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New BaRE6(Ge2O7)2(Ge3O10), BaYb6(Ge2O7)2(Ge3O10):Tm3+ and BaLu6(Ge2O7)2(Ge3O10):Yb3+,Tm3+ germanates crystallizing in the monoclinic system (S.G. P21/m) have been characterized by various methods.

The Ca(Ba)RE2Ge3O10 germanates are currently considered as a promising class of active optical media emitting in the visible and infrared spectral region. Herein, a new series of BaY2-x-yTmxDyyGe3O10 phosphors was synthesized using the solid-state reaction and characterized by X-ray diffraction, scanning electron microscopy, diffuse reflectance and luminescence spectroscopy. All the studied compounds crystallize in the monoclinic system, space group P21/m, Z = 2. Under 808 nm excitation, BaY2-xTmxGe3O10 germanates demonstrate a broad emission in 1.3–2.2 µm range owing to a series of cascade 3H4 → 3F4 and 3F4 → 3H6 transitions in Tm3+ ions. The BaY2-xTmxGe3O10 phosphors exhibit a high thermal stability over a wide temperature range and belong to promising infrared luminescence materials. The intensity of these emission bands decreases with an increase in the dysprosium content in the BaY1.97-yTm0.03DyyGe3O10 germanates. The magnetic properties measurements have been also carried out since Dy3+ ions exhibit a large magnetic anisotropy and non-collinearity of the magnetization easy axes. The magnetization curve of Tm3+-doped BaY2Ge3O10 shows the effects of saturation typical of paramagnetics, while the magnetization of germanates doped with Dy3+ ions occurs with hysteresis which is observed for single-ion magnets.

A new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, Dy) germanates and Ba2Gd2−xEuxGe4O13 (x = 0.1–0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE2O12 dimers, zigzag C2-symmetric [Ge4O13]10− tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba2RE2Ge4O13 compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba2Gd2−xEuxGe4O13 phosphors exhibit an orange-red emission corresponding to the characteristic f–f transitions in Eu3+ ions. The highest intensity of lines at 580 nm (5D0 → 7F0), 582–602 nm (5D0 → 7F1), 602–640 nm (5D0 → 7F2), 648–660 nm (5D0 → 7F3), and 680–715 nm (5D0 → 7F4) is observed for the samples with x = 0.4–0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba2Gd2−xEuxGe4O13 solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λex = 255 nm) is used as a semiconductor chip.

The phosphates LiMgPO4:RE (RE – Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm) with an olivine-type orthorhombic structure were synthesized by the conventional solid state reaction route and their optical properties were examined by the experimental and theoretical methods. In this work, we showed that the radioluminescence and thermoluminescence spectra of irradiated LiMgPO4:RE (RE = Sm, Gd, Tb, Dy, Tm) consist of lines reflecting 4f–4f transitions in the rare-earth element. For LiMgPO4:RE (RE = Nd, Ho, Er), they coincide with the spectrum of the phosphate matrix with two broad bands at 360 nm and 650 nm but are much more intense. A model of energy transfer is proposed to explain this phenomenon. It is shown that the model is confirmed by the features of photoluminescence in doped LiMgPO4 and by the results of ab initio calculations.

New apatite-type NaLa9-xTmx(GeO4)6O2 (x = 0.025–0.8) germanates (space group P63/m, Z = 1) have been prepared using both the citrate technique and the conventional solid state method. The samples were characterized by powder X-ray diffraction, scanning electron microscopy, diffuse reflectance and photoluminescence spectroscopy. Photoluminescence measurements of NaLa9-xTmx(GeO4)6O2 compounds were carried out both in the 425–850 nm range under 356 nm excitation and in the infrared region under 808 nm excitation. The most intensive lines at 453 nm, 478 nm and in 625–850 nm wavelength range are associated with the 1D2 → 3F4, 1G4 → 3H6 and a series of 1G4 → 3F4, 3F2 → 3H6, 3H4 → 3H6 transitions in Tm3+ ions. The spectra measured in the infrared spectral range consist of two broad emission bands centered at 1440 nm and 1810 nm, which correspond to the cascade 3H4°→°3F4 and 3F4°→°3H6 transitions in Tm3+. The influence of dopant content and temperature on possible mechanisms promoting the population of Tm3+ excited states has been studied.

For the first time, the Sr2La8 – xTmx(GeO4)6O2 (x = 0.1–1.0) solid solution with the apatite structure were synthesized by the solid-phase method, and their spectral–luminescence properties were studied. The prospects of using these compounds as phosphors in the visible and short-wave infrared ranges were demonstrated. The luminescence of Sr2La8 – xTmx(GeO4)6O2 germanates, which occurs under ultraviolet radiation, is characterized by the high purity of blue color; the chromaticity coordinates are close to commercially available phosphors. The Sr2La8 – xTmx(GeO4)6O2 compounds efficiently convert 808 nm laser radiation into a series of emission lines in the 1.3–2.2 μm spectral range, caused by sequential 3H4 → 3F4 and 3F4 → 3H6 transitions in Tm3+ ions. Germanate Sr2La7.6Tm0.4(GeO4)6O2 with a maximum emission intensity in the short-wave infrared region shows high thermal stability of luminescence in the 30–220°C range.

