Structural investigations of lead-strontium fluoroapatites

Solid solutions in the system Pb_(10−x)Sr_x(PO₄)₆F₂, 0?x?10, were obtained as apatitic phases from aqueous medium. They were investigated by X-ray diffraction, chemical analysis and infrared (IR) spectroscopy. The results of the structural refinements indicated that the substitution of lead by strontium induces a regular decrease of the lattice constant “a” and a preferential strontium distribution in site M(1). A progressive shift of the F⁻ ion position along the apatitic channel was detected and confirmed by IR evidence. The different character of the M-F and M-O interactions was invoked to justify the structural differences observed as a function of composition.     

Optimisation des facteurs influencant la conductivité anionique dans quelques fluorures de structure fluorine

The large influence in the M_1?xM′_xF_2+x solid solutions (M = Sr, Pb, M′ = Y, In, Sb, Bi) of the covalency of the MF₂ “starting lattice” on the electrical properties of fluorides of fluorite-type structure is clearly shown in a comparative investigation. The influence of the polarizability of the substituting trivalent ion is only significant as far as the starting lattice contains a weakly polarizable cation. Enhancement of the electrical performances of β-PbF₂ by substitution of Pb²⁺ by trivalent cations seems to be due mainly to increasing disordering within the anionic sublattice and hence the role of cationic polarizability is apparently a second-order effect.     

Structural and electrical studies of the PbF2-BiOF system

An investigation of the PbF₂-BiOF system at 600°C has allowed isolation of a Pb_1?xBi_xO_xF_2?x solid solution (0<x? 0.80). For x ? 0.66 the structure is of fluorite type and for 0.66 <x? 0.80 it undergoes a fluorite derived rhombohedral distortion. The transport properties of Pb_1?xBi_xO_xF_2?x are for x ? 0.50 as good as those of the best fluorides.     

Evolution structurale de quelques solutions solides oxyfluorées de structure pyrochlore

Pyrochlore solid solutions Cd₂Nb_2−2xM^IV_2xO_7−2xF_2x are studied with M = Ge and Sn; the domain of existence is 0 ? x ? 0.5. We can observe two cubic domains for different values of the position parameter u. With M = Zr we obtain a pyrochlore solid solution varying between 0 ? x ? 0.875. It offers a rhomboedric distortion for 0.25 < x < 0.55 due to an ordered distribution of Nb and Zr in the (111) plans of the cell.     

Contribution a l'etude des systemes Ca3(PO4)2-MSO4 (M=Sr,Ba)

In the systems Ca₃(PO₄)₂-MSO₄ (M = Sr, Ba), the series of single phase Ca_21?3xM_2xI(PO₄)_14?2x(SO₄)_2x with 0<x<0.15 forM=Sr and 0<x<0.1 forM = Ba have been prepared. These solid solutions, respectively strontium phosphosulfate and barium phosphosulfate, are isostructural with anhydrous tricalcium orthophosphate. They have been characterized by their infrared spectra and their crystallographic unit cell parameters.     

Hydrothermal processing and characterization of Ce1−xPbxO2−δ solid solutions

Nanocrystals of Ce_1?xPb_xO_2?δ (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35) were prepared by a hydrothermal reaction route. During the formation reaction, buffer solutions were explored as an effective additive to retain the initial molar ratio. With increasing the Pb²⁺ content, the average crystallite size was slightly retarded. Morphologies observed by transmission electron microscope indicated that the particles were spherical-like and highly uniformed. Pb²⁺ ions are homogenously distributed in the solid solutions. Analyses using X-ray diffraction, Raman and UV spectroscopies showed that the solid solubility limit of Pb²⁺ in CeO₂ was about x = 0.20. For x < 0.20, with increasing the Pb²⁺ content, the bulk conductivity increased, and the oxygen storage capacity was enhanced as followed by a decrease in reduction temperature.     

