Ivan I. Leonidov

Structural and Vibrational Properties of the Ordered Y2CaGe4O12 Germanate: A Periodic Ab Initio Study

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 6...

Crystal structure and optical properties of germanates Ln2Ca(GeO3)4 (Ln = Gd, Ho, Er, Yb, Y)

This paper reports on the results of structural and optical investigations of a new class of layered compounds Ln2Ga(GeO3)4 (Ln = Gd, Ho, Er, Yb, Y), which are promising for the use in photonics as converters of laser radiation.

Synthesis, crystal structure, and luminescence properties of CaY2Ge3O10:Ln3+, Ln = Eu, Tb

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.

Nd3 +, Ho3 +-codoped garnet-related Li7La3Hf2O12 phosphor with NIR luminescence

Simultaneous emission lines around 1.05μm, 1.3μm, 1.8μm, 2.1μm and 2.7μm have been observed in Li7La3-xNdxHf2O12:Ho3+ (x=0.00-0.15) under 808nm 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 2.7μm 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.

Short CommunicationNd3 +, Ho3 +-codoped garnet-related Li7La3Hf2O12 phosphor with NIR luminescence

Simultaneous emission lines around 1.05μm, 1.3μm, 1.8μm, 2.1μm and 2.7μm have been observed in Li7La3-xNdxHf2O12:Ho3+ (x=0.00-0.15) under 808nm 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 2.7μm 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.

Structure and luminescent properties of calcium and manganese rare-earth tetrametagermanates

The new group of optical materials, so called calcium and manganese rare-earth tetrametagermanates Ln<inf>2</inf>M²⁺Ge<inf>4</inf>O<inf>12</inf>, Ln = Eu - Lu or Y, M²⁺ = Ca, Mn, and solid solutions Er<inf>x</inf>Y<inf>2-x</inf>CaGe<inf>4</inf>O<inf>12</inf>, Pr<inf>x</inf>Y<inf>2-x</inf>MnGe<inf>4</inf>O<inf>12</inf>, 0 ≤ x ≤ 2, were synthesized and their crystal and optical properties were investigated. All compounds are isostructural and their crystal structure belongs to the space group P4/nbm (No. 125, Z = 2). The existence of cyclic anions [Ge<inf>4</inf>O<inf>12</inf>]⁸⁻, consisting of four GeO<inf>4</inf> tetrahedra, is the main feature of the crystal structure which consists of two alternating layers. The first layer type includes [Ge<inf>4</inf>O<inf>12</inf>]⁸⁻ anions in boat-type conformation. Between these layers octahedral and square antiprismatic cavities are formed, that are occupied by Ln and (Ca, Mn) atoms of the second layer type. Values of interatomic distances and bond angles inside cyclic anion groups [Ge<inf>4</inf>O<inf>12</inf>]⁸⁻ are nearly equal in all compounds. So the structure of tetracycles is rigid. The Ln and (Ca, Mn) atoms are placed inside elongated oxygen octahedrons with ratio 0.5/0.5. The square antiprisms are occupied only by rare-earth cations. The decreasing atomic radius of the rare earth elements leads to a change of the island structure motive to the 2D layered type.

Synthesis, crystal structure and luminescence properties of CaY{sub 2−x}Eu{sub x}Ge{sub 3}O{sub 10} (x=0–2)

The novel red emitting phosphors based on CaY{sub 2−x}Eu{sub x}Ge{sub 3}O{sub 10} (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 CaY{sub 2−x}Eu{sub x}Ge{sub 3}O{sub 10} (x=0.1–0.8, 2.0) crystallizes in the monoclinic space group P2{sub 1}/c, Z=4. The trivalent europium ions occupy three different sites (4e) with the C{sub 1} 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 CaY{sub 1.7}Eu{sub 0.3}Ge{sub 3}O{sub 10} are x=0.54 and y=0.29. - Graphical abstract: The room temperature PL and PLE spectra of CaY{sub 1.2}Eu{sub 0.8}Ge{sub 3}O{sub 10} annealed at different temperatures. Display Omitted - Highlights: • The novel red emitting phosphors based on CaY{sub 2−x}Eu{sub x}Ge{sub 3}O{sub 10} have been prepared. • Effect of synthesis conditions on the surface morphology and luminescence properties of CaY{sub 2−x}Eu{sub x}Ge{sub 3}O{sub 10} has been studied. • Optimum dopant concentrations and annealing temperature have been reported.