Manuel Gaudon

Investigation of the first-order phase transition in the Co(1-x)Mg(x)MoO4 solid solution and discussion of the associated thermochromic behavior.

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.

ZnAl2O4 as a potential sensor: variation of luminescence with thermal history

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.

Luminescence switch of Mn-Doped ZnAl2O4 powder with temperature

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.

Characterization of the Piezochromic Behavior of Some Members of the Cumo1-xWxO4 Series

The members of the CuMo(1- x)WxO4 series (0 < or = 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 (alpha) and brownish-red for the high pressure one (gamma). These oxides can open up a new market for friendly pressure indicators, particularly for the compositions (0.07 < or = x < or = 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 CuMo(1- x)WxO4 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.

New insights into crystallite size and cell parameters correlation for ZnO nanoparticles obtained from polyol-mediated synthesis.

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.

Structural transformation and thermochromic behavior of Co2+-doped Zn3(PO4)2·4H2O hopeites

The thermal history of Co2+-doped hopeite Zn3(PO4)2·4H2O was investigated combining thermogravimetric analyses, X-ray diffraction and UV-Visible spectroscopy. For the two dehydration steps, Zn3(PO4)2·4H2O → Zn3(PO4)2·2H2O → α + δ-Zn3(PO4)2, the temperatures increases linearly with the Co2+ doping rate (130–340 °C). The coexistence of α and δ phases formed after dehydration was explained considering a complex and unusual intergrowth phenomenon. Finally these phases transform into the high temperature stable phase, γ-Zn3(PO4)2, at a temperature also controlled via the Co2+ doping rate (350–900 °C). Both phase transformations are associated with variations of Zn2+/Co2+ local environments, from a distorted octahedral site in hopeite to a distorted tetrahedral site in Zn3(PO4)2·2H2O or α + δ-Zn3(PO4)2 and finally to distorted octahedral/trigonal bipyramid sites in γ-Zn3(PO4)2. The successive changes of the Co2+ coordination induce colour variation from pink (octahedral coordination) to blue (tetrahedral coordination), then violet (5 and 6-fold coordination). Co2+-doped hopeites can so be used as efficient over-heat temperature indicators.

CuMo0.9W0.1O4 phase transition with thermochromic, piezochromic, and thermosalient effects

AMoO4 compounds (A = Co, Mn, Fe, Ni, Cu, or Zn) exhibit a first-order phase transition associated with piezochromic and thermochromic phenomena, as demonstrated in previous studies. In this study, neutron diffraction patterns were collected for CuMo0.9W0.1O4 samples across the hysteresis cycle to accurately characterize the structural evolution (i.e., cell parameters and atomic positions) versus temperature. This study provides information regarding the phase-transition origin. In accordance with the Birch–Murnaghan model, the phase transition is due to the higher compressibility coefficient, despite the presence of the shorter bonds for the high-temperature form. The cell-volume difference of 13% between the high and low temperature forms leads to additional exotic properties: the thermosalient effect (“jumping crystals”) associated with a certain crystallite fracture (along the −101 plane) is shown.

Thermochromic phase transition on CuMo0.9W0.1O4@SiO2 core-shell particles.

The thermochromic phase transition of CuMo(1-x)W(x)O(4) oxide was delayed by the deposit of SiO(2) 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.

Eu(III)/Eu(II)-doped (Ca0.7Sr0.3)CO3 phosphors with vaterite/calcite/aragonite forms as shock/temperature detectors

A pure metastable Eu(III)-doped (Ca0.7Sr0.3)CO3 vaterite phase has been obtained by the precipitation method. After thermal treatment of this phase, the carbonate crystallizes with a calcite structure. Moreover, with the incorporation of 30 mol% of Sr that tends to stabilize the aragonite structure, both vaterite and calcite phases are able to transform easily into this high-pressure form by mechanical treatment. Hence, for this single composition, the three allotropic forms are achieved and can be compared. Photoluminescence studies indicate the stabilization of the Eu ions in the carbonates obtained, whatever the allotropic form, in the trivalent oxidation state. Significantly different luminescence properties have been found depending on the carbonate structure, which are associated with the various local environments of the doping element. This offers the possibility to employ these Eu(III)-doped samples as shock and/or temperature detectors. In addition, the use of a CaH2 reducing agent allowed reduction of the main part of the Eu(III) content into Eu(II) in the three different carbonate forms at low temperature. Photoluminescence studies were also performed in these Eu(II)-doped samples and show slightly different emission spectra for the three crystalline structures.

Discussion on the Structure Stability and the Luminescence Switch under Irradiation of a Ce‐Doped Elpasolite Compound

Ce-doped Rb2 KInF6 elpasolite has the potential for tunable luminescence due to an unusual reversible redox process between the cerium and indium cations. Coupled with a deep understanding of the luminescence properties, XRD analysis and DFT calculations are used to locate the doping elements in the host lattice. The origin explanation of the charge-transfer mechanism that causes a decrease or increase in the blue-green cerium emission in opposition to the red indium emission is discussed regarding the crystallographic structure, the connection of the metallic cations and their equilibrium valence. Still detectable after nineteen years, the optical contrast created under irradiation makes this material a good candidate as photosensor for data storage.