R. Gerardin

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′.

Etude du system Fe-P-O (pour FeP ⩾1) et d'une famille: les oxyphosphates de fer

Abstract In this system, for which no phases diagram was available, the investigation has been made by progressive reduction and by synthesis in sealed tubes under vacuum at 900°C. Four new oxyphosphates have been evidenced: Fe 4 (PO 4 ) 2 O, Fe 2 (PO 4 )O, Fe 9 (PO 4 )O 8 and Fe 5 (PO 4 ) 3 O, and the structures of the three first ones determined and published elsewhere recently, and the Fe 3 (PO 4 )O 3 one as well, a compound which have been reported previously, whereas for Fe 5 (PO 4 ) 3 O we give hereafter a triclinic cell. Beside these plentiful oxyphosphates, the structures of which do not derive from a common type, we emphasize the high frequence of the 5 coordination of iron (in trigonal bipyramids), the variable character of the distances and angles in the PO 4 3− ions, and the weak stability of these oxyphosphates compared to the orthophosphates as derived from the phases relations in the Fe-P-O system. Among these oxyphosphates, three are of the mixed-valence type: Fe 2 (PO 4 )O, Fe 9 (PO 4 )O 8 and Fe 5 (PO 4 ) 3 O; the d electrons are strongly localized in the first one, whereas the second shows mixed-valence sites.

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).

Reactions de CaFe2O4 avec les oxydes MO: Synthese et structure cristalline de CaCuFe2O5

Abstract The calcium ferrite CaFe 2 O 4 reacts with CoO, NiO, MgO and ZnO to give Ca 2 Fe 2 O 5 and the corresponding spinel, there is no intergrowth process like with FeO. With CuO, CaFe 2 O 4 forms two kinds of compounds: • - Ca 4 CuFe 8 O 17 , with a variable copper composition, and isotypic with Ca 4 Fe 9 O 17 • - CaCuFe 2 O 5 , monoclinic, space group C2/c with parameters a = 10,500 A , b = 6,020 A , c = 26,720 A , β = 90°; the crystal structure is characterized by the association of blocks present in Ca 4 Fe 9 O 17 and by a double layer of trigonal bipyramids which contain iron and copper.

The non-existence of iron-rich magnetites

While magnetite is known to be iron deficient at high temperature, five publications have independently claimed that iron-rich magnetites Fe3+XO4 have been observed in the range 250 to 620‡ C. The related experiments are based upon reduction of magnetite, goethite, maghemite, hematite or iron ore, and for the one of them, upon ferrous hydroxide decomposition. Another article reports an unsuccessful attempt to produce the iron-rich magnetites through hematite reduction at 535‡ C, but this temperature is considerably higher than almost all of the reduction temperatures used where iron-rich magnetite has been detected. We have therefore carried out an extensive series of experiments in order to prepare iron-rich magnetite through reductions as near as possible to the published experimental conditions. Through thermogravimetric analysis, chemical analysis, Mössbauer spectrometry and magnetic measurements, we have shown that no excess iron magnetite has been obtained. A possible explanation could be, in some cases at least, the fact that OH groups remain in the material due to preparation from solutions and annealing at moderate temperatures.

Préparations et propriétés du diarséniure de beryllium:BeAs2, et du diantimoniure de beryllium:BeSb2

These compounds are obtained through solid-state reaction between the elements in sealed tubes. They remain unaltered with atmospheric moisture. X-ray diffraction gives evidence for pseudocubic symmetry. The atomic stacking is considered to be of disordered sphalerite type. BeAs2 and BeSb2 behave as semiconductors.