A new series of CaY2–10xYb9xErxGe3O10 trigermanates (space group P21/c, Z = 4) has been synthesized using an EDTA-assisted method. Under 980 nm laser diode excitation, the samples exhibit intense upconversion luminescence corresponding to the characteristic transitions 2H9/2 → 4I15/2 (400–420 nm), 2H11/2,4S3/2 → 4I15/2 (500–600 nm) and 4F9/2 → 4I15/2 (625–725 nm) in Er3+ ions. The CaY1.5Yb0.45Er0.05Ge3O10 phosphor can be used as a good temperature sensor with high absolute and relative sensitivities Sa(max) = 0.0051 K−1 and Sr(max) = 0.0109 K−1.

A new series of BaY2−xEuxGe3O10 (x = 0.1–0.8) trigermanates has been synthesized using the solid-state reaction method. A powder XRD study has revealed that the compounds crystallize in the monoclinic system, space group P21/m, Z = 2. The crystal lattice consists of distorted Y/EuO6 octahedra, trigermanate [Ge3O10]8− groups and Ba atoms located in eight-fold coordinated irregular sites. Under UV and 465 nm light excitation, the samples exhibit reddish-orange emission corresponding to the characteristic 5D0 → 7FJ transitions in Eu3+ ions. Concentration dependence study has shown that the powders with europium content x = 0.4–0.6 possess the highest luminescence intensity under 393 nm and 465 nm excitation. It has also been revealed that the predominant mechanism causing concentration quenching in the BaY2−xEuxGe3O10 series is due to multipolar interactions. The investigations on colour characteristics, quantum efficiency and thermal stability performance of BaY2−xEuxGe3O10 phosphors were carried out for the feasibility study of their practical application. The results indicate that BaY2−xEuxGe3O10 can be considered as a promising material for blue-excited phosphor-converted LEDs.

Cubic garnets activated by neodymium and holmium, Li6CaLa2Nb2O12:Nd3+,Ho3+, were obtained by solid-state method. The main regularities of changes in the luminescence properties of Li6C-aLa2Nb2O12:Nd3+,Ho3+ solid solutions in the visible, near-IR, and short-wave IR ranges under 808 nm laser diode radiation are determined. The energy transfer mechanism between active centers, involving the participation of Nd3+ ions as sensitizers for the luminescence of Ho3+ ions, is proposed. Low phonon energy, high luminescence intensity in the range of 2.0–3.0 μm, and weak up-conversion luminescence in the region of 450–780 nm make it possible to consider the cubic lithium–lanthanum niobates Li6CaLa2Nb2O12:Nd3+,Ho3+ as promising phosphors of the short-wave IR range.

A new series of Ba

A new series of BaRE2Ge3O10 (RE = Y, Sc, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) trigermanates has been prepared using a solid-state reaction. Powder XRD study has revealed that the compounds crystallize in the monoclinic system, space group P21/m, Z = 2. The crystal lattice consists of edge-sharing REO6 octahedra, bowed trigermanate [Ge3O10]8− groups, and Ba atoms located in eight-fold coordinated irregular sites. The geometry of [Ge3O10]8− anions persisting within different trigermanates has been examined with a special focus on the values of Ge–Ge–Ge angles and the types of conformation between the central and terminal GeO4 tetrahedra. The arrangement of [Ge3O10]8− anions in BaRE2Ge3O10 (RE = Y, Sc, Gd–Lu) crystal lattices is found to be unique. DFT calculations of different polymorphs for BaY2Ge3O10 and BaLa2Ge3O10 compounds have confirmed that our fabrication protocol yields all the products in their most thermodynamically stable forms. According to the results of vibrational spectroscopy studies, diffuse reflectance and photoluminescence measurements, the BaRE2Ge3O10 trigermanates are promising compounds for the preparation of efficient lanthanide ion activated phosphors.

A promising series of phosphors based on Li7La3Hf2O12:xLn3+ (Ln = Tb, Dy) has been prepared using a conventional solid state reaction and their luminescence properties have been investigated. The powder X-ray diffraction patterns show that Li7(La1-xLnx)3Hf2O12 (Ln = Tb, Dy) solid solutions crystallize in the tetragonal space group I41/acd, Z = 8. The photoluminescence spectra of Tb3+ doped Li7La3Hf2O12 contain well-known blue-green emission lines due to the 5D4→7FJ (J = 3–6) transitions of Tb3+. At relatively low activator concentrations, the blue emissions from the 5D3 level are observed along with emissions from the 5D4 level. The intensity ratio between the emission levels of 5D3 and 5D4 depends significantly on the terbium content, which makes it possible to obtain a tunable emission from green to yellowish green color. Under UV excitation, the emission spectra of Li7La3Hf2O12:xDy3+ phosphor exhibit several peaks attributed to the 4F9/2→6HJ (J = 11/2, 13/2 and 15/2) transitions with stable CIE chromaticity coordinates (0.417, 0.420) located in the white light region.

