Luminescence Properties of Al2O3:Ti in the Blue and Red Regions: A Combined Theoretical and Experimental Study

Using jointly experimental results and first-principles calculations, we unambiguously assign the underlying mechanisms behind two commonly observed luminescence bands for the Al2O3 material. Indeed, we show that the red band is associated with a Ti3+ d-d transition as expected, while the blue band is the combination of the Ti3+ + O- → Ti4+ + O2- and VO•+e- → VO× de-excitation processes. Thanks to our recent developments, which take into account the vibrational contributions to the electronic transitions in solids, we were able to simulate the luminescence spectra for the different signatures. The excellent agreement with the experiment demonstrates that it should be possible to predict the color of the material with a CIE chromaticity diagram. We also anticipated the luminescence signature of Al2O3:Ti,Ca and Al2O3:Ti,Be that were confirmed by experiment.

Micro- and nanostructured layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2

Pure micro- and nanocrystalline powders of the layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2 have been successfully prepared and thoroughly characterised. Microstructured samples (BaZn3-MPs) have been produced by hydrothermal reaction using synthetic martyite Zn3V2O7(OH)2·2H2O as the starting reagent. Nanoparticles (NPs) with an average size of ≈ 60 nm (BaZn3-NPs-7h) or ≈ 50 nm (BaZn3-NPs-25min) have been obtained by using a coprecipitation method at ambient pressure, and by varying the stirring time. Rietveld refinements of X-ray diffraction data indicate that micro- and nanostructured BaZn3(VO4)2(OH)2 both crystallize in a R3̄m structure very similar to that of the known layered-kagome compound BaCo3(VO4)2(OH)2. Transmission electron microscopy observation of BaZn3-NPs-7h and BaZn3-NPs-25min reveals crystallized NPs with homogenous distributions of Ba, Zn, and V elements. FT-IR and Raman spectra show subtle differences between micro- and nanostructured samples which cannot be linked to any differences in the average crystal structures. The high resolution 51V MAS NMR spectrum of BaZn3-MPs shows a single isotropic line attributed to VO43- groups with C3v point group. The spectra of the nanostructured samples reveal the presence of a weak additional signal which decreases in intensity with increasing the NPs size, and which has been tentatively assigned to the presence at the surface of the NPs of a small amount of V5+ ions in a different chemical environment. Nanostructuring also impacts the optical properties of BaZn3(VO4)2(OH)2. The UV-vis absorption spectra of NPs exhibit an additional weak transition in the visible domain which is not observed for the microstructured sample.

Revisiting the Crystal Structure of Layered Oxychalcogenides Ln2O2S2 (Ln = La, Pr, and Nd)

La₂O₂S₂ was recently used as a precursor to prepare either a new metastable form of La₂O₂S by de-insertion of half of sulfur atoms of (S₂) dimers or quaternary compounds by insertion of a coinage metal (e.g., La₂O₂Cu₂S₂). A strong structural relationship exists between the polysulfide precursor and the synthesized products, which highlights the topochemical nature of these reactions. Nevertheless, the crystal structure of the precursor material is still a matter of debate. Namely, several structural models were reported so far in the literature with different space groups and/or crystal systems. All these models were built upon infinite [Ln₂O₂] slabs separated from each other by a flat sulfur layer of (S₂) dumbbells. Nevertheless, all (S₂) dimers within a given sulfur layer may rotate in phase by 90° compared to the ideal model that induces an overall atomic disorder in (S₂) dimer orientation along the stacking axis. This leads to some imbroglio and much confusion in the description of structural arrangement of Ln₂O₂S₂ materials. Herein, the crystal structures of La₂O₂S₂ and its Pr and Nd variants are revisited. We propose an alternative model that reconciles pre-existing structural descriptions of Ln₂O₂S₂ (Ln = La, Pr, and Nd) materials and highlights the strong dependency of the degree of long-range ordering of the sulfur layers on the synthesis conditions.

