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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.

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 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.

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.

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.

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 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.

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.

New germanate–vanadates with the general formula BiCa 4− x La x (VO 4 ) 3− x (GeO 4 ) x O, x =1−3, have been synthesized by the nitrate–citrate method and characterized. Structural refinement based on X-ray powder diffraction data showed that these compounds are isostructural with BiCa 4 (VO 4 ) 3 O (space group P6 3 /m (N 176), Z =2, а =9.8327–9.8755(3), and с =7.1203–7.2133(2) ). They may be viewed as continuous series of solid solutions where Ca 2+ and V 5+ cations in the crystal lattice of vanadate BiCa 4 (VO 4 ) 3 O are replaced by La 3+ and Ge 4+ cations. In the process of substitution, the La 3+ cations occupy mainly the 4f lattice sites of the germanate–vanadate oxyapatites, but the filling of the 4f and 6h lattice sites in the germanate BiCaLa 3 (GeO 4 ) 3 is equivalent. IR and Raman spectra were studied. The Raman spectra clearly show a two-mode behavior: the lines of GeO 4 and VO 4 are separated from each other, and the (V/Ge)O 4 tetrahedra exhibit a quasi-independent behavior.

Calcium yttrium tetrametagermanates Y2CaGe4O12 doped with Er3+ and Er3+/Yb3+ reveal upconversion emission in visible spectral range under near-infrared excitation, λex = 980 nm. For the solid solution ErxY2−xCaGe4O12 concentration dependencies for the green and red lines of the visible emission around 526 nm (2H11/2 → 4I15/2), 545 nm (4S3/2 → 4I15/2) and 670 nm (4F9/2 → 4I15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y2CaGe4O12:Er3+ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y2CaGe4O12:Er3+ and Y2CaGe4O12:Er3+, Yb3+ has been proposed. Conditions for visible emission occurrence under quasi-resonance λex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition 4I13/2 → 4I15/2 in erbium ions has been detected.

The crystal structure of the promising optical materials Ln{sub 2}M{sup 2+}Ge{sub 4}O{sub 12}, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y{sub 2-x}Er{sub x}CaGe{sub 4}O{sub 12} (x=0-2), Y{sub 2-2x}Ce{sub x}Ca{sub 1+x}Ge{sub 4}O{sub 12} (x=0-1), Y{sub 2}Ca{sub 1-x}Mn{sub x}Ge{sub 4}O{sub 12} (x=0-1) and Y{sub 2-x}Pr{sub x}MnGe{sub 4}O{sub 12} (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu{sub 2}MnGe{sub 4}O{sub 12} has been found. - Graphical abstract: Crystal structure of Ln{sub 2}MGe{sub 4}O{sub 12}, where Ln=rare-earth element or Y; M=Ca, Mn, Zn.

Luminescent properties of the new group of crystalline optical materials Ln 2 CaGe 4 O 12 , Ln=Dy, Ho, Er, Tm, Yb were studied under laser excitation at λ= 976 nm in the stationary mode. Er 2 CaGe 4 O 12 , Ho 2 CaGe 4 O 12 and also the solid solution Ln x Y 2− x CaGe 4 O 12 (0⩽ x ⩽2) may be used in photonics as optical elements, such as converters and resonance amplifiers.

In the Mg4−xNixNb2O9 (0⩽x⩽4) system two ranges of solid solution have been found. One of the solid solutions has a corundum-related structure type (space group P 3 ¯ c 1 ); the second one adopts the II-Ni4Nb2O9 structure type (space group Pbcn). The unit cell constants and atomic positions have been determined and refined using neutron powder diffraction data. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) from MgNi3Nb2O9 crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg3NiNb2O9, measured in the temperature range T=2–300 K, shows no indications of magnetic ordering at low temperatures, while for MgNi3Nb2O9 there is a magnetic ordering at temperatures below 45.5 K.

A new complex oxide Y{sub 2}Cd{sub 2/3}Re{sub 4/3}O{sub 7} with hexagonal cell parameters a = 7.3564(2) A, c = 17.7092(5) A (space group P3{sub 1}21, z = 6, zirkelite structure type) was synthesized from Y{sub 3}ReO{sub 8}, ReO{sub 2}, metallic Re and CdO under pressure 6 GPa and temperature 1500 deg. C. Magnetic susceptibility measured in the temperature range from 2 to 300 K depends little on temperature above {approx}50 K and is indicative of a delocalized or intermediate character of d electrons of Re{sup 5+} cations.

