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ZnO doped with [Co.sup.2} is prepared by a Pechini process and is examined in terms of ultraviolet visible properties and crystallographic structures. The anticipative method of the structure relaxations is discussed from exchanged effective charge per bond calculated with the purely ionic Brown and Altermatt model.

The structural investigation of [Zn.sub.1-x][Cu.sub.x][Al.sub.2][O.sub.4] spinels was carried out and effects relating to the [Cu.sup.2+] Jahn-Teller distortion were studied. It was seen that the position and the intensity of the various absorption bands were influenced by the distribution of [Cu.sup.2+] in tetrahedral and octahedral sites and the increasing of Cu-O bond covalency.

The influence of the tungsten rate and the thermal/pressure history of the samples of Cu[Mo.sub.1-x][W.sub.x][O.sub.4] series (x

Calorimetry, reflectivity and magnetism techniques were used for analyzing the transition temperature modifications with tungsten substitution. Findings reveal a significant increase in the temperature transition in a parallel way for the heating and cooling mode on substitution of molybdenum for tungsten.

A series of compounds of Co1-xMgxMoO4 compositions has been prepared by a conventional ceramic route. The members of the whole solid solution exhibit a reversible first-order phase transition which was probed by using thermal expansion and low-temperature reflectivity techniques. Whereas the α → β transition temperature evolves linearly on warming from 435 to 200 °C with x going from 0 to 0.9, the β → α transition temperature variation falls down on cooling from -40 °C to -140 °C going from CoMoO4 to Co0.1Mg0.9MoO4 with an asymptotic evolution. The phase transition temperatures have been explained on the basis of a crystal polarization effect under substitution of Mg for Co. Thus, from an applicative point of view, new thermochromic pigments with tunable transition temperatures are here proposed. [ABSTRACT FROM AUTHOR]

CuMo0.9W0.1O4oxide is known to exhibit two allotropic forms (α and γ forms) linked by a first-order phase transition that can be induced by pressure and/or temperature. In this work, we show that this α → γ phase transition (with a chromic effect from green to brown color) can also take place at 25 °C by a simple immersion of the powder in an aqueous solution with acidic pH. This chromic phase transition from the high-temperature/low pressure form into the low-temperature/high-pressure form is shown to be induced by surface protonation, whereas the copper-molybdate powder is dispersed in acidic solutions. Wettability measurements on both phases were performed in order to confirm the affinity of the α-form for protons in comparison with the γ-form. Then pH metric titration (addition step by step of an acid solution on the powder, controlling at each step the pH evolution) has allowed describing the kinetic aspects of the surface protonation (kinetic laws were approached and discussed). Finally, an interpretative model of the “halochromic” phenomenon based on the modification of the cations environment at the liquid−solid interface was proposed. For the first time, a superficial interaction between a solid oxide and a liquid medium, for which no partial dissolution−recrystallization sequence is occurring, is shown to be sufficient for producing a phase transition of the whole oxide bulk. [ABSTRACT FROM AUTHOR]

Zinc/cobalt aluminates with spinel-type structure were prepared by a polymeric route, leading to a pure phase with controlled grain size. The prepared pigments were characterized by powder X-ray diffraction Rietveld analyses in order to determine structural features, scanning electron microscopy for morphological investigation, helium pycnometiy and 27Al MAS NMR in order to highlight the occurrence of defects inside the structure, and UV-visible-near-IR spectroscopy to identify electronic transitions responsible for the compounds' color. The green-blue coloration of these pigments is known to be dependent on the sample thermal history. Here, for the first lime, the Zn1-x,CoxAI2O4 color is newly interpreted. The pigment is green once synthesized at low temperature (i.e., with diminution of the pigment grain size);.this variation was attributed to the appearance of a new absorption band located at about 500 nm, linked to a complex network feature involving Co ions in octahedral sites as well as oxygen and cationic vacancies. Hence, this work shows the possibility of easily getting a nonstoichiometric network with an abnormal cationic distribution from "chimie douce" processes with moderate synthesis temperature, and so various colorations for the same composition. [ABSTRACT FROM AUTHOR]