Microstructured layered-kagome BaCo3(VO4)2(OH)2 with variable crystallite size: alternative synthetic route and comparison with nanostructured samples

Microplatelets of the layered-kagome compound BaCo₃(VO₄)₂(OH)₂, which is the Co²⁺ analogue of mineral vesignieite BaCu₃(VO₄)₂(OH)₂, have been prepared with very high yield by hydrothermal reaction using synthetic karpenkoite Co₃V₂O₇(OH)₂·2H₂O as starting reagent. The Rietveld refinement of X-ray diffraction data indicates that Co₃V₂O₇(OH)₂·2H₂O is isostructural with martyite Zn₃V₂O₇(OH)₂·2H₂O. Two single-phased samples of microstructured BaCo₃(VO₄)₂(OH)₂ have been characterized using powder X-ray diffraction, FT-IR and Raman spectroscopies, thermal analyses, scanning electron microscopy, energy-dispersive X-ray spectroscopy and magnetisation measurements. Their crystallite sizes perpendicular to the c-axis are in the range of 92(3) to 146(6) nm and depend on the synthesis conditions. Results have been compared to those previously obtained for quasi-spherical nanoparticles having a crystallite size of the order of 20 nm, to explore the effect of the crystallite size on the properties of BaCo₃(VO₄)₂(OH)₂. This study highlights that the magnetic properties depend on the crystallite sizes only at low temperatures.

Impact of Nanostructuration on the Chemical Composition of Nickel Oxide Nanoparticles

Reduction of the size of a particle down to a few tens of nanometers or below may drastically affect its physical properties. That is well-known for quantum dots. Conversely, many works consider the chemical composition of nanoparticles as invariant upon reduction of their dimension. Here we demonstrate that the chemical composition of a transition-metal oxide, namely, nickel oxide, is drastically affected by its nanostructuration.

Evidence of Wolframite-Type Structure in Ultrasmall Nanocrystals with a Targeted Composition MnWO4

Here, we report a study of white-ochre powders with targeted composition MnWO₄ prepared via a coprecipitation method. Through X-ray total scattering combined with pair distribution function analysis and Rietveld refinement of X-ray diffraction data, we find that their crystal structure is similar to that of bulk-MnWO₄, despite a mean crystallite size of 1.0-1.6 nm and a significant deviation of the average chemical composition from MnWO₄. The chemical formula derived from elemental and thermogravimetric analyses is Mn_0.8WO_3.6(OH)_0.4·3H₂O. X-ray absorption and magnetic susceptibility measurements show that Mn and W have the same oxidation states as in MnWO₄. No magnetic ordering or spin glass or superparamagnetic behavior is observed above 2 K, unlike in the case of MnWO₄ nanocrystals having a mean size higher than 10 nm.

Crystal structure and chemical bonding in the mixed anion compound BaSF

BaSF: a textbook example of a layered mixed anion compound containing sulfur pairs (S₂)²⁻.

Unravelling the origin of the giant Zn deficiency in wurtzite type ZnO nanoparticles

Owing to its high technological importance for optoelectronics, zinc oxide received much attention. In particular, the role of defects on its physical properties has been extensively studied as well as their thermodynamical stability. In particular, a large concentration of Zn vacancies in ZnO bulk materials is so far considered highly unstable. Here we report that the thermal decomposition of zinc peroxide produces wurtzite-type ZnO nanoparticles with an extraordinary large amount of zinc vacancies (>15%). These Zn vacancies segregate at the surface of the nanoparticles, as confirmed by ab initio calculations, to form a pseudo core-shell structure made of a dense ZnO sphere coated by a Zn free oxo-hydroxide mono layer. In others terms, oxygen terminated surfaces are privileged over zinc-terminated surfaces for passivation reasons what accounts for the Zn off-stoichiometry observed in ultra-fine powdered samples. Such Zn-deficient Zn_1-xO nanoparticles exhibit an unprecedented photoluminescence signature suggesting that the core-shell-like edifice drastically influences the electronic structure of ZnO. This nanostructuration could be at the origin of the recent stabilisation of p-type charge carriers in nitrogen-doped ZnO nanoparticles.

Influence of electronic vs. steric factors on the solid-state photochromic performances of new polyoxometalate/spirooxazine and spiropyran hybrid materials

New supramolecular spirooxazine and spiropyran/polyoxotungstate materials have been designed. Their solid-state photochromic performances are strongly exalted compared to their isostructural Mo counterparts due to a more favorable electronic effect.

[Bi2O2]2+ Layers in Bi2O2(OH)(NO3): Synthesis And Structure Determination

Bi2O2(OH)(NO3) is the first bismuth oxynitrate containing [Bi2O2]2+ layers for which the crystal structure has been determined. The current study presents its synthesis, formula determination using powder XRD, thermogravimetric analysis and nitrate assay, and finally, crystal structure refinement from twinned crystals. Bi2O2(OH)(NO3) crystallizes in the orthorhombic system, a = 5.3878(11), b = 5.3984(10) and c = 17.136(2) Å , with the space group Cmc21. The structure refinement based on 1257 independent reflections converged to RI (obs) = 0.0321 and RIw (obs) = 0.0765. The Bi2O2(OH)(NO3) structure is based on [Bi2O2]2+ sheets in the (ab) plane. The interlayer space is composed of two slices containing OH− and (NO3)− groups, respectively. Within the nitrate layers, the disorder over two possible NO3- orientations (related by a four fold axis) involves half occupied positions for two of the oxygen atoms. An attempt to lower the symmetry and/or enlarge the unit cell, looking for an (NO3)− ordering, failed