A new rhenium (IV) dioxide crystallized in the rutile-type structure was successfully synthesized by a solid-state route and characterized by elemental analyses, X-ray and neutron powder diffraction. The structure obtained is tetragonal (space group P42/mnm) with the lattice constants a = 4.79825 ( 5 ) and c = 2.80770 ( 4 ) A . The electronic properties of rutile-like ReO2 were investigated by means of the projected augmented wave (PAW) method within both LDA and LDA+U approximations. The results revealed that the antiferromagnetic ordering is favorable for this system with the local magnetic moments about 1.0 μ B per Re atom. O 2p–Re 5d hybridization is important, and a covalent bonding between O and Re atoms occurs.

A new high-pressure, high-temperature phase, Zn4Nb2O9, has been prepared at T=1700 °C, using a pressure of P=9 GPa. An XRD study based on the Rietveld technique showed the structure to be corundum related ( P 3 c1 , Z=2) with the unit-cell parameters a=5.1864(1) A, c=14.1445(2) A. The structure consists of close-packed oxygen atoms, with 2/3 of the octahedral interstices occupied by zinc and niobium atoms. The space group was confirmed by micro diffraction and selected-area electron diffraction studies.

A new high-pressure, high-temperature modification of Mn 4 Nb 2 O 9 , has been prepared at T =1700 °C, P =9 GPa. The XRD study based on Rietveld technique suggests the structure to be a rhombohedral (R3c) modification of Mn 4 Nb 2 O 9 , (LiNbO 3 type structure) with the chemical formula Mn(Mn 1/3 Nb 2/3 )O 3 and unit cell parameters a =5.34139(2), c =14.15807(7) A. However, electron diffraction (ED) and high resolution electron microscopy (HREM) images of crystallites found in an XRD single phase sample revealed frequent disorder, but also the existence of a new monoclinic modification of Mn 4 Nb 2 O 9 often intergrown with the corundum related P 3 c1-Mn 4 Nb 2 O 9 modification stable at ambient pressure and temperature. This new monoclinic modification has the unit cell parameters: a =9.93, b =5.34, c =13.20 A, β =92.9° and space group C2/c or Cc. It is closely related to the corundum lattice since its c ∗ axis is parallel to 〈2 0 2〉 ∗ R 3 c in the rhombohedral sublattice.

Synthesis conditions, crystal structures, and magnetic properties of quasi-one-dimensional complex oxides Ca3CuMnO6 (space group P-1, z=4, triclinic cell) and Ca3Co1+xMn1−xO6 with x=0, 0.25, 1.0 (structural type K4CdCl6, space group R-3c, z=6) are presented. The crystal structures of Ca3CoMnO6 and Ca3CuMnO6 were refined using neutron and combined X-ray and neutron diffraction analysis, respectively. The interatomic distances in oxygen polyhedra were found. In contrast to ferromagnetic Ca3Co2O6 (Tc=24 K), manganese-containing phases Ca3Co1+xMn1−xO6 are characterized by antiferromagnetic interactions with Neel temperatures 18 K (x=0.25) and 13 K (x=0). For Ca3CuMnO6TN was established to be 6 K.

The electronic structure and chemical bonding of anhydrous tin and lead hexacyanoferrates (II) have been studied using the linear muffin-tin orbital method in the tight-binding minimal basis set approximation and semi-empirical Huckel method. The hybridised s- and p-states of CN-groups were shown to have a dominant influence on the electronic structure of these compounds, that results in strong splitting of iron d-states. The chemical bonding C–N and Fe–C in the [Fe(CN)6]4–-complex are mainly covalent. Bonds between Pb(Sn) and N atoms are more weak covalent and anisotropic. The low stability of tin hexacyanoferrate is due to high anisotropy of Sn–N bonding.