The thermochromic phase transition of CuMo[sub1-x]W[subx]O[sub4] oxide was delayed by the deposit of SiO[sub2] shells. The phase transition temperature was investigated by optical reflectivity versus temperature. The effect of the shell thickness on the transition temperature is established. The cyclability of the phenomenon is also discussed. [ABSTRACT FROM AUTHOR]

The members of the CuMo1-~W~O4 series (0 ~ x < 0.1) undergo a first-order phase transition that can be induced by pressure application; the thermochromic properties of such a series have already been reported. The two polymorphic forms exhibit two distinguishable colors: green for the low pressure form (a) and brownish-red for the high pressure one (y). These oxides can open up a new market for friendly pressure indicators, particularly for the compositions (0.07 ~ x ~ 0.1) for which the two polymorphs are stable at room temperature, that is, for which the color transition via pressure application is nonreversible. Within the CuMo1-~W~O4 solid solution domain, the dependence of the transition pressure versus tungsten content, temperature of measurement, and sample thermal- pressure history was studied. A large control of the transition pressure (from 5 to several 100 MPa) was brought to the fore. The transition was then studied using X-ray diffraction and transmission electron microscopy-energy dispersive X-ray analyses. This first-order transition, occurring by atomic migration inside the cell, seems to be preceded by an atomic disordering; moreover, transition temperatures may be modified by W segregation at the surface of the grains. [ABSTRACT FROM AUTHOR]

The members of the CuMo1 - xWxO4 series (0 ≤ x < 0.1) undergo a first-order phase transition at normal pressure, which can be induced by temperature. The two allotropic forms exhibit two distinguishable colors, green for the high-temperature form (a) and brownish-red for the low temperature one (γ), which opens up a new market for user-friendly temperature indicators. From X-ray diffraction and microprobe analyses as from optical properties, the tungsten substitution rate for molybdenum is limited to 12%. Beyond, a third, parasitic wolframite-type phase, CuMo0.6W0.4O6, systematically crystallizes besides the α/γCuMo0.9W0.104 compounds. Within the CuMo1 - xWxO4 solid-solution domain, the dependence of the transition temperatures was followed by calorimetry, optical reflectivity, and magnetism. On the basis of these measurements, the transition is characterized for all of the chemical compositions by a hysteresis loop of about 90 K in width with a temperature transition strongly dependent on the tungsten content. Namely, the γ → α transition can occur between 260 and 360 K, and the α → γ transition between 175 and 275 K as a function of x. The control of the α/γ transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment. [ABSTRACT FROM AUTHOR]

Zn1-xCuxAl2O4 (0 ≤ x ≤ 1) compositions have been synthesized by solid-state route, and a color scale going from white (x = 0) to brownish-red (x = 1) with intermediate colors as pale green for x = 0.10 and pale brown for x = 0.30 can be observed. XRD-data refinements on the whole solid solution have allowed defining two critical areas where structural features such as cell parameter and inversion rate as well as cation-oxygen bond distances in 8a and 16d sites of the spinel network exhibit clear unique variations. By observing the direct environment of tetrahedral and octahedral sites potentially occupied by Cu2+ Jahn-Teller cations, the two critical compositions have been estimated to x1 =1/6 and x2 = 4/7. Actually from the x1 and x2 copper contents, the probability of getting Cu2+-Cu2+ pairs involving tetrahedral-octahedral or two octahedral sites respectively is high. Then, the stabilization of Cu2+ JT ions in distorted octahedral site, identified by ESR, is in competition with the occurrence of Cu2+ in tetrahedral sites, and both electronic and steric effects of Cu2+ JT cations lead to the explanation of the evolution of the inversion rate in this series. The study of the optical absorption properties clearly shows that the position and the intensity of the various absorption bands are influenced by the distribution of Cu2+ ions in tetrahedral and octahedral sites, the creation of Cu2+-Cu2+ pairs around the x1 and x2 critical compositions, and the increasing of Cu-O bond covalency. Such an evolution of structural features correlated with electronic properties where various critical compositions have been identified can be generalized to other spinel oxides, considering local distortion around Jahn-Teller ions. [ABSTRACT FROM AUTHOR]

