Manganese-doped ZnAl2O4 phosphors were prepared by the Pechini synthesis route and treated at various temperatures from 600 to 1350 °C. The samples were characterized by TEM-EDX, XRD, EPR, and their ...diffuse reflectance and luminescence properties were investigated. The structural analysis showed the high solubility limit of manganese in this spinel matrix and allowed the determination of the global inversion rate, which characterizes the cation distribution in the A and B sites of the spinel structure. As the annealing temperature increased, this factor decreased leading to a more direct matrix. EPR analysis showed that, besides Mn3+ to Mn2+ reduction, the local environment of Mn2+ cations changed with the annealing temperature, which was also reflected in the evolution of the optical properties. As the annealing temperature increased, the red luminescence related to the presence of divalent manganese in octahedral sites faded and was replaced by a new green emission due to Mn(II) ions located in tetrahedral sites within the spinel structure. For 0.5% Mn-doped ZnAl2O4, this red to green luminescence switch occurred for samples treated between 1200 and 1350 °C. Moreover, the Al-overstoichiometric samples (Mn:ZnAl2.2O4+δ) showed that it is possible to modify the temperature range and the kinetics of this variation in emission wavelength. These tuneable properties suggest that Mn-doped spinels are potential candidates for developing stable and highly sensitive thermal sensors.
In this fundamental solid-state chemistry study, two sample series were investigated in depth: iron(III)-doped hydroxyapatite (HA) compounds obtained from a co-sintering process of hematite and pure ...HA under air and iron(III)-doped HA compounds obtained from a co-sintering process from iron(II) acetate and pure HA under an argon atmosphere. X-ray diffraction, UV–visible, Fourier transform infrared, 1H and 31P NMR, electron paramagnetic resonance (EPR,) and Mössbauer spectroscopy methods were coupled to unravel the Fe valence states, the interactions with other anionic species (OH– and PO4 3–), and finally the complex local environments in hexagonal channels in both the series. In particular, we highlighted the associated mechanism to ensure electroneutrality with a focus on deprotonation versus calcium substitution. By diverging mechanisms, Fe3+ and Fe2+ ions were found to be located in different coordinated sites: 4(+1) coordinated site for Fe3+ and 2(+3) coordinated site for Fe2+ and clearly associated with very different Mössbauer and EPR signatures as various absorption bands (leading to different sample colors).
In this work, 0%, 0.5%, 1%, and 2% Cu-doped WO
3
nanoparticles were synthesized via a polyol method. The as-synthesized materials were characterized by x-ray diffraction, scanning electron microscopy ...(SEM), electron paramagnetic resonance (EPR), x-ray photoelectron spectroscopy (XPS), and UV-Vis photochromic activity. A bond valence model was adopted to explain the relationship between the lattice parameter and Cu percentage. Additionally, two films (1% Cu-doped and un-doped samples) obtained by dip-coating from a well-dispersed suspension were optically investigated using ex situ and in situ UV-Vis spectrometry; coloring/bleaching kinetics were thoroughly studied. The Cu-doped WO
3
film was found to have a high-quality bleaching mechanism (five times as fas as the un-doped WO
3
film). Finally, the high bleaching performance of the doped films was confirmed by successive cycling, showing that the as-prepared compounds are of great interest for smart window applications, for example.
The structural and optical properties of the refractory oxides Sr2CoxGa1–xNbO6 (x = 0 and 0.02) were investigated using powder X-ray diffraction; 93Nb, 71Ga, and 59Co magic-angle spinning nuclear ...magnetic resonance; electron spin resonance; and UV–vis–NIR absorption. A flux using SrCl2 versus a ball milling strategy to stabilize rock salt-ordered Sr2CoxGa1–xNbO6 (x = 0 and 0.02) at low temperature was underlined. Sr2Co0.02Ga0.98NbO6 showed a tetragonal I4/m crystalline structure with a slight lattice distortion index in the range of 0.04. Combined spectroscopic studies, further supported by UV–vis–NIR absorption analysis, definitively indicate an unusual mixed spin state of Co3+ at room temperature in the Sr2Co0.02Ga0.98NbO6 double perovskite that might be of interest for pale-brown inorganic pigments.
Applications of vanadium dioxide, VO2, able to modulate near-infrared radiation by changing from a transparent to a reflective/absorptive state, remain limited by its shaping as transparent films. In ...this work, a V2O5@PVP core-shell structure is designed prior to the formation of island-structured VO2 films. Poly(N-vinyl-2-pyrrolidone) (PVP) is used as a surface stabilizer for the preparation of V2O5 NP suspensions; additionally, PVP acts as a reducing agent during the film post-annealing process, helping the V2O5→VO2 transformation under dynamic vacuum. Using in situ TG-FTIR characterization, the reducing mechanism is carefully discussed and analyzed. Finally, a uniform VO2 film is fabricated with an attractive gray color and excellent thermal stability. Three strategies with various sintering parameters are used to optimize the film's morphology, i.e., surface coverage. In the last strategy, the thermochromic behavior of the designed island-structure VO2 film is simulated based on Mie scattering theory in consideration of surface coverage and “island” particle size fitting with our experiments. This pioneering work provides guidance for future studies on discontinuous VO2 films.