Modelling and quantification of intergrowth in γ-MnO2by laboratory pair distribution function analysis

γ-MnO2is a material formed by random intergrowth of two phases, β-MnO2andR-MnO2. It is demonstrated here on seven γ-MnO2samples that pair distribution function analysis using a conventional X-ray diffraction setup (Bragg–Brentano geometry with a molybdenum anode) allows the quantification of this intergrowth simplyviaa simulation of the actual material by a mixture of β-MnO2andR-MnO2phases. Although this method does not take into account specifically the relaxed distances in the vicinity of the intergrowth zone, it is found to be very robust, accurate and in full agreement with the widely used quantification based on the empirical approach of Chabre & Pannetier [Prog. Solid State Chem.(1995),23, 1–130].

Photochromic properties of polyoxotungstates with grafted spiropyran molecules

The first systems associating in a single molecule polyoxotungstates (POTs) and photochromic organic groups have been elaborated. Using the (TBA)4[PW11O39{Sn(C6H4I)}] precursor, two hybrid organic-inorganic species where a spiropyran derivative (SP) has been covalently grafted onto a {PW11Sn} fragment via a Sonogashira coupling have been successfully obtained. Alternatively, a complex containing a silicotungstate {PW11Si2} unit connected to two spiropyran entities has been characterized. The purity of these species has been assessed using several techniques, including (1)H and (31)P NMR spectroscopy, mass spectrometry, and electrochemical measurements. The optical properties of the hybrid materials have been investigated both in solution and in the solid state. These studies reveal that the grafting of SPs onto POTs does not significantly alter the photochromic behavior of the organic chromophore in solution. In contrast, these novel hybrid SP-POT materials display highly effective solid-state photochromism from neutral SP molecules initially nonphotochromic in the crystalline state. The photoresponses of the SP-POT systems in the solid state strongly depend on the nature and the number of grafted SP groups.

Polarized infrared reflectance spectra of brushite (CaHPO4·2H2O) crystal investigation of the phosphate stretching modes

Polarized infrared (IR) reflectance measurements at near-normal incidence were recorded from the ac-plane of a monoclinic brushite (CaHPO4·2H2O) crystal in the 800-1200 cm(-1) spectral range (P-O stretching modes). The adjustment of these data, on the basis of a dispersion analysis (DA) model for monoclinic case, allowed the determination of oscillators parameters for the four P-O stretching observed modes of the phosphate group.

Structural and spectroscopic characterization of a series of potassium- and/or sodium-substituted β-tricalcium phosphate

In this paper, we report X-ray diffraction investigations as well as Raman and solid-state (31)P and (23)Na magic angle spinning nuclear magnetic resonance (NMR) characterization of three series of calcium orthophosphates. The general formulae of the studied compounds are Ca(10.5-x/2)M(x)(PO(4))(7), where M=K or Na and x=0, 0.25, 0.50, 0.75, 1.0; and Ca(10)K(x)Na(1-x)(PO(4))(7), where x=0, 0.25, 0.5, 0.75, 1.0. These calcium orthophosphates are found to be isostructural with β-tricalcium phosphate (β-TCP, Ca(3)(PO(4))(2)) with the substitution of some calcium sites by potassium and/or sodium cations. The unit cell parameters vary continuously with the level of substitution, a characteristic of these solid solutions. The Raman spectra show the different vibrational bands of the phosphate groups PO(4), while the NMR chemical shifts are sensitive to the non-equivalent phosphorus and sodium ions present in these substituted samples. As both Raman and NMR spectroscopies are local probes, they offer tools to distinguish between these different phosphorus and phosphate groups, according to their structural site and local environment, especially the type of cation substituent. A convenient decomposition of the Raman and NMR spectra into Gaussian-Lorentzian components leads us to propose an assignment of the main observed bands of these substituted β-TCPs.