We report the synthesis of WO 3 , TiO 2 , and TiO 2 -WO 3 nanoparticles by a polyol route, with the objective of studying the influence of the preparation method on their photochromic properties. By combining transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and diffuse reflectance experiments, we show that low W 6+ concentrations and high ripening temperatures allow the preparation of WO 3 nanoparticles with high photochromic efficiency. WO 3 -TiO 2 nanocomposites (NCs) prepared by the introduction of a TiO 2+ solution in a WO 3 nanoparticle suspension exhibit a strong coloring photochromism, which is attributed to the TiO 2 coating of the WO 3 nanoparticles as it involves the formation of W-O-Ti oxo-bonds in place of W 5+ -ν O defects. Especially, after an oxidative treatment in order to obtain an initial pale-yellow material, such WO 3 -TiO 2 NCs exhibit a fully reversible photochromism with a large contrast between the colored and bleached state. They could therefore be incorporated in hybrid smart films for solar control on building window glasses. On the other hand, while the WO 3 -TiO 2 NCs are functionalized with DPA (n-dodecyl phosphonic acid), the as-prepared nanocomposites exhibit exacerbated coloring contrast but with a nearly nonreversible photochromism (very limited bleaching), which makes them good candidates for the fabrication of smart UV-sensor devices that can indicate the cumulative UV dose which is received.

A thermal sensor was used to better understand parameters which influenced the interaction between a laser beam and a 0.5% Mn-doped ZnAl 2.2 O 4 material, especially the laser defocusing parameter. The optical properties of the material depend on whether the Mn 2+ ions occupy octahedral and/or tetrahedral sites depending on thermal history. A screen printing process is performed to obtain material thin films. Laser irradiation of the films was carried out (patterning 1 cm length single laser track) with different z defocusing heights. Luminescence properties around laser tracks led to the thermal history determination at the micrometer scale. It was shown that spatial thermal gradients defined at the micrometer scale perpendicularly to the borders of the laser tracks could be semiquantified for different z conditions. Laser defocusing leads to decrease thermal gradients as confirmed by thermal modeling studies., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

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, 1 H and 31 P 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 PO 4 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, Fe 3+ and Fe 2+ ions were found to be located in different coordinated sites: 4(+1) coordinated site for Fe 3+ and 2(+3) coordinated site for Fe 2+ and clearly associated with very different Mössbauer and EPR signatures as various absorption bands (leading to different sample colors).

Thanks to a homemade dynamic vacuum system, fully crystallized VO 2 (M) is successfully synthesized in a merged step of vanadyl ethylene glycolate (VEG) decomposition and crystallization of VO 2 at high temperatures (>500 °C). During the whole process, vanadium valence (+4) is well maintained, and VEG microstructure plays an important role in the end-product size and shape. Finally, the suggested route appears well suitable for the mass production of VO 2 nanoparticles.

Vanadium dioxide (VO 2 ) undergoes a fully reversible first-order metal-insulator transition from the M1 monoclinic phase ( P 2 1 / c ) to a high-temperature tetragonal phase ( P 4 2 / mnm ) at around 68 °C. Modulation of the phase transition of VO 2 by chemical doping is of fundamental and technological interest. Here, we report the synthesis of highly crystallized Fe-doped VO 2 powders by a carbo-thermal reduction process. The impact of Fe doping on the structural and phase transition of VO 2 is studied. The as-prepared Fe-doped VO 2 samples crystallize in the M2 monoclinic form ( C 2/ m ), which is linked to segregation of the doping ions in the V2 zigzag chains. A large increase in the transition temperature to 134 °C is observed, which does correspond to a breakthrough in VO 2 -type thermochromic materials.

