Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

Upconversion luminescence and optical thermometry behaviors of Yb3+ and Ho3+ co-doped GYTO crystal

Optical thermometry based on the upconversion (UC) luminescence intensity ratio (LIR) has attracted considerable attention because of its feasibility for achievement of accurate non-contact temperature measurement. Compared with traditional UC phosphors, optical thermometry based on UC single crystals can achieve faster response and higher sensitivity due to the stability and high thermal conductivity of the single crystals. In this study, a high-quality 5 at% Yb3+ and 1 at% Ho3+ co-doped Gd0.74Y0.2TaO4 single crystal was grown by the Czochralski (Cz) method, and the structure of the as-grown crystal was characterized. Importantly, the UC luminescent properties and optical thermometry behaviors of this crystal were revealed. Under 980 nm wavelength excitation, green and red UC luminescence lines at 550 and 650 nm and corresponding to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+, respectively, were observed. The green and red UC emissions involved a two-photon mechanism, as evidenced by the analysis of power-dependent UC emission spectra. The temperature-dependent UC emission spectra were measured in the temperature range of 330-660 K to assess the optical temperature sensing behavior. At 660 K, the maximum relative sensing sensitivity (Sr) was determined to be 0.0037 K-1. These results highlight the significant potential of Yb,Ho:GYTO single crystal for optical temperature sensors.

Structural, Spectroscopic, Electric and Magnetic Properties of New Trigonal K5FeHf(MoO4)6 Orthomolybdate

A new multicationic structurally disordered K₅FeHf(MoO₄)₆ crystal belonging to the molybdate family is synthesized by the two-stage solid state reaction method. The characterization of the electronic and vibrational properties of the K₅FeHf(MoO₄)₆ was performed using density functional theory calculations, group theory, Raman and infrared spectroscopy. The vibrational spectra are dominated by vibrations of the MoO₄ tetrahedra, while the lattice modes are observed in a low-wavenumber part of the spectra. The experimental gap in the phonon spectra between 450 and 700 cm^-1 is in a good agreement with the simulated phonon density of the states. K₅FeHf(MoO₄)₆ is a paramagnetic down to 4.2 K. The negative Curie-Weiss temperature of -6.7 K indicates dominant antiferromagnetic interactions in the compound. The direct and indirect optical bandgaps of K₅FeHf(MoO₄)₆ are 2.97 and 3.21 eV, respectively. The K₅FeHf(MoO₄)₆ bandgap narrowing, with respect to the variety of known molybdates and the ab initio calculations, is explained by the presence of Mott-Hubbard optical excitation in the system of Fe³⁺ ions.

Up-Converting K2Gd(PO4)(WO4):20%Yb3+,Ho3+ Phosphors for Temperature Sensing

Inorganic luminescent materials that can be excited with NIR radiation and emit in the visible spectrum have recently gained much scientific interest. Such materials can be utilized as anti-counterfeiting pigments, luminescent thermometers, bio-imaging agents, etc. In this work, we report the synthesis and optical properties of K₂Gd(PO₄)(WO₄):Ho³⁺ and K₂Gd(PO₄)(WO₄):20%Yb³⁺,Ho³⁺ powders. The single-phase samples were prepared by the solid-state reaction method, and the Ho³⁺ concentration was changed from 0.5% to 10% with respect to Gd³⁺. It is interesting to note that under 450 nm excitation, no concentration quenching was observed in K₂Gd(PO₄)(WO₄):Ho³⁺ (at least up to 10% Ho³⁺) samples. However, adding 20% Yb³⁺ has caused a gradual decrease in Ho³⁺ emission intensity with an increase in its concentration. It turned out that this phenomenon is caused by the increasing probability of Ho³⁺ → Yb³⁺ energy transfer when Ho³⁺ content increases. K₂Gd(PO₄)(WO₄):20%Yb³⁺,0.5%Ho³⁺ sample showed exceptionally high up-conversion (UC) emission stability in the 77-500 K range. The UC emission intensity reached a maximum at ca. 350 K, and the intensity at 500 K was around four times stronger than the intensity at 77 K. Moreover, the red/green emission ratio gradually increased with increasing temperature, which could be used for temperature sensing purposes.