A series of Sr3La2-xEux(Ge3O9)2 (x = 0.1–1.8) cyclogermanates has been prepared by the citrate method. The powder XRD study has shown that the compounds crystallize in the monoclinic space group C2/c, Z = 4. An increase in the dopant content causes a morphotropic phase transition: at x = 0.1–0.7 the samples are isostructural to Sr3La2(Ge3O9)2, while at x = 0.9–1.8 the germanates have a crystal structure of Sr3Eu2(Ge3O9)2. In all compounds, strontium and rare earth ions occupy three symmetry independent cation sites and form layers alternating along the [ 1 0 1 ¯ ] direction. The relationships between the crystal structure and the luminescence characteristics: the energy of the charge transfer transition, the values of 4f–4f/CTB and 5D0 → 7F2/5D0 → 7F1 intensity ratios have been revealed. CIE chromaticity coordinates for Sr3La1.4Eu0.6(Ge3O9)2 and Sr3La0.8Eu1.2(Ge3O9)2 phosphors under 393 nm excitation have been measured.

The phosphate LiMgPO4 with orthorhombic olivine-type structure was synthesized by the conventional solid state reaction route and its optical properties were examined by the experimental and theoretical methods. Using diffuse-reflectance (DR) spectroscopy we observed two absorption stages at 5.5 eV and at 4.4 eV, which were attributed to the fundamental band gap and energy levels of defects within the band gap, respectively. These experimental results were confirmed by ab initio calculations. We observed photoluminescence and thermoluminescence (TL) in undoped LiMgPO4. Emission components were detected in the UV, visible and NIR regions. These multicolor emissions indicate the presence of various defects in LiMgPO4 and each color is due to different electronic transitions. The thermoluminescence of undoped LiMgPO4 shows that defects in LiMgPO4 serve as traps during irradiation. The TL glow curve with a sufficiently large light sum consists of five overlapping peaks in the temperature range 300–600 K.

A promising series of infrared phosphors Ca3Y2-х-yNdxHoyGe3O12 (x = 0.0–0.3; y = 0.000015–0.15) with garnet structure has been prepared using a liquid-phase precursor method. The room temperature X-ray powder diffraction study and density functional theory (DFT) calculations show consistently that incorporated trivalent neodymium ions occupy calcium dodecahedral sites, whereas ousted part of calcium ions moves into yttrium/holmium octahedral sites. DFT calculations and results of diffuse reflectance measurements reveal that an increase in the neodymium concentration and, consequently, the degree of cation substitution leads to a noticeable band gap reduction. The infrared luminescence in range from 1.0 to 3.4 µm for Ca3Y2-х-yNdxHoyGe3O12 is observed under 808 nm laser diode excitation. The trace amount of holmium ions and a low doping ratio of Ho3+ and Nd3+ ions allow revealing of intense emission at 2.0–3.4 µm region. The highest emission intensity is attained at x = 0.15 in Ca3Y2-хNdxGe3O12:Ho3+. The concentration quenching of Ho3+ emission in the Ca3Y1.85–yNd0.15HoyGe3O12 solid solutions is observed at the activator content of y > 1.5·10−4. A very weak red upconversion emissions at 650 and 740 nm were observed upon 808 nm laser diode excitation. The proposed mechanism for multistage process of energy transfer between the active centers involves the participation of Nd3+ ions as sensitizers of infrared luminescence of Ho3+ ions. The results indicate that the Ca3Y2-х-yNdxHoyGe3O12 germanate can be considered as a promising material for near- and middle infrared phosphors.

Cyclogermanate Sr3La2(Ge3O9)2, isostructural to silicate Sr3Er2(Si3O9)2, activated by neodymium and holmium is obtained for the first time by the precursor method. Ion Nd3+ in the structure of Sr3La2(Ge3O9)2 is a sensitizer of the infrared luminescence of Ho3+. Excitation by radiation with a wavelength of 808 nm leads to a series of emission lines in the luminescence spectra of Sr3La2-xNd x (Ge3O9)2 : Ho3+ in the short-wave and middle-IR ranges (1.0–3.4 μm). The highest intensity of lines at 2.1 and 2.7 μm, associated with the 5I7 → 5I8 and 5I6 → 5I7 transitions in the Ho3+ ion, is found for compositions containing traces of holmium. Based on the analysis of the concentration dependences of the luminescence intensity, an optimal composition of the phosphor is determined, which ensures the maximum efficiency of conversion of laser radiation energy. The data obtained are interpreted in the assumption of cross-relaxation energy transfer from Nd3+ to Ho3+.