ZnO/MoO 3 powder mixture exhibits a huge photochromic effect in comparison with the corresponding single oxides. The coloring efficiency of such combined material after UV-light irradiation was studied in terms of intensity, kinetics, and ZnO/MoO 3 powder ratio. Additionally, the incidence of the pretreatment step of the ZnO and MoO 3 powders under different atmospheres (air, Ar or Ar/H 2 flow) was analyzed. The huge photochromic effect discovered herein was interpreted as the creation of "self-closed Schottky barrier" at the solid/solid interfaces between the two oxides, associated with the full redox reaction which can be pictured by the equation ZnO 1-ε + MoO 3 → ZnO + MoO 3-ε . Remarkable optical contrast between virgin and color states as well as self-bleaching in dark allowing the reversibility of the photochromism is emphasized. From this first discovery, deeper characterization of the self-bleaching process shows that the photochromic mechanism is complex with a bleaching efficiency (possibility to come back to the virgin material optical properties without any deterioration) and a bleaching kinetics, which are both dependent on the coloring irradiation time. This demonstrates that the oxygen exchange through the Schottky interface proceeds in at least two convoluted steps: an anionic surface exchange allowing a reversibility of the redox reaction followed by bulk diffusion of the exchanged anions which are then definitively trapped. An emergent "negative photochromism effect" (i.e., photochromism associated with a self-bleaching instead of a darkening under irradiation) is observed after a long irradiation time.

Cyan Ni 1- x Al 2+2 x /3 O 4 single-phase pigments with various Ni/Al atomic ratios (from 1:2 down to 1:4) have been prepared by a sol-gel route (Pechini) followed by postannealing treatments. Nickel aluminates crystallize in the well-known spinel structure ( Fd 3 m space group), where metals are located at two different Wyckoff positions: 16d (octahedron) and 8a (tetrahedron). Based on X-ray diffraction (XRD) Rietveld refinements, Ni 2+ cations are shown to be partially located in both tetrahedral and octahedral sites and, in addition, cationic vacancies occupy the Oh environment. In the pure-phase series, Ni/Al = 0.35, 0.40, 0.45, as the Al content increases, the Ni 2+ rate in the Td site decreases for Ni/Al = 0.45, thus altering the cyan color; within this series, the most saturated cyan coloration is reached for the highest Al concentration. Inorganic pigment drawbacks are their high density and hydrophilic surface, which induce sedimentation and aggregation in nonpolar media used in electrophoretic inks. Hybrid core-shell particle pigments have been synthesized from cyan pigments using nitroxide-mediated radical polymerization (NMRP) with methyl methacrylate monomer in Isopar G, leading to a dispersion of electrically charged hybrids in apolar media. Surface functionalization of the pigments by n -octyltrimethoxysilane (OTS) and n -dodecyltrimethoxysilane (DTS) modifiers has been compared. The inorganic pigments are successfully encapsulated by organic shells to allow a strong decrease in their density. Cyan inks, adequate for their use in e-book readers or other electrophoretic displays, taking further advantage of the high contrast ratio and reflectivity of inorganic pigments in regard to organic dyes, have been stabilized., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

A low content of chromium (≤5 mol %) has been incorporated into a SnO 2 cassiterite by a coprecipitation route in a basic medium, followed by an annealing step under an O 2 flow at T = 800 °C and T = 1000 °C. Accurate UV-vis and EPR spectroscopy investigations show the coexistence of isolated Cr 4+ and Cr 3+ ions as well as ferromagnetic Cr 4+ -Cr 3+ and antiferromagnetic Cr 3+ -Cr 3+ interactions. The strong purple hue is related to the isolated Cr 4+ ions stabilized in a distorted octahedral site. This is thanks to the second-order Jahn-Teller (SOJT) effect with a crystal field splitting 10 Dq value around 2.4 eV, whereas the 10 Dq value is around 2 eV for isotropic Cr 3+ ions, partially substituted for Sn 4+ ions in cassiterite. Just after the coprecipitation process, only Cr 3+ species are stabilized in this rutile network with a poor crystallinity. The isolated Cr 4+ content remains high after annealing at 800 °C for 2 days especially for the highest Cr rate (2 and 5 mol %), leading to a darker purple color, but unfortunately the Cr 3+ content also increases for a higher Cr concentration. A lighter purple hue can be reached after calcination at a higher temperature ( T = 1000 °C) for a shorter time (4 h) but with a lower Cr content to avoid Cr clusters. This is due to stabilizing a high content of isolated Cr 4+ species and limiting the Cr 4+ -Cr 3+ ferromagnetic interactions, which are optimal for a 2% Cr content and also cause the color to darken. The key roles of the Cr 4+ rate and the Cr 4+ -Cr 3+ clusters create local defects whose concentration strongly varies with a total Cr content, which have then been demonstrated to strongly influence the optical and magnetic properties.