Les phases SrLnMnO4 (Ln = La,Nd, Sm,Gd), BaLnMnO4 (Ln = La,Nd) et M1+xLa1−xMnO4 (M = Sr,Ba)

The phases SrLnMnO₄ (Ln = La, Nd, Sm, Gd), BaLnMnO₄ (Ln = La, Nd) and the solid solutions M_1+xLa_1?xMnO₄ (M = Sr: 0 ? x ? 1; M = Ba: 0 ? x ? 0.50) have a K₂NiF₄-type structure. The $\text{c}\text{a}$ ratio of the unit cell is related to the electronic configuration of the Mn³⁺ ions.     

Mechanochemical synthesis,structure and properties of lead containing alkaline earth metal fluoride solid solutions MxPb1-xF2 (M = Ca,Sr, Ba)

The paper deals with the mechanochemical synthesis of lead containing alkaline earth metal fluoride solid solutions M_xPb_1-xF₂ (M = Ca, Sr, Ba) by high-energy ball milling. Several metal precursors and fluorinating agents were tested for synthesizing M_0.5Pb_0.5F₂. Metal acetates and ammonium fluoride as precursors show the most promising results and were therefore used for the formation of M_xPb_1-xF₂ with different metal cationic ratios. The characterization of the local fluorine coordination and the crystal structure was performed by ¹⁹F MAS NMR spectroscopy and X-ray diffraction. Additional calculations of ¹⁹F chemical shifts using the superposition model allow a deeper insight into the local structure of the compounds. The fluoride ion conductivity was followed by temperature dependent DC conductivity measurements. Significantly higher conductivities were found in comparison with those of the corresponding binary fluorides. The highest values were observed for samples with high lead content M_0.25Pb_0.75F₂, bearing in mind the much higher conductivity of PbF₂ compared to MF₂.     

Präparation und Strukturanalyse der Verbindungen Ba2Pb4F10Br2–xIx (x = 0–2) mit verwandten kristallchemischen Motiven aus der Fluoritund Matlockitstruktur

Preparation and Structure of the Compounds Ba₂Pb₄F₁₀Br_2–xI_x (x = 0–2) with Related Structure Motifs of the Fluorites and Matlockites Colourless single crystals of Ba₂Pb₄F₁₀Br_2–xI_x (x = 0–2) have been obtained under hydrothermal conditions (T = 250 °C, 10 d), starting from stoichiometric amounts of BaF₂, PbF₂, PbBr₂ and PbI₂. The compounds crystallize in the tetragonal space group P4/nmm (No. 129). A complete miscibility in the region x = 0–2 has been observed. The mixed crystals follow Vegard's rule. For the compounds with the composition Ba₂Pb₄F₁₀Br₂ (a = 5.9501(2) Å, c = 9.6768(10) Å, R[F² > 2σ(F²)] = 0.022, wR(F² all reflections) = 0.059), Ba₂Pb₄F₁₀Br_1.1I_0,9 (a = 5.9899(3) Å, c = 9.7848(5) Å, R[F² > 2σ(F²)] = 0.014, wR(F² all reflections) = 0.035) and Ba₂Pb₄F₁₀I₂ (a = 6.6417(3) Å, c = 9.9216(10) Å, R[F² > 2σ(F²)] = 0.023, wR(F² all reflections) = 0.049) complete structure analyses have been performed on the basis of single crystal diffractometer data. Microcrystalline single phase compounds Ba₂Pb₄F₁₀Br_2–xI_x (x = 0–2) have been obtained by coprecipitation from aqueous solutions of KF, KBr (KI) and Ba(CH₃COO)₂, Pb(NO₃)₂ in acetic acid medium. For Ba₂Pb₄F₁₀Br_1.5I_0.5 and Ba₂Pb₄F₁₀Br_0.5I_1.5 powder data of microcrystalline samples were used for the Rietveld analyses (R_Bragg = 0.077 for Ba₂Pb₄F₁₀Br_1,5I_0,5 and R_Bragg = 0.065 for Ba₂Pb₄F₁₀Br_0.5I_1.5). The crystal structure comprises alternating structural features of fluorite related type (CaF₂) around Ba and matlockite related type (PbFCl) around Pb1 and Pb2 along the c axis. Barium shows a {8 + 4} cuboctahedral coordination of fluorine. The coordination polyhedron around the two crystallographically independent lead atoms is a monocapped quadratic antiprism built of {4 + 1} fluorine and {4} bromine or iodine atoms, respectively.     