Minimization of absorption contrast for accurate amorphous phase quantification: application to ZrO2nanoparticles

Monoclinic and tetragonal zirconia samples were characterized by X-ray diffraction, pycnometry, thermogravimetric analysis (TGA), Fourier transform (FT) IR and mass (MS) spectroscopies, and scanning and transmission electron (TEM) microscopies. The results show, for the particular case of a tetragonal zirconia sample, an X-ray-undetected subproduct identified as an amorphous organic phase by FTIR–ATR (attenuated total reflection) and TGA–MS. The observations by TEM allowed this amorphous phase to be localized on the surface as a shell coating the nanoparticles. Moreover, this amorphous phase was quantified by Rietveld refinementviathe addition of an internal silicon standard. Because zirconia and silicon have different linear absorption coefficients, the microabsorption effect was minimized by using small particle sizes. The amorphous phase was calculated to constitute 11.4 (30)% of the initial mass before Brindley correction and 10.6 (30)% of the initial mass after Brindley correction. The closeness of these values shows that the contribution of the Brindley correction can be neglected if precautions are taken on the microabsorption effect. This work has also highlighted the importance of thoroughly characterizing commercial products, which are not necessarily pure. Indeed, the presence of impurities could become a non-negligible parameter for physical and chemical properties studies related to commercial materials.

Structural investigation of coprecipitation of technetium-99 with iron phases

Technetium is a long-lived product of nuclear fission which commonly exhibits two oxidation states (IV and VII). Siderite (FeCO3), suspected to be formed as a container corrosion product in geological radioactive waste repositories, may concentrate by coprecipitation more than 90% of technetium-99, present as Tc(IV) in surrounding aqueous fluids. X-ray diffraction and transmission electron microscopy measurements indicate that technetium can be incorporated within the siderite structure, even if we note that technetium-bearing green rust phase may also be observed. These results suggest that siderite might play a beneficial role in limiting Tc diffusion to the next environment of nuclear waste repositories.

Resolving the Aluminum Ordering in Aluminosilicates by a Combined Experimental/Theoretical Study of 27Al Electric Field Gradients

The discrimination between atomic species in light-element materials is a challenging question. An archetypal example is the resolution of the Al/Si ordering in aluminosilicates. Only an average long-range order can be deduced from powder X-ray or neutron diffraction, while magic-angle-spinning NMR provides an accurate picture of the short-range order. The long- and short-range orders thus obtained usually differ, hence raising the question of whether the difference between local and extended orders is intrinsic or caused by the difficulty of obtaining an accurate picture of the long-range order from diffraction techniques. In this communication we resolve this question for the monoclinic phases of BaAl2Si2O8 and SrAl2Si2O8 on the basis of 27Al NMR measurements and ab initio simulation of electric field gradient. Although the long- and short-range orders deduced from our XRD and NMR experiments differ, they become similar when the XRD atomic positions are optimized by ab initio electronic structure calculations.

Solving modulated structures by X-ray and electron crystallography

X-ray diffraction can be used for accurately determining not only classical, ordinary structures, but also modulated ones. For structures with weak modulations, the modulation induced satellite reflections are often hard to be observed by X-ray diffraction, but they appear clearly in electron diffraction. In these cases, X-ray diffraction will give only average structures whereas electron diffraction will yield information about the modulations. Sr(1.4)Ta(0.6)O(2.9) is a complex modulated compound with weak modulation and small modulated domains. Here we demonstrate the power of combining X-ray and electron crystallography for studying modulated structures on powders. The modulations of Sr(1.4)Ta(0.6)O(2.9) were determined from electron diffraction (SAED) and high resolution electron microscopy (HREM) images. With specially developed image processing techniques, the weak modulations were enhanced, facilitating the interpretation of HREM images in terms of atomic structure.

Determination of the crystallized fractions of a largely amorphous multiphase material by the Rietveld method

The Rietveld method has proved to be a very effective means to characterize and quantify the crystalline phases and the amorphous phase in glass ceramic materials using X-ray powder diffraction data. The technique was applied to a borosilicate glass of the type used for high-level nuclear-waste containment, in order to measure the proportions of the crystallized phases after heat treatment and, thus, to qualify the thermal stability of the glass. Six crystalline phases were analysed in this way in an almost entirely (>95 wt%) amorphous material after adding a known proportion of an internal standard (TiO2). The quantitative analyses were corrected to allow for microabsorption effects resulting from grain-size and absorption-contrast effects. In addition to the quantitative data, unit-cell parameters and site-occupancy refinements revealed solid-solution and substitution phenomena in the crystal.

A second polymorphic form of N,N'-diphenyl-1,4-phenylenediamine

A new triclinic polymorphic form of N,N'-diphenyl-1,4-phenylenediamine (C(6)H(5)NHC(6)H(4)NHC(6)H(5)) has been obtained through appropriate recrystallization of the orthorhombic form. It crystallized in the centrosymmetric space group P$\overline 1$, with two half molecules as the asymmetric unit.