New inorganic pigments with intense and saturated coloration have been prepared by a solid-state route and exhibit a large color scale from magenta to yellow. Indeed, yellow and magenta are two of the three subtractive model's colors with wide application in printing or displays as e-book readers. To develop yellow and magenta hue, we focused on cobalt- and nickel-based orthophosphates thanks to the chemical stability, low density, low price, and easy preparation of such a pigment class. All of these orthophosphates crystallize with the well-known olivine-type structure (orthorhombic Pnma space group) where transition metals are stabilized in a distorted octahedral site. This paper deals with the optical absorption properties of various orthophosphates, the correlations with structural features, and their colorimetric parameters (in L*a*b* color space). The LiCo 1- x Mg x PO 4 series show near-magenta color with tunable luminosity, while the LiNiPO 4 compound exhibits a frank yellow coloration. Co 2+ ( 4 T 1 ) and Ni 2+ ( 4 A 2 ) chromophore ions occupy a more or less distorted octahedral site, leading to tuning of the intensity of the d-d electronic transitions in the visible and near-IR ranges and providing a subtractive color scale; i.e., a LiCo 1- x Ni x PO 4 solid solution possesses a very rich panel of colors between the two yellow and magenta extremes. It is worth noting that the crystal-field splitting and B Racah parameter have been estimated in a first approximation on the basis of the Tanabe-Sugano diagram and lead to the conclusion of a slightly higher crystal-field splitting of around 0.9 eV for Ni 2+ ions and similar β covalent parameters, despite the same crystallographic sites of both of these transition metals.

Highly crystallized monoclinic vanadium dioxide, VO 2 (M), is successfully synthesized by a two-step thermal treatment: thermolysis of vanadyl ethylene glycolate (VEG) and postannealing of the poorly crystallized VO 2 powder. In the first thermolysis step, the decomposition of VEG at 300 °C is investigated by X-ray diffraction and scanning electron microscopy (SEM). A poorly crystallized VO 2 powder is obtained at a strict time of 3 min, and it is found that the residual carbon content in the powder played a critical role in the post crystallization of VO 2 (M). After postannealing at 500 and 700 °C in an oxygen-free atmosphere, VO 2 particles of various morphologies, of which the crystallite size increases with increasing temperature, are observed by SEM and transmission electron microscopy. The weight percent of crystalline VO 2 , calculated using the Fullprof program, increases from 44% to 79% and 100% after postannealing. The improved crystallinity leads to an improvement in metal-insulator transition behaviors demonstrated by sharper and more intense differential scanning calorimetry peaks. Moreover, V 2 O 3 and V 2 O 5 with novel and particular microstructures are also successfully prepared with a similar two-step method using postannealing treatment under reductive or oxidizing atmospheres, respectively.

In this study, vanadium sesquioxide (V 2 O 3 ), dioxide (VO 2 ), and pentoxide (V 2 O 5 ) 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 2 O 5 , VO 2 , and V 2 O 3 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 2 O 5 , VO 2 , and V 2 O 3 are illustrated by the characterization of the electrochromic properties of V 2 O 5 films, a discussion about the metal to insulator transition of VO 2 on the basis of in situ measurements versus temperature of its electrical and optical properties, and the characterization of the magnetic transition of V 2 O 3 powder from SQUID measurements. For the latter compound, the influence of the crystallite size on the magnetic properties is discussed.