Sur de nouveaux ferrites oxyfluorés d'yttrium ou de gadolinium à structure grenat

The oxyfluoride garnets of formula Y₃Fe_5?xM_xO_12?xF_x and Gd₃Fe_5?xM_xO_12?xF_x (M = 3d transition element) result from partial substitution of O^2? by F^? in Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ oxides. The cationic charge compensation is obtained by replacing the Fe³⁺ ions by divalent ions as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ ions. The site occupied by some of these ions (Mn²⁺, Ni²⁺, Zn²⁺) is determined by magnetic or Mössbauer measurements.     

Solid-state synthesis, characterization and luminescent properties of Eu-doped gadolinium tungstate and molybdate phosphors: Gd(2−x)MO6:Eux (M=W, Mo)

Red phosphors gadolinium tungstate and molybdate with the formula Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) were successfully synthesized by the solid-state reaction at 900 and 1300 °C for 4 h, respectively. The products were characterized by an X-ray powder diffractometer (XRD), TG-DSC, FT-IR, PL, UV-vis and SEM. Room-temperature photoluminescence indicated that the as-prepared Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) had a strong red emission, which is due to the characteristic transitions of Eu³⁺ (⁵D₀→⁷F_J, J=0, 1, 2, 3, 4) for these phosphors. Meanwhile, the ⁵D₀→⁷F₂ is in the dominant position. The emission quantum efficiency of Eu³⁺ in the Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) system has been investigated. The XRD results indicate that both Gd₂WO₆ and Gd₂MoO₆ belong to the monoclinic system with space group C2/c [A. Bril, G. Blasse, J. Chem. Phys. 45 (1966) 2350-2356] and I2/a [A. Bril, G. Blasse, J. Chem. Phys. 45 (1966) 2350-2356], respectively. SEM images indicate that the shape of Gd_1.96WO₆:Eu_0.04³⁺ is aggregated small particles with a mean diameter of about 300 nm, and the shape of Gd_1.96MoO₆:Eu_0.04³⁺ is block-like structures.     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Solubility product and related thermodynamic quantities of silver and barium chromates in formamide

The values of ΔG⁰, ΔH⁰, and ΔS⁰ for the dissolution process, M₂(CrO₄)_x + solvent — 2 M^x+ (solvated) + xCrO₄^2? (solvated), where M is Ag or Ba, and x is 1 or 2, have been determined in formamide from solubility studies. The negative value of ΔS⁰ indicates that there is more order in the dissolved state than in the undissolved state.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.     

Er-doped PbF2: Comparison between nanocrystals in glass-ceramics and bulk single crystals

An appropriate annealing of a GeO₂-PbO-PbF₂:ErF₃ glass leads to the formation of a glass-ceramic, composed of Pb_1−xEr_xF_2+x nanosized crystallites dispersed throughout an amorphous oxide matrix. These nanocrystallites are compared to Er³⁺-doped PbF₂ bulk single crystals. The influence of the annealing temperature on the glass-ceramics characteristics is thoroughly investigated. For several glass-ceramics resulting from different heat treatments, the quantity of crystallized PbF₂, as well as the segregation of Er³⁺ ions into the crystallites, has been studied through two methods: first, the study of the crystallographic characteristics and second, the evolution of the optical properties. It was evidenced that, for a heat treatment over 365 °C, the whole PbF₂:Er has completely crystallized and that the segregation of Er³⁺ ions into the crystallites was total. Strong interactions between Er³⁺ ions occur in the Pb_1−xEr_xF_2+x single and nanocrystals, promoting cross-relaxation processes, under 488-nm excitation, and favouring the 660- (red) and 810-nm emissions at the expense of the 550-nm (green) one.     