The new polycrystalline samples of Li7La3Hf2O12:xCe (x = 0.00–3.30%) with tetragonal garnet structure have been prepared using a conventional solid state reaction. The phase composition, morphology and optical properties of undoped and cerium doped compounds have been characterized by powder X-ray diffraction, scanning electron microscopy, electron paramagnetic resonance, diffuse reflectance and photoluminescence spectroscopy. The absorption bands at 2.5 eV (496 nm) and 3.3 eV (375 nm) are associated with processes of light absorption in F-centers in tetragonal Li7La3Hf2O12. The origin of luminescence in Li7La3Hf2O12 can be explained by the creation and relaxation of self-trapped exciton localized in [HfO6]8- system. The Ce ions in Li7La3Hf2O12:xCe (x = 0.00–3.30%) have predominantly the Ce4+ valence state, and no luminescence of Ce3+ is observed. The increase in the cerium concentration in Li7La3Hf2O12 leads to a reduction of the photoluminescence intensity associated with intrinsic luminescence centers. Using the temperature dependence of luminescence, the activation energy of temperature quenching of intrinsic luminance was calculated.

Solid solutions CaLa2-xEu x Ge3O10 (x = 0.0–0.6, Δx = 0.1) have been synthesized for the first time. The compounds are isostructural to CaLa2Ge3O10, they crystallize in the monoclinic system, space group P21/c, Z = 4. The low-temperature X-ray diffraction studies have revealed the strain anisotropy of germanate CaLa2Ge3O10 crystal lattice in the temperature range 80–298 K, and the linear thermal expansion coefficients have been calculated. The optical properties of the activated phases have been studied, and the influence of the dopant concentration and the excitation wavelength on the luminescence characteristics of the synthesized compounds has been established.

The formation conditions, crystal structure and luminescence properties of aluminum and gallium hydroxide diformates, Me(OH)(HCOO)2, have been established. Both compounds crystallize in a monoclinic lattice (space group C2, #5 and Z = 8) with parameters: a = 8.8629(2) A, b = 9.9890(2) A, c = 10.2411(1) A, β = 106.223(1)°, and V = 870.56(3) A3 (Al) and a = 8.8746(1) A, b = 10.3291(1) A, c = 10.27870(9) A, β = 105.7565(6)°, and V = 906.81(2) A3 (Ga). The emission spectra of Me(OH)(HCOO)2 under UV excitation with wavelength λ = 285 nm exhibit maxima in the range of 440–470 nm. The emission accompanied by light-blue luminescence is greatly enhanced when going from the aluminum to the gallium compound. The origin of intrinsic emission centers responsible for the luminescence properties of Me(OH)(HCOO)2 is suggested and the higher emission of the gallium hydroxide diformate is substantiated by means of density-functional theory calculations.

The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) A, c = 7.25602(5) A, V = 614.521(7) A3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9-x-yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 μm and 2.7 μm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications.

A new series of Sr3RE2(Ge3O9)2 (RE = La, Pr, Nd, Sm, Eu, Gd) cyclogermanates has been prepared using an ethylenediaminetetraacetic acid assisted route. The powder XRD study reveals that the compounds crystallize in the monoclinic space group C2/c, Z = 4. A decrease in the crystal radius of RE3+ ions in Sr3RE2(Ge3O9)2 (RE = La–Gd) causes a morphotropic phase transition followed by a step-like change of the lattice parameters. The transition is accompanied by internal reorganization, including the redistribution of rare earth and strontium ions among three symmetry independent Sr/RE sites, a conformation change of [Ge3O9]6− rings and a variation of Sr/RE(2) atom coordination from seven-fold to nine-fold. Density functional theory calculations reveal that Sr3RE2(Ge3O9)2 compounds are indirect bandgap materials with a band gap around 3.0 eV. The magnetic property measurements indicate that Sr3RE2(Ge3O9)2 (RE = Pr, Sm, Gd) are Curie–Weiss paramagnets up to 2 K. For Sr3Eu2(Ge3O9)2 an anomaly of magnetic susceptibility is found at about 60 K. A photoluminescence study of the Sr3Sm2(Ge3O9)2 and Sr3Eu2(Ge3O9)2 samples reveals an emission in the visible spectral range, corresponding to the characteristic 4f–4f transitions in rare earth ions.

The new single-phase oxyapatites A La 9 (GeO 4 ) 6 O 2 and A La 8.93 Eu 0.07 (GeO 4 ) 6 O 2 ( A = Li, Rb, space group P 6 3 / m ) are prepared using the ethylenediaminetetraacetic acid (EDTA) assisted process and the solid state method. Detailed characterization of the fine features of electronic structure within the families of oxyapatites A La 9 (GeO 4 ) 6 O 2 and A La 8.93 Eu 0.07 (GeO 4 ) 6 O 2 (A = Li, Na, K, Rb) has been performed using photoluminescence measurements and density functional theory (DFT) calculations. The trivalent europium ions may occupy two crystallographically nonequivalent sites (4 f ) and (6 h ) with the C 3 and C s symmetry, respectively, and can be distinguished using the photoluminescence excitation on and the photoluminescence spectra from potassium and rubidium germanates, in contrast to those for lithium and sodium compounds. All the compounds should be semiconductors with wide direct band gaps of about 3.8 eV for alkali metal derivatives and about 3.2 eV for the vacancy-stabilized La 9.33 (GeO 4 ) 6 O 2 composition. The thermodynamic stability, the lattice parameters and the band gap value of CsLa 9 (GeO 4 ) 6 O 2 not yet synthesized as a single phase have been qualified using DFT calculations.