Herein, the successful synthesis of Ti-doped vanadium pentoxide from a polyol process is reported. A high Ti concentration (up to 8.5 mol % of the total metallic content) can be inserted in vanadium oxide thanks to the synthesis route leading to nanometric crystallites. X-ray diffraction patterns were refined showing the insertion of the titanium ions inside the free pentacoordinated sites in opposition to the vanadium square pyramidal sites. This crystal organization was shown in good agreement with the ab initio positioning performed from valence calculation. The nanoparticles, NPs, of Ti-doped V 2 O 5 compounds were characterized as electrochromic materials. Films elaborated from a dip-coating process from oxide particle suspensions exhibited three distinct colorations during the redox cycling in lithium-based electrolyte. These colors were associated with three distinct oxidation states for the vanadium ions: +III (blue), +IV (green), and +V (orange). The morphology of the films was shown to drastically impact the electrochromic performances in terms of electrochemical capacity and stability.

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-xTi1+xO5 (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.

A mixture of two piezochromic compounds can be used as a universal shock detector, i.e., to determine the shock pressure without knowing a priori the temperature at which the shock occurred. For this purpose, both piezochromic compounds must exhibit different temperature influences in their transition-pressure values. This demonstration uses two piezochromic compounds (CoMoO4 and CuMoO4-type oxides) that exhibit a first-order phase transition between their two allotropic forms associated with a drastic color change. The colorimetric coordinates of the mixture indicate the pressure and temperature of a shock.

Fe1-xMgxMoO4 compounds with x = 0, 0.25, 0.5, 0.75, and 1.0 were obtained after annealing under inert gas at T = 700 °C. All of the compounds exhibit a pressure-induced and/or temperature-induced phase transition between the two polymorphs adopted by AMoO4 compounds (A = Mn, Fe, Co, and Ni). For the FeMoO4 compound, for both the α and the β allotropic forms, the structural features have been correlated to the magnetic properties, the Mössbauer signals, and the optical absorption properties to gain a better understanding of the phenomena at the origin of the piezo(thermo)chromic behavior. The different contributions of the Mössbauer signals were attributed to the different Fe(2+) ions or Fe(3+) ions from the structural data (Wyckoff positions, bond distances and angles) and were quantified. Furthermore, the low Fe(3+) concentration (9 and 4 mol %, respectively, in the α and the β allotropic forms) was also quantified based on the magnetic susceptibility measurements. The net increase in the Fe(3+) quantity in the α-form in comparison to the β-form, which is associated with the occurrence of Fe-Mo charge transfer, is at the origin of the important divergence of coloration of the two forms. To design new piezo(thermo)chromic oxides and to control the pressure (temperature) of this first-order phase transition, FeMoO4-MgMoO4 solid solutions were synthesized. The optical contrast between the two allotropic forms was increased due to magnesium incorporation, and the phase transition (β → α) pressure increased steadily with the Mg content. A new generation of nontoxic and chemically stable piezochromic compounds that are sensible to various pressures was proposed.

ZnO nanocrystals were prepared from polyol-mediated synthesis. Two key parameters, that is, the zinc precursor concentration and the alcohol mixture chosen as synthesis medium, were varied. The increase of the precursor concentration and the decrease of the permittivity of the alcohol mixture were shown to favor the crystallite growth, leading to crystallite sizes ranging from 5 to 35 nm. The aggregation behavior of the nanocrystal units to form or not polycrystalline spheres, depending on the van der Walls interaction density, was shown and explained. Every sample was accurately characterized by X-ray diffraction; cell parameters were extracted from full pattern matching refinements. A clear correlation between crystallite size and cell parameters, that is, an asymptotic decrease of a and c cell parameters versus the crystallite size, was established. A simple model was also successfully developed to interpret the as-established correlation.

A series of compounds of Co(1-x)Mg(x)MoO(4) compositions has been prepared by a conventional ceramic route. The members of the whole solid solution exhibit a reversible first-order phase transition which was probed by using thermal expansion and low-temperature reflectivity techniques. Whereas the α → β transition temperature evolves linearly on warming from 435 to 200 °C with x going from 0 to 0.9, the β → α transition temperature variation falls down on cooling from -40 °C to -140 °C going from CoMoO(4) to Co(0.1)Mg(0.9)MoO(4) with an asymptotic evolution. The phase transition temperatures have been explained on the basis of a crystal polarization effect under substitution of Mg for Co. Thus, from an applicative point of view, new thermochromic pigments with tunable transition temperatures are here proposed.

