O. Evrard

Sur l'existence des oxydes rhomboe driques A(III) B(II) B′(III)O4

Abstract The symmetry group of the rhombohedral structure of the mixed oxides A(III)B(II)B′(III)O4 is R3m: A is in an octahedral coordination, B and B′ occupy the same crystallographic sites and are in a pyramidal coordination with a triangular basis. The existence of this structural type with Ni2+ and Cr3+ ions is incompatible with this coordination close to tetrahedral. The geometric conditions necessary for adoption of the rhombohedral structure have been clearly defined by comparing the cation-oxygen and oxygen-oxygen distances determined on 4 monocrystals (InCuAlO4, InFe1, 75SiO, 25O4, YbFe2O4 and YbO,5EuO,5Fe2O4). The shortest cation-oxygen distance in the pyramidal site is designated by d′: it can be calculated through the relation: d ′ = 1 2 ( r B 2+ + r B ′ 3+ ) + r O 2− by using the ionic radii of ions in tetrahedral coordination, given by R.D. Shannon. The oxygen - A3+ cation distance of the octahedron, d, is close to the sum of the ionic radii of the ions in coordinence 6. Mixed oxides ABB′O4 can crystallize in the rhombohedral system if the ratio d′/d is between 0.842 and 0.888 and these two limiting values can be refined for each couple B, B′.

Caracterisation de nouveaux ferrites d'indium: In2Fe4O9 et InFeO3

Abstract In the system Fe 2 O 3 -In 2 O 3 , two new ferrites are characterized: In 2 Fe 4 O 9 at 400°C, and InFeO 3 at 700°C; the first is isotypic of Ln 2 Fe 4 O 9 with iron coordinated by four, five and six oxygen atoms, the second is isotypic of YAlO 3 with iron in oxygen trigonal based FeO 5 bipyramids only.

Stabilisation de la structure de InFeO3 par substitution de l'indium et du fer par l'aluminium ou le gallium

InFeO3 can be synthetized between 700°C–800°C, but it is unstable at higher temperature. InFeO3 crystallizes in the space group P63/m mc with the YALO3 crystalline structure. In3+ is in an octahedral coordination and Fe3+ in a triangular bipyramidal coordination. By substituting In and Fe by Al or Ga, In2/3Fe2/3Al2/3O3 and In2/3Fe2/3Ga2/3O3 have been synthetized between 1200°C–1400°C and at 1050°C respectively. The cationic distributions are studied by X-ray diffraction and Mossbauer spectroscopy.

Structure cristalline du ferrite hemicalcique CaFe4O7

The hemicalcic ferrite CaFe4O7 crystallizes in the monoclinic system, space group C2 with the parameters: a=10.409 A, b=6.005 A, c=31.640 A, β=96°30. Its crystalline structure is related to that of the hexagonal ferrites. It is constituted by an alternating stacking process along c axis of two structural blocks with the following characteristics: • - a plane of trigonal based FeO5 bipyramids surrounded by two mixed FeCa Layers • - a triple layer of iron atoms, formed by a plane of mixed tetrahedral and octahedral polyhedra surrounded by two octahedral polyhedra; this ordering is also encountered in the spinel structure (mixed and “kagome” system).

La structure cristalline de Ca4Fe9O17: Des feuillets “hexagonaux” d'octaedres FeO6 et de bipyramides FeO5 lies par des tetraedres FeO4

The new calcium ferrite Ca4Fe9O17, belonging to the CaFe2+nO4+n family (n = 14), has not the same stacking process of “FeO” blocks in “CaFeO4” blocks, as the others terms of the series. It crystallizes in the monoclinic system, space group C2 with the parameters: a = 10,441 A, b = 6,025 A, c = 11,384 a and β = 98°80. Its structure is characterized by the presence of iron atoms in oxygen octahedra and trigonal based bipyramides stacking in hexagonal layers along c. These layers are linked by iron atoms on tetrahedral sites. Calcium atoms are hexagonaly located around each tetrahedron.