Direct aqueous synthesis of Ag2S QDs is a challenge because hydrophobic Ag2S QDs tend to aggregate to form large polycrystals. In this article, the low temperature one-pot route for obtaining highly stable aqueous colloid solutions of Ag2S QDs, using NH3 molecules as monodentate ligands, is reported. Ionic forms of all the species in solution including free ligand, free and complex silver ions, and free sulfide ions at any given experimental pH were calculated. The obtained speciation diagrams were necessary to determine synthetic parameters such as concentration ratios [Ag+]/[HS−] and [NH3]/[Ag+], and pH. The comprehensive investigation of Ag2S QDs' colloid solutions was carried out by high-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (STEM), UV–vis–NIR spectroscopy, dynamic light scattering. The Ag2S QDs possessed monoclinic (acanthite) crystalline structure with an average size about 33 nm. Estimations of hydrodynamic diameter and degree of double layer charge in colloid solutions, i.e. the ς-potential measurements, proved high stability of colloidal system of Ag2S QDs on the order of several months under ambient conditions. Combination of both electrostatic and steric stabilization mechanisms kept Ag2S QDs uniformly dispersed in aqueous medium, with NH3 molecules playing a crucial role in avoiding aggregation of as-prepared Ag2S QDs. Colloid Ag2S QDs exhibited NIR emission at 1280 nm under 808 nm excitation.

Simultaneous emission lines around 1.05 μm, 1.3 μm, 1.8 μm, 2.1 μm and 2.7 μm have been observed in Li7La3 − xNdxHf2O12:Ho3 + (x = 0.00–0.15) under 808 nm laser diode excitation. Near-infrared luminescence due to holmium ions with residual concentration in the Li7La3Hf2O12 host has been studied. The intensity of 2.1 and 2.7 μm lines associated with 5I7 → 5I8 and 5I6 → 5I7 transitions in Ho3 + depends on the neodymium codopant concentration. This result indicates that Nd3 + ions can be potentially used as sensitizers for Ho3 + ions to stimulate the intense near-infrared emission in this system. Possible energy transfer mechanisms between lanthanide ions have been briefly discussed.

The effect of V doping on the structure and optical properties of LiMgPO 4 was studied. We found that LiMg(PO 4 ) 1−x (VO 4 ) x with x 4 and demonstrates self-activated luminescence. All emission spectra under 320 nm excitation had the form of a broad band with a maximum at 490 nm and the emission intensity increased with the vanadium concentration. The luminescence with a peak at 560 nm was also observed for LiMgVO 4 . The UV–Vis diffuse reflectance spectra were recorded for LiMgPO 4 , and LiMgPO 4 with 17% V and LiMgVO 4 , and the optical band gaps were estimated to be 4.05, 3.90 and 3.43 eV, respectively. The ab initio calculations predict an absorption-edge shift to lower energies with growing vanadium concentration and reproduce very well the experimental band gaps for vanadium containing oxides (LiMgP 0.75 V 0.25 O 4 and LiMgVO 4 ), but provide a much larger value for LiMgPO 4 .

New cyclogermanates Sr 3 RE 2 (Ge 3 O 9 ) 2 ( RE =La, Y) with Ca 3 Y 2 (Si 3 O 9 ) 2 cyclosilicate structure type have been prepared by two liquid-phase precursor routes: the citrate and the Pechini methods. The phase composition, morphology and optical properties of the intermediate and final products have been characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The thermogravimetric (TGA) and differential thermal analyses (DTA) were used to study the thermal decomposition of the precursors. The powder XRD measurements indicate that the phase formation of Sr 3 RE 2 (Ge 3 O 9 ) 2 ( RE =La, Y) (S.G. C 2 /c , Z=4) takes place after 20 h annealing at 800 °C. The DRS data revealed that the cyclogermanates are promising optical hosts.

New KLa 9 (GeO 4 ) 6 O 2 and KLa 8.37 Eu 0.63 (GeO 4 ) 6 O 2 germanates with the apatite structure type (space group P 6 3 /m , Z = 1) have been synthesized by the sol-gel method using ethylenediaminetetraacetic acid as a precursor and characterized by powder X-ray diffraction, scanning electron microscopy and diffuse reflectance spectroscopy. Density functional theory calculations reveal that KLa 9 (GeO 4 ) 6 O 2 is a direct bandgap material, and near-UV excitation leads to the electronic transitions from the O 2p - orbitals to the La d - and K 4s - states. Photoluminescence measurements of the Eu 3+ -doped sample indicate emission in the visible spectral range corresponding to the characteristic transitions from the excited 5 D 0 and 5 D 1 levels in europium ion.