The crystal structure of Pb 2 Fe(CN) 6 and Sn 2 Fe(CN) 6 has been first determined and refined using the Rietveld method with combined CuKα 1 X-ray and constant-wavelength neutron powder diffraction data in space group P -3 (147, Z =1). The unit cell constants are a =7.1346(1) and 7.1805(1) A, c =5.4531(2) and 5.3639(1) A, respectively. The compounds are layered, groups of three nearest [Fe(CN) 6 ] 4− -complexes are joined to layers by means of Pb or Sn atoms. The same Pb or Sn atoms (c.n.=3+3) joint three nearest complexes from the next layer. The jointing goes through ‘nitrogenlead(tin)nitrogen’ bonds.

The results of structural refinements and magnetic properties of one-dimensional oxide Ca 3 NiMnO 6 are presented. The structure of Ca 3 NiMnO 6 was solved by Rietveld analysis of powder neutron date in space group R -3 c with a =9.1227(9) A, c =10.5811(17) A, z=6 (type of K 4 CdCl 6 ). Infinite chains of MnO 6 octahedra and (Ni,Mn)O 6 trigonal prisms sharing faces run parallel to the c axis. The chains are separated by Ca 2+ cations, which are located in a distorted square antiprismatic environment. Magnetic susceptibility obeys the Curie-Weiss law at 300–600 K with μ eff value 5.00 μ B consistent with the valence cationic combination Ni 2+ -Mn 4+ . Magnetic measurements display the antiferromagnetic ordering in Ca 3 NiMnO 6 at 16 K.

Sr4−xNb17O26 (x=0.0, 0.3) and Eu4−xNb17O26 (x = 0.3) were synthesised from SrCO3/Eu2O3 and Nb2O5 in a vacuum furnace at temperatures up to 1600°C (1500°C for Eu) using acetylene soot or Nb as reducing agents. The synthetic studies show that these phases are only formed via an intermediate disordered ‘phase’ (or phasoid). Sr4−xNb17O26 and Eu4−xNb17O26 are isostructural with Ba4Nb17O26 and crystallise in space group P 4/m (83) with the unit cell parameters for x=0.3: a=12.023(l) A, c=4.1411(4) A and a=12.015(2) A c=4.1351l) A, Z = 1, respectively. The crystal structure can be described as an intergrowth between ANbO3 and NbO. Characteristic building blocks are quadruple chains of corner-sharing Nb6-octahedra. ?The structures of Sr4−xNb17O26 and Eu4−xNb17O26 were refined using X-ray powder diffraction and neutron diffraction data (only Sr4−xNb17O26). The Rietveld refiniments and microanalyses showed x in Eu4−xNb17O26 to be 0.3(l) while the Sr analogue both x= 0.0(1) and 0.3(1) were found. High resolution electron microscopy studies showed that the compounds frequently contained structural defects. The magnetic susceptibility of Eu4−xNb17O26 (x=0.3) shows a Curie-Weiss behaviour, with a magnetic moment in good agreement with the expected μeff=7.9 μB for Eu2+/ The Sr analogie is temperature-independent paramagnetic at room temperature, Sr4−xNb17O26 (x=0.3) and Eu4−xNb17O26 (x=0.3) are metallic, with a resistivity increasing with temperature.

The infinite-layer compounds of the (Sr, Ln)CuO 2 family have been prepared by a new synthesis method from the low-pressure phase of SrCuO 2 and NdCuO 2 as starting materials. The crystal structure, microstructure and superconductivity of these samples have been studied with use of neutron powder diffraction, optical micrographs, energy-dispersive X-ray analysis. magnetic-susceptibility and electrical-resistivity measurements. The ratio of the lattice constants a/c as an empirical parameter for the existence of superconductivity in the electron-doped infinite-layer compounds is discussed.