Availability of Fe2+ ions in Cr- or Al-substituted magnetites with relevance to the process of oxidation in defect phase γ

During oxidation of the aluminum- or chromium-substituted magnetites, (Fe²⁺Fe³⁺_2?xM^3f_x)O^2?₄, with 0.4 < x < 1.8 in defect phase γ of the same spinel structure, the availability for oxidation of Fe²⁺ ions in the tetrahedral sites (A sites) of the spinel structure is much less than that in octahedral sites (B sites) and in both cases depends on the extent of aluminum or chromium substitution. The influence of cation distribution in A and B sites on the oxidation temperature is shown directly by differential thermogravimetric analysis and by electrical conductivity.     

Study of the Solid Solutions of LiNbO3and LiTaO3with Mn

Two unusual, extensive new solid solutions of LiNbO₃and LiTaO₃with MnO have been prepared, where 4Mn²⁺replace a combination of 3Li⁺and a pentavalent cation: Nb⁵⁺or Ta⁵⁺. The formulas are Li_1−xM_1−xMn_4xO₃, 0<x<0.13, forM=Nb and 0<x<0.23 forM=Ta. The solid solutions were characterized by X-ray powder diffraction and density measurements. The manganese oxidation states were determined by X-ray photoelectron spectroscopy.     

Lithium-substituted cobalt oxide spinels LixM1−xCo2O4 (M = Co2+, Zn2+; 0 ≤ x ≤ 0.4)

Substitution of Li⁺ into Co₃O₄ and ZnCo₂O₄ gives rise to the solid solution series Li_xM_1?xCo₂O₄ (M = Co²⁺ or Zn²⁺) having the spinel structure upto x = 0.4. X-Ray diffraction intensities show that the spinel solid solutions are likely to have the following cation distributions: (Co²⁺)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄ and (Zn²⁺_1?xCo²⁺_x)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄. Electrical resistivity and Seebeck coefficient data indicate that the electron transport in these systems occurs by a small-polaron hopping mechanism.     

Solid solutions of double phosphate hexahydrates in the MgNH4PO4 · 6H2O-Ni2+-H2O system and their dehydration products

Substitutional solid solutions Mg²⁺ ? Ni²⁺ of crystal hydrates Mg_{(1 ? x)}Ni _x NH₄PO₄ · 6H₂O and Mg_{(1 ? x)}Ni _x NH₄PO₄ · H₂O (where 0 < x ≤ 0.65), which have struvite structure and dittmarite structure, respectively, have been studied. Ion exchange Mg²⁺ ? Ni²⁺ influences the condition of M²⁺-H₂O (M²⁺ = Mg²⁺, Ni²⁺) coordination bonds and hydrogen bonds involving coordinated H₂O molecules, as probed by X-ray powder diffraction, thermal analysis, and FTIR spectroscopy. The coordination of water molecules to metal ions in those crystal hydrates is treated to be a factor that determines the propensity of the resulting crystal structures to polymorphism.     

Gas phase UV and IR absorption spectra of CxF2x+1CHO (x = 1-4)

The UV and IR spectra of C_xF_2x+1CHO (x = 1-4) were investigated using computational and experimental techniques. C_xF_2x+1CHO (x = 1-4) have broad UV absorption features centered at 300-310 nm. The maximum absorption cross-section increases significantly and shifts slightly to the red with increased length of the C_xF_2x+1 group: CF₃CHO, 3.10 × 10⁻²⁰ (300 nm); C₂F₅CHO, 6.25 × 10⁻²⁰ (308 nm); C₃F₇CHO, 8.96 × 10⁻²⁰ (309 nm); and C₄F₉CHO, 10.9 × 10⁻²⁰ (309 nm). IR spectra for C_xF_2x+1CHO were recorded, calculated, and assigned. Results are discussed with respect to the literature data and to the atmospheric fate of CxF2x+1CHO.