Claude Fouassier

Structural classification and properties of the layered oxides

Layer oxides with formula AxMO2 where M stands for a transition element with two oxidation states or for a mixture of tetravalent and trivalent (or eventually divalent) elements are obtained for 0.5 ≤ x ≤ 1. The lattice is built up by sheets of edge sharing MO6 octahedra between which the alkali ions are inserted with trigonal prismatic or octahedral environment. Similar structures can be found among A2MO3 oxides, the alkali ions lying between (A13M23)O2 sheets. The influence of the pressure on the stability of the various packings is discussed. Layer structures are also obtained for the compositions Li8MO6, Li7L□O6 and Li6In2□O6. Structures of these pseudo-2D materials are characterized by a packing of octahedral and tetrahedral sheets where the alkali ions and the vacancies are distributed. Transport properties of these materials have been studied.

Electrochemical intercalation of sodium in naxcoo2 bronzes

The chemical study of the NaxCoO2 system (0.5⩽x⩽1) has shown previously the existence of bronze-type phases with layer structure. In each material the lattice is built up by sheets of edge-sharing CoO6 octahedra allowing Na+ ions to be intercalated with trigonal prismatic surrounding (AABBCC or AABB oxygen packing) for small values of x and octahedral environment (ABCABC oxygen packing) for larger ones. As these materials are mixed conductors, they have been used as cathodes in Na batteries at room temperature. Discharge potentials (2.0<V<3.5V) have been measured as a function of the composition. The electrochemical intercalation of sodium in all these materials is reversible within their existence range. During the electrochemical intercalation a reversible charge is observed between phases having AABBCC and ABCABC oxygen packing. This can be explained by the relatively small displacement of the sheets. In contrast, for the material with AABB oxygen packing no transition occurs since a packing change requires breaking of cobalt-oxygen bonds, which is apparently impossible at room temperature.

Sur quelques nouvelles phases de formule NaxMnO2 (x ⩽ 1)

Abstract Several new ternary oxides have been isolated in the manganese-oxygen-sodium system for Na Mn ⩽ 1 : Na0.20MnO2, Na0.40MnO2, Na0.44MnO2, Na0.70MnO2+y (0 ⩽ y ⩽ 0.25) and NaMnO2, both with two allotropic varieties. All structures are characterized by edge sharing (MnO6) octahedra, forming double or triple chains for small sodium content and bidimensional layers when the Na Mn ratio becomes close to 1. Electrical and magnetic behaviour of the phases has been determined.

Sur de nouveaux bronzes oxygénés de formule NaχCoO2 (χ1). Le système cobalt-oxygène-sodium

An investigation of the sodium-cobalt-oxygen system allows the isolation of four new bronze type phases with the formula Naχ.CoO2 (χ 1). The structure consists of sheets of octahedra (CoO2)n between which are inserted the sodium ions. Their structures differ from one another by the stacking sequence of the oxygen layers, and by the distortions introduced as a result of the ordering of vacancies among the sodium layers. For phases containing high sodium content, the coordination of the sodium is octahedral; for phases containing smaller amounts of sodium, the coordination becomes trigonal-prismatic.

Luminescent properties of Eu3+-activated lithium rare earth borates and oxyborates

Abstract The luminescence characteristics of Eu 3+ in the borates Li 6 Y(BO 3 ) 3 and Li 3 La 2 (BO 3 ) 3 and the recently discovered oxyborates LiLa 2 O 2 BO 3 , LiLn 6 O 5 (BO 3 ) 3 (Ln=Y,Gd) and Li 2 Lu 5 O 4 (BO 3 ) 3 have been investigated and compared to those in the borates LnBO 3 (Ln=La,Y) and the oxyborates “Ln 3 BO 6 ” (Ln=La,Gd). The probability of electric dipole f–f transitions is generally markedly higher in oxyborates than in borates. This is ascribed to the highly anisotropic environment of rare earth ions and the low position of the charge transfer states. Consequently, the Eu 3+ -activated oxyborates are characterized by a deeper red emission and faster decay.