Synthesis and Characterization of Li2MgGeO4:Ho3+

In this work, the synthesis and characterization of Li₂MgGeO₄:Ho³⁺ ceramics were reported. The X-ray diffraction measurements revealed that the studied ceramics belong to the monoclinic Li₂MgGeO₄. Luminescence properties were analyzed in the visible spectral range. Green and red emission bands correspondent to the ⁵F₄,⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transitions of Ho³⁺ were observed, and their intensities were significantly dependent on activator concentration. Luminescence spectra were also measured under direct excitation of holmium ions or ceramic matrix. Holmium ions were inserted in crystal lattice Li₂MgGeO₄, giving broad blue emission and characteristic 4f-4f luminescent transitions of rare earths under the selective excitation of the ceramic matrix. The presence of the energy transfer process between the host lattice and Ho³⁺ ions was suggested.

Exploration of the Crystal Structure and Thermal and Spectroscopic Properties of Monoclinic Praseodymium Sulfate Pr2(SO4)3

Praseodymium sulfate was obtained by the precipitation method and the crystal structure was determined by Rietveld analysis. Pr₂(SO₄)₃ is crystallized in the monoclinic structure, space group C2/c, with cell parameters a = 21.6052 (4), b = 6.7237 (1) and c = 6.9777 (1) Å, β = 107.9148 (7)°, Z = 4, V = 964.48 (3) ų (T = 150 °C). The thermal expansion of Pr₂(SO₄)₃ is strongly anisotropic. As was obtained by XRD measurements, all cell parameters are increased on heating. However, due to a strong increase of the monoclinic angle β, there is a direction of negative thermal expansion. In the argon atmosphere, Pr₂(SO₄)₃ is stable in the temperature range of T = 30–870 °C. The kinetics of the thermal decomposition process of praseodymium sulfate octahydrate Pr₂(SO₄)₃·8H₂O was studied as well. The vibrational properties of Pr₂(SO₄)₃ were examined by Raman and Fourier-transform infrared absorption spectroscopy methods. The band gap structure of Pr₂(SO₄)₃ was evaluated by ab initio calculations, and it was found that the valence band top is dominated by the p electrons of oxygen ions, while the conduction band bottom is formed by the d electrons of Pr³⁺ ions. The exact position of ZPL is determined via PL and PLE spectra at 77 K to be at 481 nm, and that enabled a correct assignment of luminescent bands. The maximum luminescent band in Pr₂(SO₄)₃ belongs to the ³P₀ → ³F₂ transition at 640 nm.

Upconversion Properties and Temperature-Sensing Behaviors of Alkaline-Earth-Metal Scandate Nanocrystals Doped with Er3+/Yb3+ Ions in the Presence of Alkali Ions (Li+, Na+, and K+)

Temperature-sensing media based on the fluorescence intensity ratio (FIR) of upconversion materials that suffer from low sensitivity owing to the small energy gap still have a need for new compounds with strong upconversion luminescence (UCL). In this work, a series of MSc₂O₄:Er³⁺/Yb³⁺ (M = Mg, Ca, Sr, and Ba) nanocrystals were prepared by a hydrothermal method using NaOH alkaline solution. The structure, morphology, and UCL characteristics of materials were investigated, and the red UCL of the CaSc₂O₄:Er³⁺/Yb³⁺ sample was dramatically enhanced by a factor of ∼12, ∼23, and ∼2000 compared with SrSc₂O₄, MgSc₂O₄, and BaSc₂O₄ samples, respectively. By adjusting alkali ions (Li⁺, Na⁺, K⁺), the UCL intensities of CaSc₂O₄:Er³⁺/Yb³⁺ and SrSc₂O₄:Er³⁺/Yb³⁺ samples were further improved, especially in the presence of Li⁺ ions. Excellent temperature-sensing behaviors are realized for CaSc₂O₄:Er³⁺/Yb³⁺ and SrSc₂O₄:Er³⁺/Yb³⁺ samples in the presence of Li⁺ ions, in which the maximum absolute sensitivity S_A values are about 0.0041 and 0.0036 K^-1 at 600 K and the corresponding relative sensitivity S_R values are expressed as 1197/T² and 1129/T² (the current optimal S_R = 1289/T²), respectively. The intense UCL and excellent S_A and S_R values indicate that CaSc₂O₄:Er³⁺/Yb³⁺ and SrSc₂O₄:Er³⁺/Yb³⁺ materials are promising candidates for application in high-temperature sensors working under 980 nm excitation.

