A. Demourgues

Supercritical fluid processing: a new route for materials synthesis

Supercritical fluids exhibit a range of unusual properties that can be exploited for new reactions which are qualitatively different from those involving classical solid state chemistry. After giving a brief introduction to these fluids we describe their use in inorganic chemistry and related fields. We then present two examples concerning different areas of solid state chemistry: (i) the formation of novel inorganic nanoparticles; (ii) the preparation of new open-structure oxy(hydroxy)fluorides, thus showing the advantages of this supercritical fluid processing that can be seen as an alternative method to regular solution chemistry or solid-gas reactions.

A novel preparation method of the SrFeO3 cubic perovskite by electrochemical means

Abstract The SrFeO 3 cubic perovskite has until now been prepared under high pressure of oxygen (340 atm) at 550°C. A novel preparation method of this phase has been investigated using electrochemical oxidation of the SrFeO 2.50 brownmillerite. The starting material SrFeO 2.53 ± 0.02 has been characterized by powder X-ray diffraction, chemical analyses and 57 Fe Mossbauer spectroscopy. The electrochemical oxidation was carried out at a potential of 400 mV for 60 hours, at room temperature using a three electrode system in air with a solution of 1N, KOH as the electrolyte. The phase obtained after this electrochemical processing has been characterized. Powder X-ray diffraction shows a cubic unit cell (a c =3.845 A ) . The chemical analysis as well as Mossbauer spectroscopy data reveal the presence of only Fe(IV), leading to the formula SrFeO 3.00 ± 0.02 . Moreover the value of the quadrupole splitting ( Δ =0) accounts for a symmetrical octahedral site for Fe(IV) and obviously corroborates the cubic structure of this phase.

Transport and magnetic properties of La2NiO4+δ (0 ≤ δ ≤ 0.25)

Materials with La{sub 2}NiO{sub 4+{delta}} composition, {delta} ranging between 0 and 0.25, have been prepared either by solid state reaction or electrochemical oxidation; their electrical conductivity, thermoelectric power, and magnetic susceptibility behaviors are reported. The compounds remain semiconducting in the whole range of composition and, at low temperature, the resistivity increases with {delta}. The sign of the Seebeck coefficient changes for {delta} {approx} 0.2 and the thermal variation shows complex behavior. La{sub 2}NiO{sub 4.03} exhibits a thermally activated hopping and an antiferromagnetic ordering. When {delta} increases, variable range hopping or nearest neighbor hopping, involving at least two types of carriers, can account for the transport properties. The magnetic behavior of materials with {delta} {le} 0.18 reveals either 3D or 2D magnetic correlations whereas the susceptibility of La{sub 2}NiO{sub 4.25} (La{sub 8}Ni{sub 4}O{sub 17}) follows a Curie-Weiss law below 170 K; a change in the electronic configuration of the nickel ions arises beyond this temperature.

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.

Chemistry and key structural features of oxyhydroxy-fluorides: relationships with the acidic character, thermal stability and surface area

Abstract The stability of Al, Cr and Fe hydroxy-fluorides MF 3− x (OH) x which adopt the hexagonal-tungsten–bronze (HTB)-type structure has been discussed by considering the lability of water coordinated to metals from a kinetic point of view. Thus, in the case of Al or Fe compounds, the easy departure of water contributes to the stabilization of fluoride ions as well as isolated hydroxyl groups around the metal, leading to the formation of the HTB structure. The stabilization of the HTB structure with respect to another structural type, the pyrochlore, with a lower density, is governed by this kinetic feature as well as the ability of fluorinated salts used as precursors to attract hydroxyls. Al(III) and Fe(III) represent the strongest acidic cations and the associated HTB-type structure containing isolated OH groups can easily be stabilized. In the case of Cr, a mixture of pyrochlore and HTB-type structure is generally obtained. We have succeeded in preparing, using supercritical medium, new (Fe, Cr) oxyhydroxy-fluorides which exhibit edge-shared octahedra and large 1D tunnels. These compounds can be considered as potential candidates for acid catalysts.

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.

Crystal structure of the thallium strontium cobaltite TlSr2CoO5 and its relationship to the electronic properties

The complex cobaltite TlSr2CoO5 undergoes a first-order metal–insulator phase transition at room temperature. The structures of the high temperature (HT) and low temperature (LT) phases have been refined on powder samples using electron diffraction, X-ray diffraction and EXAFS. The HT-phase is isostructural with the so-called 1201-type cuprate. The unit cell is tetragonal (space group P4/mmm) and cobalt occupies a highly elongated octahedron. At low temperature, a commensurate modulation of the oxygen positions sets in such that two sites are available for cobalt, in the ratio 1 to 2. The sizes and the distortions of these two sites differ in such a way that a different electronic configuration of cobalt is stabilized in each. This effect is described as a spin state disproportionation (SSD). Long range ordering of these two spin states (SSO) is compared to the charge ordering (CO) effect and SSO seems to play a role as important for cobaltites(III) as CO does for nickelates or manganites.

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.

Anionic Ordering and Thermal Properties of FeF3·3H2O

Iron fluoride trihydrate can be used to prepare iron hydroxyfluoride with the hexagonal-tungsten-bronze (HTB) type structure, a potential cathode material for batteries. To understand this phase transformation, a structural description of β-FeF3·3H2O is first performed by means of DFT calculations and Mössbauer spectroscopy. The structure of this compound consists of infinite chains of [FeF6]n and [FeF2(H2O)4]n. The decomposition of FeF3·3H2O induces a collapse and condensation of these chains, which lead to the stabilization, under specific conditions, of a hydroxyfluoride network FeF3-x(OH)x with the HTB structure. The release of H2O and HF was monitored by thermal analysis and physical characterizations during the decomposition of FeF3·3H2O. An average distribution of FeF4(OH)2 distorted octahedra in HTB-FeF3-x(OH)x was obtained subsequent to the thermal hydrolysis/olation of equatorial anionic positions involving F(-) and H2O. This study provides a clear understanding of the structure and thermal properties of FeF3·3H2O, a material that can potentially bridge the recycling of pickling sludge from the steel industry by preparing battery electrodes.