Powder samples of the reduced oxoniobates Eu0.7NbO3, EuNbO3, Eu2Nb5O9, EuNb8O14 and BaNb8O14 have been synthesised. EuNbO3 and Eu0.7NbO3 adopt the perovskite type structure (space group Pm3m) with unit cell parameters a = 4.021(1) and 3.981(3) A , respectively. Eu2Nb5O9 exhibits an intergrowth structure which can be described as alternating slabs of NbO and EuNbO3. It crystallises in P4 mmm with a = 4.131(1) and c = 12.043(5) A . EuNb8O14 and the here synthesised modification of BaNb8O14 is isostructural with SrNb8O14. The structures of EuNb8O14 and BaNb8O14 have been refined from X-ray powder diffraction data with the Rietveld technique. They have the unit cell parameters a = 9.2272(1), b = 10.2647(2), c = 5.9381(1) and a = 9.3451(1), b = 10.2689(2), c = 5.9470(4) A , (space group Pbam), respectively. In all the studied compounds AO12 cuboctahedra (A = Eu, Ba) form either chains (in ANb8O14), double layers (in Eu2Nb5O9) or three-dimensional nets (in EuNbO3 and Eu0.7NbO3). Magnetic susceptibility measurements show temperature independent paramagnetism for BaNb8O14, while the susceptibilities of the Eu niobates follow Curie-Weiss laws with effective magnetic moments in good agreement with the expected μeff = 7.9 μB for Eu2+. Eu0.7NbO3 and Eu1.0NbO3 exhibit ferromagnetic ordering below 4 K. BaNb8O14 and EuNb8O14 are semiconductors, while Eu2Nb5O9, EuNbO3 and Eu0.7NbO3 are metallic.

New complex oxides Cu 2 B SbO 6 ( B = Mn 3+ , Fe 3+ , Ga 3+ ) with a body-centered cubic elementary cell (the bixbyite type structure) and parameters 9.5667(3), 9.5622(3), and 9.5237(3) A, respectively, have been synthesized in air at 950°C from simple Cu, Sb, Mn, Fe, and Ga oxides. Rietveld's method of full-profile analysis was employed to refine the structure parameters of one of the obtained compounds, Cu 2 FeSbO 6 . Sb occupies 8b sites and Cu and Fe are in 24d sites. From cation-oxygen and oxygen-oxygen interatomic distances, it was shown that Sb occurs within an almost regular SbO 6 octahedron ( R Sb-O = 2.00 A), while Fe and Cu are located inside strongly distorted octahedra ( R M-O (A) = 1.938, 2.020, and 2.311). Magnetic susceptibility χ of the compounds was measured. For Cu 2 GaSbO 6 , χ( T ) obeys the Curie-Weiss law in the temperature range 77-300 K. For Cu 2+ , the values of Weiss's constant and the magnetic moment ( c = 0.42, μ = 1.81μ B ) are close to the theoretical ones. A transition to the antiferromagnetic state was observed at 15.6 K for Cu 2 FeSbO 6 and at 5.9 K for Cu 2 MnSbO 6 .

Ba3Nb16O23 was prepared by reducing a mixture of BaCO3 and Nb2O5 with acetylene soot under vacuum at 1550–1600 °C. The structure was determined from high resolution electron microscopy images and was refined by the Rietveld technique using X-ray and neutron powder diffraction data. The unit cell parameters are a=20.9301(4) A , b=12.4785(3) A and c=4.1619(1) A , space group Cmmm, Z=2. The crystal structure can be described as an intergrowth of BaNbO3 and NbO. Characteristic building units are quadruple chains of corner-sharing Nb6 octahedra connected via columns with a perovskite-type structure to form a three-dimensional network.

The title compound has been synthesized and its structure refined by using powder diffraction data. The structure is orthorhombic, space group Immm, with cell constants a = 7.1156 (9), b = 10.2863 (18), c = 6.5756 (9) [angstrom], and Z = 4. Two types of Nb atoms are present in the structure: Nb1 atoms in Nb[sub 2]O[sub 8] clusters with one of the shortest Nb-Nb distances known so far (2.560 [angstrom]), and Nb2 atoms forming layers of connected NbO[sub 6] octahedra. The electronic structure obtained from Extended Hueckel tight-binding band calculations suggests the coexistence of Nb(1+) ions in the Nb[sub 2]O[sub 8] clusters with quadruple Nb-Nb bonds, together with Nb(4.5+) ions in the layers of octahedra. Deviations from this simplified model are analyzed. The electrical behavior, measured in the temperature range 4.2 K < T < 300 K, is metallic at low temperatures and practically temperature-independent at room temperature. A semimetallic character is proposed based on the calculated band electronic structure, with the states around the Fermi level localized mainly on the layers of octahedra parallel to the ac plane. 27 refs., 5 figs., 4 tabs.