Evolution structurale et proprietes physiques des phases AXMO2 (A = Na, K; M = Cr, Mn, Co) (x ⩽ 1)

Abstract The structures of the A X MO 2 (x ⩽ 1) phases may be classified in three main groups. The layer structures are the most common, their anionic packing varies with the nature of the alkali element and its insertion rate. The tunnel structures are observed only for manganese and small values of x. Cristobalite derived structures characterize the KMO 2 oxides for small size M 3+ cations. Whereas the A x CrO 2 and A x MnO 2 phases are semi-conductors, the A x CoO 2 oxides have a metallic behaviour, the small Co-Co distances allowing t 2g orbital overlaping through the common edges of adjacent octahedra. The influence of the structure on magnetic properties is discussed.

Crystal growth and optical properties of new neutron detectors Ce/sup 3+/:Li/sub 6/R(BO/sub 3/)/sub 3/ (R=Gd,Y)

An efficient new scintillator that contains lithium, gadolinium, and boron, all three of which possess large neutron capture cross-section isotopes for highly exothermic reactions, has been recently developed. The homologous yttrium material was also investigated. These compounds of composition Li/sub 6/R(BO/sub 3/)/sub 3/ (R=Gd, Y) can be activated by Ce/sup 3+/. The synthesis of powders was carried out by high-temperature solid-state reaction from the starting materials LiOH, H/sub 2/O, H/sub 3/BO/sub 3/, Gd/sub 2/O/sub 3/, or Y/sub 2/O/sub 3/, and Ce(NO/sub 3/)/sub 3/, 6H/sub 2/O of purity /spl ges/99.99%, under a flow of argon/H/sub 2/ (5%). Under ultraviolet excitation, they show a broadband emission peaking at 390 nm. The gadolinium emission lies near the maximum of a 4f-5d Ce/sup 3+/ absorption band, so efficient Gd/sup 3+/-Ce/sup 3+/ transfer occurs. Ce/sup 3+/:Li/sub 6/R(BO/sub 3/)/sub 3/ (R=Gd, Y) crystals were grown by the Czochralski method in a resistance heating furnace using a conical vitreous carbon crucible of 150 cm/sup 3/ under deoxygenated pure argon. Monocrystalline boules as large as 3 cm diameter and 6 cm length have been obtained. The interest of these new materials is the high scintillation efficiency-as much as six times that of Li-glass scintillators for the Gd material. Moreover, these scintillators offer the ability to tailor their response to the neutron spectrum by varying the isotopic composition of the key constituents [lithium, gadolinium (yttrium), boron].

Cathodoluminescent Properties and Energy Transfer in Red Calcium Sulfide Phosphors (CaS:Eu, Mn)

Red and orange calcium sulfide phosphors (CaS:Eu and CaS:Mn) were prepared by the flux method. Eu2+ and Mn2+ show strong concentration quenching at 0.1 and 0.8% respectively. Use of the potassium polysulfide flux leads to efficient phosphors; their efficiency reaches 60 and 100% of Y2O2S:Eu, respectively. Calcium sulfide codoped with Eu and Mn shows a strong increase of Eu2+ in addition to Mn2+ emission. The colour obtained varies between red and orange depending on the concentration rates of Mn and Eu. A luminous efficiency of about 86% was reached for CaS:Eu(0.08%), Mn(0.3%). The decay characteristics of Mn2+ in CaS:Mn, Eu show that the fast part depends on the Eu2+ content. On the other hand, the Eu2+ decay became slower in the presence of Mn2+. These observations support the resonant mechanism of the energy transfer from Mn2+ to Eu2+.

Synthesis and structural study of the new rare earth magnesium borates LnMgB5O10 (Ln = La, …, Er)

Abstract To obtain rare earth luminescent materials with weak concentration quenching, the B2O3-rich portion of the ternary diagram Ln2O3MgOB2O3 (Ln = rare earth) has been investigated. A ternary phase of composition LnMgB5O10 has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group P2 1 c . The structure has been determined on a LaMgB5O10 crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B5O105−)n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.

Les bronzes de cobalt KxCoO2 (x < 1). L'oxyde KCoO2

Abstract The reaction between potassium and cobalt oxides leads to two bronze-type phases with formula K x CoO 2 ( x = 0.50 and x = 0.67), and to a KCoO 2 oxide with two allotropic forms. The lattice of K x CoO 2 is built up by sheets of (CoO 2 ) n octahedra between which potassium ions are inserted with a prismatic coordination; they have a metallic behavior. The structures of both KCoO 2 varieties are related to the cristobalite type. The oxidation state of cobalt is discussed on the basis of structural, optical, and magnetical data.