A new series of CaYb2−xErxGe3O10 (x = 0–2) germanates has been prepared by conventional solid state reaction and an EDTA-assisted precursor route. Strong upconversion luminescence has been detected under 980 nm diode laser excitation. Concentration dependencies study shows that the highest intensity of the lines around 408 nm (2H9/2 → 4I15/2), 524 nm (2H11/2 → 4I15/2), 548 nm (4S3/2 → 4I15/2) and 659 nm (4F9/2 → 4I15/2) is observed for the sample x = 0.2 prepared by the precursor method. The measured quantum yield for this compound is more than 3%. Possible mechanisms of energy transfer between active centers have been also proposed.

A promising series of phosphors based on Li7La3−xEuxHf2O12 (x = 0.05–0.20) has been prepared using a conventional solid state reaction. Rietveld refinement of room temperature powder X-ray diffraction patterns shows that the Li7La3−xEuxHf2O12 (x = 0.00–0.20) solid solution crystallizes in the tetragonal space group I41/acd, Z = 8. The trivalent europium ions occupy two crystallographically nonequivalent sites (8b) and (16e) with the D′2 and C2 symmetry, respectively. Raman spectroscopy corroborates the X-ray diffraction results and confirms the crystal structure features of tetragonal Li7La3Hf2O12 with the maximum phonon energy of 650 cm−1. The phosphors exhibit reddish orange emission under excitation at 282 nm. Calculation of the Ω2 and Ω4 Judd–Ofelt intensity parameters and quantum efficiencies from the recorded emission spectra allows a more extensive study of the luminescence properties of Eu3+-doped Li7La3Hf2O12. The maximum light emission is revealed at the activator content of x = 0.15 in Li7La3−xEuxHf2O12. CIE chromaticity coordinates of Li7La2.85Eu0.15Hf2O12 are x = 0.643 and y = 0.354.

Solid solutions CaLa2–xNdxGe3O10:Ho3+, Er3+ have been synthesized, their optical characteristics have been studied, and the dependences of the Stokes luminescence in the 0.9–3.0 µm region on the concentration of neodymium ions under 808 nm laser diode excitation have been determined. Based on analysis of the obtained dependences, the optimal composition of the phosphor with the maximum efficiency of the energy conversion is established.

New germanate ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites, showing promise as host lattices for phosphors with emission in the visible and infrared spectral ranges, have been obtained for the first time by ultrasonic spray pyrolysis. The discussion is focused on native defectiveness and optical properties of these compounds, depending on the nature of alkaline ions. Spray pyrolysis method was consciously used to obtain the compounds with intrinsic defects as confirmed by electron paramagnetic resonance. Reviewing available EPR data, the registered EPR response was assigned to the paramagnetic centers related to germanium tetrahedra embracing the oxygen vacancies. According to quantum-chemical calculations the oxygen-deficient centers in ALa9(GeO4)6O2 are presumably stabilized by antisite defects in the cation sublattice. Diffuse reflectance and pulse cathodoluminescence spectra of the compounds were investigated in detail, establishing a distinct dependence of optical properties on the type of alkaline ion. The observed optical absorption maxima at 275 and 370 nm were attributed to the F-like centers. The inverse dependence of luminescence intensity on the degree of defectiveness was interpreted on the assumption that these centers were involved in non-radiative energy relaxation.

The novel lithium tantalates doped with samarium ions were prepared using the solid state method. The single-phase Li5+xCaxLa3-xTa2O12:Sm3+ (x = 0; 1) compounds crystallize in the cubic space group Ia 3 ‾ d (Z = 8) and belong to the garnet family which is used for various optical applications. The diffuse reflectance, excitation and photoluminescence spectra measured at room and elevated temperatures are discussed in detail. The compounds demonstrate the well-known orange-red emission associated with the 4f-4f transitions in Sm3+ ions. The optimum samarium content in Li6CaLa2-xSmxTa2O12 solid solution is equal to 3 mol%, and the quenching mechanism is caused by the dipole-dipole interaction. Li6CaLa2-xSmxTa2O12 series provides higher luminescence intensity in comparison with Li5La3-xSmxTa2O12 samples. The CIE chromaticity coordinates are almost identical for all compounds and are equal to (0.61, 0.39). The Li6CaLa2Ta2O12:Sm3+ phosphors have good thermal stability of emission which allows one to consider these lithium calcium tantalate garnets as potential orange-red-emitting phosphors.

