ho:KY(WO<sub>4</sub>)<sub>2</sub> Crystal and Ho:KGdYbY(WO<sub>4</sub>)<sub>2</sub> Epitaxial Layer

2 μm lasers are in demand for a number of practical applications, such as environmental monitoring, remote sensing, medicine, material processing, and are also used as a pump sources for optical parametric generators. Crystals of double potassium tungstates doped with ions of rare-earth elements were shown to be promising materials both for the creation of classical solid-state lasers and waveguide lasers. The aim of this work was to develop a tunable pump laser in the spectral region of 1.9 µm based on double tungstate crystals doped with thulium ions and to study the lasing characteristics of a Ho:KY(WO4)2 crystal and a Ho:KGdYbY(WO4)2 single-crystal epitaxial layer under in-band pumping.With a Ho(1at.%):KY(WO4)2 crystal, continuous wave low-threshold lasing with an output power of 85 mW with a slope efficiency of 54 % at 2074 nm was achieved. For the first time to our knowledge, continuous wave laser generation in a waveguide configuration is realized in a single-crystal layer of potassium tungstate doped with holmium ions grown by liquid-phase epitaxy. The maximum output power at a wavelength of 2055 nm was 16.5 mW.

Two-stage transient stimulated Raman chirped-pulse amplification in KGd(WO4)2 with compression to 145 fs.

We report efficient amplification of chirped supercontinuum pulses in a two-stage stimulated Raman amplifier based on double tungstate [KGd(WO4)2] crystals, pumped with 1.2 ps transform-limited pulses at a 1030 nm wavelength. The second stage demonstrates a conversion efficiency of 55% with an output pulse energy of 0.6 mJ at a 1135 nm wavelength. The amplified Stokes bandwidth is 10 times the pump bandwidth, providing 145 fs pulses after compression.

Luminescence behaviour of the synthesized erbium and thulium co-doped potassium, sodium, lithium or rubidium yttrium double tungstate nanopowders

Nanocrystalline M I Y(WO 4 ) 2 double tungstates (M I = Li, Na, K and Rb) co-doped with Er 3+ and Tm 3+ ions were synthesized by hydrothermal and Pechini techniques and characterised by means of X-ray diffraction, infrared absorption and electron absorption spectra. Their emission spectra and luminescence decay kinetics were recorded at ambient temperature. These investigations were focused on impact of structural changes and morphology inducing by the alkaline ion on the Er ↔ Tm energy transfer efficiency in the studied nanocrystalline oxide systems. The effect of the thermal treatment on morphology and spectroscopic properties of the synthesized nano-materials was studied. Furthermore, emission spectra attained under different excitation as well as study on luminescence decay curves made it possible to determine the differences and dynamics of feeding and relaxation of excited states involved in luminescence phenomena. The energy transfer mechanism between Er 3+ and Tm 3+ ions was proposed and analysed in terms its usefulness in the phosphors and optical amplifiers operating at 1.8 μm. • Li/Na/K/RbY(WO 4 ) 2 :Er 3+ ,Tm 3+ were synthesized by the Pechini and hydrothermal method in the nanocrystalline form. • Structural properties were characterised by the X-ray diffraction (XRD) and infrared spectroscopy. • Strong green emission of Er 3+ ions located at 525 and 550 nm is observed for all compounds. • The lowest intensity of this emission is observed for lithium compounds. • The efficient energy transfer from erbium to thulium takes place in KY(WO 4 ) 2 :Er 3+ ,Tm 3+ sample.