Luminescence Properties of Ho2O3-Doped Y2O3 Stabilized ZrO2 Single Crystals

Single crystals of Ho2O3-doped Y2O3 stabilized ZrO2 (YSZ) with different Y2O3 and Ho2O3 contents were grown by the optical floating zone method. XRD and Raman spectra were measured and showed that crystal samples all had tetragonal structures. Measurements of positron annihilation lifetime spectra indicated that the increase in Y2O3 concentration led to the increases of defects and mean positron lifetime, which enhanced the scattering of light and reduced the luminous intensity and the quantum yield (QY) of the crystal. Under the excitation at 446 nm, photoluminescence (PL) spectra of Ho2O3-doped YSZ crystals showed emission peaks at 540, 551, 670, and 757 nm corresponding to Ho3+ transitions from 5S2, 5F4, 5F5, and 5I4 excited states to the 5I8 ground state, respectively. At low Ho2O3-doped concentrations (0.10–0.50 mol%), the overall emission intensity increased with Ho2O3 contents, reached the maximum value at 0.50 mol%, then decreased with higher Ho2O3 contents, probably as a result of increased non-radiative relaxation caused by increased interactions between Ho3+ ions. Quenching of the PL occurred at Ho2O3 concentrations > 0.5 mol% and due to the electric dipole–dipole interaction. The calculated chromaticity coordinates (CIE) were approximately (0.307, 0.683) and the color purity achieved 99.6%. The results showed that Ho2O3: YSZ crystals were suitable for green light-emitting devices.

Generation of Pure Green Up-Conversion Luminescence in Er3+ Doped and Yb3+-Er3+ Co-Doped YVO4 Nanomaterials under 785 and 975 nm Excitation

Materials that generate pure, single-color emission are desirable in the development and manufacturing of modern optoelectronic devices. This work shows the possibility of generating pure, green up-conversion luminescence upon the excitation of Er3+-doped nanomaterials with a 785 nm NIR laser. The up-converting inorganic nanoluminophores YVO4: Er3+ and YVO4: Yb3+ and Er3+ were obtained using a hydrothermal method and subsequent calcination. The synthesized vanadate nanomaterials had a tetragonal structure and crystallized in the form of nearly spherical nanoparticles. Up-conversion emission spectra of the nanomaterials were measured using laser light sources with λex = 785 and 975 nm. Importantly, under the influence of the mentioned laser irradiation, the as-prepared samples exhibited bright green up-conversion luminescence that was visible to the naked eye. Depending on the dopant ions used and the selected excitation wavelengths, two (green) or three (green and red) bands originating from erbium ions appeared in the emission spectra. In this way, by changing the UC mechanisms, pure green luminescence of the material can be obtained. The proposed strategy, in combination with various single-doped UC nanomaterials activated with Er3+, might be beneficial for modern optoelectronics, such as light-emitting diodes with a rich color gamut for back-light display applications.

Low-temperature synthesis of NaRE(WO4)2 films via anion exchange from Layered rare-earth hydroxides (LRHs) films, phase/morphology evolution and photoluminescence

Alkaline lanthanide tungstates NaRE(WO4)2 (RE = La-Ho, and Y) films were hydrothermally synthesized via anion exchange using the electrodeposited layered rare-earth hydroxide (RE2(OH)5NO3·nH2O) films as precursor template in the presence...