The phase formation of Sr3RЕ2(Ge3O9)2 (RE = Dy, Ho, Er, Tm, Yb, Lu) cyclogermanates was studied at 1000 and 1100 °C in air. It was shown that these compounds did not exist under these conditions, in contrast to cyclogermanates with large RE3+ ions, such as La, Pr, Nd, Sm, Eu and Gd. Using the thulium-based system SrO–Tm2O3–GeO2 as an example, it was established that compounds with Sr/RE atomic ratio other than 3/2 were formed instead of the expected Sr3RЕ2(Ge3O9)2. Single phase Sr4TmGe5.667O16.834 was prepared and characterized by X-ray powder diffraction. The quantum-chemical calculations confirmed the thermodynamic instability of strontium germanates Sr3RЕ2(Ge3O9)2 with RE3+ cations smaller than Gd3+. It is suggested that the prototypic structure of mixed strontium and Dy, Ho, Er, Tm, Yb and Lu rare earth germanates stems from the crystal structure of walstromite BaCa2Si3O9.

The apatite-like NaLa9(GeO4)6O2:Nd3+,Ho3+ phosphor is prepared using the solid-state method. Rietveld refinement of high-resolution time-of-flight neutron powder diffraction measurements indicate that this compound crystallizes in the hexagonal system with space group P63/m, Z = 1 and unit cell parameters a = 9.88903(6) Å, c = 7.25602(5) Å, V = 614.521(7) Å3 at room temperature. The 4f sites are statistically occupied by La, Nd and Na, while 6h sites are occupied by La and Nd. Luminescence in the near- and middle-IR range caused by the transitions in neodymium and holmium ions is excited under 808 nm laser diode radiation. The highest emission intensity in NaLa9-x-yNdxHoy(GeO4)6O2 is attained at trace amounts of holmium, and it decreases sharply when y increases to 0.01. The IR phosphors have a good thermal stability and exhibit a very weak upconversion emission in the red spectral range upon 808 nm excitation. A scheme of excitation and emission pathways involving ground/excited state absorption, energy transfer, cross-relaxation, nonradiative multiphonon relaxation processes in Nd3+ and Ho3+ ions has been proposed. The data analysis indicates that Nd3+ ions serve as sensitizers for Ho3+ ions in these compounds, stimulating intense 2.1 μm and 2.7 μm emissions. These apatite-related germanate phosphors are promising materials for near- and middle-infrared solid-state lighting applications.

The CaY2−x Ln x Ge3O10 (Ln = Eu or Tb) phosphors have been synthesized via an ethylenediaminetetraacetic acid complexing process. Simultaneous thermal analysis has been used to study thermal decomposition of the precursor. X-ray powder diffraction measurements indicate that the well crystallized single-phase solid solutions (S.G. P21/c, Z = 4) can be obtained at temperatures above 1,000 °C. According to scanning electron microscopy data, the final products consist of dense particles with the average size of

A new series of germanates CaRE2Ge3O10 (RE = Y, La–Yb) has been prepared using an EDTA-assisted route. Rietveld refinement of room temperature powder X-ray diffraction patterns shows that these compounds crystallize in the monoclinic system (S.G. P21/c, Z = 4) and have two morphotropic transitions. The calcium and rare earth atoms are distributed among three nonequivalent sites and form layers along the [0 0 1] direction connected into the framework through [Ge3O10]. The morphotropic transitions are accompanied by changes in the site occupancy factors of metal cations and by variation in the first and second coordination spheres of Ca2+/RE3+.

The crystal structure and luminescence properties of CaY2Ge3O10:Ln3+ (Ln = Eu, Tb) germanates synthesized via a conventional solid-state reaction and an ethylenediaminetetraacetic acid complexing process are studied. The CaY2 − xLnxGe3O10 (Ln = Eu, Tb; x = 0–1.0, 2.0; Δx = 0.1) solid solutions have a monoclinic structure (space group P21/c, Z = 4), in which dopant ions occupy three nonequivalent noncentrosymmetric sites with different Ca2+/Ln3+ ratios. The effect of the synthesis methods, dopant concentrations, and excitation wavelengths on the luminescence properties of the compounds obtained is determined.

DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYSTAL09 code to describe the crystal structure and vibrational spectra of Y2CaGe4O12 cyclotetragermanate, a new optical host. Two space groups P4/nbm and Cmme have been considered. The former corresponds to a mixed (0.5 Ca + 0.5 Y) distribution at the octahedral sites found from the results of Rietveld refinement of room temperature powder XRD pattern; the latter refers to the model of crystallographically nonequivalent calcium and yttrium atomic setting in distorted oxygen octahedrons. The most accurate geometry description has been obtained with the WC1LYP and PBE (n = 6) hybrid functionals, while the B3LYP calculation provides the best agreement between the recorded infrared and Raman spectra and their computed counterparts. Assignments of most of the observed bands to vibrational modes are given. The comparison between calculated and experimental frequencies shows a general good agreement for the spectra below 600 cm-1. The relationship between selected infrared bands and Raman lines, internal vibrations of the [Ge4O12] unit, and external modes is briefly discussed. © 2014 American Chemical Society.

The tetragermanate Ba2Eu2Ge4O13 was synthesized for the first time, and its structural and luminescent properties were examined. It crystallizes in a monoclinic system (space group C2/c, Z = 4) and its crystal structure incorporates a unique linear [Ge4O13]10− anion. The photoluminescence spectrum of the sample (λex = 280 nm) contains several intense peaks in the orange-red spectral range corresponding to the 4f–4f transitions in Eu3+ ions.