Two Co(1-x)Mg(x)MoO(4) oxide compositions (x=0 and x=0.4) were investigated as potential pressure indicators. The first order phase half-transition induced by pressure application from the β to the α form, i.e. from the high temperature/low pressure form to the high pressure/low temperature form, was studied thanks to the powder diffuse reflection (color) evolution versus the applied pressure. Three key parameters were analyzed: (i) the magnesium content, (ii) the powder grain sizes, (iii) the pressure application mode (uniaxial or isostatical). It was shown these three parameters allow tuning the transition pressure in a wide range from few bars to few kbars., (© 2011 American Chemical Society)

Two sets of Ga-doped ZnO powders were synthesized via solid-state and Pechini routes with a substitution rate varying from 0 to 4 mol %. The gallium solubility limit is strongly dependent on the synthesis history. Indeed, a low temperature annealing allows incorporating about 1.5 mol % (X-ray diffraction (XRD), inductive coupled plasma spectroscopy (ICP), optical properties) whereas under 0.1% of dopant is introduced after thermal treatment at high temperature: 1500 degrees C (from XRD and pellets conductivity). The incorporation of gallium leads to an anisotropic distortion of the zincite crystal lattice (a and c parameters increase and decrease, respectively, versus the Ga content leading to a decrease of the c/a ratio) which can be explained from the valence bond model. XRD analysis, chemical titration by ICP, and conductivity measurements (on pellets obtained at high temperature) allow determining accurately the maximum Ga content in the zincite. The optical properties (IR absorption efficiency) linked to electron carriers are directly correlated to the gallium rate introduced in ZnO oxide; nevertheless, the non linear correlation between these two parameters tends to show that the concentration of charge carriers in the system is not equal to the amount of Ga(3+) atoms inserted per ZnO volume unit. A saturation regime is observed and was here explained once again on the basis of the valence band model by the increase of inhibiting p type defects with the increase of (n-type donors) Ga(3+) concentration.

The members of the CuMo1-xWxO4 series (0alpha transition can occur between 260 and 360 K, and the alpha-->gamma transition between 175 and 275 K as a function of x. The control of the alpha/gamma transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment.

Zn1-xCuxAl2O4 (0 < or = x < 0.30) compounds have been synthesized by polyesterification using metallic salts and annealing at low temperatures as well as by conventional solid state. XRD-powder data refinements (Rietveld method) have demonstrated that both compound series crystallize in the spinel structure (Fd3m) and exhibit similar inversion rates. This low-temperature route lead to metastable phases with crystallite sizes around 40 nm whereas particle sizes are larger than 1 moicrom in the case of solid-state route. This preparative method largely described in the literature allows stabilizing reduced copper states thanks to the presence of reductive organic species, which are decomposed below T = 700 degrees C. The absorption spectra of the x = 0.15 composition exhibit strong differences depending on the synthesis route. These differences can be explained by the occurrence of Cu2+/Cu+ mixed valencies in compounds prepared by the low-temperature route; 33% of monovalent copper has been identified in the x = 0.15 composition prepared by low-temperature process, whereas the solid-state compound contains only divalent copper. Reductive properties of polyesterification reaction implying citric acid and low annealing temperature (T = 700 degrees C) are mainly responsible of the occurrence of the Cu2+/Cu+ mixed valencies. Actually, the annealing under air at T = 1000 degrees C of divided zinc-copper aluminates prepared at low temperatures (T = 700 degrees C) leads to the oxidation reaction Cu+ --> Cu2+ + e- confirmed by the evolution of magnetic measurements, ESR spectra, and optical absorption properties. Defects such as oxygen vacancies in the anionic network leading to reduction in the cations coordination number could also explain the strong evolution of optical absorption spectra especially around lambda = 700 nm where intervalencies transfer (Cu+/Cu2+) as well as intra-atomic d-d transitions (Cu2+ in a 5-fold coordination) can occur. Finally the occurrence of monovalent and divalent copper at the surface of such divided oxides, probably in tetrahedral sites, has been demonstrated by FTIR spectroscopy using the co-adsorption of CO and NO as probe molecules.