Contribution a l'etude du systeme Lu2O3-Mn2O3-Fe2O3 et caracterisation de deux varietes cristallographiques de Lu2Mn2Fe2O9

Resume At 1100°C, according to the preparative method, it is possible to replace some Lu3+ and Mn3+ by Fe3+ in the hexagonal structure of LuMnO3. Two cationic distributions have been found. Firstly, we can obtain the solid solution LuMn(1−x)FexO3 (0 Furthermore, it is possible to stabilize the Lu2Mn2Fe2O9 monoclinic phase at low temperature. X-ray diffraction and 57Fe Mossbauer spectroscopy allow specification of the cationic co-ordination and distribution in these phases. Over the solid solution LuMn1 −xFexO3 the triangular magnetic structure of LuMnO3 is maintained for all x.

Structures et proprietes magnetiques des ferrites de calcium pseudo hexagonaux Ca4Fe9O17, CaFe4O7 et Ca3Fe15O25

Abstract Recent investigations of the iron-rich part of the Ca-Fe-O phase diagram at 1200°C, have allowed us to specify the crystallographic characteristics of three compounds, CaFe4O7, Ca4Fe9O17 and Ca3Fe15O25. We present here the results of magnetization and 57Fe Mossbauer spectroscopy studies made on these compounds. CaFe4O7 and Ca4Fe9O17 must be considered as antiferromagnetic, whereas Ca3Fe15O25 is ferrimagnetic. Parallel crystallographic and magnetic analyses indicate that these ferrites can be classified in the hexagonal ferrite family with the magnetoplumbite structure.

Structures cristallines des phases M2Ga2Fe2O9 (M = In,Sc) nouveaux exemples de coordinence 5 du fer

Abstract New quaternary oxides are reported : M 2 Ga 2 Fe 2 O 9 (M = In,Sc). In 2 Ga 2 Fe 2 O 9 has been studied by single-crystal X-ray diffraction analysis. Its structure is a new type with space group Pba2 and z = 8 3 : a = 19.253(3) , b = 7.2176(2), c = 3.2581(1) A , D x = 6.05 g.cm −3 , μ (AgK α ) = 9mm −1 , F(000) = 757.1, R = 0.037 for 420 independant reflexions (R w = 0.037). The indium are heptahedrally coordinated by seven oxygen atoms and the iron and gallium atoms by five oxygen atoms at the apices of a trigonal bipyramid.

Synthese et caracterisation d'une wustite tres oxydee stabilisee par des traces de calcium

By thermal decomposition of CaFe5O7 at 1125 °C in a sealed silica tube during about ten days, we obtained the oxide Fe0.832Ca0.028O1 referred to as “Fe6O7”. Like wustite, it is iron deficient, very rich in iron (III) cations (Fe2+Fe3+ = 1.97), has the rock-salt structure (a = 429.4 pm), but the x-ray diffraction pattern does not show satellite reflections around the Bragg peaks. Like wustite, “Fe6O7” is antiferromagnetic (TN = 207 K), but the room temperature Mossbauer spectrum is different. It seems that this phase is stabilized by traces of calcium and that there is no tetrahedral interstitial iron (III).

Iron affinity for the bixbyite structure type Mössbauer spectroscopy study of (ln1−xFex)2O3 and (Sc1−xFex)xO3

Abstract Solid solutions (In 1−x Fe x ) 2 O 3 and (Sc 1−x Fe x ) 2 O 3 with the bixbyite type structure are synthetized at 1500°C and studied by X-ray diffraction and 57 Fe Mossbauer spectroscopy. The maximum amount of Fe substitution varies as the inverse of the ionic radius of M 3+ in the oxide M 2 O 3 (M=Sc,In,Lu) : x=0.6 for Sc 2 O 3 , 0.33 for In 2 O 3 and almost zero for Lu 2 O 3 . Fe atoms occupy the two cristallographic sites 24d and 8b. The coordinence of these sites does not change with the iron substitution. This can explain the various limits of the iron solubility. A preliminary magnetic study of the iron richest phase (Sc 0.4 Fe 0.6 ) 2 O 3 has also been performed.