The novel red emitting phosphors based on CaY2−xEuxGe3O10 (x=0–2) have been prepared using both a conventional solid-state reaction and a synthesis route via EDTA-complexing process. Powder XRD study has revealed that CaY2−xEuxGe3O10 (x=0.1–0.8, 2.0) crystallizes in the monoclinic space group P21/c, Z=4. The trivalent europium ions occupy three different sites (4e) with the C1 symmetry. The phosphors exhibit a strong red emission under excitations at 250 nm and 393 nm. The light emission efficiency depends on the excitation wavelength, the activator content and the method of synthesis. Appropriate CIE chromaticity coordinates for CaY1.7Eu0.3Ge3O10 are x=0.54 and y=0.29.

The crystal structures of KCaVO4 and RbBaVO4, synthesized at high-pressure/high-temperature and by a conventional solid-state reaction, respectively, were determined using X-ray powder diffraction data. These compounds were found to have the β-K2SO4 structure type (space group Pnma, Z = 4) with parameters a = 7.2628(5) Å, b = 5.7258(4) Å, c = 9.6854(7) Å (KCaVO4), and a = 7.8887(1) Å, b = 5.9589(1) Å, c = 10.3958(2) Å (RbBaVO4). The unit cell volume of KCaVO4, 402.77(5) Å3, is significantly lower than for the low-temperature modification reported previously, 436.2 Å3. The difference can be explained by a pressure-induced phase transition to a more dense state resulting from the rotation of tetrahedra and the exchange of oxygen atoms from the first and second coordination spheres for potassium and calcium atoms.

A new series of promising luminescent materials, cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu, has been synthesized via a solid-state reaction. X-ray and neutron powder diffraction studies show that these oxides crystallize in the monoclinic crystal system (S.G. C2/c, Z=4) and have a morphotropic phase transition between Er and Tm compounds followed by a step-like change of the unit cell constants. Isolated [Si3O9] rings located in layers are basic building units and stack with Sr/R layers along the [1 0 1 ¯ ] direction. The rare earth atoms are distributed among three independent Sr/R sites coordinated by 8, 7 and 6 oxygen atoms, and the Sr-R populations change from mixed to 0.5/0.5 over site (1) and full occupation of sites (2) and (3) by Sr and R, respectively, at the transition. Changes of the conformation and mutual arrangement of [Si3O9] rings, as well as exchange of oxygen atoms from the first and the second coordination sphere of two Sr/R sites also feature the phase transition. Luminescence in Sr3Y2(Si3O9)2:Eu3+ under ultraviolet (UV) excitation has been discussed.

A promising series of phosphors based on Li7La3−xEuxHf2O12 (x = 0.05–0.20) has been prepared using a conventional solid state reaction. Rietveld refinement of room temperature powder X-ray diffraction patterns shows that the Li7La3−xEuxHf2O12 (x = 0.00–0.20) solid solution crystallizes in the tetragonal space group I41/acd, Z = 8. The trivalent europium ions occupy two crystallographically nonequivalent sites (8b) and (16e) with the D′2 and C2 symmetry, respectively. Raman spectroscopy corroborates the X-ray diffraction results and confirms the crystal structure features of tetragonal Li7La3Hf2O12 with the maximum phonon energy of 650 cm−1. The phosphors exhibit reddish orange emission under excitation at 282 nm. Calculation of the Ω2 and Ω4 Judd–Ofelt intensity parameters and quantum efficiencies from the recorded emission spectra allows a more extensive study of the luminescence properties of Eu3+-doped Li7La3Hf2O12. The maximum light emission is revealed at the activator content of x = 0.15 in Li7La3−xEuxHf2O12. CIE chromaticity coordinates of Li7La2.85Eu0.15Hf2O12 are x = 0.643 and y = 0.354.

Simultaneous emission lines around 1.05μm, 1.3μm, 1.8μm, 2.1μm and 2.7μm have been observed in Li

A polycrystalline sample of Ba 4 Mn 3 O 10 has been prepared and characterized by X-ray diffraction (290 K), neutron diffraction (290, 80, 5 K) and magnetometry (5≤ T (K)≤1000). At 290 K the compound is paramagnetic and isostructural with Ba 4 Ti 2 PtO 10 . Mn 3 O 12 trimers, built up from MnO 6 octahedra, are linked through common vertices to form corrugated sheets perpendicular to the y -axis of the orthorhombic unit cell (Space group Cmca , a =5.6850(1), b =13.1284(1), c =12.7327(1) A); Ba atoms occupy the space between the layers. On cooling, the magnetic susceptibility shows a broad maximum at ∼130 K, and a sharp transition at 40 K. Neutron diffraction has shown that long-range antiferromagnetic order is present at 5 K but not at 80 K, although magnetometry at 5 K has revealed a remanent magnetization (0.002 μ B per Mn) which is below the detection limit of the neutron experiment.