•Successful fabrication of uniform and discontinuous VO2 film with consumer-acceptable grey color 2.•Formation of island-structured VO2 films from reduction of the V2O5@PVP core-shell structure.•Investigation of the reducing mechanism by In situ TG-FTIR characterization.•Simulation based on the Mie scattering theory of the thermochromic behavior of the island-structure VO2 film.
ZnAl2O4 spinel powders were prepared using the Pechini or co-precipitation synthetic route and were then treated at different temperatures (600-1350 °C). These powders were characterised by X-ray ...diffraction, scanning electron microscopy (SEM), diffuse reflectance and luminescence measurements. SEM investigations and the X-ray patterns showed that the spinel crystallite size was dependent on the synthetic route and the treatment temperature. In addition, the structural evolution was investigated by Rietveld refinements. The inversion rate decrease was correlated with the temperature, leading to a direct spinel phase for the sample treated at high temperature. Furthermore, luminescence measurements showed various emissions linked to the presence of defects in the matrix structure. The two main emissions observed were attributed to oxygen vacancy and Zn in the interstitial positions (as revealed by differential Fourier maps). The luminescence spectra exhibited strong differences between 1200 °C and 1350 °C. At the higher temperature, the characteristic emission spectra can be attributed to the direct spinel phase. The indirect-direct spinel transformation can be monitored through the change in the optical properties and correlated to the thermal history of the sample.
The structural and optical properties of the refractory oxides Sr2Co x Ga1–x NbO6 (x = 0 and 0.02) were investigated using powder X-ray diffraction; 93Nb, 71Ga, and 59Co magic-angle spinning nuclear ...magnetic resonance; electron spin resonance; and UV–vis–NIR absorption. A flux using SrCl2 versus a ball milling strategy to stabilize rock salt-ordered Sr2Co x Ga1–x NbO6 (x = 0 and 0.02) at low temperature was underlined. Sr2Co0.02Ga0.98NbO6 showed a tetragonal I4/m crystalline structure with a slight lattice distortion index in the range of 0.04. Combined spectroscopic studies, further supported by UV–vis–NIR absorption analysis, definitively indicate an unusual mixed spin state of Co3+ at room temperature in the Sr2Co0.02Ga0.98NbO6 double perovskite that might be of interest for pale-brown inorganic pigments.
Because of a very low thermodynamic stability, obtaining a pure monophasic compound of ferric pseudobrookite is quite difficult to achieve. Indeed, the low reticular energy of this phase leads easily ...to its decomposition and the occurrence of the secondary phases: hematite (Fe2O3) and/or rutile (TiO2). Samples with global composition Fe2–x Ti1+x O5 (x = 0, 0.05, and 0.10) have been synthesized by the Pechini route and, thereafter, thermally treated at different temperatures. The concentrations of Fe2O3 and TiO2 secondary phases were accurately determined and correlated with the target compositions and the synthesis parameters, especially the thermal treatment temperature. As revealed by Mössbauer spectroscopy, all iron ions are at the III+ oxidation state. Thus, the formation of hematite or rutile as a secondary phase may be related to the occurrence of cationic vacancies within the pseudobrookite structure, with the amount of vacancies depending on the annealing temperature. In light of the presented results, it appears unreasonable to propose a “fixed” binary phase diagram for such a complex system. Furthermore, the occurrence of cationic vacancies induces a coloration change (darkening), preventing any industrial use of this reddish-brown pseudobrookite as a ceramic pigment.
In this study, vanadium sesquioxide (V
O
), dioxide (VO
), and pentoxide (V
O
) were all synthesized from a single polyol route through the precipitation of an intermediate precursor: vanadium ...ethylene glycolate (VEG). Various annealing treatments of the VEG precursor, under controlled atmosphere and temperature, led to the successful synthesis of the three pure oxides, with sub-micrometer crystallite size. To the best of our knowledge, the synthesis of the three oxides V
O
, VO
, and V
O
from a single polyol batch has never been reported in the literature. In a second part of the study, the potentialities brought about by the successful preparation of sub-micrometer V
O
, VO
, and V
O
are illustrated by the characterization of the electrochromic properties of V
O
films, a discussion about the metal to insulator transition of VO
on the basis of in situ measurements versus temperature of its electrical and optical properties, and the characterization of the magnetic transition of V
O
powder from SQUID measurements. For the latter compound, the influence of the crystallite size on the magnetic properties is discussed.
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